Six new deep-water sternaspid species (Annelida, Sternaspidae) from the Pacific Ocean

Abstract Most sternaspid species have been described from shallow water, and Caulleryaspis Sendall & Salazar-Vallejo, 2013 includes one deep water species: C. gudmundssoni Sendall & Salazar-Vallejo, 2013 from Iceland. In Sternaspis Otto, 1821, the most speciose genus, most species were described from shallow water and only three thrive in deep water: S. maior Chamberlin, 1919 from the Gulf of California, S. princeps Selenka, 1885 from New Zealand, and S. riestchi Caullery, 1944 from Indonesia. The study of some deep sea sternaspids from the Pacific Ocean in the collections of six research institutions resulted in the discovery of six undescribed species, and for three of them there were abundant materials showing ventro-caudal shield development. Caulleryaspis fauchaldi sp. n. is described based on specimens from Oregon and California; it differs from the known species because it has a shield with rounded anterior margins and its peg chaetae form thin, small spines. Caulleryaspis nuda sp. n. was collected off Oregon; it is unique because its shield lacks a layer of sediment particles firmly attached, but has instead a thin layer of small particles loosely attached. Four other species are newly described in Sternaspis: S. annenkovae sp. n. was collected east off the northern Kurile Islands in about 4,000 m depth; it differs from other species by having a bicolored body, with the introvert darker than the abdomen, and its ventro-caudal shield plates are divergent resulting in a divided fan. The second species, S. maureri sp. n. was found off Peru in 1296–6489 m water depths and in the Southwestern Pacific in 795–3830 m; it resembles S. williamsae sp. n. but differs because its shield has better-developed ribs, the fan has a shallow or indistinct median notch and has lateral notches well-developed. The third species, S. uschakovi sp. n., was found in the Okhotsk Sea in 592–1366 m, off California in 1585 m, Gulf of California in 1200–1274 m, and Western Mexico in 2548 m; it resembles S. africana Augener, 1918 and S. andamanensis Sendall & Salazar-Vallejo, 2013 in having shields with a denticulate posterior margin; the latter two species live in shallow water and they differ from each other and from the new species by a combination of shield and papillae features. The fourth species, S. williamsae sp. n., was found off Oregon in 1000–2400 m, and off California in 878–1246 m; it resembles S. annenkovae because both species have shields with fans narrower than the anterior margin width, but differ in the relative development of shield features and in the relative size of prostomium and mouth; as stated above it also resembles S. maureri sp. n. but its shield has poorly-developed ribs, its median notch is distinct, and the lateral notches are poorly developed or indistinct. Keys to identify all species of Caulleryaspis and Sternaspis are also included.


Introduction
In the polychaete family Sternaspidae Carus, 1863, most species have been described from shallow water, of less than 200 m depth ; however, one of the two known species of Caulleryaspis Sendall & Salazar-Vallejo, 2013 and only three species in Sternaspis Otto, 1821 were described from deeper water, of around 1000 m depth: S. maior Chamberlin, 1919, S. princeps Selenka, 1885, and S. rietschi Caullery, 1944. The former was described from the Gulf of California, Eastern Pacific Ocean, whereas the two other species were described from specimens collected in the Southwestern Pacific. Deep water sternaspids are delicate, fragile and difficult to study because they are often rare, but some abundant specimens have allowed us to study the development of the ventro-caudal shield.
The study of the Pacific Ocean sternaspid material lodged in six major research institutions resulted in the recognition of six species which are newly described: C. fauchaldi sp. n. from off Oregon and California, C. nuda sp. n. from off Oregon, S. annenkovae sp. n. from off the Northern Kurile Islands, S. maureri sp. n. from off Peru and from the Southwestern Pacific, S. uschakovi sp. n. from the Okhotsk Sea, California, Gulf of California and Western Mexico, and S. williamsae sp. n. from off Oregon and California.
First three chaetigers with about 16 falcate, tapered introvert hooks per bundle, each with subdistal dark areas (tips broken, darker areas look distal; subdistal in complete hooks). Genital papillae not seen (paratype with ventrolateral pores between segments 7 and 8). Pre-shield region with 7 segments (difficult to count because of specimens fragility); capillary chaetae along first pre-shield segment (paratype with capillaries in two segments).
Ventro-caudal shield completely covered by a thick coating of adhered particles (Fig. 1C), perforated, better preserved in paratype (Fig. 1E); suture not visible. Anterior margins clearly rounded; anterior depression deep; anterior keels not exposed. Ribs or concentric lines not visible. Lateral margins rounded, expanded medially, reduced posteriorly. Fan truncate, barely reaching posterior corners. Other features not visible.
