| MadSci Network: Zoology |
Yvonne Starks asked:
We have been studying the ecology of estuaries and other intertidal habitats in our biology class. I was wondering if these animals have any specific adaptions that they use to survive in and out of water. I am particularly interested in species of worms and snails.
Worms and snails are indeed most interesting animals. And intertidal habitats are some of the most fascinating places in the world. Of course, as a marine biologist who has been interested in these since the age of about seven or eight I might be a bit biased!
Anyway...I've chosen to answer Yvonne's question in several sections:
Intertidal habitats can be divided (broadly) into two categories: the soft and the hard. Soft habitats are environments such as mudflats and sandy beaches. The animals inhabiting these places often spend at least some of their time buried in the sediment (that is, in the sand or mud). Hard habitats are places such as rocky shores, but also include places such as the roots and trunks of mangroves, the blades of seagrass plants, and coral. Most animals inhabiting these environments live attached to the surface (e.g. of the rock or coral) or in cracks or crevices. Some however—and this includes many worms and some animals related to snails—actually burrow into the rock, coral or wood.
Biologists refer to animals living on the surface of some material as the epifauna. They refer to animals living inside some material as the infauna. (Animals in the water are referred to as pelagic and are divided as those which drift with the currents—the plankton—and those which can swim against the currents—the nekton.)
The type of environment an animal inhabits (hard or soft) and its lifestyle (epifaunal or infaunal) influences the types of stresses and dangers, as well as the types of opportunities, to which it will be exposed. For instance, mangrove forests and seagrass beds usually only grow in rather sheltered areas, so the animals (and plants) inhabiting these environments will not be exposed to rough waves (except, perhaps, during storms). Also, animals living buried in sand, mud or some other material will be less exposed to waves, the sun and predators than animals living on the surface.
With one possible exception, intertidal marine animals have no special adaptations for living in the water that other marine animals don't have. This is probably because the marine environment is, in most ways, a rather benign place in which to live—after all, it was in the oceans that life first evolved.
The possible exception is that interidal animals on exposed shores must be able to remain firmly attached when battered by big waves, particularly those caused by storms and cyclones. I don't really think, however, that this qualifies as a special adaptation because marine animals in some places (for example, where there are strong tidal currents) may be always underwater but still experience very strong currents and surges. Even so, intertidal animals on exposed shores must be able to withstand the force of waves and those living in these types of environments display adaptations for this. Examples are:
The marine animals living in intertidal habitats face several problems when the tide goes out. These can be grouped into a few categories: problems of support, problems with stress and problems with predators. Intertidal animals, like all animals, must feed and reproduce but these activities don't involve particular problems except that animals high on the shore may have only a limited time when the tide is in to do these things.
Water—fresh or salt—is a dense medium which supports much of the weight of the organisms in it. Animals on land lack this support so are either small, or have some type of support structure or skeleton. This applies to animals in intertidal habitats. In fact, most of the commoner animals in these habitats are those which had already evolved some type of support system: polychaete worms, molluscs, crustaceans and echinoderms.
These systems are needed to support the animal's weight and to allow it to move (if the animal is mobile) and function. Even animals which don't move from place to place, such as barnacles or sessile polychaetes, need to move parts of their body to feed and perform other functions (e.g. building their tube).
Probably the biggest, although not the only, stress that many intertidal animals face when the tide goes out is dessication stress; the stress resulting from getting dried out. And it is this, rather than heat, which is most important. After all, the temperature of the water in shallow tidal pools can become quite high—up to 50°C—and the animals can survive, provided that the conditions don't last for too long and there aren't other sorts of stresses (e.g. low oxygen).
Getting dried out, however, seems to be a more serious sort of problem. One reason is that it results in the chemicals inside the animal's cells become more concentrated and this can interfere seriously with the machinery of the cell. Also some chemicals may become concentrated to poisonous levels.
To avoid this problem, intertidal animals must conserve water when the tide is out. Mobile animals may be able to replenish their supply of water: crabs often do this by quickly ducking into a tide pool or their burrow. Slower moving and sessile animals don't have this option, so conserve water by becoming inactive; sheltering in a pool or shaded spot (if they are mobile); and having an outer covering (such as a shell or exoskeleton) which slows the rate of water loss.
When the tide goes out, intertidal animals are exposed to a new set of predators, some of which have evolved special adaptations for just this function. The commonest are, of course, the birds and these are abundant and efficient predators, particularly on the animals inhabiting beaches and mudflats.
Even so, some of the most voracious and effecient predators on intertidal animals are other intertidal animals! The most common are crabs, starfish and gastropods. Some of these are specialised predators; others are omnivores or scavengers, feeding on whatever they encounter.
The crustaceans, echinoderms and molluscs already have a good form of defense in their shell or skeleton. And for sessile animals this is often their only means of defense, unless, like the sea-anemones, they have some type of stinging cell. Mobile animals may try to flee from predators, or hide in crevices, under rocks, or in the sand until they have gone. Sessile animals can't flee from predators, but they can pull soft parts (such as tentacles) inside their shell (if they have one) and close it up.
This is by no means a complete description of the ways in which intertidal animals live and, indeed, I could hardly hope to do this in a few short paragraphs. For one thing, the different types of intertidal habitats have (as I briefly described) different features which affect the way in which the animals live. For another, there are many different kinds of intertidal animals living in many different places and in many different ways. Finally, biologists have not yet studied all these different kinds of animals and places, so certainly don't all the answers to all the questions we have. And that, of course, is for many people (such as myself) one of the reasons why the biology of these animals is so interesting.
Try the links in the MadSci Library for more information on Zoology.