Redescription of Parophryotrocha isochaeta ( Eliason , 1962 ) and a new species of Parophryotrocha ( Polychaeta : Dorvilleidae ) from the Skagerrak and North Sea

The original description of Parophryotrocha isochaeta (Eliason, 1962; as Ophryotrocha (?) isochaeta) was based on three specimens collected in the deepest part of the Skagerrak in 1933. The species is here redescribed from new material collected at the type locality in 1990. Its most characteristic features pertain to jaw morphology and the presence of only one type of simple chaeta in all parapodia. Several structures, such as prostomial appendages, parapodia and parapodial cirri, were not depicted in the original description, but were found to be present and are here characterised. A new species, Parophryotrocha rhadina n. sp., is described from material collected from the northern North Sea. It differs from P. isochaeta by being smaller and of slimmer size, by lacking ventral cirri and by the shape of the mandibles and maxillae. The genus Parophryotrocha Hartmann-Schröder, 1971 was originally erected for O. (?) isochaeta with lack of antennae, palps, parapodia and composite chaetae as diagnostic features. The generic definition is here emended. The status of the genus is discussed with regard to related genera. The presence of prostomial and parapodial structures implies that the difference between P. isochaeta and the species currently placed in Ophryotrocha is not as great as had previously been considered.

Palabras clave: Polychaeta, Parophryotrocha, redescripción, nueva especie, Skagerrak, Mar del Norte.Sardá, G. San Martín, E. López, D. Martin and D. George (eds.)1962) from the Norwegian part of the Skagerrak, and P. gesae Hilbig and Blake, 1991 from the US Atlantic slope.Recently, a third species of Parophryotrocha was collected during environmental monitoring in the northern North Sea.

SCIENTIFIC ADVANCES IN POLYCHAETE
The original description of Parophryotrocha isochaeta (as Ophryotrocha (?) isochaeta) was based on three specimens collected in 1933 from the deepest part of the Skagerrak (Eliason, 1962).The description is brief, and several characteristics, such as prostomial appendages, parapodia and parapodial cirri, are not depicted.In 1990, numerous specimens were collected during sampling at the type locality.The new material shows that several of these structures are present.Some of the characters are important for the diagnosis of the genus.
In this study, Parophryotrocha isochaeta is redescribed and an emended diagnosis of Parophryotrocha is given.The new species from the North Sea is described and compared with the other species in the genus.

MATERIAL AND METHODS
The new material of Parophryotrocha isochaeta was collected during a reinvestigation of the epibenthic fauna in the Skagerrak reported from Swedish expeditions in 1933-37.Several stations were revisited using a modified Rothlisberg-Percy epibenthic sledge.Samples were sieved on 0.5 mm screens and preserved in 4 % buffered formaldehyde solution.Details of the sampling, the equipment used, and the results for amphipod crustaceans have been given by Miskov-Nodland et al. (1999).
The new species was collected during environmental monitoring in the vicinity of oil installations in the northern North Sea.Quantitative samples were taken using a 0.1 m 2 van Veen grab, sieved through 1 mm round-holed screens, and preserved in 4 % buffered formaldehyde solution.Specimens were rinsed in freshwater and transferred to alcohol during sorting.Details of the sampling and results have been given by Mannvik et al. (2003).
Specimens were examined in alcohol or glycerol under a stereomicroscope or a compound microscope with phase contrast.To inspect maxillary structures and mandibles, a dorsal incision was made and muscular tissue removed using a pair of fine needles.Drawings were prepared with the aid of an eyepiece measurement graticule.
The material is deposited at the Museum of Evolution, Uppsala University, Sweden (UUZM) and the Bergen Museum, Bergen, Norway (ZMUB).

