Amphisamytha (Annelida: Ampharetidae) from Indian Ocean hydrothermal vents: Biogeographic implications

Abstract

Abstract Deep-sea polychaetes in the genus Amphisamytha are well known from numerous hydrothermal vents and hydrocarbon seeps in the Pacific and Atlantic oceans. Records from the Indian Ocean, however, have not been characterized morphologically or genetically. Here, we examined specimens taken from deep-sea vents on three mid-ocean ridges in the Indian Ocean, including Southwest Indian Ridge (SWIR), Central Indian Ridge (CIR), and Carlsberg Ridge (CR), and re-assessed phylogenetic relationships within the genus including the Indian Ocean records. Multi-gene phylogenetic analyses using a concatenated alignment of 16S rRNA, 18S rRNA, 28S rRNA, and COI genes recovered four distinct, hitherto unreported, lineages in the Indian Ocean interpreted to represent four undescribed species. One of these, formally described as Amphisamytha marisindica n. sp. herein, is characterized by relatively large body size, conspicuous nuchal organs, smooth abdominal glandular pads, and 17–19 abdominal chaetigers. It was found to span all three ridges, representing one of the most widespread species in the genus. Two further sympatric species, Amphisamytha collaris n. sp. and Amphisamytha sp. Longqi, appeared to prefer relict/inactive chimneys in the Longqi field, SWIR. Amphisamytha collaris n. sp. is unique for its V-shaped collar on the posterior lobe of prostomium. Due to the scarcity and condition of the samples available, Amphisamytha sp. Longqi was not formally described. Lastly, Amphisamytha wocanensis n. sp. was only recorded from the Wocan field, CR, and no reliable morphological characteristics could separate it from A. carldarei and A. fauchaldi, indicative of cryptic speciation. Monophyly of the genus Amphisamytha was not supported in the current phylogeny, warranting further revisions at the genus level. The four Indian Ocean species were placed into two well-supported clades: Amphisamytha sp. Longqi and A. collaris n. sp. formed sister-relationship with the Southwest Pacific A. julianeae, while A. marisindica n. sp. and A. wocanensis were recovered as crown taxa in a clade composed of their East Pacific and Atlantic congeners, indicating two separate evolutionary lineages of Amphisamytha meeting in the Indian Ocean. The sister-relationship of species from the Carlsberg Ridge and East Pacific suggests extreme long-distance dispersal of their ancestor, or points towards numerous undescribed or extinct species belonging to this clade.