Coastal waterways are some of the most productive natural systems on the planet. For example, estuaries and bays provide food and shelter that support important fisheries. However, many of the features that make estuaries so productive also make them prone to acidification. The organisms that live in estuaries must have mechanisms for dealing with acidification, and understanding how estuarine creatures adapt to low pH may help us predict ecosystem responses to ocean acidification.
Recently my colleagues and I published a paper looking at the impacts of estuarine acidification on oysters and the invertebrates that live in oyster reefs, using a case study in New South Wales, Australia. Many coastal areas in New South Wales are characterized by acid sulfate soils, which form sulfuric acid. We transplanted oysters in water of varying pH, and recorded how the oysters fared, as well as how the invertebrate community in the oyster reef changed.
Overall, oyster-associated invertebrate communities that were exposed to acidiﬁcation were signiﬁcantly different from communities with low risk of exposure. The mussel Xenostrobus securis and the snail Bembicium auratum were signiﬁcantly less abundant in oysters that were exposed to acidiﬁcation, as compared with communities from areas with low risk of exposure. Both of these species are calcifiers, meaning they build shells out of calcium carbonate, which dissolves in low pH. The snail can migrate to avoid acidified water, which is a likely reason we observed fewer snails in acidified oyster reef. However, mussels do not migrate, and may instead experience higher mortality rates in low pH water. The response of these species to estuarine acidification suggests that calcifiers may be great indicator species that can help predict the impact of acidification on a community of organisms, even in an ecosystem that has been exposed to acidification for decades, such as the estuaries in New South Wales.
An additional finding in this study involved the relationship between oyster mortality and invertebrate community composition. The mussel Xenostrobus securis and the limpet Patelloida mimula were negative correlated with
oyster mortality, suggesting that these communities are closely tied to oyster survival. Both of these species are immobile- therefore, oyster mortality may impact immobile organisms first, reducing their abundance.
This study provides possible indicator species that could be used to look at acidiﬁcation in the Hastings Estuary and other coastal systems in NSW. Identifying indicator species that are sensitive to changes in pH may help ecosystem managers to relate changes in the environment to taxonomic diversity, and allow for a scientiﬁcally sound and logistically feasible means to see the impacts of management decisions.
Finally, the results of this study suggest that the fate of this ecosystem is closely tied to that of its foundation species, the Sydney rock oyster. Because oysters and many other habitat-forming species such as seagrasses, corals, and kelp are at risk due to anthropogenic stressors, the degree to which habitat-forming species and their associated communities respond similarly or differently to the effects of environmental stressors is of concern. Resistance to acidiﬁcation in oyster-associated species in coastal NSW may depend on the ability of oysters to persist in the face of environmental ﬂuctuations or degradation.
Increased human population near the coast and associated impacts on these environments are leading to worldwide loss of coastal habitat. These human-derived stressors have resulted in substantial loss of coastal habitats, including seagrass, salt marsh, mangrove, coral, and tropical forest. Losses occur due to land reclamation, development, excess sediment or nutrients finding their way into the water supply, overfishing, damage by boats and fishing gear, logging, competition from invasive species, some aquaculture, and climate change, including sea level rise and warming that makes some regions uninhabitable for certain habitat-forming species like coral. These habitats provide inherent value to humans, which can be quantified and is normally referred to as ecosystem services. Seagrasses, salt marsh, mangrove, and coral reefs all have high value in terms of ecosystem services- that is, they serve an important function for humanity.
What do these services look like? One major service provided by coastal habitats includes protection from storms. Damages from storms like hurricane Katrina are exacerbated by the loss of salt marsh, which slow approaching waves and help mitigate flooding. Another valuable service complex habitats provide is protection of sensitive early life stages of a variety of commercially and recreationally important fish and shellfish species. These habitats act as a nursery for many species because the structural complexity they provide protects young fish from predators. Some habitats, such as coral, function as the main driver of coastal tourism in many regions. This ecosystem service brings revenue to coastal communities. As corals and other coastal habitats are lost to stressors like climate change, so too are many other species that depend on them for structure. Thus habitat loss is generally also associated with loss in species diversity.
In fact, in the 2016 Living Planet Report predicts that the top threats for global biodiversity of vertebrates are habitat and overexploitation. This study pulls together data from more than 3,000 short-term and long-term monitoring project which together track the status of 14,152 populations of some 3,700 vertebrate species. Other major threats to diversity include pollution, invasive species, disease, and climate change.
Overall it appears the cumulative effect of the stressors I’ve mentioned to this point has been a decline in species globally. There is a clear trend of declining diversity that appears to be accelerating. These are species that, while still present in the background, no longer contribute to their community the same way they once did.
The consequence of this is a loss of function, impairing the ocean’s ability to provide food, maintain water quality, and recover from disturbance. Loss in species diversity means decreased fisheries production, deceases in nursery habitat, and loss of filtration in coastal systems. The consequences of these lost ecosystem services include increases in events that harm people and their livelihoods, such as beach closures, harmful algae blooms, fish kills, fishery closures, low oxygen, floods, and invasive species. Overall, systems with high species diversity are more stable and provide more ecosystem services.
The Chesapeake Bay is no stranger to habitat loss. The dominant seagrass in Chesapeake Bay, eelgrass Zostera marina, plays an important role as nursery habitat for a variety of economically and ecologically important species in the Bay, including the blue crab. The abundance of seagrass has fluctuated over the last 100 years due to a disease outbreak in the 1930s, subsequent recovery until the 1960s, and dramatic declines post 1972, when tropical storm Agnes wiped out half of the population of eelgrass in the Bay. This was largely due to fresh water influx from heavy precipitation, and increased sediment loading from flooding, which buried beds. Eelgrass in Chesapeake Bay has never recovered from Agnes, and its range in the Bay has been drastically reduced. More recently, since 1991, eelgrass has declined 29%. This is largely due to poor water quality, from nutrient pollution and coastal development, which decreases water clarity and makes it hard for seagrass to photosynthesize, resulting in a 50% loss of deep water beds. The situation in shallow beds is exacerbated by rising temperatures, which is a source of stress for this species, which is at the southern end of its range in the Bay.
