Hubrechtella ijimai ( Takakura, 1922 ) Kajihara, 2006

Kajihara, Hiroshi, 2006, Four palaeonemerteans (Nemertea: Anopla) from a tidal flat in middle Honshu, Japan, Zootaxa 1163, pp. 1-47 : 28-37

Hubrechtella ijimai ( Takakura, 1922) comb. nov. ( Figs 15–19 )

Coeia ijimai Takakura, 1922

Diagnosis

Hubrechtella with a long ‘tail’; body­wall musculature without zigzag fibres; probos­ cis musculature with inner circular, middle longitudinal, and outer circular layers, with single muscle cross; proboscis epithelium with spherical bodies; mid­dorsal blood vessel penetrating into rhynchocoel; foregut lacunar network present.

Material examined

ZIHU­3122, female, 31 July 2003, HK coll., 55 slides, series of 6­µm transverse sections of a fragment of body containing cephalic tip; ZIHU­3124, female, 1 August 2003, HK coll., 38 + 52 slides, series of 6­µm transverse sections of a complete specimen except middle portion; ZIHU­3126, female, 1 August 2003, HK coll., 39 slides, 8­µm serial transverse sections of a body fragment containing the caudal end.

External features

The body is 3–5 cm long, about 0.7 mm wide, translucent white in colour ( Fig. 15A ). The head is much more transparent than the following portion, wider than the neck; the rhynchodaeum seen through the epidermis is white, cone shaped, tapering anteriorly. When put in a Petri dish with seawater, the animal showed searching behaviour, frequently swinging the head from side to side. The ovaries are tinged with a greyish colour. There is a transparent ‘tail’ region that contains neither intestine nor gonads; it tapers posteriorly to end in a pointed tip ( Fig. 15B ). The tail is at least 10 times as long as the diameter of its most anterior portion.

Body wall, musculature and parenchyma

The ciliated epidermis ( Fig. 16 ) is up to 30–40 µm thick in the brain region, 45–60 µm thick in the foregut region, reduced posteriorly to 10–15 µm thick in the intestinal region. Type 1 cells are confined to the anterior portion of the foregut­intestine transitional zone ( Fig. 15C ); type 2 cells predominate throughout the body ( Fig. 15C ); type 3 cells are distributed post­cerebrally ( Fig. 15C ); type 4 cells not found; type 5 cells are thinner than type 2 cells, distributed ventrolaterally in the intestinal region ( Fig. 15D ). E (b) = 0.07 (ZIHU­3122), 0.11 (ZIHU­3124); E (i) = 0.04 (ZIHU­3122; the epidermis in the intestinal region was largely sloughed off during fixation or histological preparation in ZIHU­3124 and 3126).

The basement membrane is best developed in the brain region, where it reaches a thickness of up to 4 µm thick. At the tip of the head, a thin basement membrane lies between the neuro­glandular layer and the body­wall circular muscle layer ( Fig. 17A ). This basement membrane becomes very inconspicuous in front of the proboscis insertion, while another basement membrane appears between the epidermis and the neuro­glandular layer ( Fig. 17B ). Post­cerebrally, this basement membrane continues between the epidermis and the neuro­fibrous layer ( Fig. 17C ); it becomes inverted with the latter in the anterior intestinal region, where the body wall is composed, from the surface inwards, of the epidermis, the neuro­fibrous layer, the basement membrane, and the body­wall musculature ( Fig. 17D ). In the pre­cerebral region, processes extend from inner to outer basement membranes through the neuro­glandular layer ( Fig. 17E ); these do not reach the epidermis. A mesh­like structure was not found.

The body­wall musculature consists of an outer circular layer and an inner longitudi­ nal layer, which in the foregut region attain a thickness of 7–15 µm and 12–50 µm, respec­ tively. Zigzag fibres are absent. The diagonal muscle layer is present but not obvious ( Fig. 17C). Dorsoventral muscles were not found. Radial muscle fibres connecting the body­ wall longitudinal muscle layer and the buccal/foregut wall run through the lumina of the lateral blood lacunae. The foregut and its junction with the intestine are surrounded by a longitudinal muscle layer that is only one or two fibres thick. This splanchnic longitudinal muscle layer is separated from the body­wall longitudinal muscle layer by a thin membrane of connective tissue; the lateral blood lacunae and lateral blood vessels lie outside this splanchnic layer. Posteriorly in the intestinal region, the splanchnic layer disappears except dorsally between the intestine and rhynchocoel, where it remains as a longitudinal muscle plate, terminating anterior to the end of the rhynchocoel.

Parenchymatous connective tissue is inconspicuous throughout the body, except as thin membranes surrounding various organs.

