Description of Aegialoalaimus bratteni sp. n. from Skagerrak and a review of the genus (Aegialoalaimidae, Nematoda incertae sedis)

Abstract Background The genus Aegialoalaimus de Man, 1907 includes 12 nominal species, of which three species are considered valid, two species were transferred to other genera and seven species have uncertain taxonomic status. New information New species, Aegialoalaimus bratteni sp. n. was found in Skagerrak off the west coast of Sweden. It is particularly characterized by the 1.5-1.8 mm long body, short papilliform cephalic sensilla, excretory pore opening just posterior to nerve ring level, spicules that are straight in shape, supplements and gubernaculum absent, separating it from other valid species of the genus. It can be further differentiated from Aegialoalaimus elegans in having longer body (1.5-1.8 mm in A. bratteni sp. n. vs 0.8-1.3 mm in A. elegans), shape and size of spicules (straight and 22-29 µm long in A. bratteni sp. n. vs arcuate and 34 µm long in A. elegans), absence of precloacal supplements (vs seven-eight in A. elegans), absence of gubernaculum (vs present in A. elegans); from A. setosa in having shorter tail (c´=2.6-3.1 in A. bratteni sp. n. vs c´=4.2 in A. setosa), shorter cephalic sensilla (0.5-1.0 µm in A. bratteni sp. n. vs 9 µm in A. setosa), shape and size of spicules (straight and 22-29 µm long in A. bratteni sp. n. vs arcuate and 40-45 µm long in A. setosa), absence of precloacal supplements (vs eight in A. setosa), absence of gubernaculum (vs present in A. setosa); from A. leptosoma in having longer body (1.5-1.8 mm in A. bratteni sp. n. vs 0.5-0.7 mm in A. leptosoma) and other measurements, shape of spicules (straight in A. bratteni sp. n. vs arcuate in A. leptosoma), absence of precloacal supplements (vs three-five in A. leptosoma), absence of gubernaculum (vs present in A. leptosoma). Type specimens of Aegialoalaimus cylindricauda Allgén, 1933 and A. paratenuicaudatus Allgén, 1959 are redescribed and taxonomic status of these two species is re-evaluated. A taxonomic review, tabular compendium and identification key for species of the genus Aegialoalaimus are also given.


Introduction
The genus Aegialoalaimus de Man, 1907 was originally described based on single species Aegialoalaimus elegans de Man, 1907 from the North Sea (de Man 1907). Subsequently, eleven more species were described within this genus, but only three were considered valid in the most recent review of the genus by Tchesunov (1990). Aegialoalaimus brevicaudatus Kreis, 1924 was transferred to the genus Paraterschellingia, and A. amphibulbosus Gerlach, 1955 -to Setoplectus. Two species, A. setosa Bouwman, 1981 andA. leptosoma Gagarin, 2012, are described well enough to warrant status of valid species (Bouwman 1981, Gagarin 2012, while the status of remaining seven species remains questionable. In this paper a new species of Aegialoalaimus, A. bratteni sp. n., is described from the bottom sediments collected in the Bratten area (Swedish economic zone of Skagerrak) off the west coast of Sweden.

Materials and methods
Bottom sediment samples were collected in several locations in the southern part of the Skagerrak designated as Bratten area. All samples were collected with the bottom dredge and further sieved in the lab before fixation. Nematodes were extracted from samples using a decanting and sieving method (smallest mesh sizes: 45 µm or 70 µm). Fresh water was used during sieving to induce osmotic shock in nematodes so they will detach from the substrate. Samples were immediately fixed in 4% formaldehyde. Formaldehyde-preserved specimens were transferred to pure glycerine using Seinhorst (1959) rapid method as modified by De Grisse (1969). Permanent nematode mounts on glass slides were prepared using the paraffin wax ring method. All curved structures were measured along the curved median line.
Abbreviations used in the text are as follows: L = overall body length; a = body length / greatest body diameter; b = body length / pharyngeal region length; c = body length / tail length; c' = tail length / cloacal body diameter; T = length of main gonad (excluding flexures) / body length (expressed in %).

