Effects of prey density on survival, feeding rate and development of zoeas of the red frog crab Ranina ranina (Crustacea, Decapoda, Raninidae)

Abstract

The effects of prey density on growth and survival of larval Ranina ranina were examined. Five levels of prey density (0.05−5 Artemia nauplii ml −1) were tested for their effects on survival, intermolt period and growth at all the zoeal instars. Six or seven levels of prey densities (1−10 ml −1) were examined for feeding rate at instars I, III, V and VII. Survival decreased with decreasing prey density. Survival rate dropped to around 50% in 0.05 ml −1 to instar I, 0.1 ml −1 to instar II, 0.5 ml −1 at instar V, and 1 ml −1 after instar VI. Metamorphosis rate increased with increasing prey density, from 0% at 0.1 ml −1 to 100% at 5 ml −1. Intermolt period and duration of the zoeal stages were longer at 0.05 and 0.1 ml −1 compared to 0.5−5 ml −1. Growth of several body parts was lower at 0.05 and 0.1 ml −1 than at higher prey densities for instars III, V and VII. The external morphology of several appendages was less affected than was growth. A regression equation, F= M (1 − e − kp ) ( F=feeding rate, k=constant, M=calculated maximum feeding rate, p=prey density), was derived for the relationship between observed prey density and feeding rate. The optimal prey density necessary to ensure highest survival and the shortest intermolt period calculated using this regression equation was similar to results measured from the experiment in terms of survival, intermolt period and growth: it was 0.5 ml −1 at instar I, 0.6 ml −1 at instar III, 1.8 ml −1 at instar V, and 2.3 ml −1 at instar VII.