Thenus is widely distributed throughout the Indian and western Pacific Oceans, and all documented catches of Thenus are associated with shallow waters and within an average 50Km wide coastal margin. Some of the lobster catches from areas within the range of Thenus are commercially viable, and are an important protein source to many coastal human communities of third world countries. The fact that Thenus is regularly taken at depths up to 100m in the Philippines by otter-trawling would suggest that the depth range of Thenus may extend much further out on the continental shelf areas than is currently believed. Since most Thenus catches are as bycatch from commercial prawn trawling, and otter-trawling activities for prawns are generally confined to the inner shelf areas, it is probable the potential for the commercial catch of Thenus is as yet unrealised. Similarly, the potential catch of Thenus in isolated locations such as the Andaman and Nicobar Is., Laccadive Is. and Reunion Is. is difficult to determine as these areas are outside the fishing grounds of commercial fleets.
This study investigated the taxonomic status within the genus Thenus. It is generally regarded as being monotypic with one species Thenus orientalis (see Holthuis, 1991), but Jones (1993) felt that there were two species, viz. T. orientalis and what he calls ''Thenus indicus'. A third putative species temporarily named Thenus sp.A by the author was also included in the investigation. Material was obtained and examined from a wide area, and five species are here recognised.
Four methods were used to investigate the taxonomy within the Theninae. The first involved traditional methods which included making comparisons of pereopod colour variation, spermatid development and morphometric features between groups of Thenus, which were collected from different geographical regions. The second involved seeking biochemical differences using starch gel electrophoresis on muscle and hepato-pancreas tissues of all specimens. The third methodology compared the results of direct mtDNA sequencing of periopod muscle tissue of Thenus to those of the electrophoresis investigation. All results of these latter techniques were then used to re-examine for morphometric variation between the putative species.
Using traditional taxonomic methods, specimens of the Genus Thenus and eight outgroup species of the Scyllarridae, which included Ibacus spp., Scyllarides elizabethii, Scyllarus demanii and Parribacus sp., were investigated for any consistent differences among 20 groups of Thenus collected from 11 geographic locations spread throughout the range Thenus. Results indicated that some variation between the different geographic groups of Thenus did exist, but as the amount of variation was insufficient to delineate between populations and separate species, more detailed molecular and morphometric investigations were undertaken.
Starch gel electrophoretic investigations determined the number and type of fixed allelic differences among all geographical groups, which were recorded and used as a basis in determining the genetic relationship between the groups, using UPGMA and maximum parsimony tree-building algorithms. The results of the analyses implied that the Theninae consisted of two distinct branches which were initially named the T. orientalis and T indicus groupings. Each branch was further divided into a number of genetically related groups or groups - the T. orientalis branch consisting of three groups and T. indicus containing two. The genetic relationship between members within the groups varied according to the tree building methodology. The lack of consensus in the genetic relationships between the groups as determined by the different tree building methods was probably due to missing allelic data, consequently investigations were continued using mitochondrial DNA sequencing of muscle tissue.
Using mtDNA sequencing for the 16s and CO1 gene fragments, the status of the putative species, Thenus sp.A was found to be similar to that of T indicus. In addition, the degree of genetic diversity and divergence between groups of T. orientalis and T. indicus from different geographical locations along the Queensland coastline were estimated. Genetic variation as determined by mtDNA sequencing was calculated as percentage sequence difference between members within the same specific groups.
Once baselines for genetic diversity and divergence values between populations and species of Thenus in Australia were established, CO1 mtDNA sequencing analyses were used to compare specimens of Thenus from the different groups identified by the allozyme analysis to reconstruct relationships among populations. Sequencing data were subjected to tree-building methods (genetic distance methods and parsimony methods), both of which confirmed the result of the parsimony tree produced by the allozyme analyses.
Having found strong genetic evidence of five species within the Theninae, a detailed morphological analysis between all specimens was undertaken to determine whether morphologically distinct features for each species could be recognised. All specimens of Thenus were morphometrically measured and the measurements statistically evaluated. Results were compared with those of the previous genetic findings to determine the degree of morphometric correlation among all five groups. Multivariate morphological analyses confirmed the five species diagnosed by molecular markers, which were temporarily named Thenus orientalis, T. unimaculatus, T. australis, T. indicus and T. parindicus, and a taxonomic key developed.
In conclusion, genetic divergence rates of other invertebrate crustacean species were used to estimate time since species divergence and genetic divergence rates within the Theninae (in addition to the four outgroup species of Ibacus). The relationship between eustatic lows and resulting changes in coastal topography during the Pleistocene era, and genetic divergence rates within the various species of Thenus, is offered as an hypothetical explanation for present genetic diversification. Finally, a number of recommendations are presented whereby the findings and conclusions of this study may be expanded into a comprehensive understanding of the distribution and taxonomy of the genus Thenus.