Juveniles. Juvenile ( Fig. 2A) with papillae less abundant and larger than those present in type specimens, homogeneously distributed throughout integument, eroded in introvert and arranged in transverse series as remains of erosion along dorsal surface  ( Fig. 2D). Body about one-third as large as type specimens, with introvert damaged by compression (Fig. 2B). Ventro-caudal shield with sediment particles and abundant papillae (Fig. 2C); anterior margins poorly defined, lateral margins rounded, medially expanded. About 9 lateral chaetal bundles and 5-6 posterior ones with longer, thinner and fewer chaetae than in larger specimens. Peg chaetae not visible.
Etymology. This species is named after Dr. Kristian Fauchald, long-time teacher and friend, in recognition of his many contributions to polychaete systematics and especially because of his contribution to the study of deep-sea fauna including the off Oregon species. The epithet is a noun in the genitive case.
Type locality. W off Yaquina Bay, 2800 m depth. Remarks. Caulleryaspis fauchaldi sp. n. is very similar to C. gudmundssoni Sendall & Salazar-Vallejo, 2013 because both have shields with a deep anterior depression and robust peg chaetae. These two species differ, however, in two main features. In C. fauchaldi the anterior shield margins are rounded, the introvert has longer papillae, and the peg chaetae form thin short spines, whereas in C. gudmundssoni the anterior shield margins are more angular, the introvert has shorter papillae, and the peg chaetae form thick, long spines.
First three chaetigers with about 16 falcate, tapered introvert hooks per bundle, each with subdistal dark areas, slightly paler distally. Genital papillae not seen. Preshield region with 7 segments; capillary chaetae not seen (some paratypes with capillaries in 2-4 segments).
Ventro-caudal shield completely covered by a thin, delicate coating of adhered fine sediment particles, mostly removed from shield; suture not visible (Fig. 3C). Anterior margins rounded; anterior depression deep; anterior keels not exposed. Ribs barely defined, concentric lines not visible. Lateral margins rounded, expanded medially, reduced posteriorly. Fan with a deep wide median notch, projected beyond the margins of posterior corners. Marginal chaetal fascicles include 10 lateral and only 4 short, small posterior ones (others probably broken; some paratypes with up to six posterior bundles); lateral bundles with 7-8 chaetae each, posterior bundles with 5-6. Peg chaetae not visible (one paratype with minute, barely visible peg chaetae). Additional chaetal fascicles not visible.
Variation. All paratypes have shields without sediment particles firmly adhered or concentric lines (Fig. 3D-F). The anterior depression is deep, the anterior margins are projected, and the radial ribs are progressively better developed as growth proceeds. The posterior, median notch is well developed in all stages.
Etymology. The specific name is derived from the Latin adjective nudus (a, um): naked, to indicate that unlike other species in the genus, its shield does not have firmly adhered sediment particles on it. The epithet is in the genitive case.
Remarks. Caulleryaspis nuda sp. n. is unique because its ventro-caudal shield is soft as typical for the genus, but instead of having a thick sediment particles cover, it has a very thin layer made by loosely adhered, fine sediment particles, which can be easily eroded or brushed off. However, the general shield outline of C. nuda resembles the one present in S. williamsae sp. n. (see below), and the latter can even incorporate some sediment particles, but they differ because in C. nuda, the shield does not have a thin, stiff, yellowish layer but rather a convex, delicate, pliable margin.
Distribution. Only known from off Oregon, U.S.A., in 2519 m depth.

Diagnosis.
Sternaspids with introvert hooks falcate, tapered. Pre-shield region with 7 segments. Ventro-caudal shield stiff, usually with abundant sediment particles loosely adhered; with well-developed radial ribs and concentric lines. Branchial filaments arranged in discrete branchial plates.
Prostomium hemispherical (Fig. 4C), (similar in paratypes; slightly acute in one paratype), projected, slightly larger than mouth, with same pigmentation as introvert. Eyespots not seen. Peristomium rounded with abundant papillae, extended laterally over prostomium and ventrally to margin of first chaetiger.
Anterior abdomen with 7 segments, papillae mostly eroded, some remaining in body depressions or around branchial region, but not arranged in series or groups. Capillaries not seen (two paratypes with capillaries in first two segments, two per bundle).
