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ATaleof 

 Two Seas 



When North and South America 



collided, some close families 



were divided 



by Nancy Knowlton 



Panama, the country where I now live, 

 once lay beneath the sea. North and South 

 America were separate continents, and the 

 waters and marine animals of what are 

 now the Pacific and the Caribbean min- 

 gled freely over the submerged land that 

 would become the Central American Isth- 

 mus. The movements of the earth's crust 

 and the resultant collisions of plates — 

 which eventually led to the joining of the 

 continents and the separation of the 

 oceans — were gradual. They began some 

 fifteen million years ago and by about 

 three million years ago, the land bridge 

 was complete. These events set into mo- 

 tion one of the world's greatest natural ex- 

 periments: while land animals migrated 

 north and south into new realms, the now 

 separated inhabitants of the two oceans 

 began to travel along separate evolution- 

 ary pathways. 



Today, the closest relatives of many 

 Caribbean fishes, sea urchins, snails, and 

 shrimps are still to be found in the eastern 

 Pacific. Even experts may have a hard 

 time figuring out which ocean a particular 

 animal comes from. Nevertheless, of the 

 few attempts at mating animals from op- 

 posite sides of the isthmus, most have 

 failed; even if we can't tell the difference, 

 the animals can. Once members of a single 

 species, these organisms were separated 

 geographically after the isthmus arose, be- 

 coming over time what scientists refer to 

 today as transisthmian sister species. 



My colleagues and I investigated this 

 evolutionary phenomenon by studying a 

 single, but highly diverse, group, the 

 shrimp genus Alpheiis. These crustaceans 

 look superficially like miniature cold- 

 water lobsters, and they inhabit shallow, 

 tropical seas, where they tend to hide in 

 crevices, burrows, and shelters provided 

 by other organisms, such as corals, sea 

 anemones, and sponges. Rarely seen but 

 often heard, Alpheus are commonly called 

 snapping, or pistol, shrimps for the sound 

 produced when they rapidly close the 



larger of their two front claws during ag- 

 gressive interactions. We began by simply 

 trying to identify the snapping shrimps 

 from both coasts of Panama. With a little 

 experience, we could readily recognize 

 which ones were probably sister species 

 by similarities in external form and in 

 color patterns. 



We wanted to find out if these appar- 

 ently related, look-alike shrimps were still 



enough alike genetically to interbreed. 

 Following geographical isolation, even 

 the signals that animals use to recognize 

 potential mates can change, so our tran- 

 sisthmian sister species provided an ele- 

 gant model for smdying the process of be- 

 havioral and genetic divergence that leads 

 to the creation of new species. We paired 

 snapping shrimps from the same and op- 

 posite sides of the isthmus and then 



66 Natural History 6/94 



