Melinna hamulus, Gunton & Zhang & Kupriyanova & Hutchings, 2023

Gunton, Laetitia M., Zhang, William, Kupriyanova, Elena K. & Hutchings, Pat A., 2023, New Species of Melinna (Melinnidae, Annelida) from the Australian Abyss with Comments on M. albicincta, M. cristata and M. elisabethae, Records of the Australian Museum (Rec. Aust. Mus.) 75 (3), pp. 125-154 : 131-135

Melinna hamulus sp. nov.

urn:lsid:zoobank.org:act:04B34BFC-0464-4B96-B935-EC26699040F8

Figs 1 , 3–5 ; Tables 4, 5

Melinna cf. armandia Gunton et al., 2021: 63 , fig. 4D,d. Non McIntosh, 1885.

Holotype. Australian Museum ( AM) W.50352, IN2017_ V03 , OPS 030, Australia, Bass Strait between Tasmania and Victoria, Beam Trawl (start: 39°33'07"S 149°33'11"E, 4,197 m, end: 39°29'46"S 149°35'53"E, 4,133 m), 23 May 2017; specimen posteriorly complete, although body wall damaged in a few places, 45 mm in length, 4 mm max width in thorax, muddy tube present, formalin fixed. All paratypes collected from the same location as holotype. Paratypes. AM W.50366 (1 specimen), fixed in formalin, pale yellow lacking any pigmentation, damaged in 2 pieces, anterior end 20 mm in length, 4 mm maximum width, with 14 thoracic neuropodia, plus posterior fragment to pygidium . AM W.53257 (2 specimens) fixed in formalin, one specimen 15 mm in length, plus tube, anterior portion covered in fine mud, posterior part flimsy chitinous tube, another specimen mounted for SEM AM W.53257.001 posteriorly incomplete, 25 mm in length, 4 mm maximum width . AM W.50351 (2 specimens), fixed in formalin, anterior fragment 12 mm in length, thorax 3 mm in width, plus posterior fragment, anterior fragment 5 mm in length, plus tube fragments . AM W.50732 (1 specimen), fixed in 95% ethanol, 12 mm in length, thorax complete 4 mm max width, abdomen damaged from tissue collection for molecular analysis . AM W.53871 (1 specimen), fixed in 95 % ethanol, damaged posterior thorax, max width 4 mm and anterior abdomen with posterior abdomen intact with 40 + segments, with tube . AM W.53872 (1 specimen), fixed in 95 % ethanol, sequenced, anterior thorax, max width 4.5 mm, l35+ abdominal segments . AM W.50353 (1 specimen), thorax max width 4 mm plus separated abdomen, abdomen damaged from tissue collection for molecular analysis . AM W.50382 (1 specimen), fixed in 95% ethanol, posteriorly incomplete, abdomen damaged from tissue collection for molecular analysis, max width 3 mm . AM W.50354 (2 specimens), fixed in formalin, 1 specimen complete with regenerating posterior end, but mid body damaged, 30 mm in length, 3 mm max width, tube present with damaged posterior end, other incomplete, only thorax 7 mm in length, max width 3 mm . AM W.50383 (1 specimen), fixed in formalin, thorax only, 12 mm in length, 4 mm max width . AM W.53880 (1 specimen), fixed in formalin, in two pieces, anterior part 16 mm in length, max width 4 mm, plus posterior part 13 mm in length . AM W.53881 (1 specimen), fixed in formalin, posteriorly incomplete, 12 mm in length, max width of thorax 3 mm, plus tube .

Other material examined. Melinna albicincta ( NMWZ 1989­ 125­22, NMWZ 1989­125­26); M. cristata ( AM W.21791); M. elisabethae ( AM.W.21793) for comparative purposes, details given for each under the expanded descriptions of each of these species.

