Rhamphobrachium (Minibrachium) fractum, Paxton, Hannelore & Budaeva, Nataliya, 2015

Paxton, Hannelore & Budaeva, Nataliya, 2015, Minibrachium, a new subgenus of Rhamphobrachium (Annelida: Onuphidae) from Australia with the description of three new species, Zootaxa 4019 (1), pp. 621-634 : 626-629

publication ID	https://doi.org/ 10.11646/zootaxa.4019.1.21
publication LSID	lsid:zoobank.org:pub:4638C91A-F573-4DF1-9E8F-C25C9BBABDDF
DOI	https://doi.org/10.5281/zenodo.5662758
persistent identifier	https://treatment.plazi.org/id/B31F87A5-FF99-BA26-C3A2-FA7FFB8F9D17
treatment provided by	Plazi
scientific name	Rhamphobrachium (Minibrachium) fractum
status	sp. nov.

Treatment

Rhamphobrachium (Minibrachium) fractum View in CoL n. sp.

( Figs 2 View FIGURE 2 , 5 View FIGURE 5 )

Rhamphobrachium (Rhamphobrachium) View in CoL sp.— Paxton 1986a: 19; 1986b: 87.

Type material. Holotype: AM W.198965, Victoria, 112 km S of Lakes Entrance, 39º00’00”S, 148º24’50”E, sta. 20 ‘Esso-Gipps’, sand, 95 m, coll. C. Phipps, May 1969. Paratypes: AM W.47833 (2); AM W.47834.001, W.47835.001, W.47836.001, 3 specimens on SEM pins, same data as holotype; AM W.198964 (2), Bass Strait, 39º00’00”S, 148º30’00”E, sta. 19 ‘Esso-Gipps’, 126 m, coll. C. Phipps, 7–9 May 1969.

Other material examined. MV F43626 View Materials (1), Tasmania, eastern Bass Strait, 42 km SW of Babel Island, 40º14.4’S, 148º40.0’E, sta. BSS 165 ‘Tangaroa’, fine sand, 60 m, Smith-McIntyre grab, coll. R. Wilson, 14 Nov 1981; MV F43628 View Materials (9), Tasmania, eastern Bass Strait, 63 km E of North Point, Flinders Island, 39º44.8’S, 148º40.6’E, sta. BSS 167 ‘Tangaroa’, muddy sand, 124 m, Smith-McIntyre grab, coll. R. Wilson, 14 Nov 1981; MV F43629 View Materials (13), Tasmania, eastern Bass Strait, 63 km E of North Point, Flinders Island, 39º44.8’S, 148º40.6’E, sta. BSS 167 ‘Tangaroa’, muddy sand, 124 m, WHOI epibenthic sled, coll. R. Wilson, 14 Nov 1981.

Diagnosis. Small, up to 1.0 mm wide; diagnosis and description based on holotype and paratypes wider than 0.4 mm at chaetiger 10 excluding parapodia. Unidentate distally recurved spiny hooks on chaetigers 1 and 2; four to eight compound to pseudocompound subacicular hooks with spiny hoods on chaetiger 3, five to eight simple subacicular hooks from chaetiger 4, hoods becoming gradually less spiny, resembling typical subacicular hooks by about chaetiger 20; pectinate chaetae with 8–10 long teeth. Pygidium with two pairs of anal cirri.

Description. Holotype incomplete, measuring 11.5 mm in length for 38 chaetigers, 0.8 mm in width; only complete paratype 10 mm long for 58 chaetigers, 0.5 mm wide, incomplete paratypes 0.5–0.9 mm wide. Complete non-type specimens 4–20 mm long for 35–77 chaetigers, 0.3–1.0 mm wide. Alcohol stored specimens overall cream-coloured, lacking colour pattern.

Prostomium ( Fig. 5 View FIGURE 5 A) of holotype and most paratypes anteriorly shovel-nosed, two paratypes with indented frontal margin, representing partly formed frontal lips ( Fig. 2 View FIGURE 2 C). Ceratophores of palps and antennae with one to two proximal rings and one longer distal ring; ceratostyles short and subulate, palps reaching to peristomium or chaetiger 1, lateral and median antennae reaching to chaetiger 1. Nuchal grooves curved laterally, with narrow middorsal separation. One pair of small eyespots between bases of palps and lateral antennae. Peristomium slightly shorter than first chaetiger; peristomial cirri absent.

