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

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 24 h post-fertilisation, but all were ingested at 48 h and 72 h post-fertilisation, at different frequencies. At 48 h 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 72 h 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 141 μm and 132 μm, respectively. Digestion of micro-algae by 48 h-veligers was observed 2 h 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 4 h and 8 h after the start of feeding, respectively. Complete digestion of Pavlova sp. and Isochrysis sp. took up to 12 h in larvae at both 48 h and 72 h post-fertilisation. Our results provide a basis for developing a more nutritionally informed approach to hatchery culture of T. noae.