Proboscis apparatus

The rhynchodaeal epithelium is unciliated and shows no significant regional differences in thickness throughout its length; it is generally thinner dorsally and ventrally (20– 30 µm thick) than laterally (35–60 µm thick); acidophilic glandular cells predominate, but in one specimen basophilic glandular cells also occur ( Fig. 17A ). The rhynchodaeum is innervated ventrolaterally on each side by 3–5 nerves from the sub­epidermal neuro­glandular layer; the rhynchodaeal nervous layer is located basal to the glandular epithelium. A definite rhynchodaeal sphincter was not found. A rhynchodaeal caecum is absent; in one specimen, however, a lateral bulge was found on one side, probably caused by contraction during fixation.

The rhynchocoel does not extend to the posterior end of the body. Its wall is composed of separate outer circular and inner longitudinal muscle layers. Posteriorly, it is not developed into a muscular sac. No rhynchocoel caecum was found.

The proboscis insertion is located pre­cerebrally. Four regions can be discerned in the proboscis. The anteriormost region, in retracted position, comprises about 5% of the length of the organ including the proboscis retractor muscle, and is composed of an outer glandular layer, a neural layer, an inner longitudinal muscle layer, an outer circular muscle layer, and an endothelium ( Fig. 18A ). The second portion, about 22% of the length of the organ, has three muscle layers, including an additional outer circular muscle layer between the neural layer and the longitudinal muscle layer ( Fig. 18B ). This region is not radially symmetrical, but has a single muscle cross between the inner and outer circular muscle layers ( Fig. 18C ), although the muscle cross is not always obvious throughout this region. When the muscle cross is viewed in a 12­o’clock position, there are additional glandular masses at 4 and 8 o’clock between the outer glandular layer and the neural layer; these glandular masses contain conspicuous acidophilic spherical bodies up to 10 µm in diameter ( Fig. 18B ). The third region, about 27% of the length of the organ, is still composed of three muscle layers with a single muscle cross, but lacks the glandular masses containing spherical bodies ( Fig. 18D ). The last region, about 34% of the length of the organ, has almost the same construction as the first region, but has minute acidophilic spherules near the surface of the epithelium ( Fig. 18E ). It is followed by the proboscis retractor muscle ( Fig. 19A ), 12% of the length of the organ, composed entirely of longitudinal muscle fibres; the proboscis retractor muscle posteriorly attaches to the ventral wall of the rhynchocoel.

Alimentary canal

The mouth opens just behind the cerebral sensory organs. The buccal/foregut wall is densely ciliated, with both acido­ and basophilic glandular cells; the foregut is about 3 mm long. The intestinal wall contains mainly acidophilic cells, but a small number of basophils cells are also found. The intestine has no lateral diverticula, which, however, appear to be present regionally because the gonads are deeply embedded in the intestinal wall.

Blood system

A pair of lateral cephalic lacunae meets anteriorly above the rhynchodaeal opening. After passing through the cerebral ring, they connect ventrally to form a cross­sectionally U­shaped vessel; a thick­walled mid­dorsal vessel is dorsally branched off from the upper surface of the ventral portion of the U­shaped lacuna to enter the rhynchocoel ( Fig. 19B ). The U­shaped lacuna, with its two arms surrounding the cerebral sensory organs ( Fig. 19B ), then divides into two lateral lacunae just anterior to the mouth with the bottom becoming closed between the buccal wall and rhynchocoel. In the foregut region, the thinwalled lacunae lie lateral to the rhynchocoel and above the lateral edges of the foregut, and give rise ventrally to a delicate vascular network that surrounds the foregut laterally, ventrolaterally, and ventrally. In the anterior intestinal region, the vascular network converges posteriorly with a thick­walled lateral blood vessel on each side of the body, while the thin­walled lateral lacunae remain for a short distance, flanked by the rhynchocoel, before ending blindly. The lateral vessels lie ventrolateral to the alimentary canal between the splanchnic and body­wall longitudinal muscle layers. Initially each lateral vessel, together with the bordering splanchnic muscle layer, is distinctly embedded in the gut wall ( Fig. 16 ). Posteriorly, even after the splanchnic muscle layer disappears, the lateral vessels remain in close contact with the intestine.

The mid­dorsal vessel runs inside the rhynchocoel to form a rhynchocoelic villus, then runs down between the rhynchocoel wall and the alimentary canal in the posterior foregut region. Farther backward in the intestinal region, the intestinal wall around the mid­dorsal vessel contains basophilic cells ( Fig. 19A ).

Pseudometameric transverse connections between the lateral and mid­dorsal vessels in the intestinal region were not found.

Nervous system

The brain and lateral nerve cords are situated between the epidermal basement membrane and the body­wall circular muscle layer. A single dorsal commissure, 22–28 µm thick, lies anteriad to the ventral one, 28–30 µm thick. Dorsal and ventral ganglia are almost the same size. Medially each lateral nerve contains a single giant fibre ( Fig. 15D ), about 2 µm in diameter, that can be traced forward to a neural cell body, about 5 µm across, located dorsolaterally in the dorsal commissure. The dorsal ganglion slightly forks posteriorly into upper and lower branches, the latter of which innervates the cerebral sensory organ. The upper mid­dorsal nerve originates in the dorsal commissure and extends posteriorly between the epidermal basement membrane and the body­wall outer circular muscle layer ( Fig. 17C ); farther back, in the anterior portion of the intestine, the mid­dorsal nerve rises to lie between the epidermis and the basement membrane ( Fig. 17D ); this nerve becomes indistinguishable posterior to the rhynchocoel. The mid­dorsal nerve sends numerous branches downwards to the dorsal side of the rhynchocoel wall; thus a lower mid­dorsal nerve seems to be present, but it is not continuous anteroposteriorly.

