Mediomastus duobalteus, Tomioka, Shinri, Nishi, Eijiroh & Kajihara, Hiroshi, 2014

Tomioka, Shinri, Nishi, Eijiroh & Kajihara, Hiroshi, 2014, Two new species of Mediomastus (Annelida, Capitellidae) from Tokyo Bay, Japan, ZooKeys 422, pp. 115-126: 117-119

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scientific name

Mediomastus duobalteus

sp. n.

Taxon classification Animalia Scolecida Capitellidae

Mediomastus duobalteus   sp. n. Figs 2-6

Material examined.

Holotype: CBM−ZW 1088, Haneda, Tokyo Bay, St. L3e-2-1 (35.52783203°N, 139.7884979°E, sandy mud bottom, incomplete, collected May 2012. Paratypes (six specimens): CBM-ZW 1089, Haneda, Tokyo Bay, St. L4e-1-1, 35.52949905°N, 139.7836609°E, incomplete; mounted on SEM stub, collected spring 2007; CBM-ZW 1090, Haneda, Tokyo Bay, St. L2b-2, 35.52531815°N, 139.7931824°E, sandy-mud bottom, 5 m depth, incomplete, some parts of body mounted on slides, remaining parts preserved in 70% ethanol, collected May 2012; CBM-ZW 1091, Haneda, Tokyo Bay, St. 07s-L4e-1-2, 35.52949905°N, 139.7836609°E, sandy mud bottom, incomplete, collected spring 2007; CBM-ZW 1092, St. 07s-L4e-1-3, 35.52949905°N, 139.7836609°E, sandy mud bottom, incomplete, collected spring 2007; CBM-ZW 1093, St. 07s-L4e-1-4, 35.52949905°N, 139.7836609°E, incomplete, collected spring 2007; CBM-ZW 1094, Haneda, Tokyo Bay, St. L4e-1-2, sandy mud bottom, incomplete, collected May 2012.


Holotype, anterior fragment with 73 segments; 18.0 mm in length; 0.51 mm in maximum width. Body color white in ethanol. Epithelium smooth. Nephridiopores lacking. Branchiae absent. All segments cylindrical. Sex uncertain.

Prostomium (Fig. 2A) conical, with short palpode; eversible proboscis with numerous minute papillae. Peristomium twice as long as chaetiger 1, without eyespots, achaetigerous.

Capillary chaetae unilimbate, with narrow wing (Figs 3A, 4A), present on chaetigers 1-4; noto- and neurochaetae each 8-12 in number per fascicle (Fig. 3A). Chaetigers 5-10 with hooded hooks, but without paddle-like chaetae. Notopodial hooded hooks (Figs 3B, 4B) with short, stout fang and 3 fine teeth (Fig. 4B); hood with opening (Fig. 3B); shaft without constriction (Fig. 4B); shoulder indistinct (Fig. 4B); 6-10 hooks per fascicle. Neuropodial hooded hooks (Figs 3C, 4C) with short, stout fang and 3 fine teeth (Fig. 4C); hood with small opening (Fig. 3C); shaft without constriction (Fig. 4C); shoulder indistinct (Fig. 4C); 5-10 hooks per fascicle (Fig. 4C).

Abdominal segments twice as long as wide (Fig. 2B); with hooded hooks only. Hooded hooks (Figs 3D, 4D) with stout, pointed fang and 3 coarse teeth (Fig. 4D); hood with opening (Fig. 3D); shaft without constriction (Fig. 4D); shoulder indistinct; 2-6 hooks per fascicle.

Transition from thorax to abdomen marked by change in shape of hooded hooks; hooded hooks in thorax have small fang with fine, small teeth (Fig. 4B, C), while those in abdomen have stout, pointed fang; coarse and large teeth (Fig. 4D).

Methyl-green staining.

Among seven specimens observed, methyl-green staining resulted in three patterns (Fig. 5). All patterns showed two bands of numerous, dense, minute spots: one band on segments 2 and 3, or 3 and 4; the other extending from segments 8 or 10 to segment 11. Figure 6 shows a stained worm having the pattern diagrammed in Fig. 5C.


The specific name is a noun in the nominative singular, from the Latin numeral duo ( “two”) and the Latin noun balteus ( “belt”), referring to the species’ diagnostic staining pattern, in which the staining pattern is two belt-like bands.


Among 14 congeners, Mediomastus duobalteus   most closely resembles Mediomastus warrenae   , but differs from the latter in the staining pattern with methyl green (segments 5 and 6 lack spots in Mediomastus duobalteus   but are darkly stained post-chaetally in Mediomastus warrenae   ), the shape of the thoracic capillary chaetae (unilimbate in Mediomastus duobalteus   vs. bilimbate in Mediomastus warrenae   ), and the shape of the abdominal hooded hooks (constriction absent in Mediomastus duobalteus   but present in Mediomastus warrenae   ).