1. Crabs’ Niches in Crab Cove
College of Alameda
Department of Biology
Presenter: Anne Lei

2. Introduction
• There are 2 species of crabs at Crab Cove,
Alameda CA
Bay Shore Crab
Lined Shore Crab
• Hypothesis: Since there are 2 different
species of crab in Crab Cove, they must
occupy different niches
• Gauss’s Principle: No 2 species can occupy
same niche in a community at the same time

3. Bay Shore Crab Hemigrapsus
oregonensis
• Nearly square carapace
• Often with light greenish
spots on a dark reddish-
brown background, but
may be a pale green
(photo), yellow-green,
gray-green, or even nearly
white (photo)
• The four posterior pairs of
legs are more or less hairy
• Depth range (m): 0 - 5
• Found under rocks in mud

4. Lined Shore Crab Pachygrapsus
crassipes
• Boxy shaped carapice
Boxy shaped carapice
• Black stripes along carapace and
its overall color may be any
where from red, purple, or green
• Amphibious: Can tolerate little
water in long periods of time
• More aggressive and faster
moving
• Depth Range: High and mid
intertidal
• Found in high rocky intertidal
areas under rocks and in crevices

5. Materials Used
• Transect Lines
• Mini Red Flags
• Thermometer
• Camera phone
• Pen & Paper

6. Methods
May 9th & May 20th, 2011
• Transects at low tide (early
morning) in order to find
more crabs
• Marked Transect Area with
red flag to go back to same
area on second trip
• 4 Transect Lines (50 feet)
at the areas demarcated as
red X
• Turned over all stones that
the transect line touched
• Count the #crabs for each
species

7. Transect #1 (muddy area) 50ft line

8. Transect #2 (rocky area) 50ft line
horizontal

9. Transect #3 (muddy area) 50ft line

10. Transect #4 (rocky area) 50ft line
horizontal

11. Results
• May 9th, 2011:cloudy and wind was gusty ;
temperature was 55 degrees F
Crab Transect Transect Transect Transect
Species #1 #2 #3 #4
(muddy) (rocky) (muddy) (rocky)
#Bay 42 0 37 0
Shore
Crab
#Line 0 0 0 5
Shore
Crab

12. • May 20th, 2011: Slightly windy, sunny weather;
temperature was 63 degrees F
Crab Transect Transect Transect Transect
Species #1 #2 #3 #4
(muddy) (rocky) (muddy) (rocky)
#Bay 35 0 41 0
Shore
Crab
#Lined 0 4 0 7
Shore
Crab

13. Discussion
• As expected, I found only Bay shore crabs in the
muddy areas under rocks and only Lined shore
crabs under rocks and between crevices of the
rocky areas.
• Perhaps in order to avoid competition with each
other for food, they occupy different niches (muddy
vs rocky).
• Resource Partitioning: differentiation of niches
enables similar species to coexist in community.
• Lined shore crab also has very large claws, which
can be a form of character displacement: where
characteristics are diverged in sympatric populations
of 2 species.

14. Much like the Barnacles (Campbell)
• 2 species of barnacles that grow on rocks
• Balanus fails to survive high on rocks
because it is unable to resist desiccation
when areas are exposed to air during low
tide.
• Even though Chthalamus concentrated on
upper strata of rocks, once Balanus removed
from lower strata, the Chthalamus population
spread lower.
• Chthalamus could be found on the lower
rocks if not for the competition with Balanus.

15. • I predict that the two
species of crabs have a
similar relationship like the
barnacles.
• Competition has forced one
species of crabs to live in
the rocks while the other
species live in the muddy
areas (different niches)

16. Conclusion—Different Niches
• Bay shore crab is a better
osmoregulator than the
other local shore crabs.
• It also often digs burrows
and is capable of
withstanding more hypoxic
conditions than the other
shore crabs are.
• Feeds mainly at night,
mostly on diatoms & green
algae, but will eat meat if
available.
• Predators include gulls,
raccoons, anemones and
fish.

17. • Lined shore crab is most semi-
terrestrial of the shore crabs,
living highest in the intertidal.
• Forages in and out of the water,
active during the day.
• They spend at least half their time
out of water but return
periodically to pools to moisten
their gills.
• They are osmoregulators, and can
withstand hypo- and
hyperosmotic conditions.
• Feeds on algae and diatoms.
Occasionally eats dead animals or
small intertidal invertebrates, and
has especially been noted eating
limpets(aquatic snails)
• Predators include gulls, raccoons,
anemones, and fish.

18. • Although they both feed
on algae and diatoms,
their niches are different
due to competition.
• They live in different
habitats (muddy versus
rocky), Bay shore crab
eats small green algae
(Ulva) while Lined Shore
crab can eat different
algaes and limpets
(snails)

19. Sources of Error
1. While uncovering rocks, the crabs scatter.
They might have scattered under rocks that I
will uncover next, which results in counting
the same crab twice.
2. Bias=I specifically chose muddy and rocky
areas because I knew beforehand the habitat
it likes to live in. Next time, I should choose
a transitional area with both rocky and
muddy areas to see if the overlap would lead
to bay shore crab living in muddy areas.
3. Estimation of #crabs under rocks since they
scatter quickly.

20. References
• Campbell, N. and Jane, R. 2002 Biology 6th ed, Pearson
Publications, Pg 1177
• Cowles, Dave (2005) Pachygrapsus crassipes, viewed
May20,2011,
http://www.wallawalla.edu/academics/departments/
• Encyclopedia of Life (2009), Field Guide: Crab Cove,
viewed May 20,2011
http://education.eol.org/field-guide?guidekey
=3&eol_id=317367

21. Questions?