CRYPTIC AND SIBLING SPECIES AMONG THE DECAPOD CRUSTACEA

Cryptic species are those which are difficult to recognize using traditional systematic methods. Among the decapod Crustacea, there are probably many unrecognized complexes of cryptic species because of dependence on preserved material. Electrophoretic and mating studies have been very useful in determining whether individuals that differ in ecology, behavior, life history, or color can potentially interbreed. Color pattern of living specimens is a particularly useful character, since it is easy to assess and appears to differentiate morphologically similar species in a number of decapod groups. Examples include species of Uca and various symbionts of coral reef organisms (e.g., in the genera Trapezia and Alpheus and the subfamily Pontoniinae). Although nonmorphological characters cannot be scored in most museum specimens, statistical analysis of morphometric data first collected for individuals of known identity can subsequently be used to identify preserved material. Electrophoretic analyses, such as those done for species complexes of Uca, Trapezia, and Alpheus, have also shown that cryptic species pairs may or may not be more closely related to each other than morphologically distinct congeners. For this reason the term "sibling" should be reserved for those species which have been shown to be very closely related using biochemical techniques. Various speciation patterns, including possible examples of host race speciation and speciation facilitated by sexual selection, occur within decapod species complexes. Decapod crustaceans, like most species, are primarily described and identified by morphological characters which can be studied in preserved specimens (McLaughlin et al., 1982). Although this procedure effectively delineates many species, clusters of morphologically similar but reproductively isolated taxa cannot easily be resolved using such techniques. These problematic taxa are variously called cryptic or sibling species. The term "sibling species" has traditionally been used for sets of species which are difficult to distinguish using traditional morphological characters (Mayr, 1963). "Cryptic" when used taxonomically (as opposed to ecologically) is another frequently used term for the same concept. The latter is etymologically preferable (Henry, 1985), since "sibling" implies a particularly close relationship, a feature which Mayr (1963) asserted did not differentiate sibling from other congeneric species. More recent studies have shown, however, that species which are difficult to tell apart morphologically may or may not be as distantly related to each other, on the basis of biochemical characters, as are morphologically distinct congeners (Ayala, 1975; Henry, 1985). For this reason, I will refer to all morphologically similar species as cryptic species, unless biochemical data exist to indicate that they are in fact siblings as well. Numerous nonmorphological characters have been used to distinguish cryptic species, depending on the group involved (Mayr, 1963). They include karyology, hybridization experiments to detect postzygotic incompatibility, distribution patterns, resource use, breeding season, life history and development, mating behavior (including visual, acoustical, and chemical signals), color pattern, and various biochemical characters (especially soluble proteins studied by gel electrophoresis). Those which directly assess either the presence of hybrids (e.g., allozyme or chromosome studies) or the potential for hybridization (e.g., mating experi