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Information on the nutritional requirements and preferred diets of sea cucumber juveniles is extremely limited and this has hindered development of hatchery culture methods. This study assessed ingestion, cell wall digestion and relative nutritional values of two live micro-algae ( Isochrysis aff. galbana (TISO) and Chaetoceros muelleri ) and six concentrated micro-algae products (Instant Algae®, Reed Mariculture Inc.) for early juveniles of sandfish, Holothuria scabra . Seven of the eight micro-algae tested were ingested by juveniles with the exception of live TISO. Faeces excretion times varied between ingested diets that passed through the juvenile gut in less than 1 h. The cell walls of five of the eight micro-algae tested were partially or mostly digested ( Chaetoceros muelleri , TW1200®, Palova 1800®, Isochrysis 1800® and Shellfish 1800®), while the cell walls of Tetraselmis 3600® and 3H 1800® remained intact. Juvenile growth rates were significantly different between diet treatments over the duration of a 14 day growth trial. Mean (± SE) length of early juveniles at the end of the growth trial was highest for those fed live C . muelleri (4.10 ± 0.03 mm) followed by TW 1200® (3.49 ± 0.05 mm). Juvenile survival did not differ significantly between diet treatments and was highest for those fed C . muelleri (79.33 ± 6.11%) followed by TW1200® (78.33 ± 1.20%). Pearson's correlation tests were used to identify key correlations between the levels of specific nutrients and juvenile performance (growth and survival). A significant positive correlation between growth and dietary protein content ( P < 0.05), and a highly significant positive correlation between growth and dietary EPA:DHA ratio ( P < 0.01) provide new information to inform diet selection for juvenile sandfish that will help improve juvenile performance and support improved hatchery production. Use of commercially available micro-algae concentrates as a replacement for live micro-algae in sandfish hatcheries supports development of cheaper, simpler post settlement rearing protocols for this species. [ABSTRACT FROM AUTHOR]

Information on the nutritional requirements and preferred diets of sea cucumber (Holothuriidae) larvae is extremely limited and this has hindered development of hatchery culture methods. This study assessed ingestion and digestion of two live (TISO and Chaetoceros muelleri ) and six concentrated microalgae (Instant Algae®, Reed Mariculture Inc.) by sandfish ( Holothuria scabra ) auricularia larvae of different ages using epifluorescence microscopy. This is the first study to report the use of epifluorescence microscopy with larval echinoderms and experiments were conducted using 2, 6 and 10 day old auricularia larvae. Seven of the eight microalgae tested were ingested and digested by the larvae with digestion occurring more rapidly in older larvae. C. muelleri was rapidly digested by 6-day and 10-day old larvae but our results indicate that C. muelleri is unsuitable as a food for 2-day old sandfish larvae. TISO was well ingested by sandfish larvae in both live and concentrated forms and live TISO was the most suitable of the microalgae tested in terms of ingestion and digestibility. All commercially available microalgae concentrates tested were readily ingested and digested by H. scabra larvae with the exception of Thalassiosira pseudonana (3H 1800®) which was not ingested by larvae of any of the three ages tested. Our results show potential for using microalgae concentrates as alternatives to live microalgae in hatchery culture of sandfish. However, further research should be conducted to assess the relative nutritional values of digestible microalgae as a basis for optimising a diet for hatchery culture of sandfish, and to provide further information on the nutritional requirements of sandfish larvae. [ABSTRACT FROM AUTHOR]