Ventro-caudal shield with lateral plates divergent, surface with ribs and concentric lines, the latter less pronounced, barely banded; suture visible throughout shield. Anterior margins rounded, midventral depression shallow (Fig. 4C). Lateral margins gently rounded, not expanded posteriorly. Fan markedly notched, barely projected beyond poorly developed posterior corners, margin barely crenulated.
Marginal chaetal fascicles mostly broken off ( Fig. 4B, C), 10 lateral ones with chaetae along an oblique series, and 7 posterior fascicles with chaetae in linear arrangement. Peg chaetae not visible.
Branchiae mostly lost (few remaining in paratypes, spirally bent); interbranchial papillae long, straight, often with fine sediment particles. Branchial plates observed in some paratypes, narrow, anteriorly rounded, wider than rest of branchial plate, with 8-9 filaments per series.
Etymology. This species name is after the late Dr. Nadezhda P. Annenkova, in recognition of her many publications on polychaetes, and for her efforts to build a strong taxonomic tradition in the early to mid XX century in Russia. The epithet is a noun in the genitive case.
Variation. The introvert is always pale brown, darker than the abdomen. Introvert chaetigers have 11-13 hooks per bundle. The ventro-caudal shield is pale brick red or dirty orange, ribs and concentric lines are always visible but variably developed; anterior margins are rounded to barely acute ( Fig. 4E-G); fan markedly notched, margin barely crenulated to markedly crenulated, barely projected beyond the posterior corners. The inner margins of each lateral shield plate are fused along ½ to 1/3 of its length, resulting in a divergent or markedly notched fan. The shield chaetal bundles are difficult to count in holotype; paratypes with 9 lateral, and 6-7 posterior bundles.
Remarks. Sternaspis annenkovae sp. n. is unique among the species in the genus and two features separate it. First, the body is bi-colored having a darker introvert and a pale abdomen, whereas in all other species the introvert is usually of the same color than the abdomen, slightly paler or even transparent. It is true that sometimes the sediment-filled gut can be displaced towards the introvert, making it look darker than the posterior region, but this can be noticed by transparency of the introvert's body wall, whereas in S. annenkovae the pigmentation is widespread and homogeneous in the introvert. Second, the lateral shield plates are divergent and separated throughout its posterior region such that the fan is markedly notched. Some Sternapsis species can have a more eroded shield in older specimens, whereas the younger specimens are less eroded; however, in S. annenkovae the markedly notched fan is evident even in small specimens, rendering it a consistent feature, not significantly modified during growth. The other three deep-water species (S. maior, S. princeps, and S. rietschi) do not have a median notch or, if present, as in S. maior, it is rather shallow. Further, there are two species that have shields with deeply notched fans, especially in larger specimens as shown elsewhere : S. costata von Marenzeller, 1879 and S. islandica Malmgren, 1867. These species differ by having a wider fan such that the larger ribs form an angle of 100-120° whereas in S. annenkovae the fan is narrower and the larger ribs form an angle of 85-90°. However, as indicated in the key below S. annenkovae resembles S. fossor Stimpson, 1853 because their shields have radial ribs and concentric lines, fans with a deep, median notch, and poorly defined posterior corners. They differ because in S. annenkovae the introvert is darker than the rest of the body and its shield is wider anteriorly, whereas in S. fossor the introvert is paler or with similar pigmentation than the rest of the body, and the shield is wider medially.
On the other hand, S. annenkovae resembles S. williamsae sp. n. (described below) because in the ventro-caudal shields of both species the fan is narrower than the corresponding anterior margins. They differ because in S. annenkovae concentric lines are better developed, the fan is medially discontinuous, and the prostomium is larger than the mouth, whereas in S. williamsae the concentric lines are poorly developed, the fan is medially continuous, and the prostomium is smaller than the mouth.
Description. Holotype (LACM 0000) complete. Body wall broken and inner organs lost (Fig. 5A). Body brownish, integument papillose throughout body, dark brown (eroded leaving a paler body wall in smaller paratypes; larger ones with darker body wall). Body 7 mm long, 3 mm wide, about 29 segments; left shield plate 2.5 mm long, 2.1 mm wide.
First three chaetigers with 12-14 bronze, slightly falcate hooks per bundle, each with subdistal dark areas (up to 16-18 in some paratypes). Genital papillae lost (pale, blunt, short lobes in some paratypes, from the intersegmental groove between segments 7 and 8). Pre-shield region with 7 segments, with papillae mostly eroded from segmental ridges, but present in intersegmental furrows or along some areas, homogeneously distributed. Short, about 4-5 capillary chaetae in some segments.
