Oweniidae

DIAGNOSIS (adapted from Capa et al. 2012). — Cylindrical body. Prostomium and peristomium fused forming head. Nuchal organs absent. Mouth terminal or shifted ventrally. Anterior region with one to three uniramous segments, followed by biramous segments. Notopodial chaetae emerging from body wall, capillaries resembling small imbricate scales, smooth acicular chaetae. Tori with irregular rows of bidentate uncini. Parapodial branchiae or cirri absent. Thinsecreted layer tube encrusted with foreign objects. Distinct mitraria larvae. Presence of inter-epidermal nerve cord.

REMARKS Delle Chiaje (1844) described the first oweniid genus and species, Owenia fusiformis Delle Chiaje, 1844. Owenia was proposed in honor of the illustrious British zoologist and paleontologist Richard Owen; whereas the specific epithet refers to the tapered (“ fusiform ”) shape of the worm tube. It seems that the name Owenia was popular during the 19 th century because four other homonymous genera were later proposed in molluscs (Owenia Prosch, 1847), ctenophores (Owenia Kölliker, 1853), birds (Owenia Gray, 1855) and mammals (Owenia De Vis, 1888) (see Rioja 1917), although Delle Chiaje’s name prevails currently, according to the Principle of Priority (ICZN 1999, art. 23.4). In 1846, Grube (1846) established the genus Ammochares Grube, 1846 comprising a single species, A. ottonis Grube, 1846. Subsequently, Malmgren (1867) created the new family Ammocharidae, designating Ammochares as the generic type name. Myriochele Malmgren, 1867 was later inducted into the family. Kinberg (1867) subsequently acknowledged Ammocharidae without referencing Malmgren. Claparède (1868) suggested A. ottonis and O. fusiformis, both described from the Mediterranean Sea, as the same species, hence rendering Ammochares synonymous with Owenia. Rioja (1917) proposed Oweniidae as a replacement name, but this was unnecessary as the family name is unaffected by the synonymy of its type genus (ICZN 1999, art. 40.1). Although Ammocharidae could have remained valid, Oweniidae is considered nomen protectum due to its prevailing use, consistent usage (ICZN 1999: Art. 40.2) and original authorship (ICZN 1999: Art. 40.2.1). Ammocharidae, which was likely last recognised as being valid approximately 90 years ago (Berkeley 1931), is thus categorised as nomen oblitum to ensure nomenclatural stability (ICZN 1999: Art. 40.2). Updated accounts of the anatomy, biology, ecology, morphology, reproduction, phylogeny and taxonomy of this family have recently been reported (Capa et al. 2019; Villalobos-Guerrero 2021; Parapar et al. 2021).

