76 THE BIOLOGY OF THE SEA-SHORE 



is to this apparently, says Semper {loc. cit.), " that we must 

 refer the fact that many genera of sea-creatures which are 

 known as boreal forms live in the north at great depths while 

 in tropical seas they live very near the surface." 



Correlated with the normally small range of temperature 

 variation in tropical seas is the fact that tropical marine 

 animals are much more susceptible to wide variations of 

 temperature when they occur than are forms inhabiting the 

 temperate or arctic zones. With the latter a considerable 

 range of temperature above or below the normal produces 

 relatively little difference in activity, whereas in the case of 

 tropical forms even a few degrees of cold or heat causes a 

 marked depression in movement. 



Generally speaking, tropical marine animals live within 

 5° C. of their temperature of maximum activity and within 

 io° C. or 15° C. of their upper death temperature. In order, 

 moreover, that a marine animal may live throughout the 

 year in the shallow or surface waters of the tropics it 

 must be capable of surviving at 29° C. For instance, 

 at Tortugas, Florida, when after several hot, calm days the 

 temperature of the shallow water over Bird Key Reef rose 

 to 33°-38° C, it was observed that large numbers oi Diadema, 

 Octopus, Fissurella, and other molluscs and small fish were 

 killed over extensive areas. Corals also were injured even 

 when not exposed to the air. Experiments conducted at 

 the Murray Islands (Australian Great Barrier Reef) show 

 that those corals which die at temperatures below 36*5° C. 

 are killed by being buried eleven hours under the mud, 

 but those which resist 37° and above are proportionately 

 less sensitive to the smothering effects of mud and may 

 survive burial for thirty to forty hours. This suggests that 

 high temperature produces death by causing asphyxiation, 

 the oxygen of the sea-water being insufficient to sustain the 

 increased metabolic activity of the animal (Mayer, 1914). 



Certain forms {e.g. Coelenterates and Sponges) have been 

 shown to enter on a quiescent phase during the heat of the 

 day in the tropics and to become active again later. Thus 

 Annandale (1907) finds that for some hours in the middle 



