Rhaptothyreus typicus Hope & Murphy, 1969

Leduc, Daniel, 2014, Are juveniles of the enigmatic deep-sea nematode Rhaptothyreus (Rhaptothyreida: Rhaptothyreidae) parasitic?, European Journal of Taxonomy 94, pp. 1-12 : 3-9

Rhaptothyreus typicus Hope & Murphy, 1969

Figs 1-4 , Table 1

Material examined

NEW ZEALAND: 3 ♂♂ ( NIWA 88370; two specimens processed for SEM after observations under light microscopy), 15 Apr. 2007, NIWA cruise TAN 1004, st. 4, southern Hikurangi margin, 175.6642º E, 41.6837º S, open slope habitat, 1046 m, mean particle size (geometric) 12 µm, silt / clay content 94.7%, CaCO 3 content 2.1%, organic carbon content 1.0%. One juvenile (processed for SEM after observations under light microscopy), same data as ♂♂.

Description

Male

Body cylindrical, tapering slightly towards anterior and posterior ends. Cuticle with light striations 1–2 µm apart, confined to innermost cuticle layer ( Fig. 2B ), but also visible on SEM micrographs ( Fig. 4A, D ), 2.0– 2.5 µm thick in mid-body, thickening gradually towards head and tail regions (up to 5 µm thick) due to expansion of median layer ( Fig. 1A ). Eight longitudinal rows of short somatic setae, ~ 1 µm long, beginning from anterior edge or mid-level of amphid and extending ~2 cbd posterior to nerve ring; each seta in small pit; anteriormost setae with conspicuous ducts; rest of body devoid of somatic setae (confirmed using SEM). Chords consisting of two longitudinal rows of cells, often poorly defined, with clear round inclusions ( Figs 1E , 2C ). Longitudinal muscle fibres 2–5 µm thick, beginning at level of amphids and most conspicuous in anterior body region. Metanemes not observed.

Remnant of oral aperture pore-like or not discernible. Six inner and six outer labial papillae in pairs and in one circle (presence of paired inner and outer labial papillae difficult to observe with light microscopy), ~ 1 µm long and 5–7 µm from anterior end, each pair of setae in small pit with conspicuous duct; second circle of four submedian papilliform sensillae of similar length and structure to setae of first circle, 12–14 µm from anterior end. Amphid large, oblong, ~1.3 cbd in height and 0.5–0.6 cbd wide, with strongly cuticularized outline ( Fig. 2A ). Amphideal fovea covered by a thin layer of cuticle with central longitudinal rib tapering anteriorly; thin layer of cuticle with transverse rows of minute perforations. Amphideal canal strongly cuticularized, splitting into two ducts directly underneath amphideal fovea ( Fig. 2A ). No clear buccal cavity present; pharynx with indistinct contours, remnant of pharyngeal lumen sometimes visible as continuous, longitudinal slit extending from oral region to slightly posterior to nerve ring. Nerve ring conspicuous. Secretory-excretory system not observed.

Trophosome present, tapering anteriorly, beginning 169–183 µm from anterior end, filled with numerous small, rod-shaped structures, 0.7–1.4 × 1.2–2.3 µm. No cellular structure discernible in anterior portion of trophosome; one specimen with clear globular inclusions, possibly remnant of cell nuclei. Four or five longitudinal rows of nucleated, oval-shaped turgescent cells, 7–17 × 17–25 µm, situated dorsally and subdorsally and extending from anterior end of trophosome to slightly anterior to spicules ( Figs 1E , 2B ). At ~90% of body length from anterior, trophosome appearance changes abruptly to rows of large, strongly stained, nucleated cells with minute round inclusions, ~ 1 µm in diameter ( Fig. 3B ). Cells decrease in size slightly posteriorly and form a short duct (possibly vas deferens) anterior to spicule.

Single unpaired spicule, 1.5–1.8 abd long, strongly cuticularized, tapering distally and with sharp bend at ~1/5 of spicule length from distal tip; distal tip slightly swollen ( Fig. 2D ). Proximal part of spicule contains several small glandular cells; longitudinal structures along spicule length may be ducts connecting glands and small pores (discontinuities in cuticle) near distal tip of spicule ( Fig. 1F ). Spicule almost completely surrounded by muscular tissue (sheath); at least three pairs of muscles attached to dorsal and ventral body wall. Two small ducts, apparently joining before cloacal opening, visible in two specimens ( Fig. 2D ). Tail short, conical, with conspicuous terminal cap ( Figs 2D , 4D ). Terminal or subterminal pore present; caudal glands not observed.