Oyster reef habitat has also declined in the Chesapeake Bay drastically, with a 50-fold decrease since 1900. The decline began with the advent of more efficient fishing technology, indicating the decrease in oysters is due to overfishing and habitat loss, which go hand in hand for this species, since oysters live on other oysters. With the loss of oysters, a major source of shell was also lost, as oyster shell is a limited and non-renewable resource, and shells dissolve in seawater given enough time. This shell would have provided an important structure for attachment in a system otherwise characterized by soft sediments. So between seagrass loss and oyster reef decline, the Bay is experiencing substantial reduction in complex habitat.
One group of species that may be impacted by habitat loss in Chesapeake Bay is bivalves. Bivalves serve many important roles in coastal systems like Chesapeake Bay. They are key prey resources for a variety of economically and ecologically important species, like blue crabs. Their capacity to filter feed contributes to the water quality of the Bay, as demonstrated in this photo of two tanks, one with oysters, which has clear water, and one without, which has murky water. Through filter feeding, and serving as a food resource for finfish, bivalves connect the food webs and nutrient cycles at the sea floor with those in the water column. And of course, bivalves support profitable fisheries in the Bay and in other coastal regions. In Chesapeake Bay, oysters and hard clams are the focus of aquaculture, and wild fisheries still exist for soft-shell clam and razor clams, which are mainly harvested as bait.
Bivalves in Chesapeake Bay are highly diverse. Not all bivalves serve the same role. To illustrate this, perhaps it is best to look at two common species, soft-shell clam Mya arenaria, and Gemma gemma. Mya are several cm in shell height, and can be biomass dominants where they are locally abundant, while Gemmagemma max out at only a few mm, and are often hard to distinguish from grains of sand.
How do we calculate the diversity of roles bivalves play in the Chesapeake Bay? We can caluculate something called functional diversity. Functional diversity goes further than quantifying number of species, and instead attempts to capture the roles that are filled by various species groups. Functional diversity operates on the premise that not all species are equally distinct. For instance, take these two photos.
Both communities have just four rocky intertidal species. However, it may seem as though the community on the right is more diverse, because the animals represented are more different from each other than the animals on the left. In fact, the community on the left has three species of barnacle. While different barnacle species can look very different, they are likely all serving similar roles. The community on the right has species from different phyla, and even different kingdoms, that likely serve very different roles.Functional diversity metrics are based on functional traits: that is any measurable aspect of an organisms that reflects what it actually does, and how it interacts with its surroundings.
Bivalves interact with their environments in a variety of different ways. Certain predators may or may not be able to eat them, depending on their defenses. Furthermore, bivalves themselves have different feeding modes. Thus, feeding mode is one of the traits that define how bivalves interact with their environment. Bivalve living position also serves as a functional trait. Bivalves that are on the sediment surface, such as the mussel, are easily encountered by predators, while deep-burrowing bivalves remain undetected. These three traits, based on a bivalve’s role in the food web, describe a bivalve’s function much better than taxonomic definition of species.
Once functional traits are identified, there are a variety of ways to calculate functional diversity. For two continuous traits, such as shell thickness and burial depth, you can imagine mapping out each bivalve on those two axes, and calculating the distance between each. Some indices use this graph to calculate things like the maximum area between all species as a metric of functional diversity. Others will take the distances between species and create a matrix of pairwise distances like the one here, and calculate functional diversity as the average or sum of distances. Others still will create a phenogram showing the relationships between species according the traits. Functional diversity is then calculated as a function of number of individuals in each of these categories, A B C and D, using common diversity indices like the Simpson’s index. Functional richness can also be calculated as the number of functional groups in a sample or area.
In a recent study published by myself and colleagues, we calculated functional diversity of bivalves in the Chesapeake Bay using the phenogram approach. We sorted all bivalve species into one of four groups, based on presence of an armored shell, feeding mode, and living position. These groups were deposit feeders (DF, meaning bivalves that extract nutrients from sediment), deep-burrowing and suspension (or filter) feeding bivalves (DBSF), thin-shelled and surface dwelling bivalves (TSSD), and armored bivalves (ARM). We then calculated diversity using these functional groupings, rather than individual species.
We used this measure of functional diversity of explore how habitat is related to function in bivalves. We noticed that each unique habitat in the Chesapeake Bay supported a different bivalve functional group. Deposit feeding bivalves were associated with mud, possibly because this was a good food source for them. Armored bivalves were associated with oyster shell, likely because the hard shell material provides a surface for attachment for mussels, a common armored species in Chesapeake Bay. Finally, thin-shelled and surface-dwelling bivalves were strongly associated with seagrass habitats. It is likely the roots and blades of seagrass provide vulnerable species with needed protection from predators. It appears that all habitats play a roll in maximizing functional diversity in Chesapeake Bay. This means habitat loss may lead to the loss of entire functional groups of bivalves in the Bay, with consequences for ecosystem function.
It’s not all bad news- recently seagrass in particular appears to be recovering in the coastal bays of Virginia and in some locations in Chesapeake Bay. This may bode well for ecosystem function in the Chesapeake Bay. However, out understanding of habitat and ecosystem function means persistent action to maintain healthy growth of seagrass, and maintenance of all habitats in the Bay, is necessary to maintain the natural resources for future generations.
Duarte, C.M., Dennison, W.C., Orth, R.J.W., and Carruthers, T.J.B. 2008. The charisma of coastal ecosystems: Addressing the imbalance. Estuaries and Coasts 31:233-238.
Glaspie, C.N., and Seitz, R.D. 2017. Role of habitat and predators in maintaining functional diversity of estuarine bivalves. Marine Ecology Progress Series 570:113-125. http://dx.doi.org/10.3354/meps12103 [download]
Lefcheck, J.S., Wilcox, D.J., Murphy, R.R., Marion, S.R., and Orth, R.J. 2017. Multiple stressors threaten the imperiled coastal foundation species eelgrass (Zostera marina) in Chesapeake Bay, USA. Global Change Biology. 23(9):3474-3483.
