THE MARINE ENVIRONMENT
fte field of marine biogeography has progressed much more slowly than has its terrestrial counterpart. ftis is understandable; for humans, the marine environment is much less hospitable and accessible than most terrestrial environments. We are not adapted to breathing in water, and so must have specialized equipment in order to remain under water for very long. fte tremendous physical pressure in the deep oceans has made those environments inaccessible until the relatively recent development of underwater vessels capable of withstanding such pressures. fte last few decades have seen rapid and exciting developments in the field of marine biogeography.
ftere are several differences between marine and terrestrial environments that must be taken into account to fully understand the biogeography of marine organisms, apart from the obvious one that water is wet and air is dry . One significant difference is the great importance of the vertical dimension in the oceans. On land, there is a vertical dimension associated with plant structure, flying animals, and other airborne organisms. But on land, “what goes up must come down,” and the horizontal dimension associated with the land surface, plant cover, and so forth, dominates. In the marine environment, the buoyancy provided by water makes it about as easy for many organisms to move and maintain their positions vertically as horizontally. Furthermore, as we shall see, the physical properties of water create fairly distinct vertical stratification that affects the distributions of marine organisms. Another important characteristic of the oceans is that there are fewer physical barriers to dispersal; compared to land, the marine environment is more continuous.
In general, marine organisms are much less familiar to us than are terrestrial organisms, mostly because the marine environment is less accessible to us than the terrestrial environment. fte physical properties of the marine environment also provide selective pressures for traits that often seem bizarre from our perspective. Gas exchange structures are a case in point. Because aquatic organisms are surrounded by water, desiccation is of little concern. fte body wall is generally very thin, and gas exchange often occurs directly across this surface. Even specialized gas exchange structures of marine animals are simply vascularized, thin-walled extensions of the body wall. ftese structures are usually external and in direct contact with the surrounding fluid. But terrestrial animals have a problem – the ever-present danger of desiccation in the relatively dry surrounding air. fterefore, selective pressures have favored the evolution of internal respiratory structures that are protected from the surrounding dry air. Likewise, reproduction in marine environments generally involves external fertilization, with gametes released directly into the water. In terrestrial environments, desiccation of gametes is a danger in open air, so internal fertilization is the norm. Also, marine animals generally eliminate metabolic wastes as ammonia. Ammonia is toxic; even low concentrations in the body are toxic to most animals. But marine animals can constantly eliminate ammonia because water for dilution is plentiful. Many marine animals can excrete ammonia right across the body surface. However, because desiccation is such a concern in the terrestrial environment, land animals cannot afford to expend large amounts of water diluting ammonia. Terrestrial animals generally eliminate ammonia as uric acid or urea, which require less water. ftey also require more complex excretory systems than do marine animals.
Other differences between marine and terrestrial animals relate to the greater density of water compared to air. ftis means that marine animals require less rigid skeletons, because water provides much of the support against the force of gravity. fte density of water also makes possible the free-floating existence of planktonic marine organisms, as well as the suspension- feeding stationary, or sessile, animals, such as sponges and sea anemones, that feed on these abundant planktonic organisms. On land, air just doesn’t contain enough of these planktonic organisms to support such suspension-feeding strategies, although orb-weaving spiders could be considered a notable exception.