Description

Description based on holotype with variation observed among paratypes indicated. Prostomium with well­defined anterior and posterior parts, separated by pair of transverse nuchal slits ( Fig. 3F ), anterior part distally trilobed, followed by achaetous segment forming a glandular collar dorsally with smooth margin ( Fig. 3B,F ). Buccal tentacles almost completely withdrawn with tips exposed ( Figs 3F , 4A,B ). Lateral wings of anterior body between prostomium and segment 5 highly arched ( Fig. 3A,B,C ). First four segments glandular, subsequent seven segments with discrete ventral glandular shields ( Figs 3C , 4A,C ). Four pairs of smooth branchiae on segments 2–5, although appearing to arise on a dorsal ridge on segments 2 and 3, arranged two in front and two behind. Completely separated from each other. All branchia similar in both width and length, tapering to fine tips distally and having slightly swollen bases with slightly crenulated surfaces ( Fig. 4F,G,I ). Branchia circular in cross section. Incomplete dorsal membranous fold on segment 4 ( Fig. 3B,C ). Postbranchial dorsal membrane with 13 (17) triangular lobes or projections, all similar in size, except for the slightly larger middle one, some bilobed, and middle larger one with small lateral lobes at base so appearing as tricorn ( Figs 3A,B , 4F,G ).Pair of short dorsal curved hooks, pale yellow with darker slightly curved pointed tip, base triangular (one missing on holotype), no canal inside tooth visible ( Figs 3B,b , 4H,I ). Segmentation visible dorsally in postbranchial area. No visible nephridial papillae.Capillary notochaetae present in 23 thoracic chaetigers starting from segment 4. First notopodia (segment 4) very small. Following notopodia inserted more laterally ( Figs 3A , 4C ), becoming progressively larger and reaching maximum size by segment 8 ( Fig. 3B ). Subsequent notopodia (from segment 8 onwards) continuing for 15 segments. Notopodia well­developed, each bearing paired notopodial lobes and long golden capillary notochaetae ( Figs 4D,E , 5D ). Notochaetae arranged in two tiers of capillary chaetae ( Figs 4D,E , 5D ). Abdominal notochaetae lacking. Small, rounded projections evident in notopodial positions, no cilia observed ( Fig. 3D ). Neurochaetae as an elongated row of simple acicular short spines on slightly raised glandular ridge and inserted progressively more dorsally from segment 2–5 ( Fig. 3A,B ). Neuropodia with pectinate uncini from segment 6, present in 21 pairs of thoracic uncinigers (to end of abdomen, segment 26). Numbers of pairs of abdominal neuropodia 45, based on AM W.53257. Abdominal neuropodia elongate and basally swollen, without any rudimentary notopodia ( Fig. 5E,G ). Thoracic uncini from segment 6 with 30 uncini within a row. Abdominal neuropodia with 31 uncini in torus. Dentition of uncini within a row varies slightly ( Fig. 5F,H ). Thoracic uncini avicular with two teeth in a vertical row over rostral tooth, sub­rostral process and basal prow ( Figs 3G , 5A,B,C ). Side of uncini with marked ornamentation ( Fig. 5C ). Abdominal uncini with conspicuous long tendons attaching them to body and visible through body wall. Uncini arranged on elevated neuropodia in curved row ( Fig. 5E,G ). Uncini with four to six teeth ( Fig. 5F,H ). Posterior segments compacted, none of the paratypes complete. Pygidium with two lateral lobes and 4–5 smaller lobes surrounding them ( Fig. 3H ). No anal cirri present.

Tube. Composed of fine­grained sediment and lined with a thin, stiff clear membrane. Length of tube at least 2 times specimen ( Fig. 3E ).

Methyl blue staining pattern. Use of methyl blue staining reveals strong staining of the prostomium and segments 1–4, branchiae lightly speckled at tips and transverse rings at bases. Stained band behind dorsal membrane from segments 5–6. Light staining elsewhere on thorax and on abdomen. Light staining on ventral shields, staining stronger laterally on shields.

