. The biology of marine animals. Marine animals; Physiology, Comparative. 36 THE BIOLOGY OF MARINE ANIMALS these animals, Caudina, suffer salt loss which reduces the degree of swelling. Experiments with the sea hare Aplysia punctata in dilute sea water show that these animals swell rapidly during the first two hours as water is absorbed, but the weight then falls off as salt is lost. On returning to normal sea water there is a further loss in weight since the external medium is now hyperosmotic to the animal (Fig. ). A converse experiment in which Aplysia was placed in a solution conta
. The biology of marine animals. Marine animals; Physiology, Comparative. 36 THE BIOLOGY OF MARINE ANIMALS these animals, Caudina, suffer salt loss which reduces the degree of swelling. Experiments with the sea hare Aplysia punctata in dilute sea water show that these animals swell rapidly during the first two hours as water is absorbed, but the weight then falls off as salt is lost. On returning to normal sea water there is a further loss in weight since the external medium is now hyperosmotic to the animal (Fig. ). A converse experiment in which Aplysia was placed in a solution containing part sea water and part sugar caused the animal to shrink, since the body wall is impermeable to sugar whereas salt diffuses out. Similarly, starfish and echinoids suffer. 8 W 72 74- Time (hours) Fig. Osmotic Adaptation in Different Marine Animals Percentage change in body weight of animals placed in dilute sea water. (From Bethe, 1934, and Maloeuf, 1937.) some weight increase in dilute media, but their hard skeletons restrict volume changes and considerable salt loss is believed to take place (77,107). Some of these animals are partly estuarine in habit or occur in enclosed waters of reduced salinity. Thus Arenicola marina is found in nature in concentrations down to 23% sea water, and its tissues are capable of functioning in sea water diluted to that extent (Fig. ) (144). Some freezing-point values for this species in natural and experimental media are shown in Table (cf. Fig. ). Species such as A. marina which can tolerate considerable dilutions of their body fluids are able to live in brackish water of reduced salinities (8%0 and less), but they are unable to tolerate fresh water. To invade rivers and lakes, animals have needed some powers of osmoregulation, together with an integument which resists flooding by water and prevents salt loss. During short periods some animals are able to fend off unfavourable. Please note that these images are extracted
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