42 



THE BIOLOGY OF MARINE ANIMALS 



medium. In distilled water or in certain stream waters the animals die and 

 disintegrate, and it has been demonstrated that this is due to absence of 

 calcium in the external medium (Fig. 2.10). In nature Procerodes has been 

 found in streams with minimal calcium levels of 0-5 mg/L, where they are 

 exposed to fresh water for as long as 5 days during neap tides (100). 



Much of the water that is taken up osmotically by Procerodes, when in 

 dilute sea water, passes through the parenchyma to the endoderm where it 

 is stored in large vesicles. Contrary to expectation, these vacuoles do not 

 discharge into the gut. In dilute sea water oxygen consumption is increased 

 once the worms have attained a steady state, and swelling proceeds beyond 

 normal when the animals are subjected to oxygen lack, or are poisoned 

 with cyanide (100). 



Procerodes is probably isosmotic with sea water, and in dilute media the 

 tissues at first become flooded with water. Calcium is believed to act by 



40 60 80 



Time (minutes) 



Fig. 2.10. Behaviour of Procerodes ulvae in Distilled Water 



Upper curve shows increase in volume; lower curve, loss of salts. Arrows indicate 

 dissolution of ectoderm. (Somewhat diagrammatic. Smoothed curves based on a figure 

 of Pantin's (100).) 



lowering the permeability of the body wall to water and salts, thereby 

 reducing the inflow until regulatory mechanisms become effective, while 

 excess water is stored in endodermal vesicles. In summary, active osmo- 

 regulatory processes in Procerodes appear to involve: the secretion of 

 water, by nephridia or through the body wall, against an osmotic gradient; 

 maintenance of depressed ectodermal permeability; retention of water in 

 endodermal vesicles having a low salt content. 



Polychaeta. As indicated above, the sublittoral nereid worms Nereis 

 pelagica and Perinereis cultrifera adjust to low salt concentrations by 

 dilution of the body fluids and succumb when the external medium falls 

 much below 8% . N. diversicolor, on the other hand, possesses limited 

 powers of osmoregulation and is able to tolerate brackish waters with 

 salinities as low as 0-5% o . The internal fluids of the latter species are 

 isosmotic with normal sea water, but in dilute media they maintain some 

 degree of hypertonicity (Fig. 2.7). 



