Crabs’ Niches in Crab Crabs’ Niches in Crab CoveCove

College of AlamedaCollege of Alameda

Department of BiologyDepartment of Biology

Presenter: Anne LeiPresenter: Anne Lei

IntroductionIntroduction• There are 2 species of crabs at Crab There are 2 species of crabs at Crab

Cove, Alameda CA Cove, Alameda CA

Bay Shore CrabBay Shore Crab

Lined Shore CrabLined Shore Crab

• Hypothesis: Since there are 2 different Hypothesis: Since there are 2 different species of crab in Crab Cove, they must species of crab in Crab Cove, they must occupy different nichesoccupy different niches

• Gauss’s Principle: No 2 species can Gauss’s Principle: No 2 species can occupy same niche in a community at occupy same niche in a community at the same time the same time

Bay Shore CrabBay Shore Crab  Hemigrapsus Hemigrapsus oregonensisoregonensis

• Nearly square carapace Nearly square carapace • Often with light greenish Often with light greenish

spots on a dark reddish-spots on a dark reddish-brown background, but brown background, but may be a pale green may be a pale green (photo), yellow-green, (photo), yellow-green, gray-green, or even gray-green, or even nearly white (photo)nearly white (photo)

• The four posterior pairs The four posterior pairs of legs are more or less of legs are more or less hairyhairy

• Depth range (m): 0 - 5 Depth range (m): 0 - 5 • Found under rocks in Found under rocks in

mudmud

Lined Shore Lined Shore CrabCrab  Pachygrapsus crassipesPachygrapsus crassipes • Boxy shaped carapiceBoxy shaped carapice

• Black stripes along carapace Black stripes along carapace and its overall color may be and its overall color may be any where from red, purple, or any where from red, purple, or greengreen

• Amphibious: Can tolerate little Amphibious: Can tolerate little water in long periods of timewater in long periods of time

• More aggressive and faster More aggressive and faster movingmoving

• Depth Range: High and mid Depth Range: High and mid intertidal intertidal

• Found in high rocky intertidal Found in high rocky intertidal areas under rocks and in areas under rocks and in crevicescrevices

Materials UsedMaterials Used

• Transect LinesTransect Lines

• Mini Red FlagsMini Red Flags

• ThermometerThermometer

• Camera phoneCamera phone

• Pen & PaperPen & Paper

MethodsMethodsMay 9May 9thth & May 20 & May 20thth, 2011, 2011

• Transects at low tide Transects at low tide (early morning) in order (early morning) in order to find more crabsto find more crabs

• Marked Transect Area Marked Transect Area with red flag to go back with red flag to go back to same area on second to same area on second triptrip

• 4 Transect Lines (50 4 Transect Lines (50 feet) at the areas feet) at the areas demarcated as reddemarcated as red X X

• Turned over all stones Turned over all stones that the transect line that the transect line touchedtouched

• Count the #crabs for Count the #crabs for each specieseach species

Transect #1 (muddy area) 50ft Transect #1 (muddy area) 50ft lineline

Transect #2 (rocky area) 50ft Transect #2 (rocky area) 50ft line horizontalline horizontal

Transect #3 (muddy area) 50ft Transect #3 (muddy area) 50ft lineline

Transect #4 (rocky area) 50ft Transect #4 (rocky area) 50ft line horizontalline horizontal

ResultsResults• May 9May 9thth, 2011:cloudy and wind was gusty ; , 2011:cloudy and wind was gusty ;

temperature was 55 degrees Ftemperature was 55 degrees F

Crab Crab SpeciesSpecies

Transect Transect #1 #1 (muddy)(muddy)

Transect Transect #2 #2 (rocky)(rocky)

Transect Transect #3 #3 (muddy)(muddy)

Transect Transect #4 #4 (rocky)(rocky)

#Bay #Bay Shore Shore CrabCrab

4242 00 3737 00

#Line #Line Shore Shore CrabCrab

00 00 00 55

• May 20May 20thth, 2011: Slightly windy, sunny weather; , 2011: Slightly windy, sunny weather; temperature was 63 degrees Ftemperature was 63 degrees F

Crab Crab SpeciesSpecies

Transect Transect #1 #1 (muddy)(muddy)

Transect Transect #2 #2 (rocky)(rocky)

Transect Transect #3 #3 (muddy)(muddy)

Transect Transect #4 #4 (rocky)(rocky)

#Bay #Bay Shore Shore CrabCrab

3535 00 4141 00

#Lined #Lined Shore Shore CrabCrab

00 44 00 77

DiscussionDiscussion• As expected, I found As expected, I found onlyonly Bay shore crabs in the Bay shore crabs in the

muddy areas under rocks and muddy areas under rocks and onlyonly Lined shore Lined shore crabs under rocks and between crevices of the crabs under rocks and between crevices of the rocky areas. rocky areas.

• Perhaps in order to avoid competition with each Perhaps in order to avoid competition with each other for food, they occupy different niches other for food, they occupy different niches (muddy vs rocky). (muddy vs rocky).

• Resource PartitioningResource Partitioning: differentiation of niches : differentiation of niches enables similar species to coexist in community.enables similar species to coexist in community.

• Lined shore crab also has very large claws, Lined shore crab also has very large claws, which can be a form of which can be a form of character displacement: character displacement: where characteristics are diverged in sympatric where characteristics are diverged in sympatric populations of 2 species.populations of 2 species.

