A new species of Linopherus (Annelida, Amphinomidae) from Beibu Gulf, South China Sea

Abstract Linopherus beibuwanensis sp. n. is described based on six specimens deposited in the Marine Biological Museum of the Chinese Academy of Sciences, Qingdao, collected from the Beibu Gulf (Tokin Gulf), South China Sea. The new species differs from all other Linopherus species by the shape of prostomium and caruncle. The posterior margin of prostomium is bilobed; the caruncle arises medially and confluently from the posterior margin of the prostomium, joining together with prostomium. A key to distinguish the known species of the genus is provided.

Species of the genus Linopherus Quatrefages, 1865 have been referred as members of Pseudeurythoe Fauvel, 1932. Fauchald (1972a described a new species from abyssal depths and reviewed the species of Pseudeurythoe. In 1977, he treated Pseudeurythoe as a synonym of Linopherus. Since then Linopherus has been accepted by subsequent authors and in the present work (San Martín 1986;Salazar-Vallejo 1987). Linopherus differs remarkably from the other genera of the family in having species without caruncle or with a reduced caruncle, and branchiae that are limited to the anterior part of the body. The morphology of the prostomium, the development of prostomial appendages, caruncle, parapodia, and the number of branchiae are important in species identification (Kudenov 1995). To date, six species of Linopherus have been reported from the coastal waters of China (Sun, Lei and Zhou 2008): Linopherus ambigua (Monro, 1933), L. paucibranchiata Fauvel, 1932, L. hirsuta (Wesenderg-Lund, 1949, L. oligobranchia (Wu, Shen & Chen, 1975), L. microcephala (Fauvel, 1932), and L. spiralis (Wesenderg-Lund, 1949).
When the authors examined the Amphinomidae archived in the Marine Biological Museum of Chinese Academy of Sciences (MBMCAS), Institute of Oceanology, Chinese Academy of Sciences (IOCAS), six specimens of Linopherus were separated out. Careful examination revealed that these specimens represented a new species. The new species is described here, and a key to worldwide species of Linopherus is provided.

Material and methods
The specimens were collected during the late 1950s to early 1960s from Beibu Gulf (Tonkin Gulf), northern South China Sea. All specimens are deposited in the MBMCAS, preserved in 75% ethanol solution. Microscopy observations and drawings were made using a Zeiss Stemi 2000-C stereomicroscope equipped with an AxioCam MRc 5 digital camera.
Variations. All specimens examined are incomplete lacking the posterior part. Specimen size varies from 1.9 to 2.5 mm in maximum width. Branchial chaetiger range varies from 3 to 31-40. The maximum number of branchial filaments varies from 20-29. One female paratype (MBM200143) with eggs in posterior coelomic cavity, ranging in diameter from 71μm to 90 μm.
Etymology. The species is named after Beibuwan, the Chinese name for the Beibu Gulf (Tonkin Gulf), South China Sea.
Distribution. Only known from Beibu Gulf, South China Sea. Remarks. Linopherus beibuwanensis sp. n. is referred to Linopherus because of the arrangement of branchiae and the reduced but characteristic caruncle. Linopherus beibuwanensis sp. n. is unique in this genus for the combined shape of its prostomium and the caruncle. While the posterior prostomial margin is straight (not bilobed) in most Linopherus species, their caruncles are also either absent or reduced. In the latter case, species with reduced caruncles typically exhibit a transverse groove that appears to isolate the caruncle from the prostomium, which is usually partly or completely concealed by the second chaetiger (see Langerhans 1881;Fauvel 1932;Monro 1933Monro , 1937Treadwell 1941;Wesenberg-Lund 1949;Knox 1960;Fauchald 1972a, b;Kudenov 1975;Wu et al. 1975;Kudenov and Blake 1985;San Martín 1986;Salazar-Vallejo 1987).
Linopherus beibuwanensis sp. n. is similar to L. abyssalis in having the posterior prostomial margin bilobed. The new species can be distinguished from the latter by the presence of eyes and arrangement of branchiae; the former has two pairs of eyes and more than 40 pairs of branchiae, while the latter has no externally detectable eyes and only 5-6 pairs of branchiae.
Caruncle morphology appears to be an essential character which can be used to distinguish the species of Linopherus. However, Linopherus species are small in size, and so their caruncle morphology is imperfectly known since it is usually concealed by constriction of the anterior chaetigers. Clearly, further research on this taxon is necessary.   (Fauchald, 1972b)