Orth, R.J., Denison, W.C., Lefcheck, J.S., Gurbisz, C., Hannam, M., Keisman, J., Landry, J.B., Moore, K.A., Murphy, R.R., Patrick, C.J., Testa, J., Weller, D.E., and Wilcox, D.J. 2017. Submersed aquatic vegetation in Chesapeake Bay: Sentinel species in a changing world. BioScience 67(8):698-712.
Petchey, O.L. and Gaston, K.L. 2006. Functional diversity: Back to basics and looking forward. Ecology Letters 9:741-758.
Worm, B., Barbier, E.B., Beaumont, N., Duffy, J.E., Folke, C., Halpern, B.S., Jackson, J.B.C., Lotze, H.K., Micheli, F., Palumbi, S.R., Sala, E., Selkoe, K.A., Stchowicz, J.J., and Watson, R. 2006. Impacts of biodiversity loss on ocean ecosystem services. Science 314:787-790.
WWF. 2016. Living Planet Report 2016. Risk and resilience in a new era.
WWF International, Gland, Switzerland.
Have you ever lifted a shell to your ear to hear the ocean? While it is nice to think that each shell is bringing memories of the ocean with it, what you are actually hearing is ambient sound from your environment bouncing off the surfaces of the inside of the shell. The noises are mixed together and amplified, and the result is a sound that reminds us of a wave.
While the ocean-in-a-shell is just a myth, the truth about sound in the ocean is far more amazing. Sound waves are pressure waves, and as the density of the substance they are moving through increases, the speed of the sound increases. Since water is denser than air, sound can move much faster in water than in air. That means sound is an excellent way to communicate over long distances in the ocean, and many animals have taken advantage of sound as a means to send messages.
Whales and dolphins are some of the first animals we think of when it comes to communication in the ocean. Noises made by large whales are the loudest produced by any living animal. In fact, certain low-frequency whale calls can be transmitted across an entire ocean basin. The ability to produce noise that can travel so far is very helpful in the ocean, where whales must find each other despite being separated by large distances.
Large marine mammals are not the only ocean animals that use sound. Environments with a lot of activity, such as oyster reefs, are loud. Snapping shrimp and a variety of sounds produced by feeding animals can increase sound by as much as 30 decibels over an oyster reef. That is about the same amount of sound as an average human residence (ex. the intensity of “inside voices”). These reef noises may provide a cue for drifting oyster larvae that this would be a good place to settle (see Lillis et al. 2014). Other animals use cues from their environments to avoid areas that are dangerous. Crabs can detect sound and will stop feeding when researchers play noises generated by large predators (see Hughes et al. 2014). Scientists are just starting to understand all of the ways that animals use sound to transmit and receive information.
Humans also use ocean sound to transmit and collect information. Acoustic tags are used to track movements of endangered species, such as the great white shark. These tags produce pinging noises that are picked up by receivers as the shark swims past. Receivers are often anchored at the entrances to bays and across the shallows of the ocean, but they may also be anchored or free-drifting in the open ocean. When a ping is picked up, the noise lets researchers know a shark is nearby, and specific characteristics of the noise let them know which shark it is. Through the movement of individuals we can start to understand migration patterns, behavior, and even which groups of sharks may be related to each other, based on where they go to breed. This is important to understand so we can protect the areas of the world that are most commonly used by these threatened animals, so they are still around for future generations.
In addition to understanding behavior, acoustic tags can help scientists and ocean resource managers learn if a marine protected area actually protects the animals it is designed to protect. Marine protected areas are areas where fishing and other human activities that remove or destroy marine life are prevented. These areas often support large numbers of fish that can then spill over into areas where fishing is allowed and support the livelihoods of fishermen in a sustainable manner. These areas are only effective if they protect fish long enough for them to breed and grow the population. We can track specific fish to see if they are using the areas that are protected from fishing, and how long they stay there (see Garcia et al. 2014). Depending on what we find out about fish movements, we can change the shape and size of the protected area to make it more effective.
Larger sound receivers, called hydrophones, can be used to pick up extremely loud noises, such as earthquakes. A sea-floor earthquake in 2004 caused most of the continental shelf off Sumatra to tumble into the ocean depths and resulted in a series of tsunamis that devastated coastal communities in the Indian Ocean. The sound of the earthquake (see below) was picked up by many hydrophones in the Indian Ocean, even thousands of kilometers away. Sounds from category 4 earthquakes are regularly heard across an entire ocean basin!
At certain depths sounds can travel across an entire ocean. The aptly named SOFAR channel is a depth at which sound waves, once they reach it, tend to stay and travel across entire ocean basins without losing much intensity. We put most of our hydrophones in this region so we can pick up the maximum number of sounds generated in the ocean.
Hydrophones are collecting sound signals all of the time, and sometimes they pick up very strange things. These noises are named and researchers try to determine what caused them. You can see some of the explained and unexplained noises here. As much as we would like them to be caused by some huge sea creature, often they are the result of glacial activity. For instance, take “The Bloop”. This noise was recorded in 1997 and was unexplained until very recently. People now believe it is the sound of a glacier breaking in half, because we have recordings of comparable, explainable glacial noise from the Southern Ocean. Listen to “The Bloop” here– it is not hard to understand how many thought that this was the noise of some sea creature.
This post is only the tip of the iceberg (pun intended) of all of the ways sound is used in the ocean. However, even this handful of examples is enough to develop an understanding of how important sound is in the marine realm. The idea that animals other than whales and dolphins can use sound to transmit information is fairly new, and we call it “soundscape ecology”. Soundscape ecology is bound to provide some unique insights into how animals collect information in an environment as vast and unpredictable as the ocean.
For more information:
Garcia, J., Y. Rousseau, H. Legrand, G. Saragoni, and P. Lenfant. 2014. Movement patterns of fish in a Martinique MPA: implications for marine reserve design. Marine Ecology Pogress Series 513:171-185.
Hughes, A.R., D.A. Mann, and D.L. Kimbro. 2014. Predatory fish sounds can alter crab foraging behavior and influence bivalve abundance. Proceedings of the Royal Society B 282(1802), DOI: 10.1098/rspb.2014.0715
Lillis, A., D.B. Eggleston, and D.R. Bohnenstiehl. 2014. Soundscape variation from a larval perspective: the case for habitat-associated sound as a settlement cue for weakly swimming estuarine larvae. Marine Ecology Progress Series 509:57-70.
