Intra-guild predation among early benthic phase red and blue king crabs: Evidence for a habitat-mediated competitive advantage

The apparent shift in predominance of commercial crab species from blue king crab (Paralithodes platypus) to red king crab (Paralithodes camtschaticus) around the Pribilof Islands, Alaska, USA, may be driven by ecosystem-level processes such as competitive displacement or predation during early life stages. Specifically, intra-guild predation among species during the early benthic phase might be a source of mortality limiting blue king crab population recovery. In a laboratory experiment, we evaluated how varying ratios of prey species (year-0 blue and red king crabs) and habitat type (shell and cobble) affect prey preference of year-1 red king crabs. Year-0 red king crabs were preferentially consumed over blue king crabs in shell habitat, but not cobble, likely because the smaller interstitial spaces in shell reduced predator foraging efficiency and predator-prey encounter rates. Blue king crabs had high levels of crypsis regardless of habitat type or predator presence/absence, while red king crabs were less cryptic overall compared to blue king crabs, but crypsis increased when predators were present. Preference for red king crabs in shell may reflect divergent adaptations between species for minimizing predation. Blue king crab's light, mottled coloration and relatively smooth, flat carapace is well adapted for visual and physical crypsis in shell habitat, while the monochromatic, dark coloration and pronounced spination of red king crabs may make them more conspicuous. Our results show that habitat can mediate intra-guild predation between Paralithodes species and suggest that shell material may provide blue king crabs a competitive advantage over red king crabs. The extent of shell habitat may be an important factor regulating blue king crab distribution or the recovery of depleted populations around the Pribilof Islands.