Epsilonema steineri Chitwood 1935

Epsilonema steineri Chitwood, 1935

Figs 7–10, Table 4 (morphometrics).

Material. Ten males and ten females.

Locality. White Sea (Northern Russia), Karelian Coast of the Kandalaksha Bay, vicinity of the White Sea Biological Station (66°33'N, 33°06'E), Velikaja Salma Strait between the Veliky Island and Kindo Peninsula, depth 5 m, red algae. 18–27 August 2013.

Description. Body ε-shaped and narrowest in the region of the ventral bend between pharynx and anterior gonad. Male body widest in pharyngeal region. Female body widest in region of reproductive system.

Head rostrum (cephalic capsule) small, conoid, formed by smooth, uniformly thick cuticle (Fig. 10 A, B). Somatic cuticle posterior to rostrum distinctly annulated. Annules decrease in width to mid-body and then increase again: there three annules in 10 µm on anterior body just posterior to rostrum, seven annules in 10 µm on the region of ventral bend, and four annules in 10 µm on posterior half of the body. Cuticle of a male composed of 157 annules. Annules of the pharyngeal region ornamented with one row of tiny oval vacuoles. Very tiny spines, visible distinctly only with SEM, located on the dorsal side of the body and directed backwards (Fig. 10 E). Spines located predominantly on the anterior body, from body bend to anal region; spines on dorsal side of posterior half of the body 2–2.5 times longer than those on ventral side (3 Μ m and 1 Μ m, respectively).

Labial region composed of six lips usually inverted into the head in fixed specimens (Fig. 10 A). Six inner labial papillae located on outer side of the lips. Six outer labial as short setae on anterior part of the rostrum. Four cephalic setae (3.6–8.0 Μ m long in males, 3.7–8.3 Μ m long in females) situated at level of the amphideal fovea. Amphideal foveas spirally coiled in 1.5 turns (Fig. 10 B); their width 0.4–1.7 Μ m in males and 0.8–1.4 Μ m in females; the amphideal foveas shifted a bit to dorso-lateral position. No evident difference between foveas of males and females.

Thirty to 50 thin hair-like somatic setae on either body side. Somatic setae mostly 9–11 Μ m in length, but quite a few single setae may be longer (17–19 Μ m). The ambulatory setae thick, slightly bent in the middle, their tips hooked. Ambulatory setae arranged in four rows on mid-body; all about equal in size or posterior setae slightly shorter. All ambulatory setae located anterior to the anus. Latero-ventral rows a bit longer (their ends located more posteriorly) than subventral rows.

Stoma small and unarmed. Pharynx short, muscular; terminal bulb strongly developed, oval to rounded, with thickened internal cuticle.

Male reproductive system consists of an anterior outstretched testis extending to the ventral bend. Spicules strongly curved, thick; proximal ends distinctly knobbed, distal ends pointed. Gubernaculum a small bar at distal end of spicules. Small and sometimes nearly imperceptible copulatory thorns present as local ventral projections of cuticular annules just posterior to the rows of ambulatory setae. Position of the thorns varies from 5–7th to 10– 13 th, more often from 6th–10th and from 7th–11th. Four copulatory thorns (Fig. 9, 10 C).

Female reproductive system didelphic-amphidelphic; ovaries antidromously reflexed and located ventrally or left ventro-laterally of the intestine. Vulva not bulging, sometimes indistinct, situated in the middle of the ambulatory setae rows (five latero-ventral and seven subventral setae anterior and five latero-ventral and four subventral setae posterior to vulva on either body side).

Tail short, conical, with up to 10–11 somatic setae. Caudal glands terminating with three separate outlets on tail tip.

Diagnosis. Epsilonema. Body length of adults 276–454 Μ m. Body curved epsilon-like, with narrowest region between pharynx and anterior gonad levels, just anterior to the ventral bend. Body cuticle composed of nearly 150– 160 annules. Annules of the pharyngeal region ornamented with one row of tiny oval vacuoles. Very minute spines discerned only with SEM, located on annules predominantly on mid- to posterior part of body. Amphideal fovea coiled in ~1.5 turns, 0.4–1.7 Μ m wide, situated at base of the conical rostrum. Ambulatory setae arranged in four rows between ventral bend and anal opening; eight to 16 setae in latero-ventral rows and nine to 13 in subventral rows. Spicules 26–51 Μ m (arch). About four copulatory thorns present on ventral body at or just posterior to posterior end of rows of ambulatory setae.

Discussion. The first species of Epsilonema was found at Salerno (Italy, Tyrrhenian Sea) on littoral algae and described by Metschnikoff (1867) under the name Rhabdogaster cygnoides. The description was based on a single female and lacks significant details used nowadays for morphological discrimination of species. However, the species described by Metschnikoff fits the genus Epsilonema well in its current understanding (Gourbault & Decraemer 1996; Tchesunov 2014). In fact, the original diagnosis of Epsilonema cygnoides by Metschnikoff (1867) consists of generic but not species characters; hence this species would not be recognized today if found away from its type locality. The species was recorded later in papers on collections from the North Sea, Canary Islands and Mediterranean without illustrations or remarks on morphology. Schepotieff (1908) found Rhabdogaster cygnoides abundant on hard substrates in Bergen (Norway), Neapel, Rovigno and Brindisi (Mediterranean), from shallow to depth 250 m, and gave a description in more detail and even with cross-sections. Unfortunately, Schepotieff did not indicate where specifically the figured and described specimens had been sampled. Since the vulva may be situated in different females within or posterior to the row of ambulatory setae as depicted on Figs 3 and 6 and noted in the text, it is possible the author dealt with a mixture of species. Having the pharynx swollen and forming two distinct consecutive bulbs anterior to the nerve ring is a very unlikely feature, not known among epsilonematid nematodes.

Steiner (1916) described and depicted a female Rhabdogaster cygnoides from the Barents Sea. Our White Sea female specimens fit this description well, although it lacks some fine but important characters currently used for species diagnostics. Later, Steiner (1931) considered the Barents Sea specimen to belong to a separate species. Chitwood (1935) named the Barents Sea species “ Epsilonema steineri ” but gave no grounds for doing so. Later, the species Epsilonema steineri was transferred to the genus Epsilonematina Johnston, 1938 (see Gerlach & Riemann 1973). However, Lorenzen (1973) found no significant differences between descriptions of Metschnikoff and Steiner, and therefore accepted steineri as a junior synonym of cygnoides; he also rejected validity of Epsilonematina. Both synonymies were accepted in the review of Epsilonematidae by Gourbault & Decraemer (1996). Here, we identify the White Sea species as Epsilonema steineri Chitwood 1935 based on two considerations: (1) no differences between our specimens and the description of Steiner (1916); (2) the White Sea locality is close to that of the Barents Sea. This is a preliminary decision; it may be verified or falsified by morphological re-descriptions and molecular genetic studies of Epsilonema cygnoides and E. steineri from their type localities. We also qualify Epsilonema cygnoides (Metschnikoff, 1867) as species inquirenda until it can be thoroughly re-described based on specimens from the type locality, i.e. the vicinity of Salerno, Tyrrhenian Sea.