Effects of Temperature on Tropical Marine Animals. 5 



at about 7.75° C, while those from Nova Scotia continue to move at a 

 temperature of —1.4° C. with ice floating in the water above them. If 

 frozen solidly into the ice the Tortugas medusae are killed, but (as was 

 observed by Romanes, 1885), our northern Aurellia withstands this treat- 

 ment without serious injury. 



Aurellia aurita is more abundant in cold than in warm seas, however, 

 perhaps on account of the more abundant zooplankton and food-supply in 

 cold waters; yet it may be due to a better adjustment to environmental 

 temperature in the medusae of the cold waters, for at Halifax the surface- 

 temperature in summer is about 14° C. and the range in activity of the 

 medusa is then about 15° C, both above and below this temperature, the 

 animals being most active at from 18° to 23° C. On the other hand, at 

 Tortugas the average surface-temperature in summer is about 29° C, 

 being practically identical with that of the animal's greatest activity; but a 

 rise of 8° C. would cause all movement to cease and is generally fatal to the 

 medusae, while the same species at Tortugas can still pulsate even at a 

 temperature of 21° below that of the ocean in which they live. It appears 

 probable that Aurellia is a boreal or arctic animal which has wandered into 

 the tropics and become fairly well acclimated, although living in these warm 

 waters within 9° C. of its death-temperature. Thus, in comparison with 

 their northern relatives, these Aurellias in the tropics are poorly adjusted 

 to their temperature environment, and a change of even 2° or 3° above their 

 normal temperature causes a decided lassitude and loss of rate in pulsation. 

 We see, therefore, that the medusa has become adapted to a tropical environ- 

 ment, but this has been accomplished at the expense of its factor of safe 

 adjustment to temperature changes. 



Harvey (191 1, p. 34) showed that Cassiopea, which is strictly confined 

 to the tropics, can not withstand a higher degree of heat than can Limulus 

 at Woods Hole, Massachusetts. Thus, Cassiopea is an animal living 

 constantly within 15° C. of its death-point, yet it is not adapted to with- 

 stand higher temperatures than the heart of a northern animal {Limulus) 

 living at 25° to 30° C. from its death-point. 



In general, one finds that tropical marine animals normally live at tem- 

 peratures much nearer their death-point than do northern forms. For 

 example, the most resistant reef coral, Siderastrcea radians, which lives at 

 about 28° to 30° C, is killed at 38.5° C, and the West Indian reef Madre- 

 pores are killed at about 35.8°. The tropical sea-urchins, Diadema (Centre- 

 chinus) setosum and Toxopneustes {Lytechinus) variegatus, and the brittle- 

 star, Ophioderma appressa, are killed at 37.4° to 37.7°. In these forms the 

 temperature of the sea-water in summer is only from 7° to 10° C. below the 

 death-temperature. By way of contrast, we may cite the cases of the 

 arctic forms Cyanea arctica and Bero'e cucumis, which are killed at about 27° 

 and 30° C, respectively, and which live in an ocean not warmer than 14° C, 

 thus being at least 13° to 16° C. from their death-points. 



This low factor of safety in tropical marine animals may at times become 



