From lecithotrophy to planktotrophy: ontogeny of larval feeding in the Amazon River prawn Macrobrachium amazonicum

During downstream transport from rivers to estuaries, early larval stages of the Amazon River prawn Macrobrachium amazonicum (Heller 1862) are likely exposed to planktonic food limitation. In the laboratory, we studied the effects of presence or absence of food on larval survival, moulting and biomass (dry mass, and content of carbon, hydrogen and nitrogen). Unfed larvae developed successfully from hatching of the Zoea I (Z I) to the third zoeal stage (Z III). Complete absence of starvation effects indicated obligatory lecithotrophy in Z I, while significantly delayed moulting and reduced biomass in unfed Z II proved that this stage is facultatively lecithotrophic. Although unfed Z III did not develop any further (obligatory planktotrophy), they still showed a high endotrophic potential, surviving for up to another 10 d (in total 2 wk from hatching), and utilizing about two-thirds of their initial biomass. Dramatically decreasing C:N ratios in unfed larvae suggest an almost complete metabolic degradation of internal lipid stores (visible as fat droplets in the hepatopancreas region), but little protein mobilization. Larvae obtained from 2 different females differed significantly in biomass at hatching and, correspondingly, also in maximum survival time, delay of moulting to the Z III and average rates of biomass utilization. In conclusion, the early larval stages of M. amazonicum depend very little on food, shifting from completely non-feeding behaviour (Z I) through facultative lecithotrophy (Z II), to planktotrophy (later stages). This ontogenetic pattern may be an adaptation to initial food limitation associated with larval export from limnic to estuarine environments.