Today a paper on acidification in blue crabs and soft-shell clams, written by myself, my intern, and my PhD advisor, was published, so I thought I’d celebrate by sharing the lab setup we used.
We acidified seawater by bubbling CO2 into tanks. This mimics the process of ocean acidification, which is caused by excess CO2 in the atmosphere (see my blog post about ocean acidification).
We added only enough CO2 to maintain the pH of the water at 7.2, which is about 4 times more acidic than today’s pH in the York River (a tributary of Chesapeake Bay). pH was controlled using a pH meter paired with a solenoid valve. This valve only opened to let CO2 in the tank when pH increased above 7.2.
Juvenile soft-shell clams were either acidified or held in non-acidified tanks (as a control) for 30 days. After this period of time, acidified clams had lighter shells than clams held under the control. This is likely because it is harder to build shell in acidified water, which tends to dissolved calcified matter like shell material.
After 30 days, clams were placed in sand and we tested their ability to avoid detection by predators by moving a probe slowly towards the clam, and seeing when the clam would exhibit “hiding” behavior. The behavior we were looking for was retraction of the siphon, which is the fleshy opening at the top of the clam that draws in water for feeding and respiration. Acidified clams allowed the probe (simulating the approach of a predator) to get closer before hiding than non-acidified clams. This may indicate that soft-shell clams will be less successful avoiding predation when they are exposed to acidification.
Finally, clams were exposed to real predation by blue crabs. Crabs were held in acidified or control tanks for 48 h before feeding trials to acclimate them to reduced pH conditions. Then they were allowed to feed on 4 soft-shell clams (which were allowed to burrow in sand) for 48 hours. After the trial was over, we searched for any remaining clams and calculated clam mortality.
In normal conditions, if a crab detects there is food available, it is typical for that crab to continue searching until all clams are found. Alternatively, another example of normal behavior is for crabs to fail to detect any prey, forego foraging entirely, and laze about for 48 hours, leaving all clams alive. What is not normal is for a crab to consume a portion of clams, and then stop searching. This abnormal behavior never happened for non-acidified crabs, but over half of the acidified crabs ate only a portion of the prey. Furthermore, crabs found at least one clam in all of the acidified trials, possibly because clams are not that good at hiding when they are in acidified water.
We also collected video of crabs during the feeding trials to identify behaviors that could ave led to the observed differences in feeding and foraging behavior. From this video, we calculated: (1) search time (h), which is the total cumulative time spent exhibiting foraging behavior, such as probing the sediment with legs or claws or lifting
items to mouth parts; (2) encounter rate (hr−1), which is the number
of encounters (picking up and consuming a bivalve) divided by the
search time; and (3) handling time (h), which is the total cumulative time spent manipulating and/or eating a bivalve, divided by the number of encounters.
Compared to non-acidified crabs, crabs in acidified trials had higher encounter
rates, which also supports the conclusion that acidified clams are bad at hiding. However, acidified crabs also had lower search time, which means they spent less time foraging, even though they were able to detect prey.
It is unclear why this may be happening to acidified crabs, but one thing is clear: acidification has interesting and unexpected consequences for interacting species. An understanding of the indirect impacts
of acidification mediated by predator-prey interactions is necessary to
make viable predictions and take conservation actions that may preserve these species.
Glaspie, C.N., Longmire, K., and Seitz, R.D. 2017. Acidification alters predator-prey interactions of clue crab Callinectes sapidus and soft-shell clam Mya arenaria. Journal of Experimental Marine Biology and Ecology 489:58-65. http://dx.doi.org/10.1016/j.jembe.2016.11.010 [download]
I am now finishing up my research projects in Australia, and since I have collected so much video while I’ve been working on my projects, I made a short video to explain some of what I have been doing at Macquarie University in Sydney, Australia.
My goal is to use oysters that have grown up in areas acidified by acid sulphate soils (a natural process that occurs in some parts of Australia) to understand how ocean acidification may impact an oyster’s ability to protect itself from crabs. Ocean acidification is expected to dissolve shell material, in much the same way chalk dissolves in vinegar, or teeth dissolve in soda. However it will be a slow process of acidification, so animals such as oysters may have time to get used to the extra acid in the system. In Australia, the oysters that grow up in areas with acid sulphate soils have had generations to adapt or acclimate to changes, so this is a great place to start. Will oysters that grew up in acidic conditions be eaten faster by predatory mud crabs? Or have they figured out some way to compensate? How might oysters respond to gradual changes in acidity, as is expected to happen world wide with climate change and ocean acidification? Hopefully after I analyze all of my data, I will have some answers.
The Great Barrier Reef is arguably the largest living organism in the world. Arguably, because it is actually made up of billions of tiny organisms, that are themselves part animal, part plant, and part mineral. Coral polyps, the base unit of a coral reef, are colonial animals that look like teeny tiny anemones. Inside their skin they have algae that help them by making some extra food from sunlight. These algae are called xoozanthellae. The coral also secret a hard calcium carbonate skeleton, which (for hard corals at least) forms the base of the colony and helps the coral grow. Over thousands of year (8,000 to be exact) these tiny creatures have built a structure that can be seen from space. I have dreamed of seeing the reef ever since I was a child, and am so excited I got to dive on the reef as part of the first few weeks of my 10-week stay in Australia.
After all of the time I have spent sitting in marine biology classes, I am well aware of all of the problems facing the Great Barrier Reef, including disease, pollution, and climate change, which causes the corals to lose their friendly algae, causing the corals to turn white, or “bleach”. However, after talking to some scientists at the Australian Institute of Marine Science this week, I have learned about another threat to coral reefs- starfish. A particular species of starfish, the crown of thorns starfish, is a very efficient predator of coral. In addition, it has a rather nasty habit of popping up every couple of decades in epidemic proportions. Nobody quite knows why, but it seems every 10 or so years the starfish show signs of increasing in numbers, and then are found everywhere on the reef, consuming coral and sending the Australian tourism business into episodes of head-hanging and hand-wringing anxiety.
