A new genus and species of Sternaspidae (Annelida: Polychaeta) from the deep eastern Atlantic

Based on specimens recently collected in sediments from 2700 m depth off Mauritania (Northwest Africa; type locality) and from 2700–4400 m depth off Angola (Southwest Africa), a new genus and species, Mauretanaspis longichaeta gen. et spec. nov., is described. The new genus and species are characterized by a unique combination of characters: ventro-caudal shield covered by fi rmly adhering sediment, lateral margins strongly bent and merging into integument; introvert hooks tapering; eight pre-shield segments; absence of peg chaetae; exceptionally long posteriormost lateral chaetae equaling body length; posterior shield chaetae equaling shield length. A comparative table of characters for all currently recognised sternaspid genera and a key to all species with ventro-caudal shield covered by fi rmly adhering sediment are provided.


Introduction
represents an enigmatic polychaete taxon characterized by the presence of a unique bilaterally symmetrical ventro-caudal shield. The taxonomy of Sternaspidae has received specifi c attention during recent years following the fi rst revision of the group by Sendall & Salazar-Vallejo (2013). Their morphological study resulted in the distinction of three genera (Sternaspis Otto, 1821, Caulleryaspis Sendall & Salazar-Vallejo, 2013and Petersenaspis Sendall & Salazar-Vallejo, 2013) and a rapid increase of the number of species from 13 to 44, including the species described herein (Fiege 2016(Fiege , 2019. With Sternaspis monroi Salazar-Vallejo, 2014, recently synoymized with S. sendalli Salazar-Vallejo, 2014 (Drennan et al. 2019), the actual number of species comes to 43.
Twelve of these are known to occur in deep waters (Table 1). Our present knowledge of the morphology, anatomy, biology and phylogeny of the group was summarized by Fiege (2016Fiege ( , 2019. A recent study by Drennan et al. (2019) investigated the phylogenetic relationships using molecular markers both within Sternaspidae as well as their position among the Polychaeta as a major taxon. Their molecular data of three genes (16S, 18S and CO1) places Sternaspidae next to Scalibregmatidae Malmgren, 1867 and these two families as a sistergroup of Cirratuliformia Fauchald, 1977 while other phylogenetic analyses using a higher number of genes paired Sternaspidae with Fauveliopsidae Hartman, 1971 (e.g., Rousset et al. 2007; for details see Fiege 2019 andDrennan et al. 2019). The latter authors also revealed a genetic connectivity among some species of Sternaspis across the Pacifi c. These molecular results also cast some doubt on the signifi cance of the shield morphology for species distinction.
Below, we present a morphological description of a new genus and species, Mauretanaspis longichaeta gen. et spec. nov., recently collected in sediments from 2700 m depth off Mauritania (Northwest Africa) and from 2700-4400 m off Angola (Southwest Africa). Unfortunately, the material was not suitably preserved for a molecular analysis, but the unique combination of morphological characters allows a clear distinction from other sternaspid genera and species described to date. As mentioned by Drennan et al. (2019), the genera Sternaspis and Caulleryaspis are in need of revision, but we believe that the characters observed in our specimens are suffi cient to establish a new genus and species.
The diagnostic characters of all sternaspid genera described so far, including the new genus established herein, are summarized in Table 2 and a key to all sternaspid species with a ventro-caudal shield covered by fi rmly adhering sediment is provided below.

Material and methods
The specimens were collected in deep-sea mud in a depth of about 2700 m during a benthic survey to the Block C9 area off Mauritania and in depths between about 2700-4400 m off Angola. Sampling was carried out in 2018 by the French environmental consultancy CREOCEAN on behalf of TOTAL. Specimens were sampled using a 0.25 m² box core (Grey O'Hara), sieved on a semi-automatic table, fi xed in 4% formalin in seawater and later transferred to 70% ethanol for preservation. Morphological studies were carried out using a Leica MZ8 and Leica M 125C stereo microsope. Photographs were taken with a Canon EOS 5D Mark II camera equipped with a 65 mm MP-E lens and a DMC 4500 Leica camera attached to the stereo microsope. Measurements were taken using an ocular micrometer. Counting of segments starts with chaetiger 1, i.e., pro-and peristomium are not included.The holotype is deposited at

