Does vulnerability influence trade-offs made by whelks between predation risk and feeding opportunities?

In the northern Gulf of St Lawrence, eastern Canada, the whelk Buccinum undatum occasionally aggregates near the feeding asteroid Leptasterias polaris to obtain food. This is surprising considering that whelks are frequently preyed upon by L. polaris and have developed a violent escape response to this predator. Almost all whelks in aggregations near feeding asteroids are large. This study suggests that small whelks do not approach feeding L. polaris because they are more vulnerable to this predator. Predation experiments demonstrated that the vulnerability of whelks to L. polaris decreases with increasing whelk size. Sampling in the field showed that 97% of whelks measuring more than 5 cm in shell length are found in the sediment zone, where L. polaris extracts large endobenthic bivalves, but whelks less than 5 cm are equally abundant in the rocky (49.5%) and sediment (50.5%) zones. Field experiments showed that whelks within 50 cm of large bivalves being ingested by L. polaris were larger than those within 50 cm of bivalves placed by themselves on the bottom. Finally, laboratory measurements showed that the influence of predator threat on the time whelks spent feeding decreased with whelk size, but the influence of feeding motivation on the time whelks spent feeding increased with whelk size. In the situation where motivation to feed was high and size-related differences in vulnerability were large, the impact of the predator decreased significantly with increasing whelk size. Whelks show threat-sensitive decision making adjusted to potential feeding benefits and vulnerability to predation. (C) 1996 The Association for the Study of Animal Behaviour