Abstract:

Aim: Because of its unique paleogeographical history and geographical location, Hainan Island is a major diversity hotspot in China. Previous research has shown that the earthworm fauna on Hainan Island is distinctive, and migration between Hainan Island and mainland China has occurred. However, how the earthworm fauna on this island formed is still unknown. Thus, this study aims to: (1) systematically assess earthworm diversity and their geographical distribution on Hainan Island; (2) improve understanding of the differentiation and dispersal of earthworms on Hainan Island using phylogenetic analysis.
Method: First, we collected earthworms on Hainan Island, especially from nature reserves, between 2006 and 2009 either by digging by hand or using an electric machine. We used both morphology and molecular techniques to identify earthworm species. Then, the molecular phylogenetic analysis was conducted using 5 mitochondrial genes—COI, COII, ND1, 12S rRNA and 16S rRNA using MEGA 5, jModelTest 2.1.10 and MrBayes 3.2.7. Their ancestral distribution area was reconstructed, and their differentiation time was speculated using the RelTime method and GTR model.
Results: (1) There were a total of 122 earthworm species, including 103 endemic species, belonging to 9 genera and 6 families. Megascolecidae is the dominant family, and all of the Hainan earthworms in this family belonged to pheretimoid species. The number of species varied by altitude, initially increasing before decreasing at higher altitudes. The highest number appeared in the altitude range of 800‒1,000 m. (2) The changes of genetic distance of the Hainan pheretimoid earthworm in species, subspecies and population levels were basically the same as those on mainland China. The shortest genetic distances appeared between Amynthas scitulus* and A. zonarius at the species level. The genetic distance between subspecies Metaphire magna magna and M. magna minuscula was close to that between two species. Furthermore, the genetic distances between A. homosetus populations were comparable to those at the subspecies level. (3) The pheretimoid earthworms of Hainan Island could be grouped into 7 clusters. The results suggested that they may have originated from the Diaoluo Mountain about 68.26 Ma years ago. All the earthworms appeared in the Cenozoic era.
Conclusion: Our results suggest that the earthworm fauna on Hainan Island including their composition, geographical distribution and genetic distances are unique, which indicate that they are affected by the environmental factors, such as altitude. This study not only provides information on earthworm biodiversity in China, but also serves as a reference for future genetic analyses of earthworms on islands.

Key words: earthworm, Megascolecidae, species composition, phylogenetic analysis, Hainan Island