Ingestion and digestion of micro-algae concentrates by veliger larvae of the giant clam, Tridacna noae

Abstract

Knowledge of ingestion and digestion of micro-algae by bivalve larvae is critical for provision of appropriate larval nutrition supporting maximal growth and survival. However, little is known about the ingestion and digestion of micro-algae by giant clam larvae. This study determined the rates of ingestion and digestion of commercially available micro-algae concentrates by Tridacna noae larvae of different ages using epifluorescence microscopy. The micro-algae used were Isochrysis sp. (Isochrysis 1800®), Pavlova sp. (Pavlova 1800®), Tetraselmis sp. (Tetraselmis 3600®) and Thalassiosira weissflogii (TW 1200®). None of the four micro-algal concentrates were ingested by T. noae larvae at 24h post-fertilisation, but all were ingested at 48h and 72h post-fertilisation, at different frequencies. At 48h post-fertilisation, Isochrysis sp. and Pavlova sp. were ingested by 77% and 70% of veligers, respectively, while T. weissflogii and Tetraselmis sp. were ingested by 10% and 30% of veligers, respectively. Similar rates of ingestion were observed for each micro-alga by larvae at 72h post-fertilisation. Larvae capable of ingesting micro-algae concentrates were significantly larger than those that were empty and the minimum antero-posterior shell length of T. noae larvae capable of ingesting Pavlova sp. and Isochrysis sp. was 141μm and 132μm, respectively. Digestion of micro-algae by 48h-veligers was observed 2h after the start of feeding in 26.1% and 14.3% of larvae that had ingested Isochrysis sp. and Pavlova sp., respectively, but digestion of Tetraselmis sp. and T. weissflogii was not observed until 4h and 8h after the start of feeding, respectively. Complete digestion of Pavlova sp. and Isochrysis sp. took up to 12h in larvae at both 48h and 72h post-fertilisation. Our results provide a basis for developing a more nutritionally informed approach to hatchery culture of T. noae.