Materials
Holotype: Male. (Fig. 1) Body slender, cylindrical over most of its length, tapering anteriorly in the anterior half of pharyngeal region and posteriorly on tail; usually straight or weakly ventrally curved upon fixation; maximum body diameter is 46-49 µm. Cuticle weakly annulated externally, but with distinct transverse striation of inner layers; annules are 1 µm wide, without external ornamentation. Lateral field absent. Crystalloids absent. Body pores and epidermal glands absent. Somatic sensilla present, small papilliform in shape, most distinct along the tail. Labial region rounded, continuous with the body contour, 12.5-13.0 µm wide; lips fused. Inner labial sensilla indistinct. Outer labial sensill pore-like, their nerve endings are distinct. Cephalic sensilla small papilliform, 0.5-1.0 µm long; their bases are located 5-6 µm from the anterior body end. Subcephalic and cervical sensilla absent. Amphidial fovea circular, 10.0-10.5 µm in diameter, with distinct sclerotized margin; its anterior end is located posterior to cephalic sensilla bases, 14-16 µm from the anterior body end. Ocelli absent.
Pharyngeal region is 174-189 m long. Nerve ring surrounds the pharynx at 55.0-58.2% of its length. Hemizonid located at the level with the nerve ring. Secretory-excretory system present; renette cell located on ventral and ventrosublateral sides of the body along the posterior part of pharynx; it extends anteriorly and forms small excretory ampulla just posterior to the nerve ring level. Excretory canal absent, excretory ampulla opens to the exterior on ventral side of the body, at the level with the nerve ring, at 52.9-66.6% of the pharyngeal region length.
Oral opening terminal. Buccal cavity is weakly developed; cheilostom is a narrow tube 3-5 µm long; pharyngostom is indistinguishable from the sclerotized lumen of the pharynx. Pharynx divided into two sections: anterior tubular part and posterior (basal) bulb. Tubular part of the pharynx is 138-151 µm long and 3-4 µm wide; it is surrounded by thin sheath of tissue. Basal bulb is strongly muscularized; 30-33 µm long and 24-28 m wide. Nucleus of the dorsal pharyngeal gland is visible in the middle of the dorsal sector of the basal bulb. Subventral pharyngeal glands indistinct. Pharyngeal gland orifices indistinct. Cardia is 15-18 µm long, its posterior part is embedded in the intestine.
Reproductive system is diorchic, both anterior and posterior testes are outstretched anteriad. Spicules are 20-29 µm long, paired and symmetrical, straight in shape; with conoid shaft and small funnel-shaped manubrium. Gubernaculum absent. Supplements absent. Tail is 114-126 µm long, conoid to subcylindrical in shape, straight or weakly curved ventrad; with bluntly rounded terminus. Caudal glands present, they open to the exterior through common spinneret. Caudal gland nuclei are incaudal.
Female. Not found.

Diagnosis
Aegialoalaimus bratteni sp. n. is particularly characterized by 1.5-1.8 mm long body, short papilliform cephalic sensilla, excretory pore opening just posterior to the nerve ring level, straight spicules, supplements and gubernaculum absent.

Description
The only type specimen is preserved but its internal structures are poorly visible.

Taxon discussion
The original description is based on single female specimen (Allgén 1933: 62-63, Fig. 35). In general morphology, this specimen resembles members of the family Microlaimidae ( Fig. 2A): amphid circular in shape; stoma funnel-shaped with small dorsal tooth; pharynx is muscular along its entire length, cylindrical in its anterior part and with well developed basal bulb; female reproductive system didelphic, gonads outstretched. This species is considered species inquirenda et incerta sedis within the family Microlaimidae.

Description
The only type specimen is poorly preserved, its anterior end is dried out.

Taxon discussion
The original description is based on single female specimen (Allgén 1959: 132, Fig. 134). In general morphology, this specimen resembles members of the family Comesomatidae (Fig. 2B): amphid is multispiral; pharynx is muscular along its entire length, cylindrical in its anterior part and with weak basal swelling; female reproductive system didelphic, gonads outstretched. This species is considered species inquirenda et incerta sedis within the family Comesomatidae. a b Figure 2.
Photomicrographs of the anterior ends of type specimens of two Aegialoalaimus species described by C. Allgén (scale bar a-b = 10 µm).  Boucher & Helléuët, 1977op. Jensen, 1978 Cuticle smooth or finely annulated. Lateral alae absent. Epidermal glands absent. Somatic sensilla present. Labial region rounded, continuous with body contour. Inner labial sensilla indistinct. Outer labial sensilla pore-like or papilliform, located on outer surface of lips. Cephalic sensilla papilliform or setiform, located at base of labial region, anterior to amphid. Amphidial aperture circular. Subcephalic and cervical sensilla, deirid and ocelli absent. Secretory-excretory system present; renette cell located opposite to ventral side of pharynx, cardia or anterior part of intestine. Excretory ampulla present. Excretory duct very short, opens to exterior at level with cephalic setae in females and at level of nerve ring in males. Stoma small, undifferentiated. Pharynx divided into anterior long tubular section with strongly sclerotized lumen and weakly developed tissue, and strongly muscularized oval basal bulb. Cardia cylindrical or conoid, partly enveloped by intestinal tissue in its posterior part. Female reproductive system didelphic, amphidelphic, ovary branches reflexed antidromously. Spermatheca present, axial. Vulva equatorial, transverse. Vagina thick; pars refringens vaginae absent. Male reproductive system diorchic, both testes outstretched. Spicules symmetrical, straight or arcuate; gubernaculum present or absent. Copulatory apparatus composed of a row of midventral precloacal sensilla (absent in one species). Postcloacal sensilla absent. Tail similar between sexes, conoid or subcylindrical with rounded terminus. Three caudal glands present, open via common spinneret.

