Distribution of dimorphic males of three species of Nematoscelis (Euphausiacea)

Abstract

Samples collected from the eastern Atlantic Ocean between 40° and 4° N with rectangular and ring nets contained male specimens of Nematoscelis atlantica, N. microps, and N. tenella (Euphausiacea) with saddle-shaped thickenings of chitin on the dorsal surface of some abdominal segments, and with enlarged photophores on the abdominal segments immediately posterior to those with thickenings. Two forms of each species are distinguished by the number and the position of chitinous thickenings and enlarged photophores. Except in a zone off W. Africa (approximately between 22° and 16° N) each form of N. atlantica and N. microps occurs as a distinct population. These populations appear to be characteristic of either the North or South Atlantic Central water, and the overlap zone corresponds approximately with the physical boundary between these water masses and with a faunal transition zone found in this region by other workers. For N. tenella it appears that only one form occurs as a separate population south of 18° N in South Atlantic Central water. Between 28° and 18° N the two forms co-exist.