50 



THE BIOLOGY OF MARINE ANIMALS 



keeps its blood hyperosmotic to the external medium when the concentra- 

 tion of the latter drops. In G. pulex the blood concentration is relatively 

 low but is maintained above that of the external medium. The nephridium 

 is larger and more complicated in G. pulex (fresh water) than in G. locusta 

 (marine) and this suggests a functional role in salt resorption in the fresh- 

 water species (4, 5, 132). 



Vertebrates. Among cyclostomes the myxinoids are exclusively salt- 

 water forms. The blood is about isosmotic with sea water, and the osmotic 

 pressure is due largely to inorganic ions. In dilute or concentrated sea 

 water (salinity, 25-40% o ), blood chloride of Polistotrema follows that of 

 the medium. Some lampreys are anadromous in habit, ascending rivers 



— Fresh water — »j i. 

 Tidehead Spr/nrfsl 



-Brackish water- 



-Sea water - 



Neaps 



Mouth 



\dammarus locusta^ 



Gammarus zaddachi salinus 



Oammarus z. zaddachi 



Gammarus chevreuxi (W.Britain) 



Gammarus pulex 

 Gammarus duebeni 



Fresh 



Ma rinogamma ru§ 

 obtusatus 





Ma r i nogammarus warmus 



Salt 

 .Spring 



rSs 





Salinity 

 gradient 



Fig. 2.15. Diagram Representing the Relative Distributions of some 

 Related Gammarids in Estuarine and Contiguous Regions of Rivers 

 (Suggested by a figure of Bassindale's, 1942) 



from salt water to spawn. In the sea they are markedly hypo-osmotic 

 (blood of Petromyzon marinus from the Mediterranean, A 0-586°C). In 

 fresh water, lampreys regulate by mechanisms similar to those of teleosts 

 {see later section). Fresh-run lampreys (Lampetra fluviatilis) can maintain 

 their plasma chloride constant only in media more dilute than half sea 

 water. Presumably, some change takes place in their capacity to osmo- 

 regulate during up-stream migration. Chloride excretory cells, resembling 

 those of teleosts, occur in the gills of lampreys (see p. 54). It is these 

 cells which are responsible for the excretion of salt across the gills when 

 the animals are in sea water (22, 29, 9\a, 916, 114, 146a). 



There is evidence for believing that both major groups of extant fishes, 

 the elasmobranchs and teleosts, are fresh-water in origin and during their 

 successful invasion of the sea they have had to accommodate themselves 

 to a hypertonic medium. Fishes are homoiosmotic to a considerable degree 



