Detection of sex and sex ratio in Calanus finmarchicus early stage fifth copepodites

Ability to determine sex ratios from developing copepodites collected in th e field has, in the past, been limited to observations of overall gonad sha pe and size. This approach has been useful in distinguishing developing mal e and female fifth copepodites (C5s), but is limited to individuals in the final stages of gonad development. We have refined the approach by identify ing and describing five distinct gonad types found in C. finmarchicus C5s a nd categorizing the early stages of sexual differentiation in the gonad on the basis of gonad shape and cell size. In Calanus, the first meiotic divis ion of cells in the gonad occurs well before it has begun to take on the ch aracteristic shape of an ovary or testis. Cell size can be seen easily afte r staining the gonads and clearing the integument and somatic tissues. Deve loping testes and ovaries can be recognized at an early stage by observing relative cell size anterior and posterior to the zone of first meiotic divi sion. We used this technique to make preliminary determinations of the sex ratio in developing C. finmarchicus C5s on and around the Georges Bank duri ng the 1998 GLOBEC Broadscale Survey. These data are compared with monthly sex ratios calculated from adult abundance from the same sample set. Result s show that the seasonal patterns of C5 and adult sex ratios are similar, w ith a temporal offset attributed to developmental timing. In all:samples, t he proportion of developing male C5s is higher than the proportion of adult malts, which we attribute to differential mortality between adult males an d females. (C) 2000 International Council for the Exploration of the Sea.