The Florida stone crab and the gulf stone crab were once one
species. Changes in climate and sea level probably kept two
populations separate over time until they became genetically
Two species of stone crabs
exist in the Southeastern United States: Menippe
mercenaria in the peninsula of Florida, and Menippe
adina in the northern and western Gulf of Mexico. The two
species differ in coloration and genetic makeup; they also differ
somewhat in reproduction, salinity and temperature
tolerance, and ecology. The table and pictures below show the
differences in color pattern and habitat.
|Body and Claw Color
||Tan to light or medium gray
||Deep chocolate to maroon
|Body and Claw Markings
||Black spotted, spots usually small but uniform
||Usually solid, occasional mottling of light brown
|Leg Color and Markings
||Dark brown, distinct white bands
||Solid deep chocolate to maroon
||Limestone sand, rocky outcrops, seagrass beds
||Muddy bottoms, rocky outcrops, seagrass beds
These two species of stone crab are only partially physically
separated in habitat; in fact, where their ranges overlap, they interbreed to produce
hybrid offspring. Scientists believe these two forms were once
one species that diverged to become two. When and how did the two
To determine when the species diverged from a common ancestor,
scientists calculate "genetic distance." Using known mutation rates,
scientists look at the number of differences in the DNA and back-calculate
how long it took for that many mutational differences to occur
between the two forms. For stone crabs, this genetic distance is
2.9-3.2 million years. In other words, two groups of stone crabs
that probably were geographically separated started to become
genetically different during this time.
For genetic differences to occur, two populations must be
reproductively isolated from one another so that mutations (new
genetic forms) are only passed within one group and not between
groups. This isolation may be due to a physical barrier or to
differences in the timing of reproduction. In the case of stone
crabs, reproduction occurs near the same time in each species
(during summer), but we can look back and find that a physical
barrier did once exist.
Southeastern United States during the Miocene period
In the late Miocene period (about 2-4 million years ago), when
sea level was higher than it is now, the peninsula of Florida was
once a group of islands separated from the mainland by the deep
water of the Okeefenokee Trough. Because they are shallow-water
animals, juvenile and adult stone crabs would have a difficult time
crossing this barrier to mate with stone crabs on the other side.
Stone crab larvae (the very young stage of stone crabs) can be
carried by ocean streams; however, scientists hypothesize that the
water current through the Okeefenokee Trough was very swift, like
the Florida Straits current is today. In situations like that,
currents can entrain small floating
animals like stone crab larvae and limit their dispersal to other
shores. In this way, the Florida population (M.
mercenaria) could have been reproductively isolated from the
mainland population (M. adina). As mutations generated
characteristics favorable to different environments-a light body
with spots that would blend in with sand and sea grass (M.
mercenaria) versus a dark body that would blend in with muddy
bottoms (M. adina)-the genes that carried these
characteristics would be passed through generations within each
population, but not between populations. Over time, the populations
would become increasingly different, until they were different
enough to be considered two species.
When sea level dropped, the Okeefenokee Trough was exposed (it
is now the Okeefenokee Swamp), and Florida once again connected
with the mainland. Two things resulted: 1) the Florida population
and the mainland population were no longer separated and could
interbreed, and 2) the western mainland population (northern and
western Gulf of Mexico) was separated from the eastern mainland
population (Atlantic Ocean).
The pattern of distribution we see today has large areas of
hybridization, where crabs show characteristics that are a mix of
each species. One area is in northwest Florida, where both species
have migrated, met, and mated. The other area is along the Atlantic
coasts of Georgia and South Carolina, where the remnants of the
mainland population were invaded by Florida crabs.
Distribution of stone crabs
at present. Blue = Menippe adina range, red = Menippe
mercenaria range, purple = hybrid zones.
What does the future hold for stone crabs? Will the hybrid zones
expand until all stone crabs are once again genetically similar
enough to call one species, or will the different habitats keep
most of the pure species within their current ranges? Only time
Bert, T.M. 1986. Speciation in western Atlantic stone crabs
(genus Menippe): the role of geological processes and
climatic events in the formation and distribution of species. Mar.
Bert, T.M and R.G. Harrison. 1988. Hybridization in western
Atlantic stone crabs (genus Menippe): evolutionary history
and ecological context influence species interactions. Evol.
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