Much like the Barnacles Much like the Barnacles (Campbell)(Campbell)• 2 species of barnacles that grow on rocks2 species of barnacles that grow on rocks• BalanusBalanus fails to survive high on rocks fails to survive high on rocks

because it is unable to resist desiccation because it is unable to resist desiccation when areas are exposed to air during low when areas are exposed to air during low tide. tide.

• Even though Even though Chthalamus Chthalamus concentrated concentrated on upper strata of rocks, once on upper strata of rocks, once Balanus Balanus removed from lower strata, the removed from lower strata, the ChthalamusChthalamus population spread lower. population spread lower.

• ChthalamusChthalamus could be found on the lower could be found on the lower rocks if not for the competition with rocks if not for the competition with Balanus.Balanus.

• I predict that the two I predict that the two species of crabs have a species of crabs have a similar relationship like similar relationship like the barnacles.the barnacles.

• Competition has forced Competition has forced one species of crabs to one species of crabs to live in the rocks while live in the rocks while the other species live in the other species live in the muddy areas the muddy areas (different niches) (different niches)

Conclusion—Different Conclusion—Different NichesNiches• Bay shore crabBay shore crab is a is a better osmoregulator better osmoregulator than the other local than the other local shore crabs. shore crabs. 

• It also often digs burrows It also often digs burrows and is capable of and is capable of withstanding more withstanding more hypoxic conditions than hypoxic conditions than the other shore crabs the other shore crabs are. are. 

• Feeds mainly at night, Feeds mainly at night, mostly on diatoms & mostly on diatoms & green algae, but will eat green algae, but will eat meat if available.meat if available.

• Predators include gulls, Predators include gulls, raccoons, anemones and raccoons, anemones and fish. fish.

• Lined shore crabLined shore crab is most is most semi-terrestrial of the shore semi-terrestrial of the shore crabs, living highest in the crabs, living highest in the intertidal. intertidal. 

• Forages in and out of the Forages in and out of the water, active during the day. water, active during the day. 

• They spend at least half their They spend at least half their time out of water but return time out of water but return periodically to pools to periodically to pools to moisten their gills. moisten their gills. 

• They are osmoregulators, and They are osmoregulators, and can withstand hypo- and can withstand hypo- and hyperosmotic conditions.  hyperosmotic conditions. 

• Feeds on algae and diatoms. Feeds on algae and diatoms. Occasionally eats dead Occasionally eats dead animals or small intertidal animals or small intertidal invertebrates, and has invertebrates, and has especially been noted eating especially been noted eating limpets(aquatic snails)  limpets(aquatic snails) 

• Predators include gulls, Predators include gulls, raccoons, anemones, and fish. raccoons, anemones, and fish.

•Although they both Although they both feed on algae and feed on algae and diatoms, their niches diatoms, their niches are different due to are different due to competition.competition.

•They live in different They live in different habitats (muddy versus habitats (muddy versus rocky), Bay shore crab rocky), Bay shore crab eats small green algae eats small green algae (Ulva) while Lined (Ulva) while Lined Shore crab can eat Shore crab can eat different algaes and different algaes and limpets (snails)limpets (snails)

Sources of ErrorSources of Error1.1. While uncovering rocks, the crabs While uncovering rocks, the crabs

scatter. They might have scattered under scatter. They might have scattered under rocks that I will uncover next, which rocks that I will uncover next, which results in counting the same crab twice.results in counting the same crab twice.

2.2. Bias=I specifically chose muddy and Bias=I specifically chose muddy and rocky areas because I knew beforehand rocky areas because I knew beforehand the habitat it likes to live in. Next time, I the habitat it likes to live in. Next time, I should choose a transitional area with should choose a transitional area with both rocky and muddy areas to see if the both rocky and muddy areas to see if the overlap would lead to bay shore crab overlap would lead to bay shore crab living in muddy areas. living in muddy areas.

3.3. Estimation of #crabs under rocks since Estimation of #crabs under rocks since they scatter quickly.they scatter quickly.

ReferencesReferences

• Campbell, N. and Jane, R. 2002 Biology 6Campbell, N. and Jane, R. 2002 Biology 6thth ed, ed, Pearson Publications, Pg 1177Pearson Publications, Pg 1177

• Cowles, Dave (2005) Cowles, Dave (2005) Pachygrapsus crassipes, Pachygrapsus crassipes, viewed May20,2011,viewed May20,2011,  http://www.wallawalla.edu/academics/deparhttp://www.wallawalla.edu/academics/departments/biology/rosario/inverts/Arthropoda/Ctments/biology/rosario/inverts/Arthropoda/Crustacea/Malacostraca/Eumalacostraca/Eucarustacea/Malacostraca/Eumalacostraca/Eucarida/Decapoda/Brachyura/Family_Grapsidae/rida/Decapoda/Brachyura/Family_Grapsidae/Pachygrapsus_crassipes.htmlPachygrapsus_crassipes.html

• Encyclopedia of Life (2009), Encyclopedia of Life (2009), Field Guide: Crab Field Guide: Crab CoveCove, viewed May 20,2011 , viewed May 20,2011 http://education.eol.org/field-guide?guidekeyhttp://education.eol.org/field-guide?guidekey=3&eol_id=317367=3&eol_id=317367

Questions?Questions?