Feeding behaviour of larval greenback flounder Rhombosolea tapirina

Understanding the feeding behaviour of larval fish is critical for determining optimal management protocols in aquaculture and understanding larval prey selectivity patterns of wild fish populations. This study examined the feeding performance of cultured greenback flounder larvae Rhombosolea tapirina, using the live feed organisms Brachionus plicatilis (rotifers), and Artemia sp., to determine the primary sensory modality involved in feeding, the relationship between mouth dimensions and prey size selected, and the effect of previous exposure to a prey species on subsequent prey selection. The proportion of larvae that fed on rotifers in the light (light intensity of 5-6 µmol.m-2 .s-1 ), increased significantly from 66% to 96% from day 12 to day 27 post-hatching, respectively. In comparison, the proportion of larvae that fed on rotifers in total darkness (0 µmol.m-2 .s-1 ) never exceeded 5% during the same period. This indicated that greenback flounder larvae were primarily dependent upon vision (a light dependent behaviour) to feed, with a lesser reliance upon non-visually mediated detection of prey. Internal horizontal dimensions of the mouth of greenback flounder larvae determined from serial histological sections, increased in a linear fashion with both increasing body size and age of larvae. Examination of the ratio of Artemia prey size (total length, carapace width, and carapace width with appendages) to larval mouth width, suggested that prey carapace width, not prey width with appendages, or prey length, limited the size of prey ingested, indicating that larvae must visually orient to ingest the prey head-on. When greenback flounder larvae were offered one of three discrete size fractions of Anemia prey, both prey size and larval age significantly effected larval feeding response. On days 12 - 17 post-hatching, larvae ingested Artemia prey of a single size range (100 - 200 µm screened size), which was considerably smaller than they were capable of ingesting. From 19 - 20 days of age and thereafter, there was a marked change in feeding performance, with larvae able to ingest all three Artemia prey size ranges offered (100 - 200 µm, 300-390 pm and 450 - 560 µm screened size, respectively), probably reflecting an ontogenetic increase in larval sensory capability, swimming speed and prey handling ability. Prior feeding experience of greenback flounder larvae to either rotifers only (Rotifer Treatment) or a mixed diet of Artemia and rotifers (Artemia and Rotifer Treatment), significantly effected subsequent prey selection when larvae were offered a mixed diet of Artemia and rotifers, but did not effect the temporal onset of selection of the novel prey species (Artemia) by Rotifer Treatment larvae. Therefore the differences in prey selection by larvae with or without prior exposure to Artemia prey, was not due to the inability of larvae to handle and ingest a novel prey species, but reflects positive selection for familiar prey species. The latter indicates a learned component in the feeding behaviour of fish larvae. This has implications for the timing of the introduction of new live prey species during intensive culture of marine fish larvae.