POSTMARSUPIAL DEVELOPMENT AND GROWTH OF PAGURAPSEUDES LARGOENSIS MCSWEENY (CRUSTACEA, TANAIDACEA)

Abstract

Postmarsupial development of the gastropod shell-inhabiting tanaidacean Pagurapseudes largoensis McSweeny is described and compared with that of Heterotanais oerstedii (Kr0yer) and Neotanais micromopher Gardiner. This is the first such study of an apseudomorphan based on experimentally reared animals. P. largoensis is gonochoristic; two manca instars are typical of all tanaidaceans so far investigated. In females, the mancas are followed by two or possibly three juvenile instars (=neutra), one or two preparatory instars characterized by rudimentary oostegites, and a copulatory stage with complete marsupium. Laboratory maintained animals pass through up to three copulatory stages, each separated by a preparatory stage of one or two instars. A preparatory instar, characterized by very small rudimentary oostegites often restricted to thoracopods VI, sometimes occurs between the juvenile and normal preparatory stages. The small number of such field-collected specimens suggests that it is an abnormality. Males develop either directly from the manca 2 instar or from the first juvenile instar. Pleopods and heterochelae either accompany initial appearance of genital cones or appear after one or two additional molts. Males retain a full set of mouthparts, feed, and molt throughout their lives. Variability in the pattern of instar succession may be construed as a primitive trait and supports the position of the Apseudomorpha as least derived of the three tanaidacean suborders. Males alone are heterochelous; the right chela is always major and its growth is allometric with respect to carapace length. Intermolt period increases with age, from a 2-week mean for mancas 1 to about 4 weeks for preparatory and copulatory females and 7 weeks for large males, although minimum intermolt periods are similar for all stages (11-29 days). Longevity is estimated at up to 15 months for males and 9 months for females. The sex ratio for field-collected specimens is about 1:1. Females bear 4-17 offspring per marsupium. Investigations into the development of the Tanaidacea have treated a very small percentage of the several hundred known species. Most are reconstructions of life histories based on preserved material. Biickle-Ramirez' (1965) experimental rearing of the tanaidomorph Heterotanais oerstedii (Kroyer) is the only detailed work based on live animals. The current investigation represents the first study of postmarsupial development of a member of the suborder Apseudomorpha to be based on live animals as well as preserved material. Pagurapseudes largoensis McSweeny, 1982, belongs to the family Pagurapseudidae Lang, 1970 (see also Gutu, 1972) and occupies vacant gastropod shells as do hermit crabs. It dwells in a variety of shallow, inshore, bottom communities in the Florida Keys but appears to prefer carpets of algae dominated by the red alga Laurencia poitei (Lamouroux) that occur in beds of turtle grass (Thalassia testudinum Konig). It is also frequently found clinging to the undersides of tussocks of the calcareous red alga Jania rubens (Linnaeus) and to bases of colonies of the inshore finger coral Porites divaricata Lesueur, both of which are found in Thalassia beds (Messing, 1979). This tanaidacean has both advantages and disadvantages as an experimental animal. It does not swim and can be kept in open dishes without becoming inextricably and fatally caught in the surface tension, a problem with tanaidaceans that can swim. It appears to be an unspecialized detritivore and is easily maintained in the laboratory. On the other hand, it occupies opaque or, at best, translucent shells that do not permit monitoring of ovarian maturation, egg deposition,