96 Mr Potts, Observations on the changes 



Observations on the changes in the Common Shore- crab 

 caused by Sacculina. By F, A. Potts, M.A., Trinity Hall. 



[Bead 22 February 1909.] 



Professor Alfred Giard, whose recent death all zoologists must 

 deplore, established a great reputation by the brilliance and versa- 

 tility of his researches. Perhaps his most noteworthy discovery 

 w^as that the Rhizocephala, a family of aberrant parasitic Cirri- 

 pedes, produce an extraordinary modification of the sexual 

 characters of the Decapod Crustacea they infect*. The evident 

 results of the association are the dwindling of the reproductive 

 glands of the host and the effacement of already existing secondary 

 sexual characters (such as copulatory styles) and assumption of 

 those of the other sex. 



Since Giard wrote, the nature of these effects has been clearly 

 set forth by Geoffrey Smith f for the case of the spider-crabs 

 (Brachyura oxyrhyncha). Perfect sterility and the disappearance 

 of the gonads results. The abdomen of the male becomes trough- 

 shaped, often completely resembling the female type. The first two 

 abdominal appendages modified as copulatory styles dwindle, and 

 on the other segments a varying number of swimmerets, charac- 

 teristic of the female, are developed. Modification in the converse 

 sense is not observed in the female, but that of the male evidently 

 indicates the development of a hermaphrodite condition : for male 

 crabs, which have undergone these changes, freed from the para- 

 sites and allowed to recover, may regenerate an ovotestis with 

 mature sexual products of both kinds. 



This, then, is an example of extreme modification. The common 

 shore-crab {Carcinus moenas) when attacked by the same parasite 

 offers a case of slight or incipient modification. The short descrip- 

 tion we owe to Giard is vague on several important points, and 

 the common association of the shore-crab and Saccidina on our 

 own coasts;]:, also renders it desirable that another account of the 

 phenomenon should be given. 



* "La Castration Parasitaire, " Bull. Sc. Dep. Nord. Ser. 2, Tome xviii. 1887, 

 p. 1, and Ser. 3, Tome i. 1888, p. 12. 



t "Ehizocephala," Mon. 29, Fauna u. Flora des Golfes von Neapel, 1906. 



X In Plymouth Sound a considerable proportion of the shore-crabs are found to 

 harbour Saccidina. It is curious to note that in many localities, e.g. Liverpool 

 Bay and the Isle of Man, the parasite is rarely or never found. 



