Reproductive periodicity in the euryalinid snake star Astrobrachion constrictum in a New Zealand fiord

The euryalinid snake star Astrobrachion constrictum (Farquhar), a normally deep-water species, exists within SCUBA diving range in New Zealand flords, and thus presents a unique opportunity for study. A population of A. constrictum in Doubtful Sound was investigated from February 1990 to July 1994 to determine the reproductive cycle. Unlike most ophiuroids, 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 was calculated for each individual based on the ratio of cross-sectional area of the gonads to cross-sectional area of the arm. Maturity indices were calculated based on stage of development of the gonads of each individual. A. constrictum has a period of enhanced breeding activity in April–June, after which both the gonad and maturity indices decline. The increase in gametogenic activity during these months was correlated with rising sea temperature within the fiord. Although the females showed an annual pattern of gonad development, oogenesis was continuous throughout the year, with oocytes at different stages of development present in the gonads. Males also showed continuous spermatogenesis and, although the same annual pattern of gonad development was evident 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.