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Fishery Bulletin 95( 1 ), 1997 
Figure 2 
Minimum-length parsimony network of red grouper mitochon- 
drial DNA haplotypes. Numbers refer to mtDNA haplotypes listed 
in Table 1. Hatch marks represent the number of restriction site 
changes among individual haplotypes. The haplotype designated 
by “a” refers to a haplotype assumed to exist, but not detected in 
the survey. Shaded and solid circles refer to haplotypes unique to 
a locality (West Florida Shelf and Campeche Banks respectively). 
Open circles are haplotypes found in both localities. 
Discussion 
Homogeneity in mtDNA haplotype frequency and 
absence of phylogeograhic structure among haplo- 
types are consistent with the hypothesis that red 
grouper from the west Florida shelf and the 
Campeche Banks represent a single unit stock. There 
are caveats to this hypothesis. First, genetic homo- 
geneity does not unequivocally establish occurrence 
of a unit stock, in part because proof of a null hy- 
pothesis is impossible. Genetic homogeneity in this 
case is simply consistent with the hypothesis that 
samples are drawn from a single population with the 
same parametric haplotype frequencies. In addition, 
small amounts of gene flow are sufficient to homog- 
enize populations genetically (Allendorf and Phelps, 
1981), even though geographic samples may be dis- 
continuous demographically. Another caveat is that 
observed homogeneity may reflect historical rather 
than current events. Present-day populations could 
be isolated spatially but have had enough contact in 
the recent past such that haplotype frequencies are 
overshadowed by historical gene flow. Examination 
of a more rapidly evolving nuclear marker (e.g. 
microsatellite loci) may provide data that suggest 
such a scenario. 
Within-population mtDNA diversity among red 
grouper from the Campeche Banks was lower than 
that reported previously for red grouper from 
the west Florida shelf, and overall, red grouper 
have among the lowest levels of mtDNA diver- 
sity reported for marine fish species (Table 2). 
Levels of intrapopulational mtDNA diversity 
are thought to reflect evolutionary-effective 
population sizes of females (Avise et al., 1988), 
although there is some evidence (Gold et al., 
1994) that intrapopulational mtDNA diversities 
may also reflect contemporary (female) popu- 
lation sizes as well. The latter is of interest given 
that some of the species with low intra- 
populational mtDNA diversities (e.g. weakfish, 
orange roughy) have experienced significant 
reductions in population sizes over the past sev- 
eral years (Graves et al., 1992b; Smolenski et 
al., 1993). The mtDNA diversity observed in red 
grouper may thus indicate that red grouper 
warrant immediate attention in terms of man- 
agement regulation. Alternatively, red grouper 
and black sea basses, the species with the low- 
est reported mtDNA diversities (Table 2), are 
protogynous hermaphrodites (Manooch, 1988), 
and it is possible that this mode of reproduc- 
tion may affect estimates of mtDNA diversity. 
Estimates for black sea bass (Bowen and Avise, 
1990), however, may be somewhat compromised 
by the low sample sizes, given that a significant pro- 
portion of the sampling variance for estimates of 
nucleotide diversity stems from population sampling 
(Lynch and Crease, 1990). Further study of mtDNA 
diversity in other hermaphroditic fishes and in sea 
bass is clearly warranted. 
Present-day gene flow 
The observed genetic homogeneity in E. morio from 
west Florida and Mexico was surprising because, a 
priori, we expected gene flow between the two areas 
to be minimal and the two populations to be diver- 
gent in mtDNA haplotypes. This expectation was 
based on available information about the life history 
of red grouper and on reported discontinuity in red 
grouper distribution. Observations from divers and 
aquaria personnel have shown that juvenile red grou- 
per are fairly sedentary, preferring to hide in crev- 
ices or shells of shallow nearshore habitat (Moe, 
1969). Adult red grouper also are important mem- 
bers of the benthic community, frequently occupying 
crevices, ledges, and caverns formed by rugged lime- 
stone reefs. There is, however, evidence that red grou- 
per do migrate at least to some extent, and tagging 
data suggest “developmental” migration from shal- 
low coastal waters to depths greater than 36 m at 
approximately 5 years of age (Moe, 1966, 1967; 
