Spawning and sexual reproduction in the octocoral, Sarcophyton elegans (Cnidaria: Alcyonacea), from Lizard Island, Great Barrier Reef

Two study sites were chosen at Lizard Island, in shallow areas with a high abundance of the soft coral, Sarcophyton elegans. Site A was located on Loomis Reef at the western entrance to the Lizard Island lagoon and Site B was located on a small wave-exposed patch reef near South Island. At each site, three 1x10 m belt transects were laid down end to end. Transects were located parallel to the direction of the current in order to examine the effects of spawning by upstream colonies on the reproductive behaviour of downstream colonies. The position of each Sarcophyton elegans colony within the belt transects was recorded and in December 1991, the basal circumference of the stalk of each colony within the transects was measured to determine colony size distribution.

In October 1991, prior to the spawning season, the mesenteries of each colony within all transects were examined by making a small incision in the polypary and the sex of each colony was recorded. The reproductive state of each colony was recorded monthly between October 1991 and January 1994. Spawning activity was monitored between October 1991 and February 1992, August 1992 and February 1993 and August 1993 and January 1994. The spatial distribution of immature, mature and spawned mesentaries in individual polyparies was investigated by making incisions at several points along the radius of the polypary and examining the exposed mesentaries. A colony was recorded to have released its gametes when ripe gametes were recorded as present during a sampling event and absent in the next sampling event. Detailed studies of the timing of gamete release within transects was also carried out at Site A only.

Fifty colonies of different size classes were collected outside the transects prior to the spawning season in 1992, to determine the minimum colony size at which sexual maturity is reached. The basal circumference of the stalk of each colony was measured and each colony was sectioned in the laboratory and examined under a light microscope to detect the presence of gametes. Histological samples of the smaller colonies were also prepared and examined for gametes.

To determine the length of the gametogenic cycle, small pieces of tissue were excised from the polypary of 10 large male and 10 large female marked colonies, located outside the transects. Tissue samples were collected on an approximately monthly basis between October 1991 and January 1994. The samples were preserved, decalcified and the diameters of the oocytes and spermaries were measured under a compound microscope. Paraffin sections from each sample were stained with hematoxylin and eosin for histological examination to determine the stage of gonadal development.