As I mentioned, crown of thorns starfish are predators of coral. In fact, they are extremely effective and efficient predators of coral, perhaps even the perfect predator. Adult crown of thorns starfish start producing eggs when coral are getting ready to reproduce. Just as the corals are storing away extra nutrition for spawning, crown of thorns starfish come through and munch away at the extra-fat coral polyps, siphoning the food directly into their eggs. Then they release their eggs at the perfect time so that when the eggs hatch, the coral is spawning, releasing eggs and sperm into the water column that will serve as a buffet for the young crown of thorns starfish, which spend the first few days of their lives as plankton, drifting in the water and feeding on small things like coral eggs. Then when the crown of thorns starfish are adults, they continue to mow down the coral at a rate of 6 square meters per year. This doesn’t seem like much, but since they have millions of baby starfish each year, very few natural predators, and can live 6-8 years, they can consume quite a bit of coral. It is clear that this creature has evolved to become a very effective predator of coral, and it is understandable that people who depend on the reef for their livelihood are concerned about an outbreak of crown of thorns starfish.
What can be done? Some scientists are focusing on understanding what causes the starfish to reach epidemic proportions, so they can reverse or at least prevent outbreaks. One leading hypothesis is that pollution from land, especially extra nutrients, causes more young crown of thorns starfish to survive to adulthood, because there is more food for them. However, other scientists believe that crown of thorns outbreaks are simply natural phenonmenon that are caused by a slight change in water flow around the reef, which keeps more young starfish on the reef, instead of flushing them out to sea. Other researchers are focusing on an immediate fix for the crown of thorns starfish problem. They focus on developing a way to kill crown of thorns on the reef, and they have developed a lethal injection that will kill the starfish, but this method is expensive and would require a lot of man power. Other researchers are looking for deterrants or even attractants so we can keep starfish out of some areas or trap and remove them in large numbers. The trouble is that they will have to find an attractant/deterrant that works only for the crown of thorns, and not on any other starfish, because we are still concerned about keeping the other starfish that do not destroy the reefs healthy.
If this is likely a natural phenomenon, why should we care? While reefs may be able to bounce back from an outbreak of crown of thorns starfish, it is also just as likely that the other problems that plague the reef, including pollution, climate change, and other natural phenomena like storm events, may make it too difficult for coral to repopulate the reef. Certain coral species, and even certain reefs, may be lost.
So how likely is it that crown of thorns starfish will eat the Great Barrier Reef? The best coral reef scientists believe that the crown of thorns starfish numbers are increasing, and the last time this happened there was a severe outbreak within a few years, so many people are very concerned. I, for one, am glad I made it out to see the reef this year, and not a couple of years from now. The reefscape may look very different in just a few short years.
This morning I am making my last few travel plans for my next big adventure. In less than three weeks I will be boarding a plane for Sydney, to spend 10 weeks exploring and studying one of the most unique places on earth. Since I will be posting a lot about my research in Australia, this week’s topic will be an introduction of one of my main research interests, which also happens to be a very complex and serious issue facing the world’s oceans: ocean acidification. Get ready, I’m about to bust out some chemistry.
Ocean acidification is the process of the oceans becoming gradually more acidic, due to increasing levels of carbon dioxide in the atmosphere. This is how it works. As we hear almost every day, atmospheric concentrations of CO2 are increasing at an alarming rate. This is the driver of climate change. More CO2 in the atmosphere means that more CO2 will come in contact with ocean surface waters, where gas exchange occurs. After CO2 is dissolved in water, it takes part in a number of reactions. The products of these reactions are carbonic acid, bicarbonate, and the carbonate ion. There is, however, one other important byproduct of these reactions. At each step, hydrogen ions are lost into the water. More hydrogen ions in a liquid means the liquid become more acidic. The result is a decrease in seawater pH, because a lower pH means a solution is more acidic. Under some scenarios, if the current rate of CO2 emissions continues, we can expect a decrease in pH of 0.4 units by the end of the century. While this may not seem like a significant decrease in pH, it actually represents a three-fold increase in hydrogen ions.
Acid can harm many organisms in the ocean. Fish gills are very sensitive to acidity, and more acid in the blood stream is, as you can imagine, very stressful. The main concern is usually for young life stages of many ocean species, because young animals are usually more sensitive to changes in the environment. In addition to the direct effect of acid, there are other, potentially more harmful consequences of ocean acidification. For example, carbonate ions are a natural buffer for seawater. They are the Tums of the ocean- just as Tums help relieve extra acid in your stomach, carbonate ions help eliminate extra acid in the ocean. As ocean acidification continues, the result is a loss of carbonate ions from the ocean. By the end of the century, there may be 60% fewer carbonate ions in the ocean. This means there will be less “Tums” to rid the ocean of extra acidity.
I addition to acting as Tums, carbonate ions are one of the building blocks for shell material. As carbonate ions get used up to buffer acid in the ocean, there are less carbonate ions available for the animals that need them to build hard shells, such as crabs, clams, oysters, corals, and some plankton which form the base of the ocean food web. Not only will there be less material for them to use to build shells, but if the oceans get acidic enough shell material may even begin to dissolve away, just as Tums dissolve in your stomach. Hard shells protect and provide structure for these animals. If the shells weaken or dissolve, these animals cannot grow and are less likely to survive an attack by a predator.
Despite the importance and complexity of this issue, relatively little research has been done on ocean acidification. Besides coral, which has been studied extensively, few species have been included in ocean acidification experiments. Many studies experiment with extremely low pH values which have dramatic effects on animals, but are unlikely to be seen in the oceans in the next century. Perhaps the greatest problem is the lack of long-term ocean acidification studies. Most of the research that has been done covers a period of less than 7 days. Such a short-term study is not adequate to understand the effects that ocean acidification may have over an organism’s lifetime, let alone the effect it might have on generations of animals that have a chance to adapt to change.
