Quantitative study on diagnostic characters of two pelagic amphipods, Themisto japonica and T. pacifica, along their growth
1991
Semura, H. (Hokkaido Univ., Hakodate (Japan). Faculty of Fisheries) | Minoda, T. | Kawamura, A.
Qunatitative study on diagnostic characters was made to elucidate developmental differences between Themisto japonica and Themisto pacifica, which are closely to each other and abundant in the northern North Pacific. In females, mean value of the length ratio of antenna to antennule is higher in T. japonica than in T. pacifica of the same size, although there are overlaps. The length ratio of antennal flagellum to head is always higher in T. japonica than in T. pacifica. Overlapping is observed in juveniles smaller than 5 mm. In females and juveniles, number of long setae on the inner margin of the carpus of fourth pereopod is one or two higher in T. japonica than in T. pacifica of the same size. the numbers of setae on inner and outer margins of propus of sixth pereopod of T. japonica are always higher than those of T. pacifica. T. japonica has always one more segment on the exopod of first pleopod than in T. pacifica in individuals longer than 2 mm. In adult males, these species are distinguishable by the numbers of setae on inner margin of the propus of sixth pereopod and of segments of the exopod of first pleopod. Whereas, in juveniles smaller than 5 mm, characteristics separating both species were not found in the present study
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