Maternal and nutritional factors affecting larval competency in the spiny lobster, Jasus edwardsii.
thesisposted on 2023-05-26, 07:46 authored by Smith, GG
The major impediment to closing the life-cycle of Jasus edwardsii Hutton (Decapoda: Palinuridae), a spiny lobster being assessed as a candidate for aquaculture, lies with larval rearing. Larval development in J. edwardsii phyllosoma is a complex and protracted phase taking in excess of 300 days in captivity. Often the initial phyllosoma competency (survival and growth) is poor and thought to be associated with parental influences. To improve survival in culture a method to assess larval competency at hatch was determined and environmental, physiological and nutritional influences on competency assessed. The development of an activity test to assess larval competence involved short-term exposure of phyllosoma larvae to temperature and salinity stresses, responses monitored (stress indices) and later correlated with survival in starvation tests and in culture. The initial activity test (1 h in 23¬¨‚à´C at 10‚ÄövÑ‚àû) was refined (1 h in 21¬¨‚à´C at 10‚ÄövÑ‚àû) during an examination of nutritional influence on larval competency allowing the most competent larvae to be selected for culture, a significant advantage for any potential aquaculture species. The effect of exposing broodstock to elevated temperatures (17 and 19¬¨‚à´C) during embryonic development was assessed in relation to the larval competence of newly-hatched phyllosoma. Elevated temperature provided out-of-season larvae by accelerating embryonic development by up to 2 months however; the larvae produced were smaller and less competent than those incubated under ambient conditions (9.5-13¬¨‚à´C). This was a significant finding as it ended the use of this technique to provide out-of-season phyllosoma larvae and stimulated research into alternate methods to provide phyllosoma at different times during the year. Larval competence was also assessed in relation to broodstock physiology. Larger females produced larger phyllosoma, which demonstrated a greater potential for survival in culture, ascertained by stress indices and LD-50. Viable fecundity (phyllosoma number at hatch) in captivity was 45% of that expected from fisheries estimates and may indicate a reduction in fecundity associated with captivity, an area identified as requiring further dedicated studies. Size at onset of maturity was correlated with morphometric changes to male leg length and female abdominal segment width, providing non-destructive correlates for determining maturity in both males and non-ovigerous females. The ability of juvenile Artemia, the predominant food source of cultured J. edwardsii phyllosoma larvae, to assimilate a range of enrichment products was assessed. The nutritional factors targeted were those credited with improving larval competency in crustaceans and included lipids and the development of a protocol for enrichment with ascorbic acid (AA). These live feed studies demonstrated that both lipid and AA (particulate form) are assimilated by juvenile Artemia (at >1% of Artemia dw for AA) concomitant with enrichment concentrations, and maintained with short-term starvation. The lipid assimilation ability of juvenile Artemia was examined in relation to the loss of gut content, a common occurrence during the external mastication process of J. edwardsii larval feeding. Gut content did not make a significant contribution to total Artemia profiles shortly after the cessation of enrichment meaning any subsequent loss that may occur during feeding would not significantly alter Artemia nutritional profiles. It is considered that juvenile Artemia, like smaller metanauplii are powerful a tool for the delivery of specialized enrichment products. Dietary influences on the maternal lipid and fatty acid digestive gland (storage), ovary (maturation) and tail muscle profiles were examined during starvation and ovarian maturation and their roles examined in relation to the production of competent larvae. The digestive gland lipid content was high (70% dw) and dominated by triacylglycerol (energy source); it showed evidence of utilization during starvation but not during ovarian maturation. The digestive gland fatty acid profile resembled that of the diet, a fact that could be used to identify preferred wild lobster diets, i.e., through the use of dietary lipid signatures. A potential method to ascertain a range of beneficial nutritional products that may assist larval competence in wild caught larvae. The ovary and tail muscle were resilient to dietary changes in lipid or fatty acid profile, with the tail muscle approximating that of newly-hatched phyllosoma. This study demonstrated that J. edwardsii are effective in obtaining their lipid requirements in spite of minimal dietary input and reduces the emphasis on lipids and fatty acids in the nutrition of J. edwardsii broodstock. A study to ascertain the benefit of supplementary AA during broodstock maturation found that ovarian AA concentration was easily saturated, and in the absence of supplementation was sequestered from the tail muscle, the first time this has been considered as a significant AA store. There was minimal AA utilization during embryonic development, no improvement to broodstock fecundity or phyllosoma competency suggesting that there are no reproductive or larval competency benefits to providing supplementary AA to broodstock. In contrast, the delivery of mega-doses of AA to newly-hatched phyllosoma via juvenile Artemia resulted in large increases to larval AA content (161,2250g, g-1) and concomitant increases in larval survival (74%). This is the first time Artemia enrichment protocols have resulted in an improvement to larval survival. This thesis has identified factors which contribute to the production of competent phyllosoma larvae; a dietary mix of live and pellet feed is sufficient to supply the lipid and AA requirements of broodstock, large females producing large eggs and phyllosoma should be targeted as broodstock, while incubation should be conducted under conditions of ambient temperature. The development of an activity test is an effective tool that provides a quick and easy determination of larval competency at hatch, capable of enhancement by feeding with juvenile Artemia supplemented with a particulate form of AA. Adoption of these procedures should ensure the most competent larvae are available to improve the success of phyllosoma larval rearing in the spiny lobster J. edwardsii.