Developmental and genetic studies of the genus Macrobrachium bate
Macrobrachium as a genus, has evolved comparatively recently from the marine into the freshwater environment. Biogeographic and morphological analysis suggested that the genus has evolved in the present-day Indoâ€”West Pacific area and spread subsequently to West Africa, Eastern America and Western America. The present thesis investigated the larval development and genetics of the species and related the results to both evolution and aquaculture of the genus.
M.lanchesteri was found to have nine zoeal stages before metamorphosing to the postlarval stage. This is similar to the other species in the genus with an extended type of larval development. However, it is one of the few species of this group that can complete the larval phase in freshwater. M.hainagense, on the other hand, has the abbreviated type of larval development, all pereopods and pleopods (non-larval forms) are already present in the first post-embryonic stage. Phylogenetic trees constructed from allozyme data suggest that the â€™hainanenseâ€™ group (species with similar larval development to M.hainanense) is directly related to a primitive group within the genus, the â€™rosenbergiiâ€™ group, rather than to other species in the Indo-West Pacific. The comparatively fewer species with intermediate type of abbreviated larval development, coupled with inference from allozyme data, suggests that the genus has probably â€™invadedâ€™ the freshwater environment in more than one wave. Phylogenetic trees drawn from larval developmental data and allozyme data also have a high degree of congruence. As the degree of abbreviated development relates to the history of advancement in freshwater (except â€˜rosenbergiiâ€™ group), the congruence between the larval developmental data and the allozyme data confirms the polyphyletic nature of the genus.
Larvae of a population of M.nipponense was found to metamorphose in a range of salinity, including freshwater (reduced survival rate). A relatively high realised heritability was observed for larval freshwater tolerance of M.nipponense. This suggested that some species (some populations) still have a high potential for further adaptation to the freshwater environment.
Considerable genetic differentiation was observed between populations of M.nipponense and M.rosenbergii. Hydrographic conditions are suggested to have contributed to the population differentiation in M.rosenbergii. In M.nipponense, the two pond/lake populations have much lower values of genetic diversity than the two estuarine populations.