Juvenile growth of the South China Sea whelk Hemifusus tuba (Gmelin) (Prosobranchia: Melongenidae) and the importance of sibling cannibalism in estimates of consumption

Abstract

As an adult, Hemifusus tuba (Gmelin) is a predator of bivalves. Egg capsules are laid in spring and hatch 2 to 3 months later. Hatchlings emerge at a mean length of ≈ 4.4 mm and commence feeding. This study examines growth in hatchlings from birth for 12 wk. Up to Wk 7 post-hatching, growth in total weight is principally of the shell. A large shell, the juveniles primary defence, probably enhances survival by reducing predation. Significant increases in tissue weight, to fill the shell, only begin after Wk 4. This is preceded at Wk 3 by dramatically elevated feeding rates upon bivalve prey. At this time the juveniles consume > 40% of their wet tissue weightday −1. Subsequently, consumption rates fall to adult levels of ≈ 4%. Of a mean of 1500 eggs laid in each capsule, only a mean of 8.8 emerge as juveniles. In the capsule they feed on “nurse” eggs and cannibalize siblings. Post-hatching mortality is high (≈ 14%) but declines to 0 by Wk 10. Estimates of the potential value of such dying or killed hatchlings to the consumption of surviving siblings, in relation to the bivalve diet, show that they are insignificant (<2.0% · day −1). It is concluded that cannibalism is only a feature of pre-hatched H. tuba. Post-hatching cannibalism may enhance the survival of an individual, but can play little roˆle in species survival, especially in the natural environment where hatchlings are likely to disperse randomly.