Remarks

Melinna hamulus sp. nov. is characterized by the following characters: around 45 abdominal segments, dorsal membrane with 13–17 triangular projections, four pairs of branchiae completely free, one pair of curved dorsal hooks with pointed tips lacking a canal, and a pygidium with two large lateral lobes surrounded by four or five smaller lobes, which distinguishes it from all other described species where the data are available ( Table 5). We compare the morphological characters along with type locality and depth of our new species with the other currently accepted 26 species of Melinna ( Tables 4, 5).

Gunton et al. (2021) recorded this species as M. cf. armandi McIntosh, 1885 , which was originally described from west of North Island, New Zealand in a depth of 2,012 m. However, while having a similar number of abdominal segments, M. armandi has the dorsal membrane with only eight large projections, far fewer than in the new species (13–17) and the dorsal hooks have a distinct canal which is absent in M. hamulus sp. nov. The only other Melinna species recorded in the region is M. arnaudi Parapar & San Martín, 1997 from Livingston Island in Antarctica from 104–422 m, which has 72 abdominal chaetigers and so easily distinguished from M. hamulus sp. nov., which has 45 abdominal segments and occurs below 4,000 m depth.

Type locality and habitat. Bass Strait, SE Australia, 4,133– 4,197 m, no sediment data was collected due to gear failure.

Etymology. The new species is named hamulus meaning little hook in Latin, a reference to the small dorsal hooks of this new species.

Figures

Figure 1. Map of southeastern Australia showing type locality (black dot) of the species Melinna hamulus sp.nov.collected during the RV Investigator voyage “Sampling theAbyss” (IN2017_V03), operation 030. Green polygons represent Australian Marine Parks.

Figure 3. Light microscopy images of Melinna hamulus sp. nov. (A) AM W.50366, dorsal view of anterior end showing positions of first neuro­ and notochaetae, in specimen stained with methyl blue. (B) AM W.50366, dorsal view of anterior end stained with methyl blue. Ellipsoid shows raised thoracic neuropodia.(b)AM W.53880, dorsal hook outlined to highlight shape.(C)AM W.50366, whole specimen, upper­dorsal view stained with methyl blue, lower­ventral view stained with methyl blue. (D) AM W.53257, abdomen, arrows indicate notopodia as small rounded projections. (E) AM W.50366, tube. (F) AM W.50366, ventral view anterior. (G) AM W.53257, thoracic uncini of segment 9. (H) AM W.50366, pygidium stained with methyl blue. Scale bars: A, 500 μm; B, C, F, 1 mm; b, H, 200 μm; E, 2 mm; G, 20 μm. Abbreviations: seg, segment; noto, notochaetae; neuro, neurochaetae; es, eyespots; bf, branchiae completely free; dm, dorsal membrane; dh, dorsal hooks; full noto, fully developed notopodia.

Figure 4. SEM images of Melinna hamulus sp. nov. (AM W.53257.001). (A) Ventral view of anterior end. (B) Ventral view of lips. (C) Lateral view of anterior notopodia. (D) Anterior notopodia. (E) Surface of notochaetae. (F) Lateral view of anterior end. (G) Dorsal view of anterior end. (H) Dorsal hook. (I) Lateral view of anterior end including dorsal hooks. Scale bars: A, C, F, G, I, 1 mm; B, H, 500 μm; D, 200 μm; E, 20 μm. Abbreviations: dm, dorsal membrane; dh, dorsal hook; 2t, two tiers of notochaetae.

Figure 5. SEM of Melinna hamulus sp. nov. (AM W.53257.001). (A) Neurochaeta of chaetiger 10. (B) Neurochaetae of chaetiger 13. (C) Neurochaetae of chaetiger 13. (D) Notochaetae of chaetiger 13. (E) Anterior abdominal neurochaetae. (F) Close­up view of anterior abdominal neurochaetae. (G) Posterior abdominal neurochaetae. (H) Close­up view of posterior abdominal neurochaetae. Scale bars: A, C, F, H, 10 μm; B, 20 μm; D, 200 μm; E, G, 50 μm. Abbreviations: d, dentition; t, two tiers of notochaetae.