SYSTEMATICS
Genus Parophryotrocha Hartmann-Schröder, 1971 Type species: Ophryotrocha (?) isochaeta Eliason, 1962.Emended diagnosis.Body cylindrical and slender, completely ciliated.Prostomium with simple antennae, with or without palps.Parapodia uniramous, with or without dorsal and ventral cirri.Chaetae all simple, gently curved with obtuse tips only, or fine capillaries and gently curved with obtuse tips.Maxillae composed of p-type or K-type forceps and 5-7 pairs of free denticles in four rows.Mandibles anteriorly flared, with or without an indented cutting edge.
Remarks.Hartmann-Schröder (1971) considered the lack of antennae, palps, parapodia and composite chaetae as diagnostic features of Parophryotrocha.Hilbig and Blake (1991) emended the diagnosis to encompass P. gesae which has antennae and distinct parapodia.The present redescription of P. isochaeta shows that palps and parapodial cirri are also present.The diagnosis is here emended in accordance with the new information on morphological features of P. isochaeta.(Eliason, 1962) (Fig. 1) Description.Complete specimens (preserved) 18-19 mm long, 0.8-0.9mm wide at chaetiger 10, with 130-135 chaetigers.Body becoming gradually slimmer posteriorly, width 0.4-0.5 mm at about chaetiger 100.Segments short, dorsally strongly convex, ventrally flat (Fig. 1A).Segments in middle and posterior body with a more or less recognisable transverse ridge, often drawn out into dorsolateral flaps or lobes.Segments ringed by ciliary bands, dorsally and laterally running on the transversal ridge.Pygidium simple, gradually tapering, without cirri.Prostomium (Fig. 1A) short, anteriorly flattened, wider than long.Antennae dorsolateral, short digitiform, weakly bi-articulate.Palps ventrolateral, simple papilliform.A pit or depression (in some specimens appearing as a retracted papilla) situated dorsally on the prostomium may be a nuchal organ.Eyes not visible.Peristomium wider than prostomium, in two achaetous and apodous sections.Mouth ventral with wide opening, often with mandibles and anterior maxillae clearly visible and partly protruded.Chaetigers with simple uniramous parapodia, of similar size throughout.Parapodia lateral in most anterior chaetigers, ventrolateral in middle and posterior body, with a low rounded prechaetal lobe and a low weakly bilobed postchaetal lobe (Fig. 1B).Dorsal cirri papilliform to short digitiform.Ventral cirri short papillate, often partly retracted and hardly visible.Parapodia supported by one strong medial acicula and one ventrally directed thin acicula with a curved distal end.All chaetae simple, slightly curved, with obtuse tips, without teeth or serrations (Fig. 1C).Chaetae grouped in supra-acicular and sub-acicular fascicles, 15-30 chaetae in each fascicle.Jaws situated in pharynx within peristomium and anterior chaetigers.Mandibles (Fig. 1D) composed of two triangular plates, anterior margin gently undulate, without defined teeth.Mandibles connected by a thin ligament.Maxillae (Fig. 1E) composed of strong forceps and anterior denticles in two partly coalesced rows on each side.Prongs of forceps fused with the most posterior denticle on each side, each prong forming a structure with a medially-directed branch with strong teeth followed by a cutting edge with 4-6 small teeth and an incurved hook.Prongs asymmetrical, medial branch of left prong situated posterior to the branch of right prong.Occasionally two most posterior denticles fused with forceps.Seven free denticles on each side, each denticle with an inward directed strong hook.Most posterior free denticle with small teeth or serration along inner margin.Sexual products (eggs) observed in middle and posterior chaetigers in some specimens.

