Morphometric, genetic and reproductive characteristics of mud crabs (genus Scylla de Haan, 1833) from Southeast Asia

Abstract

The edible mud crab, genus Scylla, is important to fisheries and aquaculture throughout the Indo Pacific region, but its taxonomic status has been confused for decades and a new classification has only recently been proposed. This project was undertaken to investigate the species status of mud crabs in Southeast Asia, with a view to deciding whether two sympatric morphs of Scylla found in Ban Don Bay, Surat Thani Province, Thailand, are two separate species. A further aim was to elucidate any possible pre-zygotic reproductive isolating mechanisms (RIMs) and ecological features that maintain the apparent sympatry between these two morphs. Mud crabs were collected from a primary site (Surat Thani, Thailand) as well as from six other locations in Thailand, Vietnam, Malaysia and Bangladesh. Crab samples from the latter sites were used selectively to provide a comparison to the primary study site. Descriptive taxonomy, multivariate morphometrics and allozyme electrophoresis were used to a) determine the number of species present within the crab samples collected; b) to ascertain which species they represent; c) to discover any geographical variation between locations sampled; d) to produce a possible phylogeny that summaries the relationship between Scylla species; and e) to look for pre-zygotic RIMs to explain the sympatry of the two morphs in Surat Thani. Findings from the present study reinforce the recent revision of the taxonomy of the genus Scylla into four species, S. serrata, S. olivacea, S. tranquebarica and S. paramamosain and provides new information on two of the four species which are dominant within Southeast Asia, including Ban Don Bay, Surat Thani Province, S. paramamosain and S. olivacea. Population studies showed both genetic and morphological differentiation between conspecific populations of S. paramamosain and S. olivacea, indicating stock structure for each species, although there is some disparity between morphological and genetic distances for S. paramamosain. This is discussed in relation to the effects of larval dispersal mechanisms and the subsequent recruitment of juvenile crabs. Phylogenetic interpretation of both genetic and morphological characters revealed that both S. serrata and S. olivacea are the most diverged of the four Scylla species; however, the direction of evolution is open to interpretation and the evidence for either S. olivacea or S. serrata as the more primitive species are discussed. Reproductive studies on the two mud crab species found in Surat Thani revealed no physical barrier to hybridization. Both species have a protracted breeding season which continues throughout the year. However, the size at first sexual maturity was significantly smaller for S. olivacea when compared to S. paramamosain. This and other potential mechanisms that may maintain these two species sympatrically are discussed. The clarification of four Scylla species, and the establishment of diagnostic genetic and morphological characters that can be used to identify them, means that research can now focus on both the ecology and life history of these closely related species. Such information is needed urgently with respect to fisheries management as well as to understanding the environmental requirements of each species in order to develop their potential for aquaculture.