Growth, sclerochronology and development of the Tridacnidae, with particular reference to Hippopus hippopus

In recent years there has been growing interest in the mariculture of giant clams (F: Tridacnidae) for both re-stocking of reefs and commercial farming. A lack of information on giant clam growth rates and how they can vary, both between sites of close proximity and those geographically separated, initiated this thesis. Also the importance of understanding the reproduction of species of mariculture potential, led to the first detailed study of sexual development and reproductive periodicity in giant clams being undertaken. Shell material was collected throughout the Great Barrier Reef of Australia, with routine field sampling at Orpheus Island Research Station. Growth of H. hippopus, Tridacna gigas, T. squamosa and T. maxima in the Palm Island Group, in the vicinity of Orpheus Island, was studied by tagging clams and measuring them repeatedly over a 2 year period. Also the growth and shell morphometry of H .hippopus, T. gigas, T. derasa and T. squamosa from numerous locations along the Great Barrier Reef were determined by sclerochronological examination of shells. Validation of the apparent seasonal growth 'bands seen in shell sections of giant clam was undertaken by studying shells of clams of known time of death, shell microincrements and seasonal differences in both shell chemistry and fluorescent band formation. To investigate the sexual development and reproductive cycle of H. hippopus, monthly organ indices, histology, biopsy sampling and anatomical development studies were utilised. Calculation of parameters of the von Bertalanffy growth equation for tagged H. hippopus revealed considerable differences in L(∞) and K between sites of close proximity, Iris Point Reef (L(∞) = 347 mm, K = 0.204) and Pioneer Bay, Orpheus Island (L(∞) = 414 mm, K = 0.159). Von Bertalanffy parameter values determined from tagged T. gigas (L(∞) 813-862 mm, K = 0.148-0.132), T. squamosa (L(∞) = 384, K = 0.189) and T. maxima (L(∞) = 271, K = 0.126) were similar to values elsewhere in their geographic range. Minimal seasonal growth of H. hippopus and T. gigas occured prior to the spawning season, during gametic development. Total mortality (Z) estimates were H. hippopus 0.05-017, T. gigas 0.20-0.22, T. squamosa 0.04 and T. maxima 0.28. Validation of 2 seasonal growth bands to each year enabled shells to be aged. The validation work included the first record of fluorescent bands in giant clam shells and the first analysis of seasonal shell chemistry using an electronmicroprobe. Morphometric analysis of H. hippopus revealed a trend of increasing flesh per unit shell length from south to north on the Great Barrier Reef. With regard to growth in shell length, however, there was no similar latitudinal variation; with the exception that H. hippopus from close to their southern latitudinal limit at Hayman Island exhibited the lowest Loo value. Comparison of growth rates using tag-remeasure and sclerochronological techniques from Orpheus Island were in close agreement for H. hippopus. This indicated the advantages of rapid sclerochronological investigation of shell death assemblages, which compared to the time consuming tag-remeasure method, did not result in less accurate growth estimates. Results indicated that site specific differences in growth rate within a locality, e.g. Torres Strait, Palm Islands, or between different parts of the Great Barrier Reef could be as large as any inter-country variation. Gonad indices, gonad biopsy (including ooycte diameter), gross gonad anatomy and histological results were combined to demonstrate a mid-summer spawning season for H. hippopus. With maturation, the gonad and kidney, increased relative to other clam organs. The size at first male maturity was not ascertained, but was less than or equal to 77 mm, the smallest clam sampled. Male only gonads were found in clams of up to 140 mm in shell length. The shell length at first hermaphrodite maturity was approximately 146 mm, although one clam of 112 mm in length was found to be mature. Combining growth and reproductive studies of H. hippopus, the age of male maturity (at a presumed shell length of 80 mm) was 2 years and hermaphrodite 4 years. The shell' length at full hermaphrodite maturity of H. hippopus corresponded to the inflexion point in its growth curve, indicating there may be a causal relationship between the two events. The wide variation in growth rates exhibited by all species both within and between sites, indicated that there was large scope to increase growth rates through careful site selection and selective breeding. Whilst the flesh yield of H. hippopus with age was less than that for T. gigas, further work to increase growth rate may see it join T. gigas and T. derasa as a commercially farmed species