The ecology, diet and foraging strategy of Thais orbita (Gastropoda: Muricidae) on a rocky shore of Rottnest Island, Western Australia

Abstract

A two week capture-recapture study of Thais orbita at Cape Vlamingh, Rottnest Island was conducted. Previous studies from other localities and from Rottnest suggested that T. orbita feeds on barnacles and limpets, high on the shore. We, however, confirm earlier work on Rottnest shores that T. orbita is a widely-zoned generalist predator, scavenger and cannibal, the latter two dietary components accounting for 27.3% and 4.2%, respectively, of all feeding observations. Average shell height of Thais orbita at Cape Vlamingh decreased downshore, a consequence of numerous, smaller, subtidal juveniles being recruited slowly into the intertidal population dominated by larger adults. Similarly, prey preference changed downshore. Backshore lagoon individuals preferred Cronia avellana, whereas reef crest individuals preferred Pyrene bidentata, Septifer bilocularis and conspecifics. Our study began immediately following a mass-mortality event caused by thermal stress, providing ample carrion on the shore which T. orbita consumed. Many lagoon- dwelling T. orbita were themselves subject to and killed by the same thermal stress. We have no record of interspecific predation upon T. orbita in this habitat, even by crabs. Acknowledging our limited research time available, we nevertheless tentatively conclude that population structure is primarily regulated by physical and intra-specific factors. Surprisingly, favoured prey items of Thais orbita at Cape Vlamingh were small in relation to the predator, usually providing < 0.10 g dry tissue weight.g dry tissue weight of predator⁻². T. orbita apparently feeds infrequently; only 5% of all examined individuals were feeding. Other lines of evidence suggest that feeding occurs at approximately five day intervals, although there is some evidence that juveniles feed more frequently than adults. Such observations challenge the significance and applicability of optimal foraging theory as it might apply to T. orbita at Cape Vlamingh, Rottnest Island. Our data suggest that, in this habitat, T. orbita and its potential prey are physically constrained, particularly by desiccation, wave action and occasional thermal stress. Some prey, especially barnacles, which are favoured by T. orbita in other portions of its range, are sparse or absent on Rottnest shores. Larger, lagoon-dwelling, T. orbita forage infrequently upon small prey items, carrion and conspecifics in a relatively limited area, consuming food rapidly and retreating to repose to avoid desiccation and heat stress. On the reef crest, however, juveniles, with higher energy requirements, forage more frequently and are thus more susceptible to dislodgement by waves and, as they migrate shoreward, to attacks by larger conspecifics. At Cape Vlamingh, therefore, the demanding habitat and the life history strategy of shoreward migration forces T. orbita to be a 'time-optimizing' forager as an adult. It probably never 'selects' prey optimally in the sense of energy maximisation, as the basic tenets of the optimal foraging theory would predict but, rather, survives within the constraints imposed by a severe environment and intra-specific controls.