Phenotype-environment matching in the shore crab (Carcinus maenas) |
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Authors: | P A Todd R A Briers R J Ladle F Middleton |
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Institution: | (1) Marine Biology Laboratory, Department of Biological Sciences, National University of Singapore, 14, Science Drive 4, Blk S1, #02-05, 117543 Singapore, Singapore;(2) School of Life Sciences, Napier University, 10 Colinton Rd, EH10 5DT Edinburgh, UK;(3) School of Geography and Environment, University of Oxford, Mansfield Rd, OX1 3TB Oxford, UK |
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Abstract: | The shore crab (Carcinus maenas) exhibits a range of carapace pattern polymorphisms, but little is known regarding their function or maintenance. If patterns
represent some form of crypsis, then associations between carapace colouration and substrate are expected; to determine whether
such relationships exist, frequency of crab morphs and quantity of substrate type were measured from fifteen 10×40 m2 quadrats at each of three sites along the southern shore of the Firth of Forth, Scotland. Five thousand one hundred and thirty-seven
crabs and 3.6 km of line intercept transect data were collected during a 9-week period. Crab abundance, relative frequency
of morphs and substrate type varied significantly among the three sites. Plain crabs were strongly associated with macro-algal
substrates whereas patterned crabs were associated with mussel beds. This pronounced phenotype-environment matching, as well
as various characteristics of the carapace patterns themselves, suggests that patterned crabs are cryptic on polychromatic
backgrounds. The frequency of patterned crabs and the percentage of white pigment on the carapace both declined significantly
with carapace width. The loss of pattern coincides with an ontogenetic shift in habitat use and we present evidence to suggest
that individual crabs lose their pigment, rather than larger patterned crabs being preferentially removed from the population
by predators. Throughout their life history, shore crabs encounter high variation in predation, food supply, and physical
habitat; to survive they have evolved a strategy that includes elements of pattern polymorphism, crypsis, ontogenetic shifts,
and plastic responses. |
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