AbstractThe mangrove clam Polymesoda (Geloina) erosa (Solander, 1786) is one of staple for indigenous community in northern Australia. Between June 2001 and September 2002, samples were taken to study reproduction and biochemical composition of the clam population in Maningrida Region, Northern Territory (NT), Australia to provide some basic information required for the management of this fishery resource. At the same time, the development of gonad and reproductive output of this species under several feeding regime in the laboratory using tropical microalgae was also studied in order to develop hatchery protocols for producing mature broodstock. The main findings and recommendations of this thesis are:
P. erosa is dioecious (gonochoristic) which mature at shell length of about 45 mm. Within the range of shell length 55 to 90 mm, the sex ratio in population was 1 male: 1 female. However, at the shell length lower than 55 mm males were significantly more predominant than females. Females were more common at the shell length larger than 80 mm, but not statistically significant. At the shell length higher than 90 mm, this clam was probably in the senility state when production of new gametes were reduced. Based on the gonadosomatic index (GSI) and histological study, P. erosa spawned during the middle until the late of rainy season. Males and females showed a synchrony in spawning. Females within the shell range of 65 to 85 mm produced at least 800,000 eggs per animal. The high frequency of spawning was likely related to long inundation during the rainy season. It was recommended that the present practice of not gathering mangrove clam during the rainy season should be maintained. Also, the minimum size should be increased to about 65 mm shell length.
Gross biochemical composition i.e., carbohydrate, lipid and protein, in the muscle, mantle, and gonad changed throughout the year and was strongly affected by the reproductive cycle. Based on the relative proportions of each body tissue i.e., pedal and adductor muscles, mantle and gonad, the mangrove clams rely on carbohydrate based metabolism, with foot, adductor muscles and mantle as storage organs. During the dry season, for gametogenesis clams derived most of its energy from the utilization of stored carbohydrate reserves. While during the wet season, the clam renewed its energy reserves. Females appeared to require more energy than males during the reproductive season. Females accumulated large amounts of lipid in their gonad, while males had high concentrations of protein in their gonad.
Measurements of physiological rates i.e., filtration rate, ingestion rate, assimilation rate and energy acquisition showed that this clam accepted wide ranges of microalgal diets. High rates occurred when clams fed on microalgae T-ISO, Chaetoceros sp (CS256), Rhodomonas sp (NTiS) and Ciyptomonas sp (CRFI01). Trials with other NT microalgae such as Isochrysis sp (NT 14) and Tetraselmis sp (NT 18) showed that the clam did not accept these algae. Mangrove clam was able to mature and produced viable larvae in hatchery under unialgal diet of T-ISO, CS256, NTiS and CRFJO I. However, the reproductive output in terms of number of eggs released by females was much lower than the potential fecundity of this species. This low reproductive output was probably due to drawbacks in biochemical and fatty acid composition of the microalgae. T-ISO and CS256 were good source of carbohydrateand energy but contained low percentage of essential fatty acids such as 20:5(n-3) or 22:6(n-3), while NT15 and CRFIO1 had lower energy content, even though they had high content of 20:5(n-3) and 22:6(n-3). All these microalgae contained low amounts of 20:4(n-6) and this may also influenced their nutritive value for conditioning. The results tend to suggest that the reproduction P. erosa may require diets with high energy content with sufficient amounts of essential fatty acids. This will constitute one of the hypotheses for future testing.
|Date of Award||Mar 2005|
|Supervisor||Jim Luong-Van (Supervisor) & Anthony Griffiths (Supervisor)|