Genetic Dif1ferentiation in Trans-Floridian Species Complexes of Sesarma and Uca (Decapoda: Brachyura)
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JOURNALOF CRUSTACEANBIOLOGY, 14(2): 191-209, 1994 GENETIC DIF1FERENTIATION IN TRANS-FLORIDIAN SPECIES COMPLEXES OF SESARMA AND UCA (DECAPODA: BRACHYURA) Darryl L. Felder and Joseph L. Staton ABSTRACT Most nearshore brachyurancrabs of the northern Gulf of Mexico have been treated as conspecificsof those in warm-temperateCarolinian waters on the southeasternAtlantic coast of North America. However, historicalphysiographic constraints appear to have periodically restrictedgene flow betweennorthern Gulf populationsand siblingAtlantic coast populations, and contemporarydisjuncture of ranges often persists across south Florida. The present ex- aminationof intertidalcomplexes has centeredon grapsidcrabs presently assigned to Sesarma reticulatumand ocypodid crabs assignedto Uca minax, 2 species in which we have observed markedvariations in colorationover their distributionalrange. Genetic differentiationbetween populationshas been assayed by allozyme electrophoresis,and resultantdata have been eval- uated with F-statistics and cluster analysis of genetic distance. Allozyme divergencebetween Gulf and Atlantic populationsof the S. reticulatumcomplex is at levels previously reported for speciatedpopulations, while that betweentrans-Floridian populations of U. minax is much less pronounced. In both species, minimal divergence can be measured between 2 widely separatedAtlantic coast sample localities that were compared,while more complex gradesof differentiationare evident between sample localities compared within the northernGulf of Mexico. Trans-Floridiandivergence of populationsfor both of these species is compatiblewith models for periods of contact and subsequentisolation of Gulf and Atlantic stocks duringand since peak glacial advances in North America. Less conspicuouspatterns of genetic differen- tiation between sample localities within the northernGulf of Mexico may reflecta history of glacialand postglacialalluvial events which resultedin contemporaryphysiography of northern Gulf estuaries. Prior to recent reevaluations,the coastal cylindrica (Stimpson), Platychirograpsus warm-temperateCarolinian decapod crus- spectabilisde Man) and some possible iso- tacean assemblageof the westernNorth At- lates of tropical stocks (Uca spinicarpa lantic was thoughtto include a largenumber Rathbun, U. marguerita Thurman, Calli- of widely distributed and highly adaptable nectes rathbunaeContreras). Evidence of a nearshoreand intertidalbrachyuran species few other endemic nearshore brachyurans that ranged from the vicinity of Cape Hat- could be deduced from published ranges, teras,North Carolina,into the northernand but there was no clear case for relationship northwestern Gulf of Mexico (Hedgpeth, of these or other Gulf endemic species to 1953; Williams, 1965; Powers, 1977). Even Atlantic coast siblings and no documenta- in cases where distributionsof species were tion of trans-Floridian geminate species known to break at the southern extreme of pairs in this group (Williams, 1965; Felder, peninsular Florida, close morphological 1973; Williams, 1974; Powers, 1977). similaritybetween disjunct populations led Evidence for historical disjuncture of most investigatorsto conclude that little or coastalbrachyuran stocks acrossthe Florida no differentiationhad occurredbetween such peninsula and ultimate speciation of those populations. These breaks in distribution stocks has accumulatedin the course of sev- were usually assumed to representnothing eral case studies from the early 1970s to the more than a contemporary disjuncture of present. In his separation of the Atlantic appropriatehabitats which accounted for a xanthidcrab Dyspanopeus sayi (Smith)from short break in an otherwise continuous the closely related Gulf of Mexico endemic range. While endemism of nearshore spe- species D. texanus(Stimpson), Abele (1972: cies was documented within some brachy- 269) discussedthe probablerole of Miocene urangenera in the Gulf of Mexico, evidence throughPleistocene events which alternate- was limited primarily to certain conspicu- ly submergedand exposed the Florida pen- ous cases of apparentrelict species (Uca sub- insula and which could have reduced gene 191 192 JOURNAL OF CRUSTACEAN BIOLOGY, VOL. 14, NO. 2, 1994 flow between Gulf and Atlantic coast pop- of appropriateestuarine habitats for these ulations of the Dyspanopeuscomplex (treat- species across southern Florida. There is, ed within Neopanope prior to Martin and however, some differencein early life his- Abele, 1986) and of "other species pairs tory and capacityfor dispersalbetween typ- among the Decapoda with similar distri- ical populations of the U. minax (five zoeal bution patternsalong the Gulf and