Zhang, G.; Li, L.; Meng, J.; Qi, H.; Qu, T.; Xu, F.; and L. Zhang. 2015. “Molecular Basis for Adaptation of Oysters to Stressful Marine Intertidal Environments”. Annual Review of Animal Biosciences. doi: 10.1146/annurev-animal-022114-110903
Calling All Oysters
Cockles and mussels! Clams and oysters and scallops….there are so many types of shellfish, it can make my head spin (Figure 1). The ones I’ve named so far are all mollusks – specifically, they are bivalves. A bivalve is a marine or freshwater invertebrate whose body is encased by a hinged shell. Most of these animals are filter feeders; these organisms pass water through their bodies, straining it for food particles before releasing the filtered water back into the environment. Many of them, like the oyster, are sessile – that is, they bury themselves in the sandy or muddy bottom or attach themselves to rocks, remaining immobile throughout their lives.
Figure 1: Diversity of mollusks on the New England coastline. Squids and octopi are mollusks, too!
Oysters, like the one in Figure 2, tend to have irregular shells completely made up of calcite (a highly stable form of calcium carbonate). They have long been a staple of coastal human diets. In fact, oysters were actually cultured in ponds by the Romans! They are an excellent source of zinc, iron, and calcium, all of which are required for a healthy diet. They are also high in protein. In addition to being an important part of the diet and economy of coastal areas, they also have a huge impact on the ecosystems in which they live. Filter feeders can affect the nutrient level of the water around them by assimilating phosphorus and nitrogen into their own tissue. High levels of these nutrients often lead to algal blooms which choke out other species, so oysters can actually help foster a diverse and healthy ecosystem. The Chesapeake Bay Program is now using oysters to reduce the amount of nitrogen and phosphorus entering the Chesapeake Bay. This post will focus on some of the ways oysters can deal with stress as highlighted by a review paper written by Zhang et al., 2015.
Figure 2: Chesapeake Bay oysters. While all oysters (all mollusks) can produce pearls, pearl oysters are actually quite different from the true oysters. This paper focused on true oysters – including the edible ones – like the one in the picture.
The Stressful Life of a Shellfish
So why are oysters so stressed out? Seems like they’ve got a pretty easy life, right? They just sit around all day, waiting for food to come to them! However, their lives are a lot more stressful than we might think at first glance. Marine oysters live primarily in the intertidal zone, the area of the shoreline which is constantly shifting due to the tides (Figure 3). A fluctuating environment means a life of constant change – change in temperature, change in salinity, and of course, change in water availability. All those changes can stress out an organism –after all, think about how stressful it can be to have your life change in a significant way. These animals are always dealing with this type of stress! This review summarizes some of the molecular changes within these organisms that allow them to deal with a constantly stressful environment.
Figure 3: The rocky intertidal zone, home to some of my favorite marine critters!
Oysters as Plastic Life-forms
When I talk about an organism that is “plastic”, I don’t mean that it’s a material that needs to be recycled! So, what is plasticity in a biological context? Simply put, a plastic organism is one that is able to respond to fluctuations in the environment by changing aspects of its behavior, physiology, or structure. Humans, for example, are incredibly plastic – we often change our behavior to suit different environments. When it grows cold, we put on additional clothes or move to a warmer location. Daphnia, a tiny crustacean known commonly as a water flea, can actually change its body shape in the presence of predators (Figure 4). What’s unique about plasticity is that it occurs within a single lifetime – the organism senses a change and initiates the appropriate reaction. The daphnia pictured in Figure 4 are genetically identical, differing only by the environment in which they were raised (with or without fish predators). This flexibility within a single individual allows plastic life-forms to respond incredibly quickly to environmental changes. Oysters use a variety of plastic responses to react to their constantly fluctuating intertidal habitat.
Figure 4: Daphnia in the presence (left) and absence (right) of predators. The long spines are costly to build but can protect the organism from becoming dinner!
Meet the Key Players of Oyster Stress Response
Heat-shock proteins (HSPs for short) are a class of proteins built by many cells in response to stressful conditions. As the name implies, they were first discovered in organisms that had been exposed to extreme heat, but they have since been implicated in a variety of stress responses, including exposure to extreme cold, exposure to UV radiation, exposure to toxins, and during wound healing and tissue remodeling. Scientists have measured the amount of HSPs in cells before, during, and after exposure to a stressful condition, and found that there are more HSPs in cells just after the exposure, suggesting that HSPs form an important component of the cellular stress response. These proteins are present abundantly in oysters – researchers have found 88 copies of a single HSP gene, HSP70, within the oyster genome. Humans, in contrast, have only about a dozen copies of this gene. Having many copies of these genes suggests that oysters can build more of the corresponding proteins quickly, and also suggests that these genes have been incredibly important to these organisms’ ability to survive and reproduce.
Antioxidant enzyme genes
What’s the relationship between antioxidants, free radicals, and your health? A free radical is a highly reactive atom or molecule. Because it is highly reactive, it will bind easily to other atoms or molecules, which can damage them. As this damage builds up, it can cause all sorts of problems, including (on the trivial end) wrinkles and (on the serious end) cancer. Antioxidants are molecules that bind readily to free radicals, stopping them from causing damage in the first place. Various types of stress can cause the buildup of free radicals within a cell, including extreme temperatures or changes in salinity. Oysters also carry several copies of major antioxidant enzyme genes within their genome. Many of those enzymes are expressed as proteins within oyster cells after exposure to low salinity conditions.
Summary: Bringing it All Together!
The intertidal zone, home to many marine invertebrates like oysters, changes constantly due to the tides as well as the seasons. Factors such as temperature, salinity, water level, and pollution amounts vary day to day and eve nhour to hour. Because their environment is always fluctuating, oysters are under nearly ubiquitous stress. These animals therefore need to have a strong molecular system in place to handle this stress, and they clearly do (Figure 5). This review discussed many major stress response pathways, of which I highlighted two, and showed that some of the genes involved in these pathways are copied many times in the oyster genome, allowing them to build lots of the corresponding protein quickly. They also showed that oyster cells build more of those proteins after exposure to a stressful condition, suggesting that these genes are truly important to the oyster stress response!
Because oysters are an important part of the marine ecosystem, filtering out pollution and assimilating nutrients that can lead to algal blooms, it is crucial to understand their stress response system in order to know how they will react to further climate change or pollution. This review helps to synthesize what researchers already know about oyster stress response pathways and to point out areas in need of further study.
Figure 5: Generalized picture of the oyster stress response. Oysters may be uniquely able to tolerate highly stressful conditions because they carry many copies of common stress response genes within their genomes!