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Tactics of a predator, Carcinus maenas, and morphological responses of the prey, Nucella lapillus
Hughes, R.N. (1979). Tactics of a predator, Carcinus maenas, and morphological responses of the prey, Nucella lapillus. J. Anim. Ecol. 48(1): 65-78
In: Journal of Animal Ecology. Blackwell Science/British Ecological Society: Oxford. ISSN 0021-8790 , more
Peer reviewed article  

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Keywords
    Predator prey interactions; Carcinus maenas (Linnaeus, 1758) [WoRMS]; Nucella lapillus (Linnaeus, 1758) [WoRMS]; Marine

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  • Hughes, R.N., more

Abstract
    (1) Shore crabs attack dogwhelks by attempting to crush the shell. Because of differences in shell morphology, sheltered shore dogwhelks tend to be opened by removal of the apex and exposed shore dogwhelks by breaking the columella. (2) The proportion of successful attacks is unity for small dogwhelks but decreases rapidly as the shells grow and become more robust. The attack success rate curve begins to fall at a smaller shell height in sheltered than in exposed shore dogwhelks. Large adult sheltered shore dogwhelks are immune to attack. (3) For large male shore crabs preying on sheltered shore dogwhelks, prey value (energy yielded per unit handling time) forms a steep normal curve when plotted against shell height, peaking at about 14 mm. Because of the rapidly decreasing attack success rate, the prey value of individuals between the optimum and about 27 mm height is very unpredictable. (4) When presented with an abundance of prey in each size category in the laboratory, shore crabs did not specialize on the optimal size as predicted by optimal foraging theory. All encountered prey were attacked but rejected if unbroken after 0.25 to 2.75 minutes. Very small dogwhelks were sometimes dropped accidentally. As a result of these mechanical processes, optimally sized dogwhelks were the most frequently eaten. (5) The foraging tactics seem to be the most appropriate for prey types which have unpredictable prey values and are sparsely distributed so that encounter rates are always low. Most intertidal gastropods are of this type. The foraging tactics applied to dogwhelks contrast with those applied to mussels where prey value is predictable and encounter rates are high. (6) Exposed shore dogwhelks living in the virtual absence of shore crabs are much more vulnerable to attack than sheltered shore dogwhelks suffering high crab predation. Sheltered shore dogwhelks are better protected because of a narrower mouth which excludes the chelae of attacking crabs and a thicker, more robust shell. These attributes were even more pronounced in a sublittoral-fringe population of dogwhelks exposed to the more powerful edible crab. (7) The narrower mouth of sheltered shore dogwhelks is produced by a more elongated spiral growth form. This does not reduce the relative size of the visceral mass but it does reduce the relative area of the foot. (8) The thicker, more robust shell of sheltered shore dogwhelks results in a heavier load per g flesh and may represent a metabolic cost of the anti-predator adaptation. Such a cost is possibly unimportant because sublittoral-fringe dogwhelks frequently carry an extra load of encrusting barnacles, without any reduction in relative size of the visceral mass or total flesh weight. (9) Selective deaths caused by crab predation must occur below a shell height of about 27 mm. The most vulnerable stages, below about 14 mm height, tend to be inaccessible to crabs because of their propensity to remain under stones or within crevices. The habit of foraging on the outer surfaces of stones is acquired only by large dogwhelks and since many of these will be over 27 mm in height, only a few will likely fall prey to shore crabs.

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