A new genus and species of bristle worm from Beibu Gulf, South China Sea (Annelida, Polychaeta, Amphinomidae)

Abstract Alleurythoe, a new genus with type species Alleurythoe tenuichaeta sp. n., is described and illustrated based on material from Beibu Gulf, northwestern South China Sea. The new genus is distinguished from all genera within Amphinomidae by a combination of characters: caruncle trilobed, conspicuous, attached to and confluent with the posterior prostomial lobe, which is free from the body wall and has 6-7 folds on each of the lateral lobes; both noto- and neuropodial aciculae are spinose, extending beyond the chaetal lobe. Alleurythoe tenuichaeta sp. n. is characterized by having branchiae present from chaetiger 4 and a bifurcate neurochaetae capillary. A key distinguishing the genera of Amphinominae is provided.


Introduction
The Amphinomidae, commonly known as fireworms, are typically associated with rocky and soft bottoms in shallow tropical and subtropical waters (Fauchald 1977;Kudenov 1995). It has been reported that amphinomid chaetae are hollow and filled each lateral lobe with 6-7 folds, supported by a basal plate. Branchiae present from chaetiger 4, dendritically branched. Bifurcate neurochaetae capillary. Both noto-and neuropodial aciculae spinose.
Etymology. The generic name is a combination of the prefix allo-(meaning "other" or "alternative" in Greek) and the generic name Eurythoe. The new genus is assigned to the subfamily Amphinominae and most similar to Eurythoe in morphology. Gender: feminine.
Remarks. Alleurythoe gen. n. is assigned to the subfamily Amphinominae Lamarck, 1818 because of the absence of accessory dorsal cirri, and justified as a new genus by the nature of its caruncle (Yáñez-Rivera 2011). The new genus is anatomically similar to Notopygos Grube, 1855 andChloeia Lamarck, 1818 in the shape of caruncle which is trilobed and essentially supported by a basal plate. However, in contrast to Alleurythoe gen. n., the caruncle in the latter two genera has an elevated median keel with several bilateral folds, and it is usually fused to the body wall on chaetigers 1-2 and free thereafter. In the new genus, median keel of caruncle is broadly sinusoidal, thickened, lacks bilateral folds, and is attached to, and confluent with the posterior prostomial lobe, and free from the body wall. In addition, Alleurythoe differs from most other amphinomids in having spinous rather than hastate aciculae, bifurcate neurochaetae capillary, while other amphinomids with heftier bifurcate neurochaetae.
Alleurythoe gen. n. is superficially similar to Eurythoe Kinberg, 1857 in the shape of caruncle, which in both genera consists of a flattened, pronounced median keel and folded lateral lobes; however, the caruncle of Eurythoe Kinberg, 1857 is fused to the body wall for most of its length, the median keel overlaps the lateral lobes, which are scalloped on each side and lack a basal plate (Bindra 1927;Borda 2012;Day 1967 Measurements. Holotype incomplete, with anterior fragment and posterior fragment, without posterior end. Anterior fragment with 62 chaetigers, 71 mm long, and 10 mm maximum width, posterior fragment with 50 chaetigers, 60 mm long. Paratype complete, broken into two fragments. Anterior fragment with 60 chaetigers, 75 mm long, and 8 mm maximum width, posterior fragment with 61 chaetigers, 57 mm long.
Prostomium rectangular, divided into two parts by transverse groove. Anterior lobe prominent, slightly bilobed anteriorly, with two palps and two lateral antennae, palps on ventrolateral part (Fig. 2a, b), lateral antennae subulate, emerging on posterior edge of anterior lobe, similar in shape and size to palps. Posterior lobe rectangular, slightly smaller than anterior one, with median antenna, digitiform, emerging in front of caruncle, short, extending back only to first chaetiger (Figs 1b;2a). Two pairs of eyes present. Buccal opening occupying two chaetigers (Figs 1f; 2b). Caruncle trilobed, conspicuous, about 2 times as long as prostomium, attached to and confluent with posterior prostomial lobe, free from body wall, extremity tapering, extending back to middle of fourth chaetiger, median keel broadly sinusoidal, lateral lobes plicate each with approximately 6-7 folds (Figs 1b-e; 2a), located slightly behind posterior prostomial lobe, supported by a basal plate. Pharynx unarmed, sac-like (Figs 1f; 2b).
Etymology. The name of this species refers to the slender form of its capillary neurochaetae.
Distribution. Presently known only from the type location, Beibu Gulf, South China Sea.
Remarks. Alleurythoe tenuichaeta sp. n. is anatomically similar to Eurythoe rullieri Fauvel, 1953 because the caruncle of both species is free from the body wall. For example, the relatively narrow median keel of Alleurythoe tenuichaeta sp. n., does not overlap the lateral lobes, while that of Eurythoe rullieri broadly overlaps the contiguous lateral lobes. The new species is further differentiated in having branchiae first present from chaetiger 4, rather than chaetiger 3, and lacking the thick bifurcate neurochaetae that are characteristic of E. rullieri and the genus Eurythoe (Fauvel 1953).
Alleurythoe tenuichaeta sp. n. also resembles E. paupera (Grube 1856) in having quadrangular body form, branchiae first present from the fourth chaetiger. However, caruncles and notochaetae differ in these species. The caruncle of Alleurythoe tenuichaeta is attached to and confluent with the posterior prostomial lobe, and free of the body wall, while the caruncle of E. paupera is fixed to the first two chaetigers, extending to the anterior edge of the third chaetiger. Meanwhile, A. tenuichaeta sp. n. has harpoon notochaetae and bifurcate neurochaetae, both of which are absent in E. paupera (Grube 1856