Early Life History of the Giant Clams Tridacna crocea Lamarck, Tridacna maxima (Roding), and Hippopus hippopus (Linnaeus)

Abstract

Giant clams may be stimulated to spawn by the addition of macerated gonads to the water. Individuals of Tridacna maxima collected at Anae Island, Guam, spawned from November to March. On Palau, Hippopus hippopus spawned in June and Tridacna crocea, in July. Tridacna crocea, T. maxima, and H. hippopus displayed a stereotyped development pattern in morphogenesis and rate of development. Fertilized eggs of T. crocea, T . maxima, and H. hippopus had mean diameters of 93.1, 104.5, and 130.0 um, respectively. The day-2 straight-hinge veligers of T. crocea, T. maxima, and H. hippopus had mean shell lengths of 155.0, 168.0, and 174.4 pm, respectively. Settlement occurred 12, 11, and 9 days after fertilization at a mean shell length of 168.0, 195.0, and 202.0 pm for T. crocea, T. maxima, and H. hippopus, respectively. Metamorphosis was basically complete about 1 day after settlement. Juveniles of T. crocea, T. maxima, and H. hippopus first acquire zooxanthellae after 19, 21, and 25 days, respectively. Growth rates increase sharply after the acquisition of zoo xanthellae. Juvenile shells show first signs of becoming opaque after 47 days for T. maxima and after 50 days for H. hippopus.