Analysis of the relationship between geography and body color with the genetic diversity in the Echiura worm Urechis unicinctus based on the mitochondrial COI and D-loop sequences

Urechis unicinctus is the only Echiurini species distributed in Bohai Gulf of China. The wild populations of this species have sharply declined in China due to overfishing. Over 150 samples from Bohai Gulf were collected in the present study, which were classified into five populations according to their geographic areas and body colors. The genetic diversity and population structure of these populations were investigated by mitochondiral COI and D-loop sequences. The haplotype diversity of U. unicinctus based on COI and D-loop sequences were still high. In addition, the evolution rate of D-loop region could faster than the COI gene of U. unicinctus. Meanwhile, over 99% genetic diversity was contributed by different individuals within populations. Moreover, phylogenetic trees did not show clear geographic or color cluster. Our findings indicated that this species in Bohai Gulf of China should be treated as a whole population.