Aggregation of whelks, Buccinum undatum, near feeding predators: the role of reproductive requirements

In the Mingan Islands, northern Gulf of St Lawrence (eastern Canada), the w helk Buccinum undatum displays a strong escape response to its predator, th e asteroid Leptasterias polaris, nevertheless large sexually mature individ uals occasionally approach feeding L. polaris to obtain food. In this study , we investigated the hypothesis that reproductive requirements increase th e tendency of sexually mature whelks to approach feeding asteroids. Prior t o egg laying, females (which invest more energy than males into the product ion of reproductive structures) represented 72% of the adult whelks that ap proached feeding L. polaris, but only 36% of the adults randomly collected from the study area. Furthermore, females that were attracted to feeding as teroids had smaller reproductive organs (after accounting for body size) th an females randomly collected from the study area. Similarly, prior to egg laying, females fed longer and ingested more food than males when tested in the presence of L. polaris in the laboratory. After egg laying, however, f emales and males displayed a similar tendency to feed in the presence of a predator, both in the field and in the laboratory. Predator-impact indices, computed by contrasting the feeding activity of whelks in the absence and presence of a predator, indicated that females (but not males) responded mo re boldly to predators prior to than after egg laying, despite a general de crease in feeding activity at that period. Taken together, our observations indicate that the tendency of adult whelks to approach feeding predators i s influenced by potential reproductive gains. Because such gains are presum ably more directly linked to a given feeding opportunity in sexually active individuals, whelks may be selected to display increased levels of boldnes s towards predators with the onset of sexual maturity. Thus, potential repr oductive benefits may partly explain the size-dependent tendency of whelks to approach feeding asteroids. (C) 2001 The Association far the Study of An imal Behaviour.