36 



Paratyphis parvus, Rhabdosoma brevicaudata* , Platyscelus ovoides, and 

 Eupronoe minute. Nevertheless, even these species are not found deeper 

 than 600-800 m. At the same time, it was found that most of the Platy- 



36 sceloidea perform daily vertical migrations of small amplitude. During 

 the day most of the species leave the upper 50-meter layer; popula- 

 tions thereby become less concentrated vertically and occupy a 50-200 

 (300 m) layer. At night, from 7:00-8:00 p.m. to 2:00-4:00 a.m., they 

 rise to the surface and mostly concentrate in the 0-50 (100) m layer. 

 The Oxycephalidae, except for the more eurybathic Rhabdosoma brevi- 

 caudatum and Streetsia challengeri, are more intimately associated with 

 surface waters and, as a rule, even in the day do not leave the 50-100 

 (150 m) layer, and at night concentrate in the 0-50 m layer and up to 

 100 m. In some areas of the ocean, in view of some special features 

 of water dynamics, the usual pattern of vertical distribution may change 

 somewhat. 



The Platysceloidea are exclusively warm-water crustaceans, some 

 of which never leave the tropical oceanic zone, while others may enter 

 temperate latitudes with warm currents but are confined there to the 

 warmest water regions, under the direct influence of warm tropical or 

 subtropical waters. Only one species, Tryphana malmi, may be consid- 

 ered a temperate-warm-water species since, on the one hand, it is do not 

 found in the equatorial zone and, on the other, penetrates the farthest 

 into high latitudes (to the Irminger Sea in the Atlantic Ocean: 67° 30' 

 N, to 51° S in the Indian Ocean and to 42° N in the northwest Pacific 

 Ocean). In the Irminger Sea region (to 60° N) Rhabdosoma minor is also 



38 transported with warm waters of the Gulf Stream. It is not possible to 

 clearly delimit the broad- and narrow-range tropical species, since many 

 of them are known from a single catch. 



Inadequacy of data on the distribution of many species precludes 

 the possibility of inferring with certainty the extent of the circumoceanic 

 nature of the gioup. The rarest of recently described species do not seem 

 to occur in all the oceans. It is more than probable that their absence in 

 material from any ocean is the consequence just of inadequate study of 

 the distribution of these species. Only for a small number of species is 

 the area of distribution restricted to small regions of the tropical zone. 

 Such, for instance, are Tullbergella cuspidata or Euscelus robustus, found 

 only in the seas of the Malayan archipelago and adjacent regions of the 

 Pacific and Indian oceans. 



It is noteworthy that species diversity of the fauna of the Atlantic, 

 Indian, and Pacific oceans is almost identical in this essentially circum- 

 tropical group. More than 90% of the reported genera and 80-90% of the 



Given as Rhabdosoma brevicaudatum in Russian text — ^Ed. 



