Genus Oligobregma Kudenov & Blake, 1975

Type species. Oligobregma aciculata (Hartman, 1967)

Diagnosis according to Blake (2020). Body elongate and arenicoliform with rows of elevated pads on dorsal and ventral segmental surfaces. Prostomium T-shaped with two prominent frontal horns; eyes present or absent; nuchal organs present. Peristomium achaetous, surrounding prostomium dorsally and forming upper and lower lips of mouth ventrally. Branchiae absent. Parapodia with well-developed dorsal and ventral cirri; interramal papilla present or absent. Capillaries present in all parapodia; furcate chaetae present anterior to capillaries from an anterior chaetiger: 2, 3, 4 or later; some species with short, slender, blunt, or pointed spinous chaetae anterior to capillaries of chaetigers 1, 2, or 3, representing homologues of furcate chaetae found on following chaetigers. Pygidium with four or more anal cirri.

Remarks. Kudenov & Blake (1978) erected the genus Oligobregma including initially the species O. aciculata (Hartman, 1965), O. simplex Kudenov & Blake, 1978 and O. oculata Kudenov & Blake, 1978. Oligobregma aciculata was firstly placed in the genus Pseudoscalibregma Ashworth, 1901 by Hartman (1965). However, Kudenov & Blake (1978) presented new criteria to arrange the Scalibregmatidae genera, differentiating Oligobregma from Pseudoscalibregma by the presence of acicular spines on first chaetigers in the former. These spines are mentioned in P. aciculata Hartman, 1965 description, justifying its transference to the genus Oligobregma by Kudenov & Blake (1978).

Later, Blake (1981) presented three additional taxa: O. collare (Levenstein, 1975), O. hartmanae Blake, 1981 and O. notiale Blake, 1981. Of these, O. collare was proposed as a new combination, O. hartmanae was proposed as a new name, and O. notiale was newly described. Oligobregma collare is a combination of P. collaris Levenstein, 1975, P. nr. aciculata Hartman, 1967 and Asclerocheilus nigrocirrus Hartman, 1978 provided by Blake (1981).

According to Blake (1981), Oligobregma hartmanae was considered as a new name due to P. bransfieldia collaris Hartman, 1978 be elevated to species level, and the new name P. collaris Hartman, 1978 be a junior homonym of P. collaris Levenstein, 1975. Therefore, P. collaris Hartman, 1978 was renamed to P. hartmanae (Hartman, 1978) and then transferred to Oligobregma as O. hartmanae sensu Blake, 1981. However, this species was later relocated to Pseudoscalibregma by Blake (2015), and is currently considered as P. hartmanae (Blake, 1981). Pseudoscalibregma hartmanae presents small spinous chaetae on chaetiger 1 – 2, not long and strong acicular spines as initially interpretated by Blake (1981). Thus, it was referred from Oligobregma hartmanae sensu Blake, 1981 to P. hartmanae (Blake, 1981) by Blake (2015).

Detinova (1985) and Schüller & Hilbig (2007) described additional species to the genus and agreed to the previous taxonomic accounts mentioned above. However, Blake (2015) considered O. blakei Schüller & Hilbig, 2007 as invalid.. The description of Oligobregma blakei was based on few juveniles and it could not be referred to as a Oligobregma species (Blake 2015). Finally, Wiklund et al. (2019) and Blake (2023) presented the most recent taxonomic advances on the diversity of the genus, providing the descriptions of several species and expanding its distribution to abyssal depths. Moreover, Blake (2023) transferred Asclerocheilus tasmanius Kierkegaard, 1996 to Oligobregma tasmania (Kierkegaard, 1996).

The genera Oligobregma, Pseudoscalibregma, Scalibregma and Sclerobregma (Kudenov & Blake, 1978) are considered as members of the “arenicoliform scalibregmatids” subgroup, presenting both dorsal and ventral cirri on parapodia (Ashworth 1901, Kudenov & Blake 1978). The presence of acicular spines is the unique morphological criterion used to separate Oligobregma species from Pseudoscalibregma species. Branchiae are absent in Oligobregma and Pseudoscalibregma, being reported only in Scalibregma and Sclerobregma (Kudenov & Blake 1978). As mentioned above, several characteristics overlap among them, leading to identification problems related to their ontogenetic development, with juveniles of these genera passing through a “Pseudoscalibregmalike ” cryptic stage (Blake 2015). Thus, descriptions based on few and juvenile specimens should be taken with caution, as demonstrated by Blake (2015) important observations on the validity of O. blakei description.

Surpassing Asclerocheilus Ashworth,1901, Oligobregma represents the most speciose genus of Scalibregmatidae, counting on 17 valid species (Parapar et al. 2021; Blake 2023). They are: a) O. aciculata (Hartman, 1965), from North Atlantic Ocean; b) O. collare (Levenstein, 1975), from Antarctic Ocean; c) O. simplex Kudenov & Blake, 1978 from Victoria, Australia, Westernport Bay; d) O. oculata Kudenov & Blake, 1978 from New Caledonia, Coral Sea; e) O. notiale Blake, 1981 from Southern Atlantic and Antarctica; f) O. lonchochaeta Detinova, 1985 from North Atlantic Ocean; g) O. tasmania (Kirkegaard, 1996) from Tasman Sea; h) O. pseudocollare Schüller & Hilbig, 2007 from Scotia and Weddel Seas, Southern Atlantic; i) O. quadrispinosa Schüller & Hilbig, 2007 from Scotia and Weddel Seas, Southern Atlantic; j) O. mucronata Blake, 2015 from East Antarctic Peninsula, Antarctic Sea; k) O. brasierae Wiklund et al. 2019 from eastern Clarion-Clipperton Zone, Pacific Ocean; l) O. tani Wiklund et al. 2019 from eastern Clarion-Clipperton Zone, Pacific Ocean; m) O. whaleyi Wiklund et al. 2019 from eastern Clarion-Clipperton Zone, Pacific Ocean; n) O. aristata Blake, 2023 from off eastern Australia lower continental slope and abyssal depths; o) O. bathyala Blake, 2023 from lower continental slope of Southeast Australia; p) O. profunda Blake, 2023 from abyssal depths off eastern Australia, Tasmania to New South Wales; and q) O. renuncula Blake, 2023 from Abyssal depths off eastern Australia from New South Wales to Queensland, Coral Sea. Although O. lonchochaeta is considered as a valid species (Blake 2020), its description lacks important morphological details. For this reason, we did not include this species to the key presented below. We also suggest that it should be redescribed based on the analysis of the type material, or on collected specimens from new sampling campaigns conducted in its type locality.