282 MARINE ANIMALS 



than 1000 m. Appendicularians, limited in the ocean to the lighted 

 layer, descend to a depth of over 1000 m. Pyrosomas, lacking in the 

 ocean below 400 m., have been obtained 49 at a depth of 1200 m. at 

 Naples; salpas, also chiefly surface animals, are at times found in 

 masses (Salpa maxima) among the contents of the fishermen's bottom 

 nets. 50 



The high temperature of the deep water makes it possible for many 

 stenothermal animal species of warm water to survive the winter in 

 the Mediterranean. In the Gulf of Naples, a number of fishes are not 

 caught in the cold season which are common in summer, e.g., Serranus 

 gigas, Julis turcica. Experiments in the Naples aquarium show that 

 these animals do not tolerate a decrease in temperature below 12°; 

 they become inactive, sink to the bottom, and die with further decrease 

 in temperature. 51 Such forms can survive the winter in the warm 

 Mediterranean depths. Thus tropical and subtropical forms, which 

 are carried toward the Straits of Gibraltar by the southern arm of the 

 Gulf Stream in some numbers, can survive in the Mediterranean. Such 

 forms include the scyphozoan Charybdea, the tropical jellyfish of the 

 genus Liriope, many siphonophores, many heteropods, the sunfish 

 Mola mola, the turtle Caretta caretta, and many others. 



But the deep water of the Mediterranean does not everywhere 

 contain sufficient oxygen to permit active animal life. Though in some 

 places, e.g., in the northern part of the Balearic Sea, in the southern 

 part of the Adriatic, and in the Aegean, an active descent of the 

 surface water into the deep has been demonstrated, 52 in other parts 

 the deep water is rich in carbon dioxide and is therefore so poor in 

 animal life that Forbes's investigations of this area led to the erroneous 

 conclusion that the depths of the oceans in general were without animal 

 life. 



Conditions in the western part of the Mediterranean nearer the 

 Straits of Gibraltar and the open ocean are more favorable for animal 

 life than those in the eastern half, and many animal groups such as 

 starfishes, salpas, and prosobranch snails have fewer species in the 

 eastern than in the western half. The exact cause of this eastern de- 

 crease is unknown, just as it is still an unsolved problem why many 

 pelagic animals which are taken in large numbers directly west of 

 the Straits of Gibraltar do not occur in the Mediterranean. This 

 applies, for example, to the fishes Argyropelecus olfersi and Vinciguer- 

 ria sanzoi, and to a number of eel larvae, whereas the larvae of the 

 river eel (L. brevirostris) pass through the Straits of Gibraltar in large 

 numbers. 53 



Species out of the most widely separated animal groups, benthic 



