As with wetland animals (see Section 15.3.3), simpler animals tend to be osmoconfor-mers. This is the case with most invertebrates, such as worms, mussels, and octopi. Their bodily fluids are isotonic with seawater. Therefore, they do not require special mechanisms to control their internal osmolarity.
Marine fish are hypotonic: Their body fluids are about one-third as saline as seawater. Without active mechanisms to counter it, their bodies would lose water by osmosis and absorb salt. The mechanism is the drinking of large volumes of water, coupled with the excretion of salt by special "chloride cells'' in their gills and by the discharge of a highly concentrated urine. It is because humans do not have these adaptations that makes it dangerous for us to drink seawater.
Viscosity becomes an important factor for small organisms. Viscosity is a measure of the force required to create shear, or velocity gradient, in a fluid. Motion of differing objects can be compared in terms of the Reynolds number, which is the ratio of momentum to viscosity. If a human propels herself through the water with a few strokes of the arms, she can expect to coast a short distance because her momentum is great compared to the viscosity of the water. A microscopic zooplankter, on the other hand, stops within milliseconds if it stops propelling itself.
Salt water is more viscous than fresh water. Seawater is about 25% more viscous at 0°C than at 20°C. Figure 15.28 shows how two species of one genus react to this difference. The warm-water form has more elaborate appendages to slow its sedimentation rate in the less viscous water.
Many marine animals control their position not only by swimming but by buoyancy. Several mollusks have rigid air containers. These include the genus Nautilus, the cuttlefish Sepia, and the squid Spirula. Their air chambers just balance the buoyant weight of the rest of their bodies, making them neutrally buoyant. Some of the boney fish have a swim bladder to store air. The bladder is filled either by extracting gases from the blood or by direct ingestion through the esophagus. Active swimmers, such as the tuna, or fish that live on the bottom do not have swim bladders.
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