Diagnosis
Sternaspids with introvert hooks tapering. Eight segments between gonopodial lobes and shield (i.e., preshield region). Ventro-caudal shield densely covered with fi rmly adhering fi ne sediment, which cannot be brushed off. Due to sediment cover no shield structures like ribs or concentric rings visible except median suture. Shield with lateral margins strongly bent towards dorsal side, merging into integument. Peg chaetae absent.

Etymology
The genus name is a combination of the Latin name for the type locality off the Northwest African state Mauritania (i.e., Mauretania, Lat.) and the Greek word 'aspis' (= 'shield'); gender: feminine.

Remarks
Mauretanaspis gen. nov. differs in the combination of characters from the other genera of Sternaspidae. The shield is completely and fi rmly covered by sediment, as in several species of Caulleryaspis, but it has eight pre-shield segments; peg chaetae (i.e., a dense group of short chaetae covered by a common sheath) are absent, while Caulleryaspis has seven pre-shield segments and peg chaetae are present. In Caulleryaspis nuda peg chaetae are absent, but according to Drennan et al. (2019) this species possibly belongs to the genus Sternaspis (see 'Discussion'). Petersenaspis has eight pre-shield segments and Etymology The species epithet is a combination of the Latin 'longus' (= 'long') and the Greek 'chaeta' (= 'bristle') characterizing the striking length of the posteriormost lateral chaetae of the ventro-caudal shield.
ANTERIOR ABDOMEN (PRE-SHIELD REGION). Comprising eight segments. Body papillae minute, round, forming single rows close to intersegmental folds, sometimes obscured by sediment particles (Figs 1,  4B). Most pre-shield segments of holotype with smooth median surface on ventral side, possibly abraded, and few, short, fi lamentous papillae on lateral sides; in paratype median surface papillated (Figs 1, 4A). Few fi lamentous papillae among adhering sediment particles on segments anterior to branchial plates. VENTRO-CAUDAL SHIELD. Densely covered by fi ne, fi rmly adhering sediment particles. Shield colour light brown, obscured by dense cover of sediment. Colour most intense around anterior end of median suture (sediment abraded in holotype, not abraded in paratype) (Figs 3B,4A). No structures (ribs, concentric rings) visible due to sediment cover. Anterior margins slightly rounded, anterior depression shallow, covered by sediment; anterior keels faintly visible (Fig. 3B), not exposed. Lateral margins rounded, strongly bent towards dorsal side, not forming free, stiff or pliable margins, but rather merging into integument. Posterior fan and lateral notch not discernible due to sediment cover; shallow median notch (more pronounced in paratype (Fig. 4A, C)). Suture defi ned in anterior ⅓ of shield, moderately deep with distinct lateral borders (Figs 3B, 4A).  BRANCHIAL PLATES. Brownish, forming ovoid to trapezoidal region, densely covered by fi laments, fi lamentous papillae and sediment grains (Fig. 3C).

Distribution
Northeast to Southeast Atlantic, from off Mauritania to Angola, in deep-sea mud, in depths of 2700-4400 m.