Parophryotrocha isochaeta
Remarks.The original material consists of the holotype and two paratypes, which have been selected by Eliason according to information on the labels.The designation of types was not reported in the original description, however.The holotype is illustrated by Eliason (1962) as figure 16a.The material is in generally poor condition.Palps, antennae and parapodia are hardly discernible.The jaws are partly exposed by dissection in the holotype and one of the paratypes.The shape of the visible parts, especially the forceps, agrees with the structures in the specimens in the new material.
The new material comprises about 100 specimens, of which many are complete.The main new information pertains to the presence and shape of prostomial appendages, parapodia and parapodial cirri.The original description is confirmed with regard to chaetae, mandibles and maxillary denticles, but the forceps of the maxillae have a more complicated structure than originally illustrated.
Distribution.Only known from the deepest part of the Skagerrak, Norway.Parophryotrocha rhadina n. sp.
(Fig. 2) Description.All specimens are incomplete.Largest specimen (holotype) measures 6.0 mm for 58 chaetigers, width 0.45 mm at chaetigers 10-20.Segments short, dorsally convex, ventrally flat (Fig. 2A).Body slender, peristomium and most anterior segments containing pharyngeal structures partly swollen, posterior part slightly tapering.Segments ringed by ciliary bands.Prostomium (Fig. 2A) short, anteriorly flattened, wider than long.Antennae dorsolateral, short digitiform.Palps ventrolateral, simple papilliform, difficult to see.Eyes not visible.Peristomium wider than prostomium, in two achaetous and apodous sections.Mouth ventral with wide opening, often with mandibles partly visible.Chaetigers with simple uniramous parapodia, of similar size throughout.Parapodia lateral in most anterior chaetigers, ventrolateral in the middle and posterior body.Parapodia distally with a broadly triangular acicular lobe and a low prechaetal lobe (Fig. 2B).Dorsal cirri papilliform to short digitiform.Ventral cirri lacking.Parapodia supported by one strong medial acicula and one ventrally-directed thin acicula.All chaetae simple, distally sigmoid, slightly dilated in curved part, with obtuse knob-like tip, without teeth or serrations (Fig. 2C).Chaetae grouped in supra-acicular and sub-acicular fascicles, 12-25 chaetae in each fascicle.Jaws situated within peristomium and anterior chaetigers.Mandibles (Fig. 2D) composed of two anteriorly-flared rods, each with a marked tooth on inner border of anterior margin.Mandibles have a process on left piece fitting into a socket on right piece.Several more or less irregular hook-shaped or globular processes situated posteriorly on the rods.Maxillae (Fig. 2E) of p-type, composed of the forceps and anterior denticles in two rows on each side.Prongs of forceps with anterior hook and inner cutting edge with 6-8 teeth.Medially on prongs a raised ridge with about 6 teeth.Seven free denticles on each side, outer row denticles with a strong tooth and serrated inner margin, inner row denticles with fine teeth along inner margin.Sexual products (eggs) observed in middle and posterior chaetigers in some specimens (holotype, paratypes).
Etymology.The species name is from the Greek rhadinos, slender, slim, delicate.
Remarks.Parophryotrocha rhadina differs from P. isochaeta by being smaller and slimmer in size, by the shape and dentition of mandibles and maxillary structures, and by lacking ventral cirri.The chaetae are slimmer and more distinctly curved distally.Parophryotrocha gesae is distinguished by having simple chaetae of three types and mandibles with a serrated anterior margin (Hilbig and Blake, 1991).
Distribution.Only known from the northern North Sea.

DISCUSSION
The presence of prostomial and parapodial structures implies that the difference between Parophryotrocha and species currently placed in Ophryotrocha is less than previously had been considered.Essentially, Parophryotrocha differs from Ophryotrocha mainly in the lack of composite chaetae.Ophryotrocha, however, represents a large genus with high interspecific variability.At the present state of knowledge, it has not been possible to split the genus into more homogeneous subgroups (Hilbig and Blake, 1991;Eibye-Jacobsen and Kristensen, 1994).The generic separation among Ophryotrocha-like species in the wide sense is therefore unclear.Hilbig and Blake (1991) noted that the lack of composite chaetae in Parophryotrocha separated it from Ophryotrocha.This view was largely confirmed by Eibye-Jacobsen and Kristensen (1994) in a phylogenetic analysis of the Dorvilleidae.Using the present characters for P. isochaeta, they found that Parophryotrocha formed a subgroup together with the parasitic genus Veneriserva, which also lacks composite chaetae, in a polytomic group (clade) of Ophryotrocha and allied genera.Based on the analysis, they preferred to keep the present genera separate, a view which is also adopted here.