Identification key to Nephtyidae (Annelida) of the Sea of Okhotsk

Abstract Currently, 15 species of Nephtyidae (Annelida) are known from the Sea of Okhotsk (north-western Pacific). A new user-friendly identification key is presented with a brief description for each species. The taxonomic positions of three closely related species, Nephtys brachycephala Moore, 1903, N. schmitti Hartman, 1938 and N. paradoxa Malm, 1874, are revised. The distributions of two species, Nephtys discors Ehlers, 1968 and N. assignis Hartman, 1950, are discussed.


Introduction
Nephtyids are benthic polychaetes occurring worldwide from the intertidal to abyssal depths and mainly inhabiting soft sediments. Most of them are actively burrowing carnivores, although several species may be subsurface deposit feeders (Jumars et al. 2015). The smallest species are less than 10 mm long, while others can be rather large: the largest species from the Sea of Okhotsk may grow up to 300 mm long.
Herein, an illustrated key is provided to identify species known from the Sea of Okhotsk. This key is based mainly on external morphological characters. In brief species descriptions characters of the pharynx are also included, which are easily visible by dissection and highlighted by staining. This review facilitates the creation of a valid checklist of Nephtyidae species for this region.

Remarks on the key
Nephtyids are rather similar in their gross morphology and often difficult to distinguish. The main taxonomic characters are the position of the first branchiae, their shape and the number of branchiferous chaetigers, parapodial features (shape and size of acicular lobes, pre-and postacicular lobes, characters of chaetae) and pharynx structure.
The parapodia are biramous. Both noto-and neuropodia consist of acicular, preand postacicular lobes, and dorsal (notopodial) and ventral (neuropodial) cirri. The acicular lobes are supported by one acicula and may be conical, rounded or bilobed (Fig. 1). The branchiae (also called interramal cirri), are inserted below the dorsal cirri; they may be involute or recurved, slender and digitiform, or basally inflated and foliaceous. Foliaceous branchiae may be evenly flattened or with a thick tapering midrib and thin lateral wings (Fig. 2). A small spherical papilla may be present at the base of a branchia under the notopodial cirrus. The shape and proportions of these structures vary along the body, so they should be examined on the chaetigers that are recommended in the key.
The pharynx is a large eversible muscular proboscis, covered with soft papillae located in different areas that can be seen when everted (Fig. 3) and usually with one pair of small subterminal jaws located inside (visible with dissection). The anterior margin is surrounded by 18-20 bifid terminal papillae separated dorsally and ventrally   by gaps; each gap may bear a single conical papilla. The subterminal region has 14 to 22 longitudinal rows of conical to digitiform papillae decreasing in size towards the base of the pharynx (absent in Inermonephtys). A single longer subterminal papilla may be present mid-dorsally and mid-ventrally. The proximal surface may be smooth or covered with small warts (flat outgrowths) or small papillae (conical or rounded) which slightly rise above the surface.
The prostomium is subquadrangular to subpentagonal (shape depends on whether the proboscis is everted or not). A pair of conical antennae is present in the anterior corners of the prostomium (absent in Inermonephtys). A pair of palps is inserted ventrolaterally (may be bifid in Micronephthys). A pair of nuchal organs is located dorsolaterally on the posterior margin of the prostomium (Fig. 4). Pigment spots on prostomium (if present) may fade. It is strongly recommended to examine several specimens, rather than a single individual for identification. Staining with methylene blue makes morphological characters more visible. The segment on which the branchiae begin should be checked on both sides of worm. Several undamaged parapodia from both sides of the worm should be examined.
The morphological details of the parapodia can usually be seen under the stereomicroscope without preparing slides. All parapodia are shown in anterior view. Pharynx dissection is not always necessary but may be useful to confirm identifications. It is important to mention that not all characters are developed in juveniles, and it is not always possible to identify fragmented animals without specialized training.
Each species of Nephtys is here provided with a brief description and distribution. All figures are original except for that of N. brachycephala (after Uschakov 1950)   versity, Cambridge, MA, USA; C -chaetiger. Abbreviations with numbers denote the chaetiger, i.e. C3 means the third chaetiger. All features used in the couplets are shown in the figures nearby.
No key is complete and perfect. The key given below should be used with caution and collated with descriptions of the species concerned. If you have any difficulties, do not hesitate to contact us by e-mail or by other means.
Nepthys schmitti Hartman, 1938 from Alaska was also synonymized with N. paradoxa by Imajima and Takeda (1987) tentatively as they had not examined the type material. Hilbig (1997) examined the holotype of N. schmitti and specimens of N. paradoxa from Alaska and concluded that they represented two valid species. Later, Ravara et al. (2010) examined the Alaskan specimens of N. schmitti (including the holotype) and specimens of N. paradoxa from Europe (including the type locality) and found no significant differences between these specimens. However they considered N. schmitti to belong to a N. paradoxa species complex that seemed to have a worldwide distribution, and that the taxonomic status of this species complex should be carefully revised with examination of more specimens.
