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 ProcessFor 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 ResultsForty 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

(deg

ree)

8

0

6

0

4

0

2

0

0

Surf

ace

Are

a (m

m²)

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