First two pairs of parapodia modified, projecting anterolaterally, directed ventrally ( Fig. 5 View FIGURE 5 A). Neither of parapodia 1 and 2 fully extended, anterior ends inverted, hiding parapodial lobes. Unmodified parapodia lacking distinct parapodial lobes. Dorsal cirri subulate on anterior chaetigers, more posteriorly becoming digitate. Ventral cirri subulate on chaetiger 1 and 2, replaced by rounded ventral pads from chaetiger 3. Branchiae absent.

Modified parapodia with three long, pseudocompound recurved hooks ( Figs 2 View FIGURE 2 D, 5A). Shafts of hooks with two rows of long moveable spines below pseudoarticulation and irregularly distributed small spines along whole length of shaft, but more dense above pseudoarticulation; slight swelling before distal curvature of hook. Internal chaetal sacs to chaetiger 30.

Unmodified parapodia from chaetiger 3, supported by two to three very thick aciculae. Limbate chaetae from chaetiger 3, decreasing in length from upper to lower position ( Fig. 5 View FIGURE 5 B). Spinigers absent. Four to eight compound ( Fig. 2 View FIGURE 2 E) to pseudocompound ( Fig. 2 View FIGURE 2 F) subacicular hooks with spiny hoods on chaetiger 3 ( Fig. 5 View FIGURE 5 C) (holotype with five compound and two pseudocompound hooks); five to eight simple subacicular hooks from chaetiger 4 ( Figs 2 View FIGURE 2 G, 5D) (except in juveniles, measuring less than 0.3 mm in width). Hoods initially enclosing distal part of hooks more closely, gradually opening up, becoming smoother and changing to divided hoods, one on either side of distal part of hook by about chaetiger 20, resembling typical subacicular hooks; concurrent with morphological change, number of hooks becoming gradually reduced to two by chaetiger 20–25 ( Fig. 5 View FIGURE 5 E, F). Slightly oblique pectinate chaetae from chaetiger 13; with 8–10 long teeth, one lateral tooth twice as long, as other teeth ( Fig. 5 View FIGURE 5 G). Pygidium with two pairs of anal cirri; dorsal pair longer than ventral one ( Fig. 5 View FIGURE 5 H). Fragile mucous tube without incrustations.

Mandibles decalcified, only protomandibles visible. Maxillae very delicate, elements transparent, attachment areas between plates only areas of slight sclerotisation. Maxillary formula: MxI = 1+1; MxII = 5+8; MxIII = 6+0; MxIV = 7+10; MxV = 1+1.

Remarks. Rhamphobrachium (M.) fractum n. sp. and R. (M.) talboti n. sp. are much more similar to each other than to R. (M.) nutrix n. sp. described above. They are almost twice as large as the latter, have unidentate instead of bidentate distally recurved spiny hooks, multiple subacicular hooks from chaetiger 3 instead of only two from chaetiger 5, have two pairs instead of one pair of anal cirri and have pectinate chaetae with much longer teeth. The former two species differ from each other in that the subacicular hooks of chaetiger 3 are always simple in R. (M.) talboti n. sp. ( Fig. 6 View FIGURE 6 F) but compound to pseudocompound in R. (M.) fractum n. sp. ( Fig. 5 View FIGURE 5 C). This is a consistent characteristic that is not growth dependent as the smallest R. (M.) talboti n. sp. (11 mm for 67 chaetigers, 0.45 mm wide) presented only simple subacicular hooks on chaetiger 3. Furthermore, the pectinate chaetae of R. (M.) talboti n. sp. have 9–13 very long teeth ( Fig. 6 View FIGURE 6 K), while those of R. (M.) fractum n. sp. have 8–10 long teeth instead ( Fig. 5 View FIGURE 5 G).

Etymology. The name is derived from fractus (break) in Latin and refers to the characteristic compound subacicular hooks on chaetiger 3.

Biology. No brooding specimens were encountered. The smallest specimens in the mixed samples measured 3–4 mm in length for 35–40 chaetigers, 0.3 mm in width and are considered to be recently settled juveniles. These juveniles differ from the larger specimens described above in having compound to pseudocompound subacicular hooks not only on chaetiger 3, but also up to about chaetiger 20, and thereafter only simple ones. In other characteristics they are like larger specimens, except for having even less developed frontal lips and shorter modified parapodia. The distally recurved spiny hooks are not visible externally but are present internally with the chaetal sacs reaching to about chaetiger 20.

Type locality. Pacific Ocean, 110 km S of Lake Entrance, Victoria; 39º00’00”S, 148º24’50”E.

Distribution. Bass Strait, Victoria and Tasmania, Australia; in 60–126 m depth.