There are some places in the world that are already experiencing acidification on a small scale, due to a variety of natural (and sometimes unnatural) phenomena. This acidification occurs in polluted bays, at carbon dioxide seeps, around deep-sea volcanoes, and on the East coast of Australia, where acidic soil rinses acid into coastal waters. In Australia, acidification from acid soil is a natural process that has been going on for the last 10,000 years. The acidic soils form when iron-rich sediment is exposed to seawater, which then evaporates or recedes and leaves behind sulfide, a substance that is naturally found in seawater. The sulphide combines with iron to form iron-sulphide sediments. When this sediment is dried out and exposed to oxygen, the iron sulphides undergo a chemical reaction to form sulphuric acid. Then every time it rains or the tide comes in, all of that sulphuric acid is washed into the coastal water. The animals that live there must find a way to adapt to the acid in the water, or they will die.
While I am in Australia I will be taking a look at the communities of oysters living in the acidic bays and estuaries near Sydney, to see if they have been able to adapt to the long-term acidification of Australian coastal waters. Since my interests lie in predator-prey interactions, I will be looking at how oysters and crabs interact, and whether oyster shells are weaker and result in more acidified oysters being eaten by crabs. This study in Australia will serve as a natural experiment to help scientists understand the effect of the slow process of ocean acidification on animals as they adapt to their environment from generation to generation. It is important to understand the effects of ocean acidification because if some shellfish species are going to have a hard time, commercial shellfish farmers and harvesters may find their livelihoods threatened. Fisheries aside, plenty of animals have shells. These animals are an important component of the food web, and figuring out which ones are likely to disappear from the ocean will help us to predict if there will be large changes to the flow of energy in the ocean.
In the meantime, I will share some stories about my research and my travels in Australia this summer. Australia is full of unique landscapes and creatures, so I will undoubtedly have some cool facts that I pick up along the way, and share with all of you on my blog. Stay tuned for some adventures on the reef and in the mangrove.
Last week the discovery of a goblin shark in the Gulf of Mexico made headlines. This is only the second record of a goblin shark in the Gulf of Mexico. The goblin shark is a deep-sea species that is very rare. Most specimens come from Japan, but even there it is still not common. Scientists know almost nothing about this shark, including why it looks the way it does, how old it gets, how large it gets, or anything about goblin shark reproduction. Until 2000, when the first Gulf of Mexico goblin shark was caught, scientists didn’t even know it existed in the Gulf of Mexico. This truly was an amazing discovery, but then researchers started looking closer at the fisherman’s catch.
If you look closely at the photos of the catch, you may notice some giant creatures that look like lobster tails or pill bugs. These are deep-sea isopods, and they are about the size of a house cat. Since I have an unreasonable fear of cockroaches, these isopods naturally give me the heeby-jeebies, but there is no doubt that they are cool. These isopods are not really uncommon, but they are never found in such large numbers unless a really tasty food source is around, and such decadence is hard to come by in the deep sea. This led some scientists to suggest that the fisherman’s trawl net may have passed over something very rare and very interesting- a whale fall.
A whale fall is what happens after a whale dies. Sometimes whales float for a while after they die, but they all inevitably sink. Even most beached whales are eventually dragged out to sea to sink. Whales rest at the bottom of the ocean and provide an opportunity for lots of food in a landscape that normally does not have any. As a result, whale falls draw a number of unique species, including some that live around very rare deep-sea habitats like hydrothermal vents and cold seeps. They also draw some species that specifically feed on bone, meaning they can only be found at whale falls. This video is a great artistic representation of what happens in the deep sea when a whale arrives.
Whale falls can be broken up into different stages, each with their own group of animals. The first stage attracts crustaceans such as the squat lobster, hagfish, and large sharks like the sleeper shark. This stage may last anywhere from four months to two years. The second stage invites a variety of animals that live in the soil or on the bones, eating increasingly hard-to-digest material to make use of the rare appearance of nutritious food in the barren deep sea. This stage lasts about the same amount of time as phase one (months to years). In the third stage, species that use sulphide as an energy source instead of the sun, such as those found around hydrothermal vents, appear around the whale. Sulfide is a compound made of sulphur that builds up when bacteria are very active, and causes the rotten egg smell you may notice when passing a marsh. This phase may last from six years to decades.
The following video offers some great footage of a whale fall caught on camera by a deep-sea submersible:
Whale falls are notoriously hard to find, because the deep sea is so vastly large (139,000,000 square miles). When researchers find a whale fall, it is a big deal. When they find a whale fall in an ocean as remote as the Southern Ocean (the ocean surrounding Antarctica), it is an even bigger deal. In 2013 researchers found a whale fall near Antarctica, the first discovery of its kind. The Antarctic whale was home to nine species we had never seen before. As amazing as this discovery was, scientists that study whale falls do not have the time or money to comb the deep sea, so they take a shortcut- they sink beached whales, and monitor their progress over time. Researchers in Japan tracked 12 whale carcasses over three and a half years. In the Japanese whale experiments, two new species were discovered, one a lancelet (a primitive fish, and one of my favorite critters), and one a new species of bone-eating worm. If they continue to monitor the whale falls, it is possible they may find even more unique species. Even after three and a half years, there was still some blubber and tissue present, so the decomposition process was nowhere near complete and other groups of animals are still likely to move in.
For more information:
Amon, D.J., A.G. Glover, H. Wiklund, L. Marsh, K. Linse, A.D. Rogers, and J.T. Copley. 2013. The discovery of a natural whale fall in the Antarctic deep sea. Deep-Sea Research II 92:86-96.
Fujiwara, Y., M. Kawato, T. Yamamoto, T. Yamanaka, W. Sato-Okoshi, C. Noda, S. Tsuchida, T. Komai, S.S. Cubelio, T. Sasaki, K. Jacobsen, K. Kubokawa, K. Fujikura, T. Maruyama, Y. Furushima, K. Okoshi, H. Miyake, M. Miyazaki, Y. Nogi, A. Yatabe, and T. Okutani. 2007. Three-year investigations into sperm whale-fall ecosystems in Japan. Marine Ecology 28:219-232.
In 2008 I got the chance to travel to Kodiak, Alaska for an entire summer of research on the red king crab. Researchers were concerned that global change would threaten an economically important and thriving fishery around the Aleutian Islands. The poles are warming faster than anywhere else in the world. In addition, waters are gradually becoming more acidic in a process called ocean acidification, which is caused by increased uptake of carbon dioxide in ocean waters. Ocean acidification was my specific area of research as an intern in Kodiak. I was responsible for trying to figure out the effect of acidified water on the young life stages of the red king crab. The assumption was in this area of the world, global climate change would cause problems for this cold-water species of crab. However, that is not the assumption everywhere in the world.
