Crabs’ Niches in Crab Cove College of Alameda Department of Biology Presenter: Anne Lei
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
Bay Shore Crab Hemigrapsusoregonensis • 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
Lined Shore Crab Pachygrapsuscrassipes • 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
Materials Used• Transect Lines• Mini Red Flags• Thermometer• Camera phone• Pen & Paper
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
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.
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.
• 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)
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.
• 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.
• 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)
Sources of Error1. 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.
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