84 



Part III. — Eighteenth Annual Report 



of the female." Mr. Hutton,* Customs Officer, Peterhead, stated before 

 the Crab and Lobster Commission that when the female becomes a 

 " peelert," that is a soft crab, which she does yearly in the end of July, 

 and in August and September, the male crab, which does not cast its 

 shell at this time, mates." 



The reason why some mature females are not fertilised is that when they 

 cast no mature male had been present. A crab of, say, 4J inches had 

 been before it cast probably 34 inches across. It was then in what may 

 be called for convenience the 2^-4 inches group. This group contains no 

 mature females, but it contains a few mature males (see below). There 

 is good reason for believing that the crabs will be found grouped 

 according to size in denned areas of the sea bottom. It is not 

 improbable that these are age groups. The habitat of the 24—4 inches 

 group is not known, but it is believed to be just outside low-water 

 mark. The crabs on casting out of the last immature group (2^-4 inches) 

 enter the adult group, to which belong the crabs which are caught in 

 the creels ; the latter measure from below 4 inches up to 8 inches {vide 

 Table IX. for sizes of creel crabs). In the adult group, owing to the 

 fact that practically all the males are mature, the fertilisation of the 

 female is almost certain ; but when a female in the immature group 

 casts, the chances of being fertilised by one of the few mature males 

 are of course very much less. It is not improbable that all of the 

 unfertilised females of 4j-5§ inches had before casting belonged to 

 the immature group. If this is so, a great variation in growth is 

 apparent. It is probable that the capacity for fertilisation, or, in other 

 words, the maturity of the crab, depends upon its age, not its size. The 

 greatest variation appears in the growth of the crab, and a difference of 

 even 1| inches in breadth is probably not infrequent between large 

 individuals of the same age. 



The crabs smaller than the adult group, e.g., below 4^ inches, cast at 

 least once a year. This is apparent from the fact that, with very few 

 exceptions, their shells are always clean. Any Ectozoa found upon them 

 are small, e.g., Serpula, Spirorbis, and Zoophytes. [Only rarely do 

 we find one of these small crabs with a large amount of Ectozoa. 

 A male crab measuring 2| inches across, and which had a large 

 Sacculina attached to its abdomen, was dirty, and appeared not to have 

 cast for a year or two.J Another, 3y 3 g inches across, a female, was 

 covered with large Anomia ; on the swimmerets were a large Saxicava 

 and an Anomia, which kept the abdomen away from the thorax. All 

 the hairs of the third and fourth swimmerets were rubbed off. It 

 appeared not to have cast for a considerable time, more than one year, 

 previously. A third crab, a male, inches, was very black and 

 dirty. The liver was very small, and scattered through it were small 

 black rod -like and large red spherical bodies. f These appeared to 

 be encysted parasites. A female, 3 r 5 g- inches, had evidently not cast 

 for some time, judging from the dirty condition of the shell. Casting 

 among the immature crabs, then, appears to be inhibited by parasites, 

 which interfere with the nourishment of the creature. In the mature 

 group reproduction inhibits casting, and for an analogous reason, namely, 

 the employment, for the development of the sexual organs, of 

 the nourishment which would otherwise have been used up in the 

 growth of the crab. The crabs over 4J inches which are capable of 

 reproduction, but which have not been fertilised, are to all intents and 



* Report of the Crab and Lobster Commission, 1877. Evidence for Scotland, p. 12. 



f Vide Cue'not, "Etudes physiologiques sur les Crustaces d^capodes," Archives de 

 Biolor/ie, T. xiii., 1895. 



J It is well known that crabs infested by Sacculina do not moult. (Cf. Delage, 

 "Evolution de la Sacculine," Arch. Zool. Exper. (2), II., 1884, p. 666. 



