THE COLORATION OF REEF FISHES 539 



esting difference, between the sexes in some crabs, which shows 

 that the explanation which refers the origin of countershading 

 to the direct action of sunhght is incapable of general application. 



Examination of a number of Tortugan Brachyura makes it 

 clear that their sexually dimorphic coloration is definitely cor- 

 related with other characters. It is absent in the ghost crab 

 (Ocypoda arenaria), the orchid crab (Gecarcinus lateralis), in 

 Grapsus grapsus,^ Chorinus heros, Mithrax hispidus, etc., but 

 in none of these species is the abdomen of the ovigerous female 

 exposed in dorsal view. In the Oxyrhynca mentioned the 

 sternum is deeply excavated and the eggs are virtually borne in 

 a brood chamber. In Ocypoda and Gecarcinus the abdomen of 

 the female is hinged to the thorax in such fashion that it is im- 

 possible for its exposure to occur under normal conditions — an 

 observation confirmed by examination of ovigerous specimens 

 of the former species. On the other hand, in all the Portunidae 

 which have been noticed, Portunus sayi, ventralis, depressifrons, 

 and sebae, Callinectes ornatus and marginatus, and Charybdella 

 ruber, the abdomen has greater freedom of movement, and is 

 exposed in large part by the female when carrying her eggs. In 

 all these the pigmentation is as indicated in the following para- 

 graph, though the distinctness of the difference between the sexes 

 varies as the species is dark or light, being more clearly defined 

 in the case of the former. 



The variable ground color of the gulfweed crab (Portunus sayi) 

 is always some shade of brown. This is marked with lighter 

 brown which may verge toward white. The ventral surface 

 of the thorax and the exposed face of the abdomen lying beneath 

 it are in the male dull, waxy white. The same is true of the fe- 

 male with the exception noted below. In the male three abdom- 

 inal segments visible in dorsal view are each crossed transversely 

 by a band of brown, wdth which similar bands correspond in the 

 other sex. But upon the following three segments, invisible 

 from above and uncolored in the male, the female has three 

 additional bands. These appear before sexual maturity, and 



^ The two sexes of this species are not of the same color throughout, but their 

 differences are of no importance in the present connection. 



