Wednesday, April 18, 2012

Libinia emarginata: Spider or Crab?


Spider Crabs, Libinia emarginata, are from the species of crab, stenohaline. They live in the Atlantic Ocean on the coast of North America, but can be found from Nova Scotia to the Florida Keys, and the Gulf of Mexico. 


They have long legs, round spiny bodies, and 9 spines down their back. They live at depths up to 150 feet Females can only reproduce after molting. The young hatch from eggs as zoea larvae [5]. Since predation is a concern for these crustaceans, these crabs will attach pieces of shell, seaweed, and algae to the sticky hairs on their bodies for camouflage [1]. The crabs also use the greenish-brown algae on their back to help hide it from predators [2].


Spider crabs come from the genus and species Libinia emarginata. These crabs can grow to be up to 12 inches from claw to claw [5]. They have different claws than other crabs do in order for them to scoop up algae [3]. Spider crabs and algae share a mutualistic relationship. “A mutualistic relationship is when two organisms of different species ‘work together,’ each benefiting from the relationship” [2]. Even though this is a mutualistic relationship, it is a facultative relationship for both meaning it is not obligatory for either organism. Both organisms benefit from the relationship with the other: the crab gets to blend in with its surroundings and the algae get a place to live [2]. The relationship is established when L. emarginata scoops up the algae. This relationship is not unique to spider crabs. Spider crabs are not the only animals that use algae for camouflage. Devil scorpionfish are camouflaged to their environment because they allow algae to grow on them. Also, other types of crabs use this camouflage technique by allowing algae and dirt to stick to hairs on their body [4].


Benefit for spider crabs: As previously stated, the spider crabs benefit from the relationship with algae by using it as camouflage. Because of this, the crabs have a better chance of surviving predators.

Benefit for algae: Since the crabs pick up the algae, they get a permanent place to live. It is also protected from predators now because it has a permanent home on the crab [2].

Cost for spider crab: The spider crab spends energy to find, collect, and scoop algae onto its back.

Cost for algae: Even though the algae get a place to live, the crabs possibly could take them into a new, unfamiliar environment. Therefore, it might not be as successful there.

Since this is a mutualistic relationship, there is not a great amount of cost for either the spider crab or the algae. Any “cost” is outweighed by the benefits. Even though the crab spends energy recruiting the algae, it pays off since it can hide from predators.

Since this symbiotic relationship shows strong, beneficial qualities for both organisms, it seems like this relationship will proceed. 


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