36 



THE BIOLOGY OF MARINE ANIMALS 



these animals, e.g. 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. 2.5). 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. 2.5. 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. 2.6) (144). Some 

 freezing-point values for this species in natural and experimental media are 

 shown in Table 2.2 (cf. Fig. 2.7). 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% 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 