Some species benefit from warming waters. A large part of the Antarctic continental shelf (or the shallow waters that fringe the land mass that is Antarctica), is off limits to many species because it is simply too cold. In very cold places like Antarctica, the water at the ocean’s surface is actually colder than the deep water because the air temperature cools the surface water. So in Antarctica, many species aren’t able to live and grow in the shallow waters, even though they are abundant in the deep water offshore of the continental shelf. This is the case for many species of large crabs with crushing claws, that are abundant in the colder waters near the North Pole and Alaska, like king crabs. King crabs are the crabs of “Deadliest Catch” fame.
The shallow waters of Antarctica have been too cold for crushing crabs for 14 million years. As far as crustaceans on the Antarctic continental shelf go, currently there are only five shrimp species that are able to survive in the colder, shallower waters around Antarctica. The absence of large crab predators is very important to the bottom-dwelling critters in Antarctica. Large crabs not only tear apart and crush prey they find while foraging on the ocean floor, they also produce gashes and holes in the surface of the mud at the bottom of the ocean. This is a lot of disturbance. Since the shallow waters of Antarctica have been free of many crab and fish predators for so long, they have developed some of the most diverse communities of starfish, corals, and mollusks found anywhere in the world. More than half of all the species found on the Antarctic shelf are endemic to the area, meaning they are found nowhere else in the world.
The animals that live on the floor of the Antarctic shelf have evolved to withstand fairly constant conditions, with not much disturbance. The major predators of the Antarctic shelf are slow-moving predators like starfish and urchins. That means the clams, snails, corals, and other invertebrates that live on the Antarctic shelf are relatively unprotected: thin-shelled, slow-moving, and non-toxic. Examples include thin, almost translucent snails; fragile, fleshy gorgonian corals; meaty, lumpy clams that divers can collect by hand; and white nudibranchs (shell-less snails) that do not collect poisons from their food like their coral reef-dwelling cousins. These animals would likely not do very well if a new, relatively quick predator with crushing claws and pointy, probing feet were introduced onto the continental shelf. Which, of course, is exactly what is happening.
Sea surface temperatures on the west Antarctic Peninsula have risen about 1° C since 1950. Observations made by deep-sea submersibles suggested that this temperature increase may have been enough to allow king crabs to move from the deep water surrounding the Antarctic continental shelf, up the slope, and closer to the continental shelf itself. A team of researchers traveled to Antarctica to document king crab presence on the slope, and to see how close the crabs were to reaching the shelf. This video summarizes what they found.
King crabs were spotted on the lower portion of the continental shelf. Crabs were seen actively consuming starfish and other bottom-dwelling animals, and diversity of other animals was reduced in areas where crabs were abundant. An additional survey of the Antarctic continental shelf in 2012 revealed populations of king crabs in the deeper portions of the shelf itself. In fact, a team of researchers recently discovered a healthy population in a deep basin called Palmer Deep, 120 km away from the slope, indicating that the crabs can cross the shelf already. The amount of crabs found in Palmer Deep is greater than the commercially exploited populations present in Alaska and South America (per unit area). In Palmer Deep, the researchers never saw evidence of the four species of starfish that should have been in the area. They did see an estimated 100-300 punctures and gashes in every one-meter section of seafloor mud the robot camera covered. Wherever researchers found crabs, they also found evidence of crabs changing their environment.
If the current range of king crab in Antarctica is any indicator, current trends in warming suggest that king crabs will be a common fixture on the continental shelf in 100-200 years. This has led researchers to claim that king crabs are invading the Antarctic continental shelf. Are these crabs really invading? The question is not whether crabs will really become more common on the continental shelf of Antarctica, because they most likely will, but whether the term “invasion” is the best word for what is happening in Antarctica. “Invasion” has a very specific meaning to ecologists. According to the International Union for the Conservation of Nature (IUCN), an invasive species is “an alien species which becomes established in natural or semi-natural ecosystems or habitat, is an agent of change, and threatens native biological diversity.” The uncertainty lies in the word “alien”, indicating the species comes from another place. It is difficult to tell how long crabs have been absent from the continental shelf. The fossil record is patchy at best, and scientists must rely on indirect cues, such as a fossil record of starfish that do not suffer from missing arms, which would have presumably been the case if crabs were around. Some would argue that lack of damage in fossilized starfish is not very reliable evidence. Are we just noticing more crabs on the shelf because we are spending more time looking? Have they been in Palmer Deep for much longer time than we expect?
Perhaps these crab species are undergoing a range expansion, driven by changing climate. This is expected to happen commonly around the world, as mobile animals such as fish gradually move to more suitable areas to follow changes in global climate. However, the range shift of king crabs may be more devastating for the ecosystem than a lot of other range shifts, because the Antarctic ecosystem has not experienced this kind of disturbance in many millions of years. King crabs will almost certainly be an agent of change, and a threat to biological diversity. The silver lining is that in this case, perhaps global change will create an economically important and thriving fishery for king crab on the continental shelf around Antarctica. With the remoteness of Antarctica, and the unstable weather conditions, it may be unlikely that such a fishery would ever attract investors or fisherman willing to take the risks. If it did, we better hope there will be a reality show, because this “Deadliest Catch” spinoff will be way more dangerous.
For more information:
Griffiths, H.J., R.J. Whittle, S.J. Roberts, M. Belchier, and K. Linse. 2013. Antarctic crabs: Invasion or endurance? PLOS One 8(7).
Smith, C.R., L.J. Grange, D.L. Honig, L. Naudts, B. Huber, L. Guidi, and E. Domack. 2012. A large population of king crabs in Palmer Deep on the west Antarctic Peninsula shelf and potential invasive impacts.
Sven, T., K. Anger, J.A. Calcagno, G.A. Lovrich, H. Pörtner, and W.E. Arntz. 2005. Challengin the cold: Crabs reconquer the Antarctic. Ecology 86(3):619-625. Proceedings of the Royal Society B 279:1017-1026.
