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Burying efficiency and sediment preferences in habitat choice for Dungeness (Metacarcinus magister) and red rock (Cancer productus) crabs Joe Corcoran and Rachel Merz Swarthmore College Department of Biology, Swarthmore, PA: Friday Harbor Laboratories, San Juan, WA
Introduction
Photo - J. Murray A buried Dungeness (Metacarcinus magister)
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Are the natural distributions of Dungeness and red rock crabs determined by burial efficiency and sediment preference?
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Dungeness Crab
Red rock Crab
Habitat specialist Found in sand and eelgrass habitats
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Habitat generalist Found in sand, gravel, mud and rock habitats
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Fifth legs (V.) anchor into sediment, curving medially Legs III. and IV. also anchor in the sediment Second leg (II.) curves beneath the crab and then sweeps outwards pushing sediment Chela on same side pushes sediment forward from the carapace 3rd, 4th and 5th legs stomp down and push outwards Crab is forced backwards by the chela movement and pulled down by the legs Crab rocks forward and expels sand from below, covering its carapace
Dungeness crabs have more surface area on the second distal-most segment of their digging legs. Carapace Width(mm)
Angle (degree)
For benthic organisms, being able to bury in sediment provides a feeding ground as well as a refuge from predators and currents. Particle size distribution within a sediment determines its mechanical properties such as its stiffness and the energy required to move through it. Therefore, the abilities of organisms to bury in different sediments may influence the natural distribution of a species. Question:
Why are Dungeness faster buriers? Surface Area (mm²)
Burial Process
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Anterior leg
Dungeness crabs have sharper angles formed by their carapace and abdomen conjunction presenting a narrower wedge as they back into the sediment.
*p =0.0001
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Posterior Angle
A red rock (Cancer productus) crab
Forty crabs of each species were collected on San Juan Island, WA. Burial speeds were measured at five field locations from video recordings. Species sediment preferences were recorded in replicate mesocosms.
Do Dungeness and red rock crabs prefer to choose certain sediments? Dungeness crabs tended to prefer well sorted sediments and sediments with higher proportions of smaller particles (D & E).
• Dungeness crabs bury more quickly at all sites A juvenile red rock crab
• Dungeness crabs bury equally quickly at all sites (P>0.4) but red rocks crabs bury at varying speeds (P=0.02).
Red rock crabs tended to avoid large particles and selected against the cobble mixture (A).
• The size/burial time interaction differs for the two species at sites A and C (P
