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image of Journal of Crustacean Biology

ABSTRACT Production of a thumblike projection on the second gnathopod is a common feature of male amphipods of the family Ischyroceridae. The thumb signals mating intent. In the genus Microjassa the degree of thumb development varies among species from lack of a thumb, to presence of a thumb of varying length, to possession of multiple protuberances. The purpose of this paper is to determine whether there is evidence for directional evolution in thumbing, and, if so, what this might imply. Cladistic analysis using 20 characters produced a phylogeny of species having progressively longer and more numerous thumbs. It is hypothesized that the genus originated on the Pacific coast of North America in tropical waters prior to the mid-Miocene. Thumbing evolved as surface waters cooled after the mid-Miocene and speciation progressed primarily northward and eastward. Thumbing may have developed in response to increased coloniality, which would have promoted the evolution of signaling structures.


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