Moulting juvenile

Similar to males in general appearance. Striations of innermost cuticle layer not discernible except in posterior body region; surface of cuticle with fine striations. Small granules separating outer and inner layers of head cuticle ( Fig. 3A ). A few sparse somatic setae present in pharyngeal region, each with a small pit, not in obvious rows. Chords consist of two rows of well defined oval-shaped, nucleated cells without inclusions ( Fig. 3C ).

Cephalic sensillae not observed. No amphideal aperture visible on moulting cuticle ( Fig. 4C ); amphid of similar size and shape as male amphid, located underneath moulting cuticle, barely visible, not cuticularized ( Fig. 1B ). Oral aperture minute. Small, cuticularized, stylet-like structure, 10 µm long, narrowest in middle region, situated directly underneath oral aperture; may be remnants of narrow, cuticularized buccal cavity ( Fig. 3A ). Outline of vestigial pharynx with anterior portion of lightly cuticularized pharyngeal tubes visible, with small anterior bulb surrounding stylet-like structure and with posterior bulb immediately posterior to a conspicuous nerve ring.

Intestine modified into trophosome; anterior and posterior parts of trophosome similar in structure to that of males, but with globular, not rod-shaped, structures ( Fig. 3B ). Four rows of turgescent cells present dorsally and subdorsally. Spicule similar in shape to that of male specimens, but lightly cuticularized ( Fig. 3D ). Tail short, conical, without terminal cap. Terminal pore present.

Remarks

The Hikurangi margin male specimens agree well with the description of R. typicus by Hope & Murphy (1969), although a few inconsistencies were noted. The Hikurangi margin specimens are characterised by short body length relative to the type specimens (4,484 –6,010 vs. 6,990 –12,800 µm), but within the range reported by Miljutin et al. (2006) for the same species. The longitudinal rows of turgescent cells observed in the Hikurangi specimens were not observed in the original description of the species by Hope & Murphy (1969) or the description by Miljutin et al. (2006); Riemann (1993), however, described the presence of longitudinal rows of turgescent cells in the dorsal region of the type specimens of R. typicus . In addition, the ventral transverse ridge observed on the ventral side of the tail of the type specimens by Hope & Murphy (1969) and mentioned in the description of Miljutin et al. (2006) was not present in the specimens observed in the present study; furthermore, this feature was not noted in Riemann’s (1993) description.

Figures

Fig. 1. Rhaptothyreus typicus Hope & Murphy, 1969. Line drawings. A. Lateral view of male head. B. Lateral view of moulting juvenile head, with details of surface striations on outer cuticle and outline of amphid under moulting cuticle. C. Anterior body region of male. D. Anterior body region of juvenile. E. Lateral view of male mid-body region. F. Posterior body region of male. G. Posterior body region of juvenile. Arrows show the position of the chord (c) and turgescent cells (t). Scale bar: A–B = 50 µm, C–D = 75 µm, E = 60 µm, F–G = 70 µm.

Fig. 2. Rhaptothyreus typicus Hope & Murphy, 1969. Light micrographs (♂). A. Lateral view of head showing amphid. B. Mid-body region showing cuticle, turgescent cells and portion of anterior trophosome with rod-shaped structures. C. Cells of lateral chord with clear, round inclusions. D. Posterior body region. Arrows show the position of the two small ducts apparently joining just prior to the cloacal opening. Scale bar: A, C = 20 µm, B = 18 µm, C = 30 µm.

Fig. 3. Rhaptothyreus typicus Hope & Murphy, 1969. Light micrographs (moulting juvenile). A. Lateral view of head, showing stylet-like structure. B. Sharp transition between anterior (left) and posterior trophosome (right). C. Lateral chord, mid-body region. D. Posterior body region. Scale bar: A = 20 µm, B = 40 µm, C = 28 µm, D = 25 µm.

Fig. 4. Rhaptothyreus typicus Hope & Murphy, 1969. Scanning electron micrographs. A-B. Male head. C. Juvenile head. D. Male posterior body region. Scale bar: A = 20 µm, B = 8 µm, C = 12 µm, D = 16 µm.