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Large mitochondrial DNA differences between morphologically similar Penaeid shrimp.

Sequencing of mtDNA fragments amplified with the polymerase chain reaction is used to examine genetic differences between species of Penaeid shrimp. The data show surprisingly high genetic differences among morphologically and ecologically similar species. Differences at silent sites between subgenera of shrimp are larger than between families of mammals. Between genera, Penaeid shrimp are more divergent than some orders of mammals. Because taxonomic differences are defined on morphological grounds, these results imply that the rates of molecular versus morphological evolution in shrimp are different than in mammals. Perhaps mtDNA evolution is accelerated in shrimp, or perhaps stabilizing selection over a long time period has reduced rates of morphological change. For Penaeid shrimp, morphological similarity can mask large genetic differences.