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Size Selectivity by Atlantic Mud Crabs Panopeus herbstii (Milne Edwards) Feeding on Ivory Barnacles Balanus eburneus (Gould)

ABSTRACT 
 Many crabs preferentially consume prey items that are relatively small to optimize energy gain and to minimize the risk of claw damage. The Atlantic mud crab Panopeus herbstii is an abundant predator on benthic epifauna, including oysters (Crassostrea virginica) and barnacles (e.g., Balanus eburneus) in southeastern U.S. saltmarsh ecosystems. Size selectivity by mud crab predators feeding on barnacle prey was examined using laboratory experiments in which individual mud crabs were offered a size range of ivory barnacles. Small (carapace width, <30 mm) and large (carapace width, >30 mm) crabs were offered small (test width, <7 mm), medium (test width, 7–8.9 mm), large (test width, 9–10.9 mm), and extra large (test width, >11 mm) ivory barnacles. Small crabs selectively consumed small barnacles at greater frequencies than other barnacles. Large crabs consumed proportionately smaller barnacles, relative to their carapace width, than small crabs. The relationship between predator crusher height and prey test width indicates that larger mud crabs reduce their risk of claw damage when attacking barnacles relative to smaller conspecifics. The observed barnacle critical size varied across predator size classes and was not strongly related to crab carapace width or crusher morphology. The median barnacle size of vulnerability increased with increasing predator carapace width. The observed ontogenetic size selectivity by mud crabs feeding on barnacles may be a more relevant predictor of prey vulnerability than barnacle critical size.