Burying efficiency and sediment preferences reveal complexities in habitat choice for Dungeness (Metacarcinus magister) and red rock (Cancer productus) crabs

Particle size distribution is a key physical factor in determining where organisms live in sedimentary marine habitats because it strongly influences the mechanical properties of the medium and thus the energy required to move through it. We examined burying efficiency in and preference among five natural sediments for two crab species – Metacarcinus magister (the Dungeness crab), a habitat specialist and Cancer productus (the red rock crab), a habitat generalist. Crab burial speeds and sediment stiffnesses were measured at five field sites. Dungeness crabs buried equally well in less stiff, well-sorted sands and more stiff, poorly-sorted mud/cobble mixtures. At all sites they buried more quickly than red rock crabs whose performance decreased with increasing heterogeneity of the sediment. These results are contrary to expectations based on reported habitat specificity of the two species. Burial speed decreased with increasing carapace size especially for red rock crabs. The burial success and speed of Dungeness crabs is associated with the relatively larger surface area of their propoduses and the sharper angle formed by the posterior border of the carapace and abdomen. In replicate mesocosms, crabs explored five substrates and then buried in one. Dungeness crabs tended to select well-sorted sediments similar to where we found them in the field. In contrast, red rock crabs were least likely to bury in the mud/cobble sediment in which they had the most difficulty burying although this sediment was typical of the field site where they were most common. The discrepancies among burial performance, sediment preference, and natural distribution reveal the complexity of habitat use between these co-occurring species and are in contrast to the pattern shown by burying fish and some other crab species