CRABS (BRACHYURA) OF THE GULF OF MEXICO

18 Lawrence W. Powers
Atlantic faunal affinities with West Indian elements occur as a result of direct
recruitment, bypassing the Gulf of Mexico.
Specific genera that have been recentlj' investigated and summarized with
respect to zoogeography include Sesarma (Abele, 1973), Ovalipes (Williams,
1976), CalUnectes (Williams, 1974a), and Uca (Crane, 1975). Abele (1973)
divided the six Florida species of Sesarma into those with Carolinian and West
Indian affinities. Although Hedgpeth (1953) had emphasized salinity and temperature
tolerances as reasons for the absence of Sesarma cinereum and S. reticulatum
from southern Florida, Abele (1973) believes that competitive exclusion
by S. ricordi may account for the absence of the first and possibly the second
disjunct species. Interspecific competition is an unknown factor in the distribution
of virtually everj'' brachj^uran species, especially the aquatic crabs. An
exception is the genus Uca, where intensive studies by several authorities over
the past half century have produced the largest amount of biological data on anj'
one crab genus. Crane (1975) summarized the zoogeographical and evolutionary
information on Uca distribution. She remarked on the relatively depauperate
Atlantic fauna of this genus, compared to the rich and diverse Pacific assemblage,
also known for many other groups of invertebrates. The differences are attributed
to cooling during the Oligocene to Pleistocene, which was far more severe in the
Atlantic than in the Pacific, resulting in the extinction of many sensitive tropical
species. The isolation of the Caribbean at the end of the Pliocene, due to the
emergence of Panama, resulted in the Atlantic assemblage, each species of which
has a Pacific analogue. Only U. subcylindrica, a poorly-known Gulf endemic,
has no eastern Pacific counterpart. Other Gulf species of Uca, five of which are
endemic, may be in the process of rapid speciation. The most recentlj^-described
species, Uca panacea (Novak and Salmon, 1974), may have diverged from U.
pugilator behaviorally and ecologically with only minimal morphological change.
Many authorities are reluctant to consider this form a separate species without
further evidence of isolation or distinction. The same applies to U. virens and
U. longisignalis (see entries in systematic section), two forms similar to U.
pugnax of the Atlantic coast. An analysis of courtship displays and habitat requirements
reveals the divergence of the species not detected from preserved
specimens alone. This evidence is substantiated by preliminary comparisons of
isozyme patterns for Atlantic and Gulf populations of U. panacea, U. pugilator,
U. pugnax, U. virens, and U. longisignalis (Selander, Johnson and Avise, 1971;
Selander, pers. comm.). Unfortunately, this glimpse of speciation in progress is
a rare exception to the usual tj^pe of information available. Most accounts of
zoogeography depend on collection records derived from preserved specimens.
Misidentifications, erroneous locality labels, and gaps in collection efforts are
familiar difficulties.
This contribution is offered as a preliminary step toward compiling the necessary
information for a synthesis on the distribution and evolution of crabs in the
Gulf region. It should facilitate the collection of data on particular groups of
crabs and the comparison of specific lines of evidence relative to ecological,
behavioral, physiological, or developmental aspects. It can also indicate the rela-