Dungeness and red rock crabs differ in their sediment preferences and efficiency of burial. Dungeness crabs prefer smaller sand particles and bury faster and more consistently across sediment types compared to red rock crabs. Red rock crabs are more generalists, found in various sediment types but avoid large particles like gravel. Differences in leg and body morphology between the species contribute to Dungeness crabs having an advantage in burial speed and ability to choose sediments based on preference and efficiency.

1. Dungeness
Crab
Habitat
Specialist,
found in
sandy areas
Faster
burial in
every
sediment
Speed-size
interaction
changes
depending
on site
Prefer
sediments of
small sand
particles
Distribution
explained by
preference but
not efficiency
Red rock
Crab
Habitat
Generalist,
found in
cobble,
sand, mud
and gravel
Slower
burial in
every
sediment
Speed-size
interaction
does not
change by
site
Preference
against large
particles
(gravel and
cobbles)
Distribution not
explained by
preference or
efficiency
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
A buried Dungeness (Metacarcinus magister)
A red rock (Cancer productus) crab
False Bay, Friday Harbor, Washington
Argyle Point, Friday Harbor, Washington
A juvenile red rock crab
1. Fifth legs (V.) anchor into sediment,
curving medially
2. Legs III. and IV. also anchor in the
sediment
3. Second leg (II.) curves beneath the
crab and then sweeps outwards
pushing sediment
4. Chela on same side pushes sediment
forward from the carapace
5. 3rd, 4th and 5th legs stomp down and
push outwards
6. Crab is forced backwards by the chela
movement and pulled down by the
legs
7. Crab rocks forward and expels sand
from below, covering its carapace
Burial Process
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:
Are the natural distributions of Dungeness and red rock crabs
determined by burial efficiency and sediment preference?
Introduction
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).
Red rock crabs tended to avoid large particles and
selected against the cobble mixture (A).
Burial times of Dungeness (blue symbols) and red rock (red symbols) crabs of different sizes at Sites A, B, C, D and E. Pie charts display particle size classes by percent weight with white representing silt (smallest particles) and
increasingly dark colors (moving clockwise) representing increasing particle size. Sediment stiffness was taken by dropping a weighted rod into the sediment, measuring the depth of penetration and taking the slope of the interaction.
Percent mass by particle size (mm) Site A. Site B. Site C. Site D. Site E.
Methods and Results
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.
• Dungeness crabs bury more quickly at all sites
• Dungeness crabs bury equally quickly at all sites (P>0.4) but red rocks crabs bury at
varying speeds (P=0.02).
• The size/burial time interaction differs for the two species at sites A and C (P<0.02).
• The interaction of size and burial time also differs by sediment for Dungeness crabs
(P=0.044) but is uniform regardless of sediment for red rock crabs (P=0.0893).
Dungeness Crab Red rock Crab
Habitat specialist
Found in sand
and eelgrass
habitats
Habitat generalist
Found in sand,
gravel, mud and
rock habitats
Conclusions: Preferences and Efficiencies
Dungeness crabs have more
surface area on the second
distal-most segment of their
digging legs.
Dungeness crabs have sharper
angles formed by their carapace
and abdomen conjunction
presenting a narrower wedge as
they back into the sediment.
Posterior Angle
Anterior leg
Angle(degree)
8
0
6
0
4
0
2
0
0
SurfaceArea(mm²)
Carapace Width(mm)
Acknowledgements
We thank Ian MacDonald and Sylvia Yamada for their time, thoughts and
counsel. Also, thanks to Justin Sui his generously lent muscle when it came
down to transporting sediment. Steve Wang was invaluable in the
interpretation of statistics. This work was made possible by the FHL REU
program, an HHMI grant to JC and additional support from the family of
Walter Kemp. Additionally, we would like to thank Friday Harbor
Laboratories for the use of their facilities and equipment.
Silt
Muds
Sands
Gravel
Cobbles
‘Stiffness’ 26.1 ‘Stiffness’ 50.0‘Stiffness’ 30.4‘Stiffness’ 47.2‘Stiffness’ 25.1
Sites Sites
Count
*p =0.0001
Photo - J. Murray
Why are Dungeness faster buriers?
Burial speeds