Atlantic stages)and S. reticulatum(three zoeal stages) coasts." Subsequently, additional closely complexes, and some differencein habitat related species pairs have been identified preferencebetween U. minax (low salinity that may have divergedfrom stocks isolated marshesand banks)and S. reticulatum(me- to either side of historical or contemporary dium to high salinity marshes and banks) distributionalbreaks in south Florida;these complexes. include such Gulf/Atlantic pairings as Uca Much as color differencesfirst prompted panacea Novak and Salmon/U. pugilator further study and eventual recognition of (Bosc) (Novak and Salmon, 1974), Ovalipes sibling species in the Menippe mercenaria/ floridanus Hay and Shore/O. stephensoni M. adina complex (Williams and Felder, Williams (Williams, 1976), Panopeussimp- 1986), our observationof differencesin col- soni Rathbun/P. herbstii Milne Edwards oration between trans-Floridian popula- (Williams, 1984a), and Menippeadina Wil- tions of both the U. minax and the S. re- liams and Felder/M. mercenaria(Say) (Wil- ticulatum complexes has prompted us to liams and Felder, 1986). Even where con- undertakegenetic characterizations,despite temporaryranges for some of the above may our finding of otherwise striking morpho- now overlap in the northeastern Gulf, as logical similarities between Atlantic and in the case of Uca panacea/U. pugilator Gulf of Mexico populations.However, since (Novak and Salmon, 1974; Barnwell and in both complexes some color variations Thurman, 1984), or where, as in the Menip- were also observed between geographically pe complex,introgressive hybridization may separated populations within the Gulf of occur (Bert, 1986; Williams and Felder, Mexico, an efforthas been made to compare 1986; Bert and Harrison, 1988), historical populations of both species from through- events which separated stocks across the out their ranges in the Gulf of Mexico. Fi- Florida peninsula may have facilitated ge- nally, our interestin genetic examination of netic isolation and subsequent speciation. the S. reticulatumcomplex has also been Recognition of divergent features be- promptedby recent studies of reproductive tween closely relatedtrans-Floridian sibling biology (Zimmerman and Felder, 1990, species and populations of the Brachyura 1991) and osmoregulatoryability (Staton has in various instances been facilitatedby and Felder, 1992) that have uncovered ev- comparison of morphology,coloration, be- idence of functional divergence between havior, physiology, hemocyanin electro- Gulf of Mexico and Atlantic populations of morphs, allozyme frequencies,and nucleic this group. acid sequences(as referencedabove, see also Fielder et al., 1971; Selanderet al., 1971a; MATERIALSAND METHODS Salmon et al., 1979; Sullivan et al., 1983; Animals for genetic analyses were collected over a Neigel et al., 1991). In the presentstudy, we 2-year period (16 May 1989-1 August 1991) from lo- undertake allozyme analyses in trans-Flo- calities at intervals along the southeastern Atlantic and ridian that are treated Gulf of Mexico coasts of the United States (Fig. 1). complexes presently Within this area, we have concluded that the Sesarma as two brachyurantaxa, the fiddler crab reticulatum complex is reliably documented to range (Ocypodidae) Uca minax (Le Conte) and in the Gulf of Mexico from Barra del Tordo, Tamau- the marsh crab (Grapsidae)Sesarma reti- lipas, Mexico (Rabalais et al., 1989; Zimmerman and culatum(Say). The selection of these species Felder, 1991) to Sarasota County, Gulf coast of Florida (Abele, 1973), and from Volusia County, Atlantic coast is based upon (i) their classical and contin- of Florida, to Woods Hole, Massachusetts (Abele, 1992). ued treatment as Carolinian species that We have concluded that the Uca minax complex ranges range into the Gulf of Mexico (Hedgpeth, in the Gulf of Mexico from near the Neches River at 1953; Barnwelland Thurman, 1984; Abele the eastern extreme of the Texas coast (Wurtz and and the in dis- Roback, 1955) into northwestern Florida, perhaps to Kim, 1986), (ii) disjuncture Yankeetown, Gulf coast of Florida (Salmon, 1967); tribution of both of these species across precise records for both eastern and western extremes southern Florida, and (iii) the disjuncture of the Gulf range are questionable, since the species FELDER AND STATON: BRACHYURAN GENETICS 193 Sesarma reticulatum complex PINC - Pivers Island, North Carolina (28) 24 Oct 89 NBGA - north of Brunswick, Georgia (28) 16 May 89 SUFL - Suwannee, Florida (13) 26 Apr 90 CAFL - west of Carabelle, Florida (30) 14 Nov 90 WBFL- West Bay, Florida (30) 26 Apr 90 DIAL- Dauphin Island causeway, Alabama (30) 9 Aug 90 PINC PRMS - tributary of Pascagoula River, Mississippi (30) 27 Oct 89 COLA - Cocodrie, Louisiana (30) 24 Jul 89 CPLA - Cypremort Point, Louisiana (30) 22 Jun 89 SPTX - Sabine Pass,