26 U.S. NATIONAL MUSEUM BULLETIN 292 



Glyptograysus jamaicensis, which is beheved to have marine larvae, 

 is a Jamaican endemic, and its closest relative, G. impressus, occurs on 

 the Pacific coast of southern Mexico and Central America. Since 

 Jamaica has been a haven for several endemic grapsids, an observer is 

 tempted to conclude that G. jamaicensis is a relict of a tertiary stock 

 that occurred in the Central American region during the submergence 

 of the isthmus. That segment occurring on the Caribbean side became 

 segregated from that giving rise to the Pacific stock with the develop- 

 ment of the Panama Land Bridge during the late Tertiary or early 

 Pleistocene (Woodring, 1966). Woodring noted, in relation to his 

 study of Tertiary moUusks, that "The rise of the bridge also is in- 

 ferred to have led to impoverishment of the present Caribbean prov- 

 ince. . . . " The ancestors of G. jamaicensis could well have found a 

 refuge on Jamaica prior to this time, whereas the remainder of the 

 Gulf coastal stock perished along with other forms following the dis- 

 appearance of the interoceanic seaways. 



Three of the four Antillean endemic members of the genus Sesarma 

 are also found only on Jamaica (S. bidentatum, S. jarvisi, and S. 

 verleyi). A single specimen of the fourth, S. hanseni, is known, recorded 

 simply from "The West Indies." Prior to Hartnoll's (1964) studies, the 

 only crabs that have "been shown to pass their entire life cycle in 

 fresh water are the Potamonidae [including Pseudothelphusidae]." 

 He demonstrated that the three Jamaican endemics also complete 

 their life cycle in fresh water as does also the closely related, bromeliad- 

 inhabiting Metopaulias depressus. Almost certainly, these four Ja- 

 maican crabs had their origins on Jamaica from a common ancestor, 

 and the diverse habitats that they occupy suggest that ecological 

 factors have played an important role in their origins. Of the four, S. 

 bidentatum probably most closely approximates both in its morphol- 

 ogy and ecology those of the ancestral stock. That portion of the 

 stock that gave rise to *S'. jarvisi moved to higher elevations, that 

 leading to S. verleyi invaded subterranean streams, while that from 

 which Metopaulias depressus arose found a niche in the lentic habitat 

 of bromeliads. Once the necessary adaptations had been made so that 

 the life cycle could be completed in fresh water, it is not surprising 

 that three of the stocks invaded habitats that are unique for grapsid 

 crabs. Therefore, with the additional evidence of morphological simi- 

 larities that are not knowoi to have been affected by a transition in 

 habitat, there is every reason to agree with Hartnoll's (1964) con- 

 clusion that "all the freshwater crabs of Jamaica have been the result 

 of an invasion by a single stock derived from the subgenus Sesarma." 

 There still remains, however, the question as to the source of the 

 invading stock. Only one other member of the subgenus, S. cura^ao- 

 ense, occurs in the West Indies, ranging from southern Florida to 