Ventro-caudal shield dark orange, with ribs partly eroded, concentric lines poorly developed; suture distinct throughout shield (Fig. 5A, C). Anterior margins rounded (broken in Fig. 5A), anterior depression shallow; anterior keels not exposed, barely developed. Lateral margins rounded, reduced posteriorly. Fan truncate, one-half to twothirds as wide as anterior margins width, slightly projected beyond posterior posterior shield corners, median notch shallow, barely developed, lateral notches deeper, better developed, fan margin smooth to barely crenulated (paler in some paratypes).
Branchial plate missing (one paratype (Sta. 72) with one plate left but no branchial or interbranchial filaments left; branchial plate ovoid, tapering anteriorly; another paratype from the same station with branchial plates anteriorly converging).
Variation. There are several ontogenetic changes in the ventro-caudal shield, although the anterior depression remains shallow throughout their development (Fig.  6). Smaller specimens (Fig. 6A, D) have yellowish or pale orange shield, with ribs barely visible. Medium-sized specimens have shields slightly darker, with ribs better defined but the main, diagonal rib is still poorly developed (Fig. 6B, E). In larger specimens, the shield becomes thicker, stronger and its ribs, including the diagonal one are better defined (Fig. 6C, F) and the lateral notches, over its posterior margin or fan, become deeper.
Etymology. The species name is after Dr Don Maurer in recognition of his many publications, mostly on benthic ecological studies and especially dealing with polychaetes. He also studied the material employed for this description. The epithet is a noun in the genitive case.
Type locality. W off Trujillo, Perú, 6156-6489 m depth. Remarks. Sternaspis maureri sp. n. resembles S. annenkovae sp. n. and S. williamsae because they all have ventro-caudal shield fans narrower than the width of the anterior margin. However, the shield in S. maureri sp. n. is more similar to the one found in S. williamsae because both are medially fused. Their main differences are in the relative shield's size, radial ribs' development, and on the relative depth of the fan's lateral and median notches. In S. maureri the shield is proportionally larger (body length: shield length 7.0:2.5 mm, one paratype Sta. 77, 6.5:2.5 mm), the ribs are better developed, the median notch is shallow or very slightly developed, and the lateral notches are better developed, whereas in S. williamsae the shield is smaller (7.5:1.6 mm), the ribs are poorly developed, its median notch is deep or well defined, and the lateral notches are poorly developed or indistinct.
Distribution. Abyssal sediments off Peru, Eastern Pacific, in 1296-6489 m water depths, and in the Southwestern Pacific in 795-3830 m.
Prostomium hemispherical (Fig. 7C); projected, with same pigmentation as introvert. Eyespots not seen. Peristomium round with abundant papillae restricted to peripheral areas around the mouth, barely reaching margin of first chaetiger.
Anterior abdomen with 7 segments, lateral lobes well-defined by contraction, dorsal area bare, converging posteriorly, ventral area bare, more or less parallel; papil- lae mostly eroded, some remaining in branchial region, but not arranged in series or groups. Capillaries not seen.
Ventro-caudal shield with lateral plates slightly bent dorsally, making them look quadrate in frontal view (Fig. 7D) but each plate wider than long (Fig. 7B); suture visible throughout shield. Ribs barely developed, concentric lines poorly developed but present. Anterior margins barely rounded, midventral depression shallow. Lateral margins gently rounded, expanded posteriorly. Fan projected medially beyond posterior corners, margin denticulate.
Marginal chaetal fascicles mostly broken off (Fig. 7A, B, D), 9 lateral ones with chaetae along an oblique series, and 6 posterior fascicles with chaetae in linear arrangement. Peg chaetae not visible (some paratypes have them).
Etymology. This species is being named after the late Dr. Pavel V. Uschakov as a modest homage to his monographic publications, especially those regarding the Okhotsk Variation. The introvert is markedly swollen, like in the holotype, in one paratype and in some non-type specimens. Introvert chaetigers with 9-10 hooks per bundle. The ventro-caudal shield is dirty orange to pale brick red, ribs and concentric lines are always visible but variably developed; anterior margins rounded to barely acute (Fig.  7E-G); fan markedly projected medially, with shallow lateral notches, margin denticulate rarely reaching the level of the posterior corners. The inner margins of each lateral shield plate are fused along most of its length. The shield chaetal bundles are difficult to count but there are 9 lateral and 7 posterior bundles in better preserved paratypes. On the other hand, the specimens from the Northeastern Pacific have bodies with larger, more abundant papillae (Fig. 8A), and they are also evident in the mouth region (Fig.  8B); the ventro-caudal shield (Fig. 8C), however, is very similar to the specimens from the Northwestern Pacific. We regard the apparent difference in integument papillation as a result of different sampling and sorting procedures and not as a significant, diagnostic difference. Therefore, we conclude they are conspecifics.
