Reproduction in the giant clams Tridacna gigas and T. derasa in situ on the north-central Great Barrier Reef, Australia, and Papua New Guinea

Abstract

Small groups of T. gigas and a single individual of T. derasa have been observed to spawn sperm in nature during the austral summer on the Great Barrier Reef. In T. gigas diel periodicity to the spawning generally coincided with incoming tides near second (full), third and fourth (new) quarters of the moon phase. Intense spawning ranged from 30 min to 2 1/2 h, with spawning contractions occurring every 2–3 min. Eggs were never observed being spawned in nature. A new technique, an egg catching device, was used for sampling eggs of T. gigas which were observed spawning sperm. Results indicated that the egg phase of spawning did not follow spawning of sperm. Some clams which do not respond to a spawning neighbour may be reproductively inactive. In 70% of observations of nearest spawning neighbours the clams were within 9 m of one another. A hypodermic extraction technique was used for clam gonadal material. Examination of tissue suggested a potential early to mid-austral summer spawning for T. gigas, and to a lesser degree for T. derasa.