Two new species of Mediomastus (Annelida, Capitellidae) from Tokyo Bay, Japan

Abstract Two undescribed species of polychaetes in Mediomastus (Annelida: Capitellidae) were collected from intertidal to shallow habitats in Tokyo Bay, Japan. These are M. duobalteus sp. n. and M. hanedaensis sp. n. Mediomastus duobalteus sp. n. is distinguishable from all congeners by the following characters: 1) segments 3, 4, 8–11 stainable with methyl green, 2) thoracic capillary chaetae unilimbate, 3) abdominal capillary chaetae absent, 4) paddle-like chaetae in the thorax absent, and 5) abdominal hooded hooks not flared. Mediomastus hanedaensis sp. n. is similar to M. warrenae Green, 2002, but differs from the latter in the shapes of the thoracic capillary chaetae and the abdominal hooded hooks, and the staining pattern with methyl green. In addition, a key to all Mediomastus species is provided.


Introduction
Capitellids in the genus Mediomastus Hartman, 1944 are benthic polychaete worms that occur in marine and brackish water. Mediomastus is distinguishable from other genera in Capitellidae by the following characters: 1) peristomium (segment 1) without chaetae, 2) thorax with 10-12 segments, 3) segments 2-5 with capillary chaetae in both rami, and 4) remaining thoracic and abdominal segments with hooded hooks (Hartman 1944, Warren et al. 1994, Green 2002. The genus was originally established for M. californiensis Hartman, 1944(Hartman 1944) and now contains 14 species . In Japan, species in the genus have been reported from eight localities ( Fig. 1, references therein), but only two of these records were identified to species: M. californiensis from Sagami Bay (Imajima 2006) and M. opertaculeus from Hokkaido (Abashiri, Monbetsu, Rishiri Island, and Oshoro) (Tomioka et al. , 2014. Open circles, previous studies; closed circle, this study. Sources: Rishiri Island (Kato et al. 2003, Tomioka et al. 2014; Sarufutsu (Imajima 1992); Monbetsu (Tomioka et al. 2014); Abashiri ; Oshoro (Tomioka et al. 2014); Tokyo Bay Tanaka 2007, Nishi et al. 2009); Sagami Bay (Imajima 2006); Ariake Bay (Suyama et al. 2003, Niki et al. 2006). Nishi and Tanaka (2007) reported the occurrence of representatives of Mediomastus in Tokyo Bay but did not identify their material to species. Our capitellid specimens from Tokyo Bay were collected during an environmental assessment for the Haneda Airport re-expansion project (Nishi et al. 2009). They turned out to comprise two undescribed species, which we describe and illustrate in this paper. In addition, we provide a key to all species in Mediomastus.

Materials and methods
Worms were collected from sandy mud sediment off Haneda, at the mouth of the Tamagawa River, Tokyo Bay, Japan. All specimens were fixed in 10% formalin in seawater and were later transferred to 70% ethanol after rising in deionized water. Morphological observation and methyl-green staining were performed as described by . All specimens have been deposited in the Natural History Museum and Institute, Chiba, Japan. Morphological terminology follows that of Warren et al. (1994). Description. 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.
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.
Etymology. 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.
Abdominal segments 2.5 times as wide as long (Fig. 7B), with hooded hooks only. Hooded hooks with long fang and 3 teeth (Figs 8D, 9D); fangs in abdominal hooks longer and sharper than those in thoracic hooks; opening of hood wider than that of thoracic hooks; shaft with distinct constriction (Fig. 9D); 2-5 hooks per fascicle (Fig. 8D). Transition from thorax to abdomen marked by alteration in shape of segments (longer in abdomen), shape of shaft of hooded hooks (with constriction in abdominal hooks), and length of fang of hooded hooks (longer in abdominal hooks).
Etymology. The specific name is an adjective, referring to the type locality. Remarks. Mediomastus hanedaensis is similar to M. warrenae in the shape of the thoracic hooded hooks, but differs from the latter in the staining pattern with methyl green (segments 8 and 9 uniformly spotted in M. hanedaensis vs. post-chaetally spotted in M. warrenae) and in the shape of the thoracic capillary chaetae (unilimbate in M. hanedaensis vs. bilimbate in M. warrenae).