A Proposed Evolutionary History of Stone Crabs

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.

Species	M. mercenaria	M. adina
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
Habitat	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 species develop?

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.

The 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 will tell.

References:

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. Biol. 93:157-170.

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. 42(3):528-544.

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