616 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL. 5 



While the waters of the Galapagos Islands support 23 indigenous 

 brachyuran species, but one genus, Ectaesthesius, which is not found else- 

 where, occurs in the archipelago. However, the Galapagos Islands share 

 with adjacent Pacific coastal waters several other monotypic genera, 

 LipaesthesiuSj Lophoxanthus, and Eriphtdes, which are excluded from the 

 Atlantic. Thus the single species of each genus has no Atlantic analogue, 

 a fact which precludes its having been present in the Bay of Panama, 

 which it now occupies, as recently as Pliocene times. Yet having seen 

 that since late Pliocene only those species have evolved, the geminate 

 species, which are less distinct from one another than are ordinary species, 

 the question naturally arises : in what region have these organisms existed 

 while evolving differences which separate them generically as well as 

 specifically from all other crabs? It would seem that the only possibility 

 for such isolation within the territory they now occupy is the Galapagos 

 Islands. Are not they, then, to be considered Galapagos endemic genera 

 like Ectaesthesius which, unable to reach American mainland shores 

 against the westerly-directed current of the Panama Portal, have reached 

 them since its closure? According to this view, the Galapagos Islands 

 have existed from times sufficiently remote to have witnessed the differenti- 

 ation of genera as well as of species, and have served as a reservoir supply- 

 ing adjacent coastal waters with new and highly specialized brachyuran 

 forms. 



DISTRIBUTION OF FAMILIES 

 Fifteen families of Brachyura are represented in Galapagos waters. 

 These families are encountered in waters of the American mainland, but 

 two others of particular significance are also present: the families 

 Goneplacidae and Gecarcinidae, the latter including the land crabs. 

 Members of the first named family, mostly dredged, may well be excluded 

 from the Galapagos by ecological factors, the absence of the detritus-rich 

 mud bottoms to be found on the opposite mainland, or of the increasingly 

 fresh water to be found as one approaches the mouths of great rivers, 

 such as the San Juan. There is no apparent reason, however, why 

 members of the second family, and particularly, Gecarcinus planatus, 

 should be absent from the red gumbo which borders many Galapagos 

 lagoons, or why Aratus pisoniij a member of the sesarmid section of the 

 Grapsidae, should be excluded from their mangrove-fringed margins. 

 The former is the dominant west coast land crab, while the latter is the 

 ubiquitous mangrove crab of the American tropics. The presence of 

 these species in the Galapagos would not in itself be conclusive evidence 



