Clearance rates of cultured microalgae were significantly higher than those of 6 and 20 μm beads, suggesting that feeding is also dependent on chemical properties of food particles.
The net uptake rates of three DFAA, alanine, glutamic acid and arginine, by larvae were measured to estimate their potential contribution on the larval nutrition. Larvae selectively took up alanine (neutral amino acid) over glutamic acid (acidic) and arginine (basic). The net uptake rates of alanine from the initial concentrations of 148, 150 and 465 nM were 8.9, 15.4 and 36.9 pmol larva-1 h-1, respectively. The alanine uptake at these substrate concentrations could account for 13.1, 23.1 and 55.3% of the basic metabolic demand of a larva. Neutral DFAA are a potentially important food source, however, their ambient concentrations (trace - ca. 200 nm) generally seem too low to make a significant contribution to the larval nutrition.
Phytoplankton and DFAA were identified as potential food sources for larvae. Comparison between potential contributions of these food sources to the larval nutrition, in terms of ambient concentrations and the ability of larvae to utilize these food sources, indicates that nanoplankton are the major food. Because the availability of phytoplankton (chlorophyll a concentration) in the GBR waters is lower or marginal to the critical range, larvae are usually food limited. Food limitation is likely to have a significant impact not only on larval survival by extending the vulnerable planktonic period, but also on juvenile survival by reducing juvenile size at metamorphosis. This study confirms that food is a crucial environment factor controlling the growth, development and survival of A. planci larvae in the GBR waters.
