The feeding behaviour and competition for carrion between two sympatric scavengers on a sandy shore in Hong Kong: the gastropod, Nassarius festivus (Powys) and the hermit crab, Diogenes edwardsii (De Haan)

Field and laboratory experiments compared the feeding behaviours of two mar ine scavengers, the gastropod, Nassarius festivus and the hermit crab, Diog enes edwardsii, sympatric on the lower intertidal shore at Starfish Bay, Ho ng Kong. Field trials demonstrated that they both preferred bivalve and fis h to other carrion. N. festivus arrived at bait in greater numbers and more quickly than D. edwardsii. Numbers of the former species attracted to fish bait (1 cm(3)) peaked at a mean of 41 after 30 min, whereas the latter pea ked at a mean of only 2.3 after 55 min. Laboratory investigations revealed that N. festivus was able to detect and move fast towards carrion from a di stance of > 80 cm, feed quickly (x = 13.20 min) and, after 14 days starvati on, had a high consumption rate (0.124 mg wet weight or 0.034 mg dry weight of food individual(-1) min(-1), or 1.4% and 1.3% of its wet and dry body w eights min(-1),respectively). In contrast, D. edwardsii had a shorter food detection distance (< 10 cm), took longer to arrive at bait, a variable fee ding time ranging from 0.28 to > 119.75 min and a lower consumption rate (0 .003 mg wet weight and 0.001 mg dry weight of food individual(-1) min(-1), or 0.1% of its wet and dry body weights min(-1) respectively). Manipulative experiments provided evidence for interspecific competition for carrion be tween the two species. A morphological advantage, i.e. an extendible probos cis allowing feeding at a distance, and chemoreceptors that permit long dis tance food detection, combined with numerical superiority on the shore, are mechanisms by which N. festivus outcompetes D. edwardsii and dominates fee ding clusters. Interactive behaviour studies revealed the occurrence of int erspecific interactions between the two species and intraspecific interacti ons among D. edwardsii but not N. festivus. The number of interspecific int eractions fell when the numbers of N. festivus increased and those of D. ed wardsii decreased, and vice versa. The aggressive D. edwardsii did not sign ificantly affect the feeding behaviour of N.festivus. When the number of D. edwardsii increased and the number of N.festivus decreased, the number of intraspecific interactions in the former increased. The feeding time of N.f estivus, however, decreased with increasing numbers of conspecifics and a d ecreasing number of D. edwardsii. This suggests the possibility of intraspe cific competition in N.festivus. (C) 2000 Elsevier Science B.V. All rights reserved.