44 



THE BIOLOGY OF MARINE ANIMALS 



the secretion of water. It is interesting, in this regard, that the nephridial 

 canal of N. diversicolor is longer and more convoluted than that of P. 

 cultrifera, suggesting nephridial participation in osmoregulation. 



Isolated tissues of N. diversicolor continue to function in low dilutions, 

 and display spontaneous activity in 5-10% sea water. In P. cultrifera, on 

 the other hand, the lower limits of salinity that will still permit muscular 

 activity lie between 20-25 % sea water (144). 



A seemingly racial difference has been described in the osmoregulatory 

 performance of N. diversicolor. Animals at Bangor and Plymouth 

 require more than 100 hours to complete their weight regulation in 20% 



-to 



."a 



C 

 o 



Co 



Time (hours) 



Fig. 2.12. Osmotic Adjustment in Nereis diversicolor 



Salt loss accompanying water intake in dilute sea water (concentrations shown 

 against each curve). (From Ellis, 1937.) 



sea water, whereas worms at Roscofftake only half that time (Fig. 2.1 1). 

 Racial differences of this kind might be expected to develop in animals 

 inhabiting waters of low salinity, as adaptations to new or altered environ- 

 mental conditions. 



Freshwater molluscs have well-developed osmoregulatory powers, and 

 this rather suggests that brackish-water forms may be able to regulate 

 their internal media. The bivalve Mercenaria mercenaria shows some hyper- 

 tonicity in dilute sea water (blood 374 mM CI, in sea water 319 mM CI). 

 Potamopyrgus jenkinsi ', sl brackish-water gastropod, has invaded fresh water 

 in recent historic times. On the other hand, certain freshwater pulmonates 

 have penetrated into brackish waters, e.g. Limnaea pereger, and several 

 species are found near high-tide mark on the shore. Littoral prosobranchs 



