608 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL. 5 



system of water circulation observed in the Galapagos Islands, plus that 

 known to exist in the greater Pacific area, including demonstrable irregu- 

 larities, are together capable of accounting for the present recognized 

 distribution of brachyuran species within the archipelago, if the oceanic 

 transportation of larval stages is assumed. 



EXTRA-GALAPAGAN DISTRIBUTION 



(See also Table of Distribution) 



Galapagos species which enjoy the greatest longitudinal range, practi- 

 cally encircling the globe, are confined to the zone of warm equatorial 

 waters, which in the Eastern Pacific is compressed into a comparatively 

 narrow belt found mostly north of the Equator, or between 4° S. and 

 24° N. Latitude. Four Galapagos species are common to the Atlantic, 

 American- and Indo-Pacific Oceans. Grapsus grapsiis, Geograpsus lividus, 

 and Planes minutus are members of the family Grapsidae, among the most 

 highly evolved of the Brachyura, the first two found in the spray zone, 

 the third as a log rider. Domecia hispida, a member of the family 

 Xanthidae, is a commensal of the Pocillopora coral. 



Four species are common to the Galapagos Islands and the Western 

 Pacific and Indian Oceans which are not represented in the Atlantic. 

 Three of these are members of the family Xanthidae, which reaches its 

 greatest development in the tropics. The fourth is a grapsoid, Plagusia 

 immaculata, commonly known as the Pacific Log Rider. While the great 

 majority of the Xanthidae are free living species, those common to the 

 Galapagos and Indo-Pacific are all obligatory commensals in corals. 

 Carpilodes cinctimanus. Trapezia cymodoce ferruginea, and T. digitalis 

 are found only in coral of the genus Pocillopora, while two other xanthids 

 with strong Indo-Pacific ties, Maldivia galapagensis and Quadrella nitida, 

 are found only in coral of the genus Pavona and on the gorgonian, 

 Muricea miser, respectively. This is taken to mean, not that the com- 

 mensal species arrive in relatively greater numbers than the free living 

 but that the accustomed habitat is greater assurance of survival. The fact 

 that the Galapagos Maldivia is very close to, if not conspecific with, the 

 Maldivia of Palmyra, which lies in approximately the same Latitude as 

 Cocos but south of Hawaii, would seem to indicate the Equatorial 

 Countercurrent as the means of dispersal involved. The speed of this 

 current approaches 2 knots (Sverdrup, 1942, p. 709). 



The seven species common to the Galapagos and the Atlantic, but not 

 the Western Pacific, include three grapsoids: Pachy grapsus transversus, 

 Percnon gibbesi, and Eucliiro grapsus americanus, the latter reported from 



