COMMENSAL ASSOCIATION OF A SPIDER CRAB 



AND A MEDUSA. 



JULIAN D. CORRINGTON, 



ZOOLOGICAL LABORATORY OF THE LIBERAL ARTS COLLEGE OF 

 SYRACUSE UNIVERSITY. 



During four successive years the writer was associated with 

 a week-end biotal survey of the coast of South Carolina, and on 

 two of these field studies encountered a curious commensal re- 

 lationship between a spider crab and a medusa. The animals con- 

 cerned were Libinia dubia Milne-Edwards and Stomolophus mele- 

 agris L. Agassiz, both of them common and typical inhabitants of 

 the littoral zone of the Austroriparian Subregion. 



Libinia dubia is a spider crab of moderate size with a rounded 

 carapace averaging 6 cm. in diameter. It has long and slender 

 walking legs and six median dorsal spines. Adult stages are con- 

 fined entirely to the bottom zone where they crawl about in search 

 of food. They are scavengers and members of the benthos. 



Stomolophus melcagris is a rhizostome medusa of a diameter 

 of 18 to 20 cm., hemispherical, without marginal tentacles, with 

 eight rhopalia, and with fused oral lobes which form a thick cylin- 

 der, at the bottom of which are eight pairs of frilled lobes and a 

 small central mouth opening. The margin of the umbrella is 

 colored with a dark reddish-brown material, fading out aborally, 

 which comes off freely and stains the hands when the specimens 

 are picked up for inspection. They do not appear to possess any 

 nettling organs, or at least none which could be employed defen- 

 sively. This form is by far the most abundant Scyphozoan of 

 the South Carolina coast, and is one of the more conspicuous 

 planktonic organisms of the littoral zone. 



Here we have to do then with a commensal relationship be- 

 tween a member of the benthos on the one hand and of vertically 

 the most distant life zone, the plankton, on the other certainly a 

 most unusual arrangement. By means of this alliance the crab 

 becomes a transient component of the plankton. The association 



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