27 



6. Species with a bipolar area of distribution interrupted in the trop- 

 ical part of the ocean: Lanceola clausi gracilis and Scina antarctica. To 

 this group must be added S. wagleri abyssalis, which is found only in 

 catches from the abyssal and ultra-abyssal depths of the Kuril- Kamchatka 

 and Izu-Bonin trenches in the Northern hemisphere and Kermadec Trench 

 in the Southern hemisphere. 



Several species are distributed throughout the entire tropical part 

 of the Indian Ocean but are nevertheless absent in the northern and 

 northeastern part of the Arabian Sea. Fairly extensive material from the 

 Arabian Sea enables us to speak with a fair degree of conviction about 

 the absence of some rather common Physosomata in its waters. 



In the above region, in intermediate waters at depths from 100-200 

 to 1,000-1,500 m, a sharp oxygen minimum is established wherein the 

 oxygen content is less than 0.1 ml/1 and at a depth of 200-800 m free 

 HjS appears (Ivanenkov and Rozanov, 1961). In the upper part of the 

 minimum oxygen layer, in the 100-200 m layer, and especially in the 

 200-500 m layer, the quantity of plankton sharply decreases, sometimes 

 to almost nil; several oceanic species inhabiting depths corresponding 

 to the minimum oxygen layer do not enter regions covered by it (Vino- 

 gradov and voronina, 1961). 



Among hyperiideans, the majority of species of the upper subzone 

 of the deepwater zone (200-2,000 m) also do not enter regions defi- 

 cient in oxygen and free H2S. Such, for example, are many species of 

 the Scinidae: S. crassicornis, S. borealis, S. spinosa, the typical form of 

 S. rattrayi, S. submarginata, S. similis, S. vosseleri, and some others. 



As a rule, species living at greater depths (for example, 

 Scypholanceola agassizi, Lanceola loveni, Microphasma agassizi) 

 or conversely in surface layers {Lanceola felina, Scina tullbergi, 

 S. marginata) and highly eurybathic species found in all water depths 

 {Lanceola sayana, L. clausi, Scina rattrayi keilhacki), enter the north part 

 of the Arabian Sea in the region of sharp oxygen deficit. However, here 

 they are found in layers above or below the zone of oxygen minimum. 

 For example, Microphasma agassizi was found in the 970-1,920 

 and 1,900-3,750 m layers; Lanceola loveni, 970-1,920 m; L clausi, 

 1,000-2,040 m; L sayana, 106-213 m as well as 97-1,920, 1,520-2,050, 

 2,(X)0-3,400 m; and finally, Scina rattrayi keilhacki, 100-200 m. Rarely, 

 stray specimens do enter directly into the oxygenless* layer. 

 32 Quite possibly, specific hydrological conditions in the northern part 



of the Arabian Sea restrict the distribution of many hyperiideans. Evi- 

 dently, this is also true of the Bay of Bengal; nevertheless, available 



"Oxygen-deficient" is meant here — Eds. 



