Prey selection by inexperienced predators: do early juvenile snails maximize net energy gains on their first attack?

Abstract

Through encounters with prey, a predator may learn to become more selective and thus forage more efficiently. This is the case for several muricid gastropods. When searching for their first prey, however, early juveniles have no such foraging experience. Prey species and prey size preferences of inexperienced Nucella emarginata (Deshayes) hatchlings from Barkley Sound, British Columbia, Canada, were therefore examined to determine if selective feeding occurs in the absence of foraging experience and, if so, whether the preferences are consistent with the energy maximization hypothesis of optimal foraging theory. When given a choice between 5 prey species (small Mytilus spp., Balanus glandula (Darwin), Chthamalus dalli (Pillsbury), Pollicipes polymerus (Sowerby) and Lasaea spp.), 80% of the hatchlings attacked Mytilus spp. on their first attack. When offered 5 size classes (1, 2, 3, 4 and 5 mm shell length) of Mytilus spp., ≈ 73% of the hatchlings attacked 1-mm or 2-mm size classes. Hatchlings were, in fact, more selective than late juveniles and adults. Hence, strong prey species and size preferences can exist without prior foraging experience even in species which use such experience later in life to make foraging decisions. But hatchlings feeding on Mytilus spp. for 25 days did not grow faster than hatchlings feeding on Balanus glandula or Chthamalus dalli, and there was no difference in energy content between Mytilus spp. and Chthamalus dalli. Thus, the strong preference for small Mytilus spp. can not be explained by considerations of energy gain. In the field, small Mytilus spp. are mostly located in microhabitats which provide hatchlings with substantial protection from mortality factors. The preference for small Mytilus spp. should keep the young snails within protective microhabitats once these sites have been reached, and might be an adaptation to help the hatchlings locate these havens from a distance, in an otherwise highly dangerous environment.