Arenicolidae (Annelida) in Norwegian waters: Species occurrence, bathymetric distribution and identification of juvenile specimens

The two species of Arenicolidae, <i>Arenicola marina</i> and <i>Arenicolides ecaudata</i>, are common along the Norwegian coast. Juveniles and anterior fragments are often encountered when grab sampling, making traditional morphological characteristics insufficient for species identification. This study examined the robustness of the characteristics and development of branchiae in juvenile specimens of <i>Arenicolides ecaudata</i> (<i>n</i> = 109) and <i>Arenicola</i> (<i>n</i> = 44) (<i>A. marina</i> and <i>Arenicola</i> sp.) and additional specimens to assess geographic and bathymetric distribution. Neuropodia were discovered to be efficient in distinguishing the species. Branchiae of <i>A. ecaudata</i> appear when specimens measure between 7–13 mm, while in <i>A. marina</i> they appear in specimens measuring between 3–5 mm. The ring formula in anterior chaetigers is a key characteristic in differentiating between <i>Arenicola</i> species. This characteristic alone is insufficient for reliable species identifications, as study findings corroborated error rates in identification based on the ring formula. The bathymetric range of both species has increased considerably. <i>Arenicola marina</i> was documented at 194 m depth and <i>A. ecaudata</i> at 387 m depth. DNA barcodes with the cytochrome c oxidase subunit I (COI) gene were produced for three specimens of <i>Arenicolides ecaudata</i>, representing the first publicly available COI barcode sequences for this species. Five specimens of <i>Arenicola marina</i>, with similar sequences to specimens from other north-eastern Atlantic localities were also barcoded in BOLDsystems.