COMMENSALISM BENEFICIAL TO CRUSTACEA. 897 



39. Experimental Evidence that Comniensulism may be 

 beneficial to Cmstacea. By Edward B. Poulton, 

 D.Sc, F.R.S., E.Z.S., Hope Professor of Zoology and 

 Fellow of Jesus College in the University of Oxford. 



[Received November 7, 1922 t Read November 7, 1922.] 



The special associations of Crustacea with organisms believed to 

 be disliked by their enemies — with Sponges, Ascidians, and Sea- 

 anemones — has probably been derived from a more generalized 

 use of animal and vegetable growths for the purpose of 

 concealment. The following experiments proved that two con- 

 spicuously coloured forms associated with Hermit-crabs were 

 intensely distasteful to fishes, and that one of the crabs, when 

 deprived of its associate, was greedily devoured. The experi- 

 ments were carried out, with the help and advice of Prof. W. 

 Garstang, in the Marine Biological Laboratory at Plymouth in 

 the summer of 1890. The results have often been described, but 

 never published. 



(1) Pagurus hernJtardus and its Sea-anemone, Sagartia para- 

 sitica. — These Crustacea were commonly dredged up at Ply- 

 mouth, and large specimens bore on the shells — generally whelks 

 — from two as to as many as six of these Jarge brightly coloured 

 actinians. Prof. Garstang had already obtained much indirect 

 evidence of the value of the association, for he had often found 



. the young Hermit-crabs, too small to carry a Sea-anemone, 

 in the stomach of gurnards and other fishes. They had been 

 swallowed entire, their borrowed shells and all. He had, however, 

 never found in the fishes any of the larger crabs living in shells 

 suited for carrying Sagartia *, 



I first tested the nematocysts of the Sagartia by touching it 

 with the tip of my tongue, and at once experienced a sharp smart 

 which endured for many hours. Pieces of about the size and 

 shape of the bits of meat on which the fish in one of the tanks 

 were accustomed to be fed were then cut from a Sea-anemone 

 and thrown into the tank. Misled by this, a few fishes seized 

 pieces of the Sagartia ; but no sooner had one been received into 

 the mouth, than it was shot out again with much force, and the 

 fish shook its head violently from side to side, apparently feeling 

 the same smart which I had experierced myself. After these 

 first trials not one of the fish would touch the pieces, and it was 

 obvious that the great majority saved themselves by yielding 

 to the stimulus provided by the behaviour of the others. 



(2) Pagu7'us cuanensis and the Sponge, Suberites do7mtncula. — 

 This small Hermit-crab, also common at Plymouth, inhabits a 

 cavity in the sponge, the tip of its tail being fixed in a small 



* These facts are recorded in " Colours of Animals," Internat. Sci. Ser., Lond. 

 1890, p. 203. 