REMARKS Owenia unipinnata n. sp. is the smallest and thinnest species recorded to date (TL: 3 - 10 mm; BW = 0.030 - 0.065 mm). Among all valid Owenia species, the most similar to O. unipinnata n. sp. are O. bassensis Ford & Hutchings, 2005 (from Australia) and O. vieitezi Díaz-Díaz, Parapar & Moreira, 2020 (from north-western coast of the Gulf of Venezuela) based on the presence of three pairs of tentacles. However, O. unipinnata n. sp. differs from O. bassensis mainly because of the presence of one major ramification, the absence of a peristomial collar, the presence of methyl green speckles concentrated on the thorax, and larger body size (length and width) in the Australian species (Parapar & Moreira 2015). Owenia unipinnata n. sp. closely resembles the Venezuelan O. vieitezi species in having three pairs of tentacles and three major ramifications. Nevertheless, each tentacle with the three ramifications is located firstly very close to the crown base, secondly at mid-length, and thirdly very close to the distal end (Díaz-Díaz et al. 2018), which is totally different from that observed for O. unipinnata n. sp. The O. unipinnata n. sp. straightforwardly has three minor and simple ramifications, with the first solitary one situated at the mid-ventral paired tentacle, and another two ramifications located at the dorsal most paired tentacles. In addition, the brown colouration is present in the dorsal and lateral tentacles of the branchial crown of O. vieitezi, and is absent in the O. unipinnata n. sp. Appendix 2 provides more information on the similarities and differences among Owenia species worldwide. REMARKS All oweniid species recorded from the South China Sea are included, with the exception of Galathowenia eurystoma (Caullery, 1944) which remains taxonomically unresolved. The species originally described in the shallow to deep waters of Indonesia (32 - 1570 m) was proposed to be Myriochele (Caullery 1944). However, Capa et al. (2012) stated that it belongs to Galathowenia based on the diagnostic features that characterise this genus. The species was too briefly described using incomplete specimens, hempering its comparison with other species. In addition, considering the broad depth of the distribution range, with specimens presenting pigmentation differences (Caullery 1944), it is likely that more than one species is involved here. Owenia fusiformis (Delle Chiaje, 1844) was scarcely described in samples from the Mediterranean Sea but the type material was never deposited. The species has been recorded at all sea depths and latitudes explored to date (Ford & Hutchings 2005). Nevertheless, detailed morphological analyses of worldwide populations have demonstrated differences that confined its distribution to the original region and the recognition of new species (Koh et al. 2003). Hence, it is likely that O. fusiformis recorded in the SCS represents undescribed species. Owenia collaris (Hartman, 1955) from Southern California, USA, was redescribed by Blake (2000) using type material and its distribution was delimited to that area. Other worldwide records of the species particularly those from the SCS need to be reassessed. Galathowenia lobopygidiata (Uschakov, 1950) was initially described as Owenia based on specimens collected from the deep waters of the Okhotsk Sea, Russia. However, it was later reclassified under the genus Galathowenia because of the remarkable similarity in the head structure within the genus (Koh et al. 2003). Galathowenia lobopygidiata was also reported from Banda Trench, Indonesia. The presence of G. lobopygidiata in the Banda Trench region is possible because the maximum depth of the trench (7440 m) is in the hadopelagic zone, while the Okhotsk Sea of 3372 m is in the abyssopelagic zone. Both regions have similar extreme conditions of high pressure, low temperature and complete darkness. The range of salinity which contributes greatly to the growth and survival rate of polychaetes, may also explain the presence of this species, as the salinity range of the Banda trench and Okhotsk Sea does not differ much (Banda Trench: 34.60 to 34.65 ‰, Okhotsk Sea: 32.5 ‰ and lower) (Radchenko et al. 2010; Suyadi et al. 2018).

TYPE GENUS. — Owenia Delle Chiaje, 1844.

Species: Galathowenia oculata (Zachs 1923) References: Souza (2008) Environment: Bay Depth: 1 – 4 m Coordinates: 27 ° 35 ' S / 48 ° 35 ' W Species: Owenia fusiformis delle Chiaje 1844 References: Amaral & Morgado (1998), Marenzi (2002), Pagliosa (2005), Souza (2008), Becker (2009), Oortman (2010), Almeida & Vivan (2011) Environment: Bay, inner continental shelf Depth: 1 – 11 m Coordinates: 27 ° 35 ' S / 48 ° 35 ' W, 27 ° 38 ' 33 " S / 48 ° 31 ' 41 " W, 27 ° 29 ' S / 48 ° 30 ' W, 27 ° 17 ' 17 " S / 48 ° 33 ' 46 " W, 27 ° 14 ' 58 " S / 48 ° 35 ' 18 " W, 27 ° 17 ' 13 " S / 48 ° 35 ' 09 " W, 27 ° 14 ' 50 " S / 48 ° 33 ' 44 " W, 27 ° 14 ' 52 " S / 48 ° 32 ' 02 " W, 26 ° 46 ' 10 " S / 48 ° 49 ' 10 " W, 27 ° 13 ' 08 " S / 48 ° 33 ' 25 " W, 26 ° 46 ' 10 " S / 48 ° 38 ' 45 " W, 26 ° 46 ' 10 " S / 48 ° 47 ' 45 " W, 26 ° 12 ' 49 " S / 48 ° 36 ' 51 " W, 26 ° 13 ' 29 " S / 48 ° 37 ' 13 " W, 26 ° 11 ' 59 " S / 48 ° 37 ' 41 " W, 26 ° 12 ' 08 " S / 48 ° 37 ' 32 " W, 26 ° 10 ' 21 " S / 48 ° 46 ' 26 " W, 26 ° 11 ' 27 " S / 48 ° 37 ' 50 " W Note: Owenia fusiformis is clearly a species complex. Local records of O. fusiformis probably correspond to Ammochares brasiliensis Hansen 1882 or Ammochares tegula Kinberg, 1866 or to a new species, yet to be described.