From April 17th 2014 through May 1st 2014, the National Oceanographic and Atmospheric Administration (NOAA) has brought together a team of scientists and engineers to explore the bottom of the Gulf of Mexico using their ship, the Okeanos Explorer, and their remotely operated underwater vehicle (ROV) named the Deep Discoverer. To the delight of science lovers across the nation, NOAA has been streaming live camera feed from their ROV for anyone to watch, greatly reducing the productivity of marine scientists and nautical archaeologists. On Thursday April 27th the Okeanos Explorer visited two shipwrecks in the Gulf of Mexico (click here to see the highlights). They discovered a variety of “encrusting biology” (animals that live on the hard structures of shipwrecks), archaeological treasures, and even the chronometer (timepiece) and the octant (a navigation device) of the early 19th century ship, which has led me to coin my new favorite phrase for use when facing impossible odds, “It’s like finding an octant in a shipwreck.”
If you watched the feed from the Monterey C shipwreck, you may have noticed one major thing missing from much of the ship’s debris field- wood. Since wood from a shipwreck is pretty much the only organic material in such a harsh environment as the deep sea, a lot of the wood from the ship had long since been eaten away by animals that feed on wood. Wood is not an easy thing to eat. It is full of fiber and difficult for the gut to break down. Even animals that have special guts for digesting plant matter, such as cows and other ruminants, cannot break down wood. In the ocean there is one fairly common critter that can eat wood. They are called shipworms.
Shipworms are not worms at all. They are actually bivalves, a name that comes from the latin class Bivalvia. All of the members of the class Bivalvia have two (bi) shells (valves) that are hinged. Examples would be clams, oysters, scallops, and mussels. However, shipworms do not look like clams at all. They look more like worms, with long, fleshy, cylindrical bodies. They use their shells not for protection, like a clam or oyster, but to bore through wood. They can be a few centimeters long, or as long as a meter depending on the species, and they live in the burrows that they carve out of wood. The wood shavings are also consumed as food. The shipworm uses substances called cellulases to break down the wood, allowing them to extract nutrition from the tough fiber. This ability allows shipworms to join only a few known species that are capable of digesting wood, including protists, fungi, bacteria and a handful of species in several different invertebrate groups, including termites.
Just as termites can cause problems for homeowners, shipworms can cause problems for people whose livelihoods depend on the sea. Shipworms are responsible for destroying ships and piers around the world. They also eat shipwrecks, which in many parts of the world are an important part of history and heritage. The shipworm’s tendency to nibble away at nautical history means they are mostly viewed as pests that need to be eradicated. There are a few marine environments in the world where shipworms are not found. One is the Antarctic, which is protected from shipworms by the circumpolar currents which act as a barrier to shipworm movement. The Baltic Sea, which is too fresh for shipworms to survive, has been protected from shipworm feeding in the past. This shipwreck oasis is now in trouble because recently shipworms have begun to invade the Baltic. An eradication program has begun to prevent the shipwrecks from becoming mollusk food.
I understand the need to protect historical wrecks. However, far from considering shipworms as a pest, I propose we celebrate the shipworm as a genius. In fact, I suggest we award the shipworm an honorary degree in engineering. Here’s why. The shipworm has played a major role in helping humans discover solutions to two major engineering problems over the last two hundred years. Have you ever wondered what it takes to make a tunnel under a river? I know I have. Turns out it is not easy. Tunnels in soft sediment, like the mud under a river, tend to collapse as they are built. In the early 1800s, Marc Isambard Brunel solved this problem with a little help from the shipworm. He observed a shipworm’s tunneling behavior. The shipworm bores through the wood and as it makes its tunnel, it encases the tunnel walls in shell material to prevent the wood from swelling, closing the tunnel and crushing the worm. Brunel designed a tunneling shield, which was an iron cylinder that was pushed further and further into the tunnel as it was excavated, preventing the walls from collapsing in on the workers. In this way he was able to make the first tunnel under the Thames River, and it was all thanks to the shipworm.
A more recent example of the shipworm’s prowess as an engineer is the quest for the fuel that will replace fossil fuels. The energy crisis has led us to look at biofuels, or fuels derived from living organisms, such as plants. The production of biofuels involves collecting sugars from plants and fermenting those sugars to make ethanol. However, plants store glucose in a tough fiber called cellulose. Getting the glucose out of cellulose is costly, making biofuel production from plants costly as well. A lot of recent biofuel research has aimed at reducing the costs associated with removing glucose from plant tissue. I am sure you can see where this is going. Shipworms have already solved this problem by developing an association with bacteria that live in shipworm gills and digest cellulose. In fact, some shipworms appear to live off of wood alone, and do not grow any faster when provided with extra food. These bacteria are being cultured for use in biofuel production. Perhaps in a few years the shipworm will have helped us solve another complicated engineering problem, improving human lives.
Is the shipworm an irritant or an inspiration? Shipworms have experienced an interesting evolutionary path to fill a unique niche, and they have developed the ability to eat a substance that is extremely abundant, but useless as a food source for the vast majority of other animals. This path has unfortunately led the shipworm to torment ship-owners and owners of waterfront property. It has also led to some of the most amazing achievements in engineering in the last two hundred years. This is the reason I think the shipworm deserves an honorary degree in engineering, and I hope this interesting animal will continue to inspire the great thinkers of my generation as we tackle increasingly difficult environmental issues.
For more information:
Honein, K., G. Kaneko, I. Katsuyama, M. Matsumoto, Y. Kawashima, M. Yamada, and S. Watanabe. 2012. Studies on the cellulose-degrading system in a shipworm and its potential applications. Energy Procedia 18:1271-1274.
Asfa-Wossen, L. 2012. Tunnel vision. Materials World, 20(5):10.
Tanimura, A., W. Liu, K. Yamada, T. Kishida, and H. Toyohara. 2013. Animal cellulases with a focus on aquatic invertebrates. Fisheries Science 79:1-13.
Miller, R.C., and L.C. Boynton. 1926. Digestion of wood by the shipworm. Science 63(1638):524
Gregory, D. 2010. Shipworm invading the Baltic? The Nautical Archaeology Society 431.