Previous (workers who have examined the gut contents of members of the genus Parathemisto, and who have kept and fed specimens in laboratory conditions, have concluded that they are chiefly carnivorous in habit, feeding largely on zooplankton, and to a lesser extent on phytoplankton (Conover, 1960; Baker, 1963; Kane, 1967).The gut of P. gaudichaudi has been found to contain almost intact copepods, especially Calanus and occasionally Temora (Bigelow, 1926 – Gulf of Maine); young specimens of Euphausia superba (Hardy & Gunther, 1935 – Antarctic); and copepods (Nemoto & Yoo, 1970). Siegfried (1965 – west coast of S. Africa) found P. gaudichaudi to be a rather indiscriminate feeder, its gut contents reflecting the composition of the plankton.
All groups of meso- and macro-zooplankton in the North Sea off Northumberland, at a depth of 53 m, were studied during a 15-year period (1969–83); copepod productivity was estimated from biomass and growth rates. Phytoplankton were seasonally bi-modal with peaks in April and August–October; copepods were uni-modal peaking in June–July. The predatory zooplankters: larval fish, decapods, ctenophores, medusae (the summer-autumn predators) peaked between May and September, while chaetognaths and euphausiids (the winter predators) peaked in December–January. Copepods and the summer-autumn predators were seasonally and inter-annually positively correlated, and declined in abundance from 1974 to 1980. Euphausiids and chaetognaths on the contrary increased in abundance during these years, and were seasonally and inter-annually negatively correlated to the copepods. The mean annual abundance of copepods was positively related to the previous winter's minimum, and inversely related to the abundance of chaetognaths and euphausiids. Annual copepod productivity averaged 1260 kJ m-2 year-1, and showed no relationship to other groups of plankton.
The results of morphometric and developmental studies made to elucidate the relationship between Parathemisto gaudichaudi (Guerin) and P. gracilipes (Norman) show them to be one and the same species. Apparent differences are due to differences in the size of specimens at maturity, and to the degree of development of the bispinosa-compressa condition. P. gaudichaudi takes precedence over P. gracilipes.The species is highly variable, existing in two extreme forms, the bispinosa form and the compressa form, together with all possible intermediates. The full range of the bispinosa-compressa condition is not expressed until a body-length of 10 mm is reached. Specimens are capable of changing towards either extreme at moults, but the factors causing the change are unknown.A key to the genus Parathemisto Boeck is included.
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