Aegialoalaimus conicaudatus Allgén, 1959
Type material of this species could not be located in the collection of C. Allgén. Description and illustrations of this species do not provide sufficient information to be able to assign this species to any nematode genus or family (Allgén 1959: 132, Fig. 135a-b). As drawn, pharynx appears to be muscularized along its entire length, cylindrical in its anterior part, with basal bulb; it can match a number of nematode families, but is dissimilar to the pharynx of Aegialoalaimus. This species is considered species inquirenda et incerta sedis within Nematoda.

Aegialoalaimus elegans sensu Bussau, 1993
Population of Aegialoalaimus elegans described by Bussau (1993) differs from the typical A. elegans in having shorter body (0.4-0.6 mm in Bussau specimens vs 0.8-1.3 in A. elegans), shorter spicules (15 µm in Bussau male vs 34 µm in A. elegans), presence of a small notch on the ventral side of spicules (vs absent in A. elegans). It is possible that specimens found and described by Bussau (1993) belong to a different, new species of Aegialoalaimus, but without examining actual specimens I refrain from naming it.

Aegialoalaimus punctatus (Allgén, 1929)
Type material of this species could not be located in the collection of C. Allgén. Aegialoalaimus punctatus (Allgén, 1929) was originally described as Kreisia punctata Allgén, 1929. Description and illustrations of this species do not provide sufficient information to be able to assign this species to any nematode genus or family (Allgén 1929: 461, Fig. 21). As drawn, pharynx appears to be muscularized along its entire length, cylindrical in its anterior part, with basal bulb; it can match a number of nematode families, but is dissimilar to the pharynx of Aegialoalaimus. There are two separate circles of setae and the amphid is multispiral -two more characters not found in the genus Aegialoalaimus. This species is considered species inquirenda et incerta sedis within Nematoda.

Aegialoalaimus sabulicola Allgén, 1933
Type material of this species could not be located in the collection of C. Allgén. Description and illustrations of this species do not provide sufficient information to be able to assign this species to any nematode genus or family (Allgén 1933: 64-65, Fig. 37). As drawn, tail appears to be conoid with well developed spinneret and is dissimilar to tail shape typical for valid species of the genus Aegialoalaimus. This species is considered species inquirenda et incerta sedis within Nematoda.

Aegialoalaimus tenuicaudatus Allgén, 1932
Type material of this species could not be located in the collection of C. Allgén. Description and illustrations of this species do not provide sufficient information to be able to assign this species to any nematode genus or family (Allgén 1932: 414-415, Fig. 5). As drawn, pharynx appears to be muscularized along its entire length, cylindrical in its anterior part, with basal bulb; it can match a number of nematode families, but is dissimilar to pharynx of Aegialoalaimus. Similarly, population described as A. tenuicaudatus by Allgén (Allgén 1933: 63-64, Fig. 36) does not match the diagnosis of Aegialoalaimus. On the other hand, both show superficial resemblance to the family Microlaimidae in the shape of the amphid, pharynx and tail. Subsequent redescription of A. tenuicaudatus by Allgén (1935) was positively identified as Molgolaimus turgifrons Lorenzen, 1972 by Jensen (1978a). Therefore, we consider this species to belong to the family Microlaimidae.

Aegialoalaimus tenuis Kreis, 1928
Aegialoalaimus tenuis Kreis, 1928 most probably should be placed in the family Microlaimidae based on the following characters: pharynx cylindrical in its anterior part, with basal bulb, muscularized along its entire length; reproductive system didelphic, gonads outstretched (Kreis 1928).