Seasonal variability in gonad and jaw development in Calanus finmarchicus fifth copepodite: comparisons with oil sac fullness

J. Crain, C. Miller and M. Miller

The ability to determine sex ratios from developing copepodites collected in the field has been limited in the past to observations of overall gonad shape and size. This approach has been useful in distinguishing developing male and female fifth copepodites, but is limited to individuals in the final stages of gonad development. We have refined the techniques by categorizing the early stages of sexual differentiation in the gonad based on gonad shape and cell size. In Calanus, the first meiotic division of cells in the gonad occurs well before the gonad has begun to take on the characteristic shape of an ovary or testis. Cell size can be easily seen after staining the gonads and clearing the integument and somatic tissues. We have been able to recognize developing testes and ovaries at an early stage based on observations of relative cell size anterior and posterior to the zone where the first meiotic division takes place.

We have used this technique to make preliminary determinations of the sex ratio in developing C. finmarchicus fifth copepodites on and around Georges Bank during the 1998 GLOBEC Broadscale Survey sampling period. These data are compared with monthly sex ratios calculated from male and female abundances from the same sample set. Comparisons between sex ratios generated from early versus late stages of gonad development and insights into the timing of sex determination in Calanus are briefly discussed.