Reproduction, juvenile growth, consumption and the effects of starvation upon the South China Sea whelk Hemifusus tuba (Gmelin) (Prosobranchia: Melongenidae)

Abstract

The South China Sea whelk, Hemifusus tuba (Gmelin, 1791) (Melongenidae), is a predator of bivalves. This paper describes copulation, egg capsule laying, egg capsule structure, and hatchling morphology. Hatching took place 89 days post-laying at 21 ± 2°C. Hatchlings are born at a mean shell length of 5.53 mm and a total wet wt of 0.025 g and almost immediately commence feeding on living bivalve tissue. Growth of the juvenile has been followed for 62 wk post-hatching and can be divided into four phases. The first phase (Weeks 0–10) was a period of dramatic shell growth in length (from 5.53 to 28.68 mm) with some increase in wet tissue wt to 0.119 g after 8 wk and 0.750 g after 12. In Phase two (Weeks 10–20), growth proceeded at a slower rate to a shell length of 37.35 mm and the wet tissue increased to 1.489 g. In Phase three (Weeks 20–42), shell length and wet tissue weight remained relatively constant but total weight increased, suggesting that the shell was gaining weight but not length. This growth check at least in length and tissue weight appears to be endogenous. In Phase four (Weeks 42–62), growth in all parameters resumed and it is believed that following Phases 1–3 (essentially juvenile growth characteristics) adult growth will be a typical balance between somatic and gonadal growth. Adult consumption rates are ≈4% wet tissue wt · day −1. In the 4-wk old juvenile, daily consumption rates were calculated to be 36.51% wet tissue wt. Estimates of consumption ·g total wet wt. −1 for Week 2, however, were 26.89%. Using these data for an estimate of consumption/g wet tissue wt at Week 2, a figure of 92% is obtained, i.e., 2-wk-old H. tuba hatchlings ate virtually their body weight each day. Although this may be an over-estimate, it would help account for the dramatic increases in shell length up to Weeks 10–12. Following Week 2, consumption values progressively decreased to typical adult consumption rates of ≈3–4%. The effects of starvation upon H. tuba juveniles are typical, all parameters of growth declining proportionately. There was some evidence of sustained tissue weight increases but not shell growth, implying direct absorption of DOM. Surprisingly, moreover, no mortality took place until Week 27, and three animals survived 39 wk without food. In comparison with data from other predatory snails this is very long (especially for a juvenile). It is concluded that H. tuba is an opportunistic predator of shelf bivalves. The growth plan suggests urgent feeding to build up shell size, presumably to reduce predation pressure and that only later is the shell filled up and strengthened. Internal fertilization, the laying of egg capsules on the shells of others, rapid growth and an ability to survive long periods of starvation all can be used to explain the success of these whelks.