Globally, mangrove coverings are disappearing at the rate of 1–2% per annum and 35% have been lost in the last 20 years. Changes in climate and human activities are affecting the mangrove habitats significantly. When the mangroves were transplanted artificially 25 years ago in the Vellar estuary, no mangrove-associated crabs were found. We sampled this mangrove ecosystem and intent to estimate the diversity, species abundance, composition and phylogenetic relationships of barchyuran crabs. We also intend to evaluate the efficacy of DNA barcoding technique in precisely identifying species of brachyuran crab associated with mangroves. Mangrove species such as, Avicennia marina, A. officinalis, Rhizophora apiculata, R. mucronata and R. annamalayana, Acanthus ilicifolius and salt marshes; Suaeda maritima and Prosopis juliflora constituted the artificially created mangrove ecosystem. A total of 2844 crabs were collected, representing 35 species belonging to 20 genera within 8 families. The four species of brachyuran crab, that is, Uca lactae, U. triangularis, Selatium brockii, and Neosarmatium asiaticum contribute >70% of total abundances. The present study recovered an estimated 87.5% of crab species. The maximum association index value (97.7%) was observed between Uca lactea and Uca triangularis. Cluster analysis, grouped the sampled stations according to the type of mangrove species present. It was clear that the type of mangrove species influences brachyuran crabs’ structure and species composition. Clustering analysis also clearly distinguished the mangrove stations and salt-marsh station (control) based on the composition of the brachyuran crab species. In general, the abundances of all collected species of crabs, and particularly Neosarmatium asiaticum, prefers vegetative cover composed of multiple species of mangroves. DNA barcoding analysis shows that 40% of the species collected in this study was barcoded for the first time. In near future, the advent of new high-throughput sequencing technologies will dramatically change bio-monitoring applications and surveys. This will make reference datasets such as ours important. Using the array of diversity and species estimator indices we presented useful data on brachyuran crab diversity associated with artificially created mangrove ecosystem, which will be useful for marine policy makers, coastal ecosystem designers and climate researchers.

Environmental DNA (eDNA) barcoding technology is finding innovative applications in monitoring terrestrial and marine biodiversity. DNA barcode reference libraries containing barcodes for wide range of species is vital for the success of monitoring any environments using eDNA barcodes. Since amphipods are used as bio-indicators for monitoring environmental quality and its health, we used DNA barcodes of amphipods to test the efficacy of present DNA libraries for species identification. We barcoded 22 amphipod species belonging to 17 genera of 13 families. 50% of barcodes produced in the present study was generated for the first time, as the sequences were absent in GenBank and Barcode of Life Databases (BOLD). Tree based identification method used in the present study precisely clustered the generated sequences with reference sequences. Besides exploring the distribution of recorded amphipod species, we show that with advent of next generation sequencing technologies, reference datasets such as ours will become essential for assessing health and monitoring various environments using amphipod barcodes.