PREDATOR-PREY INTERACTIONS AMONG SOME INTERTIDAL GASTROPODS ON THE GREAT-BARRIER-REEF

On an intertidal reef Aat at Orpheus Island on the Great Barrier Reef, the gastropods Turbo brunneus and juvenile Trochus niloticus share th e same habitat with the predatory gastropod Thais tuberosa. To determi ne if the two prey species differed in their antipredator behaviour an d interactions with the predator, we examined: (1) the distributional pattern of the three species in the field; (2) the proportion of morta lity attributable to non-crushing predators (i.e. Thais tuberosa) in T . brunneus and T. niloticus, determined by the frequency of freshly de ad and undamaged shells; and (3) the response of T. brunneus and T. ni loticus to T. tuberosa in laboratory and field experiments. We compare d the responses of hatchery-reared and wild juvenile T. niloticus to d etermine if lack of previous exposure to the predator affected the beh aviour of cultured juveniles. Finally, (4) we studied prey choice by T hais tuberosa. We found that: (1) the field distribution of all three species showed high overlap and prey and predator were often found in close proximity; (2) the proportion of recently-killed, undamaged shel ls was 28% for T. brunneus and 10% for T. niloticus; (3) T. brunneus a nd T. ni(oticus responded very differently to the predator: Turbo brun neus showed a conventional flight escape response, moving nine times f aster than normal when close to T. tuberosa. The flight response was o bserved in all trials with T. brunneus in the laboratory, but only in 52% of trials in the field. In contrast, T. niloticus did not change s peed but instead released a white mucus in the presence of the predato r. Response in the field was also less intense than in the laboratory. Cultured and wild T. niloticus showed the same response when exposed to T. tuberosa, although cultured juveniles were, on average, slightly more active than wild juveniles. Lastly, (4) Thais tuberosa showed a strong preference for T. brunneus as prey. Food value, expressed as dr y flesh weight, did not explain this preference. Capture rate of the p referred species T. brunneus fell to zero in water containing mucus re leased by T. niloticus. The results indicate that predation by T. tube ,osa is more intense for T. brunneus than for T. niloticus and that a likely cause for this difference lies in the antipredator responses of the two prey species. The mucous response of T. niloticus appeared to be more effective for avoiding predation by T. tuberosa than was the flight response of T. brunneus.