First three chaetigers with 12-14 bronze, slightly falcate hooks per bundle, each with subdistal dark areas. Genital papillae lost, eroded from the intersegmental groove between segments 7 and 8 (not visible in paratypes either).
Pre-shield region with 7 segments, with papillae abundant in protected areas (some paratypes with papillae arranged in single transverse series per segment). Short, few capillary chaetae present in one segment (in up to three segments in paratypes).
Ventro-caudal shield pale orange (Fig. 9C), with ribs, concentric lines poorly developed; suture distinct throughout shield. Anterior margins rounded; anterior depression deep; anterior keels exposed (also exposed in two paratypes). Lateral margins rounded, reduced posteriorly. Fan truncate, half as wide as anterior margins width, slightly projected beyond posterior shield corners, median notch moderate, fan margin smooth (barely crenulated in two paratypes).
Branchiae lost in holotype (paratypes with abundant filaments, some 2-3 times thicker, and half as long as the others). Branchial plates parallel, anteriorly rounded.
Variation. There are several modifications in the shield development with roughly defined ribs and the general outline has lateral margins rounded, medially expanded, some distortions were noticed probably due to sample handling since the body is rather delicate. The shield pigmentations is dark yellow in smaller paratypes (Fig. 9E) with radial ribs barely visible and no concentric lines; the posterior margin is smooth with the fan not projected beyond the posterior corners. Slightly larger paratypes have a better defined diagonal rib (Fig. 9F), but concentric lines appear later (Fig.  9G), together with slight lateral notches. The largest paratype was severely damaged (Fig. 9D), and its shield is almost completely detached and bent dorsally, such that its outline is apparently different (Fig.9H); it has a well-developed diagonal rib but concentric lines are barely visible, and the lateral notches are not as distinct as in smaller specimens.
Early shield development. Very small juveniles show a transition in pigmentation and development of the ventro-caudal shield (Fig. 10A). The smallest specimen, being 0.7 mm long (Fig. 10B) has no pigmentation but chaetae are already arranged in lateral and posterior bundles, each with 1-3 chaetae. When the body doubles its size, being about 1.5 mm long (Fig. 10C) the shield has some sediment particles and becomes slightly more pigmented than in smaller specimens. The following stage, when the body reaches about 3 mm long (Fig. 10D), is not markedly different from the previous stage. Darker pigmentation and better defined shield margins are attained when the specimens are 3.5 mm long (Fig. 10E, oblique illumination resulted in a darker shield) but the ribs or the median notch are not well defined. These last two modifications apparently appear soon afterwards because in a similar-sized specimen (Fig. 10H), the shield (with a more incident angle) shows a dark yellow shield bordered by a paler region, and with better defined ribs and median notch (Fig. 10G).
Etymology. This species is named after Susan Williams, in recognition of her taxonomic work on trichobranchids, and because she left some notes indicating that she regarded some of the materials herein included for this description as representing a different pattern which deserved a name. The epithet is a noun in the genitive case.
Type locality. W off Yaquina Bay, Oregon, 2800 m depth. Remarks. Sternaspis williamsae sp. n. is very similar to S. maior Chamberlin, 1919 as redescribed elsewhere  because both have shield with ribs and poorly-developed concentric lines. They differ, however, because of the relative development of some shield features. In S. williamsae the main ribs are moderately divergent, and the fan is narrower than that of the anterior margin, being up to half as wide as the anterior margin, whereas in S. maior the main ribs are markedly divergent, and the fan is as wide or wider than that of the anterior margin. On the other hand, S. williamsae is similar to S. annenkovae because both species have shields with fan narrower than that of the anterior margin. These two species differ because in the shield of S. williamsae the concentric lines are poorly developed, the fan is medially continuous, and its prostomium is smaller than the mouth, whereas in S. annenkovae concentric lines are better developed, the fan is medially discontinuous, and the prostomium is larger than the mouth. As stated above, S. williamsae resembles S. maureri because of their shields. They differ, however, in three main features. In S. williamsae the ribs are poorly developed, its median notch is deep or well defined, and its lateral notches are poorly developed or indistinct, whereas in S. maureri the ribs are better developed, its median notch is shallow or very slightly developed, and its lateral notches are better developed.
Distribution. Only known from Oregon to California, 1000-2800 m depth.