DNA Barcoding: Molecular Positioning of Living Fossils (Horseshoe Crab)

Living fossils are the important components of biodiversity. They represent the connection between the extinct and extant species. A living fossil is a living species that appears to be similar to a species otherwise known only from fossils, typically with no close living relatives and the extant species. The study of primitive species gives an idea about the ancestors from which they diverged and bring out many surprising facts which are unknown to the world. In a case study, species belonging to the phylum Arthropoda, Brachiopoda and Molluscs were collected from Sundarbans where Horshshoe Crab was the living fossils (Carcinoscorpius rotundicauda). Lingula sp and some crab species were included as the outgroup. To position this living fossil, DNA barcoding approach was employed as per standard protocol. COI sequencing and subsequently nucleotide analysis of all the species were done and also the molecular clock was constructed to locate their position along with their divergence time in correspondence with the other sequences of the allied taxa viz, Limulus polyphemus, Tachypleus gigas etc. It is found that the Carcinoscorpius rotundicauda are more closely related to its allied taxon Tachypleus gigas as compared to Limulus poluphemus and their divergence period is calculated which is supposed to be the 550 million years ago. Thus, DNA barcoding approach is a useful technique to properly identify species and to construct phylogenetic relationship among the species and subsequent assessment of the species divergence time.