WATER, SALTS AND MINERALS 



75 



jelly coat. Eggs denuded of jelly swell or shrink in anisotonic media, and 

 disintegrate in sea water of salinity less than 14% , whereas eggs protected 

 by jelly will develop even in distilled water. Larvae show volume regulation 

 in hypotonic sea water (down to 22% ) presumably due to salt loss (Fig. 

 2.19). Sex cells and larvae of the California mussel Mytilus californianus 

 are susceptible to dilutions below 29-6% , and survival declines below this 

 concentration. This species is characteristic of coasts with high salinity, 

 and the narrow salinity tolerance of the larvae is one factor restricting its 

 distribution (75, 149). 



There is space to examine only certain aspects of water and salt relations 

 in fish eggs and larvae. Oviparous selachians produce what Needham has 

 termed a cleidoic type of Qgg which is independent of the medium for its 



20 



40 



60 80 100 



Time (minutes) 



Fig. 2.19. Volume Regulation in Larvae of a Polychaete, Marphysa gravely i 



Smoothed curves showing percentage increases of volume in waters of lowered salinity. 

 (After Krishnamoorthi (75).) 



supply of water and salts. The egg membrane is largely impermeable to 

 urea, and the accumulation of this substance as the result of protein 

 metabolism gives rise to final concentrations approaching those of the 

 adult. 



The eggs of marine teleosts are at first permeable to water and salts, 

 but during the course of development osmoregulatory ability develops 

 and the chloride content of the embryo is reduced. In contrast, eggs of the 

 euryhaline killifish Fundulus heteroclitus are nearly impermeable to water 

 and salt, and display ionic and osmotic regulation from earliest stages. 

 The genital products, eggs and sperm, are initially in osmotic equilibrium 

 with the parents' fluids. Plaice eggs (Pleuronectes platessa), which are 

 hypotonic to sea water when laid (eggs A 0-70, sea water A 1-91), will 

 develop in 20% sea water, and herring eggs (Clupea pallasii and C. 



