CLINAL VARIATION IN THE FREQUENCY OF ONE- AND TWO- SPINED FORMS OF THYSANOESSA INERMIS (KRØYER, 1849) (EUPHAUSIACEA) IN THE NORTH PACIFIC BY TAKAHISA NEMOTO 1, 2), MASAKUNI ARAKI 2) and EDWARD BRINTON 1) The typical spine or denticle on the 6th abdominal segment of Thysanoessa iz2ermi.r (Kroyer) is often referred to as a good taxonomic character (Hansen, 1911; Boden, Johnson & Brinton, 1955). The two-spined form which has an additional spine on the 5th abdominal segment was collected off Cape Cod (Hansen, 1.915). Hansen noted further that in specimens from the North Atlantic the 5th abdominal segment usually has no dorsal denticle but that this denticle seems to exist in all specimens from his North Pacific localities (Hansen, 1915). Results obtained by Einarsson (1945) in the Atlantic also showed that none of his numerous specimens bore even the slightest sign of a spine on the 5th segment. Recently, further study of specimens in the North Atlantic was carried out by Jones, Forsyth & Cooper (1967). Their results show seven two-spined individuals among a total of 147 males, and four among 77 females in materials collected in the North Sea. As for T'hy.ranoe,r.ra inerzrei.r in the Pacific, Banner (1950)
Crustaceana – Brill
Published: Jan 1, 1973
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