Breeding activity of marine phytal harpacticoid copepods from Cook Strait

Abstract

The reproductive status of 20 species of alga‐dwelling harpacticoid copepods at Island Bay, Wellington, New Zealand, was investigated from April 1973 to March 1974. These data form the first comprehensive account of the breeding regime in natural populations of truly phytal species.

In most species the breeding season, as defined by the presence of ovigerous females in the population, was either continuous or protracted: distinct maxima occurred at different times in different species, but a large number (40%) reached maximum breeding activity in the autumn. However, the proportion of cope‐podites appears to be a better indicator of the effects of reproduction, because increases in this index generally lead to the observed maxima in population density. By the relationship between the yearly mean number of eggs per female and the mean percentage of ovigerous females in the population, the 20 species fall into 5 identifiable groups: Group 1 of 4 species, where 57–78% of the female population are ovigerous, each with a low number of eggs per female (7–13); Group 2 of 6 species, where 32–42% of the female population is ovigerous, each with 10–19 eggs per female; Group 3 of 7 species, with a low percentage of ovigerous females (12–30%) carrying 15–25 eggs; Group 4 of 2 species, with 30–32% of the females ovigerous and carrying a large number of eggs per female (38–45); Group 5 of 1 species, with a high percentage of ovigerous females (60%) and with a large number of eggs per female (50).

In general, breeding is not correlated with temperature and food supply; continuous breeding is perhaps related to the small summer—winter temperature differential in Cook Strait, and to a suspected unlimited availability of food. Differing physiological responses to temperature and food results in reproductive maxima being reached at different times of the year in various species. Onset of reproduction in the supralittoral pool species Robertsonia propinqua (T. Scott) may be determined by a combined temperature/salinity regime of 13–14° c at about 30–35‰.

Sex ratios varied noticeably both within and between species from season to season, probably because of a homeostatic mechanism related to population density. Females predominate during low population densities, but males predominate during high abundance. The most abundant species occurring on Corallina have their maxima of reproductive activity temporally separated. Such a succession may reduce competition between them.

The strictly phytal species Porcellidium dilatatum Hicks (Porcellidiidae) has longevity and egg maturation rates which are similar to those of level‐bottom meiobenthic species. In general, reproductive capacity is higher in alga‐dwelling species than in interstitial species.

Notes

Present address: Wellcome Marine Laboratory, Robin Hood's Bay, North Yorkshire YO22 4SL, United Kingdom.