Phylogeography and Posterior Regeneration of the Bearded Fireworm, Hermodice carunculata (Annelida: Amphinomidae)

Abstract

The bearded fireworm, Hermodice carunculata, is an amphinomid polychaete occupying rocky habitats in temperate and tropical waters throughout the Atlantic Ocean and adjacent basins. The genus Hermodice was long regarded as monotypic but recently a formerly synonymized species, H. nigrolineata, has been resurrected based on differences in filament abundance and anal lobe morphology between East and West Atlantic Hermodice populations. The objectives of this study were 1) to examine the genetic diversity within and among Hermodice populations throughout most of the genus’s distribution range while re-evaluating morphological variation and 2) to document posterior regeneration, including anal lobe morphology. The phylogeographic analysis was based on mitochondrial cytochrome c oxidase subunit I (COI) and a fragment of 16S rDNA (16S) and included specimens from the Atlantic Ocean, the Mediterranean Sea, the Caribbean Sea, the Gulf of Mexico and the Gulf of Guinea. Additionally, sequence data from the nuclear internal transcribed spacer 1 (ITS1) was collected from specimens in the Gulf of Mexico and the Mediterranean Sea to estimate genetic distances between East and West Atlantic Hermodice populations.

Phylogeographic analysis resulted in two main clades within Hermodice: a private lineage found only in the Mediterranean and a large clade containing specimens from all sampled populations. While the Mediterranean population shows significant fixation, genetic distances (COI) among all populations are relatively low (< 3.0%). Branchial filament abundance is not diagnostic for either of the two clades.

H. carunculata is capable of regenerating large numbers of segments and lost body parts, and it is possible that there is an asexual component to their reproductive cycle. Posterior regeneration was documented by amputating the posterior segments of living H. carunculata specimens from the Gulf of Mexico and sampling regenerating tissues over a six-month period. Histological analysis of the tissue revealed that the anal lobe is the first structure to emerge during posterior regeneration, and its morphology remains constant during regeneration. Furthermore, the ventral nerve chord, which is responsible for initiating the regeneration process, terminates within the anal lobe. These results suggest that the anal lobe may play a role in posterior regeneration in H. carunculata.