General Features of the External Morphology of Marine Planktonic Copepods
Sex-ratio
The sex ratio is rarely close to 1, but most often in favour of females. Of the 2167 species of pelagic copepods, both sexes are known in 65% of the total, 29% for females only and 6% for males only. This may sometimes be due to experimental bias (different vertical distribution, insufficient sampling, different growth rates between males and females in older stages, and primarily different life spans between the sexes).
The proportion of females in Tisbe sp. (Harpacticoida) is higher when the rate of heterozygosity increases (Battaglia et al., 1958, 1963).
Various environmental factors may be involved, including temperature and food (the latter through their effects on the metabolism and energy balance of each sex, Omori, 1970). According to Paffenhofer (1970), an ontogenetic selection takes place in Calanus pacificus to the detriment of males in relation to food type.
Under certain environmental conditions, genotypic males may become phenotypic females in Acartia tonsa (Heinle, 1970).
In Tigriopus japonicus (Harpacticoida) the sex of the adult can be determined experimentally according to the chlorine concentration.
Dussart (1969) notes that the number of males is higher when the pH departs from neutrality in Macrocyclops albidus (Cyclopoida). Ultraviolet radiation may have a lethal effect on female development.
In brackish water, an increase in chlorinity acts in the same direction in a harpacticoid at the second generation (Raibaut, 1967).
Coker (1933) found that in cultures of Cyclops vernalis, at high temperatures mainly males are obtained, at low temperatures mainly females, and a mixture of both sexes at intermediate temperatures.
In Monstrilloida, sex appears to be determined by the conditions of host infestation. If the host harbours two or three parasitic larvae, they remain small and all produce males, whereas if there is only one, it produces a male or a female.
Finally, the action of cytoplasmic factors has been evoked, the sex ratio being linked to the duration of egg incubation (Igarashi, 1963).
As can be noted, interesting information comes from research on non-pelagic copepods, knowledge of which remains necessary for possible extrapolation to the group as a whole. The case of resting eggs or the diapause phenomenon, well known in freshwater copepods, was only demonstrated much later in the case of certain pelagic marine copepods.
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