88 



Part III. — Eighteenth Annual Report 



The evidence as to the maturity of the female crab may be summed 

 up as follows : — A female crab becomes potentially mature when it 

 measures about 4 J inches across the broadest part of the back. At that 

 size the vagina and spermatheca are for the first time sufficiently 

 developed to admit of the introduction of the sperms into the sperma- 

 theca. The actual maturity or reproductive activity of a crab depends 

 upon its being fertilised at the moment of casting. Of crabs between 4 J 

 and 5 1 inches in greatest breadth, a considerable proportion (vide 

 p. 82) are not fertilised, and are consequently functionally immature. 

 All crabs over 5| inches which were examined were found to be 

 fertilised. While, then, all females over 5§ inches are probably active in 

 reproduction, a proportion, possibly greater than the half, of those 

 between 4^ and 5| inches, are also mature. With regard to spawn- 

 ing, a crab, as a rule, carries at least two batches of eggs in succession. 

 It then casts, possibly soon after the hatching of the last lot of 

 eggs, and may again spawn within the following four or five months, or 

 not until after an interval of about 14 months. The latter seems to be 

 more often the case. And as to casting, certain facts seem to point to 

 the possibility of the crab not casting until a considerable time after 

 hatching has taken place. We thus have a reproductive period of at 

 least two successive years, followed by an interval of growth to be 

 succeeded by another reproductive period — annual spawning ; biennial 

 or triennial growth. 



Times of Spawning and Hatching. 



The crab, according to Wilson,* spawns in November, December, 

 and January. The observations of Meek and Cunningham confirm 

 this statement. A crab extruded its eggs in the beginning of 

 December in the tank at the Bay of Nigg, another which had been 

 kept in the creek at Dunbar was found to be berried in December. 

 The crab spawns offshore in deep water, and in summer migrates to the 

 warm water near the shore. In June, Mr. Buglass, Berwick-on-Tweed, 

 informs me, berried crabs are caught in considerable numbers in creels 

 shot on a sandy bottom near the shore. According to information 

 supplied by fishermen of Dunbar, the)' are to be found in July on the 

 sandy bottom of Belhaven Bay. A proof of the inshore migration of 

 the berried crab is mentioned by Meek.f Mr. Douglas of Beadnell, 

 marked a berried crab, and set it free 3 miles from the shore in March. 

 It was re-caught about a quarter of a mile offshore in July, and was 

 then clean hatched. 



The newly-spawned eggs of the crab are orange in colour. In May 

 and June the orange colour has given place to a more or less reddish 

 colour. The large black eyes of the embryo are in most cases already 

 prominent. When the eggs are ready to hatch they have a dirty 

 grey appearance. Of 24 berried crabs which were kept in the tanks at 

 Dunbar, 17 hatched out in July, and two during the first half of August ; 

 the remaining five died. When the larva hatches it is not very active; 

 within fifteen minutes after its escape from the egg it moults, and 

 appears as a zoea of the second stage, which is represented in Fig. 4, 

 PI. I. The zoea of the shore crab (Carcinus nicenas) resembles much the 

 larva of the edible crab, but differs from it in not possessing the large 

 lateral spine of the carapace (Fig. 2). Meek concluded that on the 

 Northumberland coast hatching takes place in July and August. The 

 same seems to hold good for the Dunbar district. The period of 

 incubation would thus be seven to eight months. 



When the eggs are ready to hatch the female moves its abdomen 



* Op. cit.,1893, pp. 5, 6, 7. 

 f Op. cit., 1899, pp. 52, 53. 