Remarks
Mauretanaspis longichaeta gen. et spec. nov. can be distinguished from all other known species of Sternaspidae by the following combination of characters: introvert hooks tapering, tips transparent; pre-shield region with eight segments; ventro-caudal shield densely covered with fi ne, fi rmly adhering sediment obscuring shield structures except median suture. Mauretanaspis longichaeta gen. et spec. nov. is the only sternaspid with chaetae in posteriormost lateral fascicles equalling body length. In one specimen collected off Angola (MNCN 16.01/18763), posteriormost lateral chaetae even measure about 4 × the length of the body.
Several specimens from off Angola (SMF 28062, MNCN 16.01/18762, MNCN 16.01/18763, DBUA0002349.01) show black pigment presumably from surrounding sediment included in sediment cover and among sand grains in the branchial region. Black particles are scattered on the shield forming spots of different sizes.
We compared our material to a specimen collected off Northwest Africa in 1320 m depth and identifi ed as Sternaspis scutata (Ranzani, 1817) by Kirkegaard (2001: 396). The specimen, deposited in the collections of the Natural History Museum of Denmark (NHMD 662048), was not available for study, but pictures of it were kindly made available to us by S. Salazar-Vallejo. Unfortunately, the animal is in very bad condition: anterior hooks tapering to fi ne tips, number of pre-shield segments and gonopodial lobes not distinguishable, shield completely covered by fi ne sediment obscuring surface structures, shield without free margins, length of lateral and posterior shield chaetae indeterminable. It remains uncertain whether the sediment can be removed from the shield without damaging it, but with regard to shield characters visible in the pictures available, the specimen appears incorrectly identifi ed as Sternaspis scutata. However, due to its bad condition we cannot identify it any further.  Kobayashi et al. (2018) and Drennan et al. (2019) used molecular techniques to study the taxonomy and systematics of Sternaspidae, but the defi nition of most sternaspid taxa still relies on morphological characters with the enigmatic ventro-caudal shield being the most prominent. Three genera and respective body patterns were distinguished by Sendall & Salazar-Vallejo (2013) in their revision of Sternaspis.

Discussion
Their defi nitions were based on morphological characters of the shield, the introvert hooks and the number of pre-shield abdominal segments. Ventro-caudal shields are described as "stiff" for Sternaspis and Petersenaspis with loose sediment cover easily brushed off, while shields in Caulleryaspis are described as "soft" and easily bent or pliable with sediment fi rmly adhered. In this case the sediment cover obscures surface structures from observation, i.e., ribs and concentric lines as described for Sternaspis and ribs only for Petersenaspis.
The ventro-caudal shield consists of a framework of fi bres and mineralized iron (Bartolomaeus 1992), providing it with a certain degree of fi rmness. According to Sendall (2006: 19), the distinction beween its stiff-or softness is best tested by trying to bend its margin. In Mauretanaspis longichaeta gen. et spec. nov. the shield margins merge into the integument without forming free margins and thus the shield cannot be tested for its fi rmness. We consider this character of limited value for a generic distinction, supporting a view that has recently been suggested by Drennan et al. ( (Zhadan, Tzetlin & Salazar-Vallejo, 2017) were described with ventro-caudal shield free of sediment particles leaving weakly developed shield structures visible. Therefore, the placement of these two species in Caulleryaspis appears questionable and consequently they are not included in the identifi cation key presented above.
Due to their small size and the lack of indications of sexual maturity, all our specimens of Mauretanaspis longichaeta gen. et spec. nov. might be considered juveniles. However, sexually mature females in specimens of comparable size were reported for Caulleryaspis nana, a species occurring in shallow waters of the South China Sea. A progenetic origin was hypothesized accordingly (Zhadan et al. 2017). A comparison of the chaetation of Mauretanaspis longichaeta gen. et spec. nov. with other sternaspids shows that number and development of chaetae is comparable to larger specimens of other species and that the striking length of the posteriormost lateral chaetae is unique. Typically, juvenile characters for Sternaspidae appear to be a distinctive papillation of the body with longer papillae including the surface of the ventro-caudal shield as shown by Zhadan et al. (2017) for Sternaspis papillosa Zhadan, Tzetlin & Salazar-Vallejo, 2017. Our animals show only minute body papillae, comparable to those found in presumed adults of other sternaspid species, another argument in favour of establishing the new genus and species. We only consider the smallest specimens examined, i.e., from stations S48-3A (MNHN-IA-PNT 120), S48-8A (SMF 28061), S48-14C (MNCN 16.01/18762) and S48-23B (DBUA0002349.01) off Angola, as juveniles because of the absence of gonopodial lobes which we see as an indication that these specimens are indeed not yet sexually mature.