Uptake of dissolved organic matter by larval stage of the crown-of-thorns starfish Acanthaster planci

Abstract

The life-history of the crown-of thorns starfish (Acanthaster planci) includes a planktotrophic larva that is capable of feeding on particulate food. It has been proposed, however, that particulate food (e.g. microalgae) is scarce in tropical water columns relative to the nutritional requirements of the larvae of A. planci, and that periodic shortages of food play an important role in the biology of this species. It has also been proposed that non-particulate sources of nutrition (e.g. dissolved organic matter, DOM) may fuel part of the nutritional requirements of the larval development of A. planci as well. The present study addresses the ability of A. planci larvae to take up several DOM species and compares rates of DOM uptake to the energy requirements of the larvae. Substrates transported in this study have been previously reported to be transported by larval asteroids from temperate and antarctic waters. Transport rates (per larval A. planci) increased steadily during larval development and some substrates had among the highest mass-specific transport rates ever reported for invertebrate larvae. Maximum transport rates (J _max ⁱⁿ) for alanine increased from 15.5 pmol larva⁻¹ h⁻¹ (13.2 pmol μg⁻¹ h⁻¹) for gastrulas (J _max ⁱⁿ=38.7 pmol larva⁻¹ h⁻¹ or 47.4 pmol μg⁻¹ h⁻¹) to 35.0 pmol larva⁻¹ h⁻¹ (13.1 pmol μg⁻¹ h⁻¹) for early brachiolaria (J _max ⁱⁿ just prior to settlement=350.0 pmol larva⁻¹ h⁻¹ or 161.1 pmol μg⁻¹ h⁻¹) at 1 μM substrate concentrations. The instantaneous metabolic demand for substrates by gastrula, bipinnaria and brachiolaria stage larvae could be completely satisfied by alanine concentrations of 11, 1.6 and 0.8 μM, respectively. Similar rates were measured in this study for the essential amino acid leucine, with rates increasing from 11.0 pmol larva⁻¹ h⁻¹ (or 9.4 pmol μg⁻¹ h⁻¹) for gastrulas (J _max ⁱⁿ=110.5 pmol larva⁻¹ h⁻¹ or 94.4 pmol μg⁻¹ h⁻¹) to 34.0 pmol larva⁻¹ h⁻¹ (or 13.0 pmol μg⁻¹ h⁻¹) for late brachiolaria (J _max ⁱⁿ=288.9 pmol larva⁻¹ h⁻¹ or 110.3 pmol μg⁻¹ h⁻¹) at 1 μM substrate concentrations. The essential amino acid histidine was transported at lower rates (1.6 pmol μg⁻¹ h⁻¹ at 1 μM for late brachiolaria). Calculation of the energy contribution of the transported species revealed that larvae of A. planci can potentially satisfy 0.6, 18.7, 29.9 and 3.3% of their total energy requirements (instantaneous energy demand plus energy added to larvae as biomass) during embryonic and larval development from external concentrations of 1 μM of glucose, alanine, leucine and histidine, respectively. These data demonstrate that a relatively minor component of the DOM pool in seawater (dissolved free amino acids, DFAA) can potentially provide significant amounts of energy for the growth and development of A. planci during larval development.