Ways of Adaptation of Animals to Life in Estuarine Environments

The estuarine environment is an area where bodies of water and shoreline meet, where rivers join the ocean or other large masses of water (Barnes, R.S.K. & Green, J. 1972). The estuarine environment is very dynamic and presents many difficulties for survival, animals have found many ways to adapt and deal with the pressures. The combination of both flows of water creates what is known as brackish water, this is a mixture of salty and fresh water (‘The Dynamic Intertidal Environment’, 2013). The area at the shoreline that is most affected by tidal movements is known as the intertidal zone, the salinity concentrations and water levels are affected (Gracey, A. et al., 2008). Salt is moved in with the ocean and a constantly changing water level results in the habitat being under water half the day and exposed to the atmosphere the rest. Wave exposure is another problem that animals face living in the intertidal zone, the force of waves crashing onto the shore can be powerful. Although organisms that live in these harsh habitats face many challenges, they have adapted to create a very productive ecosystem, nutrients flow in from both bodies of water and this allows for the support of a large diversity of animals (‘The Dynamic Intertidal Environment’, 2013). This essay will discuss the ways that barnacles, fiddler crabs and ochre sea stars have adapted to survive in these conditions.

Barnacles are bivalves that have adapted to live in estuarine environments, they are found in the intertidal zone on rocks and other solid surfaces (‘Basic Biology’, 2018). The life cycle consists of two larval stages and the adult stage, the first larval stage is dispersed to drift with the zooplankton because the intertidal zone is too harsh an environment to support the young (Anil, A.C. et al., 2010). This first larvae stage feeds until it is ready to undergo a transformation into the second larval stage, which is known as a cyprid. Once in this second larval stage it is developed enough to be able to support itself in the intertidal zone and must find a hard substrate to attach itself to, where it finally develops into an adult (Anil, A.C. et al., 2010). Barnacles are sessile which means they will spend their entire life in one position, they have a foot which has is used to attach themselves to the surface (‘Basic Biology’, 2018). Barnacles have also developed glands that can release a cement like substance to secure themselves into position, this stops the force of the waves from washing them away. Barnacles have a calcareous outer shell that, like all bivalves, is made up of five or more plates. The intertidal zone is exposed to a lot of force from heavy waves crashing and this calcareous shell helps absorb the impact of the waves which allows barnacles to live in some very hazardous places (Hoch, M. & Reyes, K., 2015). Barnacles do not have gills or lungs, instead they respirate via gas exchange through a modified leg known as a cirri. The cirri are feather-like legs that are also responsible for feeding, it extends out of the shell and sifts through the water filter feeding on particles (Hoch, M. & Reyes, K., 2015). Barnacles are only able to use their cirri when they are submerged underwater and so when the tide recedes, and the barnacles are exposed to the atmosphere it presents a major problem. To cope with this, they have adapted the outer shell to allow movement in two of the plates, meaning barnacles can open and close. When it is high tide, they will open their shells to release their cirri, and at low they will close their shells. By closing their shell barnacles can retain water inside an impermeable surface, this stops water loss when exposed to light and air long enough to survive until the water returns with the tide. Barnacles have adapted to the estuarine environment by a change in morphology that provides protection and prevents water loss.

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Fiddler crabs are another animal that lives in the intertidal zone and has adapted to survive high tide. Fiddler crabs are a terrestrial species that has evolved to limit their activities to brief terrestrial outings when it is low tide (Takeda, S. & Murai, M., 2003). They cannot breathe underwater, when it is high tide, the crabs dig burrows to hide and wait in. When it is low tide and they are exposed to the atmosphere they will emerge from their burrows to participate in everyday happenings such as feeding, maintenance of the burrow, combat and ritualized courtship (Hilke, A. et al., 2015). When the tide starts to come back in the crabs will return to their burrows, they have what is known as a ‘biological clock’ which allows them to anticipate the tidal return and get back to their burrows before they are flooded (Barnwell, F., 1968). Once inside their burrow the fiddler crabs will plug up the entrance to stop water breaching and so creating a little oxygen tank (Hilke, A. et al., 2015). It can be many hours that the tide is high and there is only a limited amount of oxygen that be stored in the burrow. By shutting down and reducing the metabolic requirements fiddler crabs have adapted to restrict the amount of oxygen that they consume, prolonging the limited supply inside the burrow (Barnwell, F., 1968). Fiddler crabs have physically and behaviorally adapted to survive high tide by developing techniques to avoid it.

Ochre sea stars, commonly known as the purple sea star, are predators that are a key stone species which means they play a major role in the stability of the surrounding ecosystem (Ferrer, R. et al., 2015). Mussels are the sea stars main source of food. They have developed a technique in which they invert their stomach inside of a mussel shell and then digest the mussel inside. It is a very effective technique and can wipe out populations of mussels, this is important because they can cover large areas inhibiting the growth of other species and by clearing the mussels it allows for a larger diversity of species (Ferrer, R. et al., 2015). Ochre sea stars live in a wide range of environments, they can be found anywhere from sheltered lagoons to the most wave exposed rocky intertidal shorelines. By manipulating their morphology ochre sea stars can survive in some of the most wave exposed shorelines, when found in wave exposed areas their arms are thinner, and they weigh less (Hayne, K. & Palmer, A., 2011). By narrowing their arms and becoming lighter it decreases lift and drag from the crashing waves. When ochre sea stars are found in sheltered areas, they have wider arms and are heavier, this can help prevent overheating by increasing its insulation and it takes advantage of its increased body volume for more gametes (Hayne, K. & Palmer, A., 2011). Sea stars can adapt to short term variation in wave exposure, if moved from sheltered areas to wave exposed areas in three months they will have changed their physiology to adapt to its environment. Ochre sea stars are radially symmetrical and they move using a series of tubes that run along the underside of its arms called a hydrostatic skeleton. Surrounding water can be moved in or out of the system through the madreporite, a porous plate located on the underside of the star (University of Puget Sound, 2019). The arms are moved using hydrostatic pressure by moving the fluid within the hydrostatic skeleton, once it is low tide water cannot enter and exit the system and restricts the sea stars movements. When exposed to the atmosphere the tissue will become very stiff, this stiffening prevents water loss and allows it to anchor itself in shade or rock crevasses to reduce light exposure and hide from predators (University of Puget Sound, 2019). Ochre sea stars have adapted to this atmosphere exposure and are able to survive whilst exposed for up to 12 hours.

The estuarine environment is a hazardous habitat that presents many challenges for the animals that live in these areas. The intertidal zone is in estuaries at the shoreline and is heavily affected by tidal movements and wave exposure. Many species have had to adapt and develop techniques to survive, barnacles have done this by adapting morphologically to developed glands to provide an anchor and a moveable shell to prevent water loss (Hoch, M. & Reyes, K., 2015). The fiddler crabs have made behavioral and physical changes to avoid the high tide completely (Takeda, S. & Murai, M., 2003). Ochre sea stars have adapted morphologically to withstand atmospheric exposure but can also exhibit short term physiological changes in response to a difference in environmental conditions (Hayne, K. & Palmer, A., 2011). There are many ways that animals have found to adapt and overcome the pressures of living in an estuarine environment.