Nereis Linnaeus, 1758

Bergamo 1, Gilberto, Carrerette 1, Orlemir, Shimabukuro 1, Mauricio, Santos, Cinthya S. G. & Sumida 1, Paulo Y. G., 2024, Revealing a new eyeless Nereis (Nereididae: Annelida) clade from deep-sea organic falls, Zoological Journal of the Linnean Society 201 (201), pp. 1-31 : 6

Genus Nereis Linnaeus, 1758

Type species: Nereis pelagica Linnaeus, 1758 , by subsequent designation.

Remarks: Nereis is the most species-rich genus of nereidids and includes ~252 species and subspecies ( Bakken et al. 2022). Although the last attempt to list all Nereis species was conducted by Hartman (1959), it is still the best overview of described species ( Bakken and Wilson 2005). The diagnosis of the genus is too broad, and synapomorphies are sometimes homoplastic, such as the presence of homogomph notopodial falcigers, also present in Eunereis , for example ( Bakken and Wilson 2005). The presence of homogomph falcigers in posterior notopodia also distinguishes Nereis from Neanthes ( Fauchald 1977a) , which causes problems of species delimitation, considering the small number and the fragility of these chaetae. Besides, other situations can compromise the identifications, such as incomplete original descriptions ( Fauchald 1972), descriptions from a single specimen or only a few heteronereidid specimens, and modifications on posterior parapodia and chaetae, making heteronereidid specimens difficult to identify ( Bakken and Wilson 2005), in addition to descriptions based on incomplete specimens.

These taxonomic issues hamper both a revision of the genus and a phylogenetic analysis considering both morphological and molecular data. In fact, morphological phylogenetic analyses of nereidids are scarce and include one or a few species of Nereis , such as the study of Santos et al. (2006), who aimed to understand the cladistic relationship of the subfamilies. Bakken and Wilson (2005) conducted an analysis of nereidids with pharyngeal paragnaths including five Nereis species and pointed to the potential paraphyly or polyphyly of the genus, although checking the monophyly of Nereis was not the objective of that study. Interestingly, similar results were achieved with molecular phylogeny studies aiming to solve relationships between nereidid genera, thus considering only one taxon of Nereis (the only mitochondrial genome available for Nereis , at the time) ( Alves et al. 2020) or reinforcing the lack of monophyly of the genus ( Liu et al. 2013). The recent study by Alves et al. (2023) was important to confirm, through molecular data, the boundaries of the subfamily Nereidinae , to which the genus Nereis belongs, but elucidating interspecific relationships within the genus was not the focus. These results and the lack of studies focused on Nereis could also be explained by the few DNA sequences of Nereis species available in GenBank in comparison to the total number of species.

Considering a unique character, such as absence of eyes, there is also a knowledge gap regarding phylogenetic analysis, with no study focusing on the systematics of eyeless species of Nereis . In fact, morphological features of these eyeless species are compared only during species description, and Nereis tricirrata is the only one of the formerly described eyeless Nereis species with molecular data available.