All three closely related species N. brachycephala, N. schmitti and N. paradoxa, have foliaceous branchiae and similar parapodial features. However, in contrast to N. paradoxa, the two other species (N. brachycephala and N. schmitti) have leaf-like branchiae with a thick tapering midrib that runs through its centre. Our examination of material from the Arctic, North Atlantic and north-western Pacific (497 specimens) has indicated that the foliaceous branchiae of N. paradoxa specimens lack the tapering midrib.
2. Nephtys discors Ehlers, 1868 was originally described from Maine, USA and its distribution appears to be restricted to the north-western Atlantic. Specimens of N. discors from the west coast of Kamchatka (Sea of Okhotsk) (Imajima 1961) were examined and synonymized with N. assignis Hartman, 1950by Banse (1972. We examined the type material of N. discors (MCZ IZ 700 and MCZ IZ 91707) and came to the same conclusion as Banse (1972). Nephtys discors sensu Imajima and Takeda (1987) from the east coast of Hokkaido (off the Notsuke Peninsula, on the southern edge of the Sea of Okhotsk, north-western Pacific) appeared to belong to the same species as the one from the west coast of Kamchatka, therefore was also considered as N. assignis (Ravara, 2010). Thus we excluded N. discors from our key and included N. assignis.
Nevertheless, both species remain valid. Nephtys assignis is a Pacific species with the initially minute branchiae increasing in size through segments 12-20; it has a proximally smooth pharynx and posterior parapodia with well-developed branchiae. Nephtys discors is a West Atlantic species with the branchiae best developed on the anterior third of the body and rudimentary in the posterior half; the pharynx is covered with proximal warts.  (Rainer 1991). Branchiae from C7-C12, minute at first, gradually increasing in size to C25-C27, in median chaetigers often (but not always) more or less foliaceous, rounded fleshy, without tapering midrib or thin lateral wings. Parapodial preacicular lobes rudimentary. Anterior notopodial acicular lobes sometimes slightly bilobed, posteriorly always rounded-conical. Postacicular lobes of anterior and median parapodia subequal in length to or slightly longer than acicular lobes, posteriorly shorter than acicular lobes. Pharynx with short mid-dorsal subterminal papilla; in large worms proximal region of pharynx sometimes covered with small conical papillae. Arcto-boreal lower shelf. Body length up to 90 mm, approx. 100 chaetigers (Hilbig 1997). Branchiae minute at first, increasing in size gradually; welldeveloped branchiae with thick tapering midrib and thin lateral wings reaching half way along midrib from C10-C15 to C35. Exactly from C36 branchiae without lateral wings, long and digitiform, posteriorly decreasing in size gradually, absent from last 8-9 chaetigers. Parapodial preacicular lobes low throughout, poorly developed. Acicular lobes of anterior parapodia rounded (notopodial acicular lobes sometimes slightly bilobed), roundedconical in median region and conical in posterior chaetigers. Anterior parapodial postacicular lobes (before C30) subequal in length, or slightly longer than acicular lobes, posteriorly shorter than acicular lobes. Both dorsal and ventral cirri of anterior and median chaetigers short, broadly conical, tapering to pointed tip, posteriorly decreasing in size to small and conical. Pharynx proximal region wrinkled, without papillae; elongate mid-dorsal subterminal papilla absent. Boreal slope and upper bathyal. Body length more than 64 mm, more than 60 chaetigers (Moore 1903). Branchiae minute at first, increasing in size gradually; well-developed branchiae (after C15) broadly foliaceous with thick tapering midrib and thin lateral wings reaching almost to distal end of midrib except for slightly projecting tip. Posteriorly branchiae decreasing in size very gradually: posterior to C41, branchiae rounded fleshy, without wings; from C51, only small cylindrical midrib present. Branchiae absent after C55-C58. Parapodial rami widely separated, noto-and neuropodia subequal in size. Pre-and postacicular lobes poorly developed, subequal in length to or shorter than acicular lobes. Acicular lobes of anterior parapodia rounded (in notopodia sometimes slightly bilobed), rounded-conical in median region and conical in posterior chaetigers. Elongate mid-dorsal subterminal papilla absent. Subtropical-boreal shelf. Body length up to 140 mm, up to 90 chaetigers (Rainer 1991). Branchiae decreasing in size to minute knob (shorter than dorsal cirri) after C75-C85 and then completely absent. Parapodial preacicular lobes low throughout, poorly developed. In anterior and median chaetigers acicular lobes deeply bilobed, posteriorly indentation of acicular lobes becoming shallower, but may be visible up to last chaetigers. Postacicular lobes rounded, in anterior and median parapodia slightly longer or subequal in length to acicular lobes, in posterior chaetigers equal in length to or shorter than acicular lobes. Pharynx with long mid-dorsal subterminal papilla, proximal region in adults with flattened distally rounded papillae (conical in juveniles). Arcto-boreal upper shelf.