A flexible response to a major predator provides the whelk Buccinum undatum L. with nutritional gains

Abstract

We examined the association of the common whelk Buccinum undatum with the predatory asteroid Leptasterias polaris in the Mingan Islands, northern Gulf of St. Lawrence. Quantitative sampling, using SCUBA diving, of surfaces of the sea floor in the vicinity (within 150 cm) of feeding and non feeding L. polaris and surfaces devoid of asteroids showed that both immature and mature whelks generally avoid L. polaris. Surprisingly, densities of mature whelks up to 12 times the average were occasionally encountered near asteroids when they were consuming a large prey, usually the surf clam Spisula polynyma. Comparisons of the stomach-content mass of whelks approaching L. polaris feeding on S. polynyma with that of whelks which were departing from the same type of feeding bout demonstrated that whelks obtained significant food gains by approaching L. polaris. Previous studies failed to identify the food resources that could account for the high biomass of whelks in the Mingan Islands. The association with L. polaris potentially explains a substantial part of the whelk's diet. Whelks of the Mingan Islands have evolved a flexible behavioural response to L. polaris which permits them to make adaptive and risk-sensitive decisions when choosing between avoiding a major predator and taking advantage of food resources it makes available.