292 THE BIOLOGY OF MARINE ANIMALS 



urine of elasmobranchs is only 10% of that in the blood, and this suggests 

 that it is actively retained by the fish, presumably as an osmoregulatory 

 agent (11, 12, 17, 28). 



Conditions in Littoral Invertebrates 



As previously noted the mode of nitrogen excretion has adaptive signific- 

 ance in connexion with osmotic stress and the water relations of the organ- 

 ism. Littoral animals might be expected to show specialized trends in 

 nitrogen excretion corresponding to the degree of exposure and desiccation 

 to which they are subjected, but unfortunately only a few species have 

 been investigated. Gastropods show some tendency for gradation from 

 ammonotelism to uricotelism when species are ranked from aquatic to 

 terrestrial forms. At the one extreme are sublittoral and lower littoral 

 marine gastropods, such as Aplysia, which excrete much ammonia and 

 little uric acid. At the other are the strictly terrestrial snails and slugs, the 

 excreta of which contain more uric acid. Terrestrial and freshwater gas- 

 tropods show a tendency to uricotelism. The excretion of uric acid in 

 lieu of ammonia is of value to animals living under conditions of restricted 

 water supply and producing cleidoic eggs. 



The uric acid content of the nephridia of a long series of molluscs 

 examined by Needham shows nice correlation with habitat (Table 7.4.) 

 Uric acid levels, low in sublittoral and low-littoral lamellibranchs and 

 gastropods, are higher in littoral periwinkles and show maximal values in 

 terrestrial snails and slugs. Further analyses of the partition of non- 

 protein-N in the excreta of marine molluscs are required to substantiate 

 these correlations (cf. Table 7.2). 



The different species of periwinkles listed in Table 7.4 show marked 

 zonation on the shore. Two species, the flat periwinkle Littorina 

 littoralis and the common periwinkle L. littorea, are found on the lower 

 half of the shore. Occurring at higher levels, from the middle shore to 

 high-tide mark, is the rough periwinkle L. saxatilis, while the small peri- 

 winkle L. neritoides inhabits crevices above high-tide mark. In apparent 

 agreement with the degree of exposure which these different species 

 encounter is an increase in the amount of uric acid excreted, which reaches 

 its maximum in L. neritoides, the species subjected to greatest exposure. 

 There appears to be no correlation between breeding habits and uric acid 

 excretion, as has been suggested, however. Thus, both L. neritoides and 

 L. littorea produce free-swimming larvae; L. littoralis produces large eggs 

 in which the larvae develop to the crawling stage ; whereas L. saxatilis re- 

 tains eggs and larvae in a brood pouch. 



A pulmonale Onchidella celtica, which has returned to the sea and which 

 seeks shelter in moist crevices during low tide, contains little uric acid. 

 Potamopyrgus jenkinsi, an estuarine species which has invaded fresh water 

 during recent historic times, produces little uric acid (Table 7.4). 



Aquatic and terrestrial amphipods and isopods are essentially am- 

 monotelic, and more than 50 % of the total non-protein-N of the excreta 



