Abstract
Dungeness crabs (Metacarcinus magister) are ecologically and economically important in the coastal Northeast Pacific, yet relatively little is currently known about their feeding behaviour in the wild or their natural diet. Trophic biomarkers, such as fatty acids (FA), can be used to reveal trophic interactions. We used two feeding experiments to assess differences in FA composition of juvenile crabs fed different known foods to evaluate how they modify and integrate dietary FA into their own tissues and determine whether crab FA reflect diet changes over a six-week period. These experimental results were then compared with the FA signatures of wild caught juvenile crab with undetermined diets. We found that juvenile Dungeness crabs fed different foods assimilated dietary FA into their tissues and were distinct in their FA signatures when analysed with multivariate statistics. Experimentally fed juvenile crabs contained greater proportions of the most abundant long-chain polyunsaturated fatty acids (LCPUFA, >C20) than their foods. Crabs fed foods lacking in LCPUFA, particularly DHA (22:6ω3, docosahexaenoic acid), did not survive or grew slower than crabs fed other foods. This suggests that LCPUFA are physiologically important for this species and indicates biosynthesis of these FA does not occur or is not sufficient to meet their needs.This article is part of the theme issue ‘The next horizons for lipids as ‘trophic biomarkers’: evidence and significance of consumer modification of dietary fatty acids’.
Highlights
Determining diets of wild animals is key to understanding organismal ecology and pathways of energy flux through ecosystems [1]
When compared with the FA profile of their diets, juvenile crabs preferentially assimilated into their tissues greater proportions of 18:1ω9, 18:1ω7, and all abundant >C20 polyunsaturated fatty acids (PUFA), except 22:5ω3, which was found in a higher proportion in crab food
The FA profiles of juvenile Dungeness crabs fed distinct foods became strongly differentiated in their resulting integrated FA signatures in both feeding assays
Summary
Determining diets of wild animals is key to understanding organismal ecology and pathways of energy flux through ecosystems [1]. Previous studies of juvenile Dungeness crab diets were based on inspection of stomach contents [18,19], but these methods can be biased to only recently consumed items and prey with recognizable hard-parts, and may miss important soft-bodied sources of nutrition [24], especially in very small individuals Crabs use their chelae and mandibles to tear food into small pieces, further impeding accurate assessment of dietary components. Six foods were selected based on their abundance and availability in the estuary at the time of Dungeness crab recruitment: (1) bivalve (mostly Clinocardum nuttallii, some juvenile Macoma sp.); (2) ghost shrimp (Neotrypaea californiensis); (3) mysid shrimp (Neomysis mercedes); (4) megalopae (M. magister); (5) polychaete (Owenia spp.); and (6) detritus (including sand, organic particulate matter, and any fauna within the upper 1 cm of benthic substratum) In this experiment, we focused on foods that were likely to be included in the diets of crabs in the particular estuary where we were collecting wild crabs. Results are presented as mean ± s.d. unless otherwise indicated
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