Development of a formulated diet for mud crab, Scylla sewata, larvae

This study aimed to develop a suitable formulated diet to replace live food in mud crab, Scylla serrata, larval culture. A microbound diet (MBD) was formulated based on known requirements of other crustaceans and was assessed for ingestion and retention by the various larval stages of S. serrata. Ingestion and retention was determined by labeling the MBD with ¹⁴C, and subsequently measuring ¹⁴C in larvae fed these diets. After it was shown that the larvae readily accepted MBD, different types of binders were tested to determine those best suited for MBD prepared for S. serrata larvae. The diet particle size preference and optimal feeding ration were then determined for the various larval stages. Finally, the diet was tested in combination with different ratios of Artemia as a food source for Megalopa. Fundamental to the success of the study was the development of methods that would ensure routine and reliable production of mud crab larvae. Larval production runs were tried using protocols from various authors and procedures that showed positive results after several trials were adopted. A progressive improvement in survival was achieved towards the end of the study and the rearing protocol that was finally adopted has now formed the basis for hatchery production of S. serrata at James Cook University. A technique for evaluating fish larvae using dietary ¹⁴C was refined and adopted in this study for the measurement of ingestion and retention of the MBD by S. serrata larvae. Several studies were cond ucted to serve as basis to refining this technique. Based on an experiment to determine factors that could affect the measurement of the ¹⁴C content of larvae fed ¹⁴ C labelled MBD, it was found that S. serrata larvae do not absorb the ¹⁴C that leaches from the diet but MBD particles that stick to the larvae were the major source of potential error. As such, it was found necessary to include a control treatment with dead larvae when running an experiment so that the radioactivity reading of the dead larvae can be used to correct the radioactivity readings in the treatments when measuring ingestion. Newly hatched S. serrata zoea readily ingested the MBD and ingestion increases with larval age. Ingestion of MBD did not vary significantly between Zoea I and Zoea II and also between Zoea III and Zoea IV; ingestion by other larval stages were significantly different from each other. Ingestion of MBD by Megalopa was found comparable to previously reported ingestion of live food (8 Artemia larva⁻¹ h⁻¹). Studies were also conducted to determine the duration of diet exposure that resulted in maximal ingestion of the MBD. Results showed that for Zoea I to Zoea III and Megalopa, feeding for 1 h resulted in maximal ingestion as there was no further increase in ingestion with longer diet exposure. For Zoea IV and Zoea V, at least 2 h was required for maximal ingestion as there was no further increase in ingestion after 2 h. The gut residence times (GRT) of MBD for the various larval stages of S. serrata were determined in order to know the required period that should be allowed for the larvae to empty their gut when measuring retention of the MBD. In Zoea I, GRT was found to be 1 h as retention of the MBD significantly decreased after 1 h following removal of available MBD. In Zoea III and Zoea IV, GRT was found to be 2 h; retention of the MBD significantly decreased after 2 h following food removal and there was no further significant decrease in retention after this time. Results did not clearly indicate the GRT of Zoea V and Megalopa, but there were indications suggesting longer GRT of around 4- 5 h for these stages. Studies to test the suitability of different binders (agar, alginate, carrageenan, gelatin and zein) for MBD showed that there were no significant differences in ingestion and retention of MBD resulting from binder type. Further evaluation of these binders, based on leaching of radioactivity from diets, showed that least leaching was found in zeinbound MBD. Since the greater leaching of nutrients from die ts with other types of binders did not make these diets more attractive (i.e. result in significantly greater rates of ingestion), the minimal leaching of nutrients from zein-bound MBD made zein the more desirable binder. Unnecessary leaching wastes important dietary components and can result in deterioration of water quality. The particle size preference and optimal feed ration were determined for the various larval stages of S. serrata based on larval ingestion of ¹⁴C labeled MBD. The results provided important information for feeding management of S. serrata and allowed recommendation of the most suitable MBD particle size range and ration for each of the larval stages of S. serrata (i.e. for Zoea I,