38 



THE BIOLOGY OF MARINE ANIMALS 



conditions by shutting themselves off from the outside world, e.g. oysters, 

 mussels and barnacles. In dilute media lamellibranchs such as Mytilus and 

 Scrobicularia retract their siphons or close their shells, thus protecting 

 the blood from extreme changes in concentration (34a). The acorn barn- 

 acle Balanus improvisus is reported to be markedly euryhaline ; specimens 



TABLE 2.2 

 Arenicola marina 



of B. balanoides and B. tintinnabulum can survive for many days in 

 fresh water, or the atmosphere, shutting their valves except for a minute 

 breathing aperture (2a). These, of course, are behavioural mechanisms for 

 avoiding inclement conditions. 



Osmoregulation 



A number of mechanisms are available to euryhaline animals which 

 attempt to keep their internal medium constant in the face of changing 

 external salt concentrations, and these have been utilized in variable degree. 

 Some animals show only very limited powers of osmotic regulation, and 

 from this level there range all degrees of regulatory ability to that possessed 

 by truly homoiosmotic species which are able to maintain their internal 

 fluids relatively constant, irrespective of wide external fluctuations in 

 salinity. 



In waters of lowered salt concentration, such as estuaries, animals can 

 passively oppose reduced salinities by means of membranes having reduced 

 permeability to waters and salts. They can also react functionally by actively 

 pumping out the water which tends to flow into the organism, and by 

 absorbing salts from the surrounding medium so as to keep up the internal 

 salt level in the face of various conditions tending to cause internal dilu- 

 tion. Waters of greatly raised salt content are not often encountered by 

 marine animals and are most liable to be found in tidal pools high on the 

 shore which are subject to evaporation during neap tides. Mechanisms 

 utilized against high salinities involve membranes with lowered perme- 

 ability to water and salts, active absorption of water against a salinity 

 gradient, and secretion of excess salts. All processes involving active trans- 

 fer of water and salts against osmotic gradients necessitate the expenditure 

 of energy by the organism in the form of osmotic work, and theoretically, 

 should be capable of detection in the form of a corresponding increased 

 level of oxygen consumption. 



