Predator‐prey interactions among some intertidal gastropods on the Great Barrier Reef

Abstract

On an intertidal reef flat at Orpheus Island on the Great Barrier Reef, the gastropods Turbo brunneus and juvenile Trochus niloticus share the same habitat with the predatory gastropod Thais tuberosa. To determine if the two prey species differed in their antipredator behaviour and interactions with the predator, we examined: (1) the distributional pattern of the three species in the field; (2) the proportion of mortality attributable to non‐crushing predators (i.e. Thais tuberosa) in T. brunneus and T. niloticus, determined by the frequency of freshly dead and undamaged shells; and (3) the response of T. brunneus and T. niloticus to T. tuberosa in laboratory and field experiments. We compared the responses of hatchery‐reared and wild juvenile T. niloticus to determine if lack of previous exposure to the predator affected the behaviour of cultured juveniles. Finally, (4) we studied prey choice by Thais 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 shells was 28% for T. brunneus and 10% for T. niloticus; (3) T. brunneus and T. niloticus responded very differently to the predator: Turbo brunneus showed a conventional flight escape response, moving nine times faster than normal when close to T. tuberosa. The flight response was observed 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 speed but instead released a white mucus in the presence of the predator. 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 dry flesh weight, did not explain this preference. Capture rate of the preferred species T. brunneus fell to zero in water containing mucus released by T. niloticus.The results indicate that predation by T. tuberosa 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.