358 



significantly longer than pereopods VI and VII; and the inner distal angle 

 of the basipodite of uropods III is only slightly stretched. 



Although recendy subjeced to a special revision (Bowman, 1960), 

 the systematics of the genus is quite confused since investigations con- 

 tinue to appear which change the concept of the validity of some species 

 (Sheader and Evans, 1974). At present the genus includes six species. 



KEY TO SPECIES OF GENUS PARATHEMISTO 



1 . Pereopods V and VI approximately equal in length or pair VI slightly 

 longer than V 2. 



— Pereopods V significantly longer than pair VI 5. 



2. Maxillipeds with row of setae in distal part of basal plate 3. 



— Maxillipeds without row of setae in distal part of basal plate 



1. P. (P.) abyssorum Boeck. 



3. Antennae I in females narrow, straight. Claws of pereopods V-VI 

 bear setae in proximal part of anterior margin 4. 



— Antennae I in females broad, curved. Claws of pereopods V-VI 

 smooth, without setae 6. P. (E.) australis (Stebb.). 



4. Antennae I and II in females equal in length. Length of adult females 

 4.5-8.5 mm 3. P. (P.) pacifica (Stebb.). 



— Antennae 11 in females longer than antennae I. Length of adult 

 females 9-17 mm 2. P. (P.) japonica Bov. 



5. Claws of pereopods V-VII bear setae in proximal part of anterior 

 margin 4. P. (E.) libellula Licht. 



— Claws of pereopods V-VII smooth, without setae 



5. P. (E.) gaudichaudi (Gu6rin). 



All representatives of the genus Parathemisto lead a free mode of 



life. They inhabit the surfacial and medium-deep layers of the pelagic 

 zone in cold-water and moderately cold-water regions. Some species play 

 a significant role in the plankton of cold-water and moderately cold-water 

 regions of the peripheral seas and oceans where they hold 3rd and 4th 

 place in terms of biomass, after copepods, chetognaths and euphausiids; 

 sometimes they form huge congregations. For example, in the northwest- 

 ern part of the Pacific Ocean, within 10 minutes of trawling the surface 

 water with a small pleusiton net we were able to catch more than 1 .5 kg 

 of minute Parathemisto pacifica. In Antarctic waters concentrations of 

 P. gaudichaudi sometimes clog the water-intake strainers of ships, while 

 293 in New Zealand dead crustaceans thrown up by storms have covered 

 large areas of the coast (Gray and McHardy, 1967). Expectedly, Parath- 

 emisto play a significant role in the food of several plankton-feeding 

 fishes. 



