Diplocirrus Haase 1915

Diplocirrus Haase, 1915, restricted

Diplocirrus: Salazar-Vallejo & Buzhinskaja, 2011 (partim).

Type species: Trophonia glauca Malmgren, 1867, by original designation.

Diagnosis (emended). Flabelligerids with body swollen anteriorly, median and posterior regions cylindrical, often with constrictions between successive segments (moniliform). Cephalic cage made by chaetiger 1, with three or less chaetae per bundle, fragile. Integument thin, usually with short papillae, with mud particles or free of them. Branchial plate with eight branchial filaments of two types, anterior filaments cirriform and posterior filaments depressed, with longitudinal ridges. Parapodia never projected laterally. Notochaetae and neurochaetae sparse, usually smaller than body width. Neurochaetae completely multiarticulated.

Remarks. Støp-Bowitz (1948: 7) used the size of cephalic cage chaetae and papillae for separating Diplocirrus species, such that the type species, D. glaucus (Malmgren, 1867) has a few chaetae along the first few chaetigers, and short papillae, whereas D. hirsutus (Hansen, 1882) and D. longisetosus (von Marenzeller, 1890), have more chaetae, and longer papillae. Because specimens can be damaged, especially by breaking chaetae, Støp-Bowitz approach was not followed in the revision of the genus (Salazar-Vallejo & Buzhinskaja 2011). As benthic sampling is being done more carefully, better preserved specimens would allow for an extended use of the cephalic cage development. However, the size of chaetae along median chaetigers is more emphasized because they are less frequently broken by sieving or similar processing of benthic samples.

Saphobranchia Chamberlin, 1919, reinstated, resembles Pherusa Oken, 1807 by having very long cephalic cage chaetae (2–3 times longer than body width), and scarce papillae along body; it also resembles some species of Lamispina Salazar-Vallejo, 2014 by having very long chaetae in median chaetigers (twice as body width). However, it differs from Pherusa because Saphobranchia has neurochaetae basally anchylosed, and medially and distally articulate, whereas Pherusa and Lamispina have them completely anchylosed, shorter and falcate in the former, longer and distally foliose in the latter. Saphobranchia resembles Diplocirrus by having multiarticulate neurochaetae, but in Saphobranchia the cephalic cage has more chaetae, and in median segments chaetal basal section is anchylosed, whereas in Diplocirrus there are a few chaetae in cephalic cage, and neurochaetae are completely articulated. The type of branchial filaments, size of chaetae, together with the presence of a basal anchylosed section in neurochaetae, and the development of the cephalic cage chaetae are the main characters that support the removal of some Diplocirrus species, and the recognition of Saphobranchia as a distinct genus.

As currently understood, Diplocirrus includes 22 species having cephalic cage chaetae variably developed, abundant small body papillae, or scarce ones along body, branchial filaments of two types and neurochaetae usually multiarticulate along their length, or with a basal section anchylosed (Darbyshire & Mackie 2009, Salazar-Vallejo & Buzhinskaja 2011, Teixeira et al. 2015, Jimi et al. 2017). Six of the species are herein transferred to Saphobranchia and are newly combined by having long chaetae, neurochaetae basally articulate, usually well-developed cephalic cage, and branchial filaments of one type: S. acafi (Teixeira, Rizzo & Santos, 2015) n. comb., S. hirsuta (Hansen, 1882) n. comb., S. longisetosa (von Marenzeller, 1890), S. micans (Fauchald, 1972) n. comb., S. normani (McIntosh, 1908) n. comb., and S. octobranchia (Hartman, 1965) n. comb. Consequently, Diplocirrus is restricted and below is a key to species. Saphobranchia species can be separated as indicated in the separate key below.

Distribution. Arctic, Antarctic, and deep water environments in the northwestern and southwestern Atlantic, and central eastern Pacific.