Chaetozone Malmgren 1867

Genus Chaetozone Malmgren, 1867

Type species: Chaetozone setosa Malmgren, by monotypy.

Diagnosis. (Modified after Blake 2015). Prostomium blunt to conical, usually lacking eyespots, with a pair of small nuchal slits or depressions at posterior edge; with a single pair of grooved dorsal tentacles arising from posterior edge of peristomium, or sometimes more posterior on an achaetous anterior segment, or rarely an anterior setiger. First pair of branchiae arising from an achaetous segment or first setiger; sometimes with first two pairs of branchiae on a single anterior segment. Body thick in anterior segments, sometimes distinctly swollen or fusiform, rarely with middle or posterior body segments beaded or moniliform. Chaetae include capillaries on most setigers and acicular spines in neuropodia and notopodia, spines typically concentrated in posterior segments, forming distinct cinctures with spines emerging from elevated membranes; cinctures with few to many spines alternating with capillaries; some species with posterior noto- and neuropodial sigmoid acicular spines numerous, encircling entire posterior parapodia; bidentate spines sometimes present in juveniles or occasionally in ventral-most position of far posterior setigers of adults accompanying unidentate spines in cinctures; some species with long, natatorylike capillaries, these sometimes limited to gravid individuals. Pygidium a simple lobe, disk like, or with long terminal cirrus.

Remarks. Chaetozone, with more than 50 species is the largest genus in the Cirratulidae (Blake & Magalhães 2017). This modified generic definition includes additional characters identified in recently described species as being important in species differentiation. These include details of where the dorsal tentacles and branchiae begin, the presence or absence of an achaetous segment between the peristomium and setiger 1, and details of the arrangement and morphology of the posterior acicular spines.

Historically, the name of the type-species, Chaetozone setosa, originally described by Malmgren (1867) from Spitzbergen in the Norwegian Arctic, was applied globally to cirratulid species having posterior cinctures of spines. New investigations have demonstrated that the majority of these records are actually distinct species (see Blake 1996; 2006, 2015; Chambers 2000; Doner & Blake 2006), with C. setosa itself limited to arctic and subarctic sites in the eastern North Atlantic (Chambers 2000; Blake 2015).

Species of Chaetozone are characterized by having posterior acicular spines arranged in spreading fascicles that either partially or completely encircle the posterior end of the body, producing distinctive cinctures with the numerous protruding spines forming a conspicuous armature. The parapodia are often enlarged with high membranes and a group of these segments may be shaped like an accordion. The number of spines in the cinctures has proven to be important in distinguishing one species from another. The acicular spines are typically unidentate and either blunt-tipped or pointed and accompanied by alternating capillaries. One group of Chaetozone species has spines with a fine tip that curves and attaches to the concave side of the shaft. In a few species, one or two bidentate spines occur with the acicular spines.

Other characters now known to be important in distinguishing species or groups of species of Chaetozone include the nature of segment 1 and whether it is achaetous or represents the first setiger. The first segment may be partially or completely fused with setiger 1; in some species the dorsal tentacles and/or branchiae arise from segment 1. These characters and others in Table 1 help differentiate the twelve species now known to occur in the North Atlantic.

Due to the increasing number of species of Chaetozone being described, Blake (2015) listed a series of characters important for their identification and suggested that these could also be used to separate certain species into groups. The new species from the Bay of Biscay has several identifying characters including the presence of a distinct dorsal crest on the peristomium, a separate achaetous segment with branchiae anterior to the first setiger, a swollen fusiform-shaped anterior body region, and numerous subdermal glands that produce an elaborate and complex Methyl Green staining pattern.