REPRODUCTIVE PERIODICITY IN THE EURYALINID SNAKE STAR ASTROBRACHION CONSTRICTUM IN A NEW-ZEALAND FJORD

The euryalinid snake star Astrobrachion constrictum (Farquhar), a norm ally deep-water species, exists within SCUBA diving range in New Zeala nd fiords, and thus presents a unique opportunity for study. A populat ion of A. constrictum in Doubtful Sound was investigated from February 1990 to July 1994 to determine the reproductive cycle. Unlike most op hiuroids, the gonads of A. constrictum are positioned along the aboral coelom in the proximal portion of the arms. The proximal 2 to 3 cm of the arms of individuals were thus removed on an approximately monthly basis for histological examination of the gonads. Image analysis was used to determine oocyte size-frequency distributions. A gonad index w as calculated for each individual based on the ratio of cross-sectiona l area of the gonads to cross-sectional area of the arm. Maturity indi ces were calculated based on stage of development of the gonads of eac h individual. A. constrictum has a period of enhanced breeding activit y in April-June, after which both the gonad and maturity indices decli ne. The increase in gametogenic activity during these months was corre lated with rising sea temperature within the fiord. Although the femal es showed an annual pattern of gonad development, oogenesis was contin uous throughout the year, with oocytes at different stages of developm ent present in the gonads. Males also showed continuous spermatogenesi s and, although the same annual pattern of gonad development was evide nt as seen in the females, it was less marked. The presence of females with large oocytes and ripe males throughout the study indicates that breeding in the Doubtful Sound population is prolonged, and possibly continuous, as found in many deep-sea ophiuroids.