Frontal organ and cephalic glands A frontal organ and cephalic glands are lacking.

Sense organs On each side of the head there is an epidermal indentation 60 µm long in the antero­ posterior axis and 80–100 µm long in the dorsoventral axis, lined with very long (about 20 µm) cilia, but no glandular cells. From each indentation, a ciliated canal leads posteroobliquely inward, narrowing from 40–45 µm to about 30 µm in external diameter (10 µm internal diameter), before turning medioventrally to enter a cerebral sensory organ on its dorsolateral surface 1/3 of the way from its anterior end; the canal runs posteriorly inside the organ without branching for the posterior 5/6 of the length of the organ, then terminates in a blind end. A bundle of nerve fibres, innervated from the ventral branch of the posterior end of the dorsal ganglion, runs along the medial side of the canal. Each cerebral sensory organ is an ovoid mass of neuro­glandular cells, oval in cross section, about 70–80 µm wide by 90–100 µm high, and about 120–150 µm long; it lies in the cephalic blood lacuna ( Fig. 19B ).

There are neither eyes nor lateral sensory organs.

Excretory system Not found.

Reproductive system

All the three specimens examined were mature females. The ovaries, up to 170 µm in diameter, each containing a single egg, are embedded in the intestinal wall ( Fig. 19A ). A gonoduct leads from each ovary, passing above the lateral nerve cord, then opens dorsolaterally in the epidermis.

Systematic remarks

Hubrechtella (= Coeia ) ijimai ( Takakura, 1922) comb. nov. has not been redescribed since its original description ( Takakura 1922), which provides an account mostly of the features of internal morphology used in the modern diagnosis for the genus Hubrechtella , but not of the characters currently used in distinguishing between species placed in this genus. Unfortunately, the type material of Coeia ijimai Takakura, 1922 is considered to be lost ( Kajihara 2004); thus, it is impossible to identify this species using its internal morphology. Accordingly, I identified my specimens as conspecific with the nominal species Coeia ijimai Takakura, 1922 on the basis only of external characters. However, the resemblance of the shape of the head in living material to Takakura’s (1922) illustration, and the nearness of my sampling site to the type locality, lend support to the identification of my material as Coeia ijimai Takakura, 1922 .

One difference between Takakura’s description and my material is the size of the body; Takakura’s specimens measured over 20 cm in length, while mine do not exceed 5 cm. This difference may be due to the age of the worms. Eventually a neotype must be designated for Coeia ijimai Takakura, 1922 ; however, this will best be done after thorough study of newly collected material from the type locality, or at least closer to it.

The combination of the characters summarised in Table 3 enables Hubrechtella ijimai ( Takakura, 1922) comb. nov. to be distinguished from all other species currently placed into Hubrechtella .

Characters and character states:

A: Muscle layers in proboscis: (0) two; (1) three.

B: Bodies in proboscis epithelium: (0) lacking; (1) nematocyst­like, rhabditous; (2) spherical.

C: Mid­dorsal blood vessel: (0) does not penetrate the rhynchocoel; (1) penetrates rhynchocoel.

D: Zigzag fibres in body­wall musculature: (0) absent; (1) present.

Figures

FIGURE 15. Hubrechtella ijimai (Takakura, 1922) comb. nov. A, photograph of a body fragment containing the head, taken in life (ZIHU­3122, female). B, photograph of a body fragment containing the tail, taken in life (ZIHU­3123, female). C, transverse section through the anterior intestinal region showing some of the epidermal glandular cell types (ZIHU­3122, female). D, transverse section showing the lateral nerve cord; arrowhead indicates the giant fibre (ZIHU­3122, female). BM, basement membrane; EP, epidermis; LN, lateral nerve cord; NF, neuro­fibrous layer; OC, body­wall outer circular muscle layer; T1, type 1 glandular cell; T2, type 2 glandular cell; T3, type 3 glandular cell; T5, type 5 glandular cell.

FIGURE 16. Hubrechtella ijimai (Takakura, 1922) comb. nov., ZIHU­3122, female, drawing of a transverse section through the anterior intestinal region showing the arrangement of internal organs. BM, basement membrane; DV, mid­dorsal vessel; EP, epidermis; IN, intestine; LL, lateral blood lacuna; LM, body­wall longitudinal muscle layer; LN, lateral nerve cord; LV, lateral blood vessel; NF, neuro­fibrous layer; OC, body­wall outer circular muscle layer; PR, proboscis; UD, upper mid­dorsal nerve.

FIGURE 17. Hubrechtella ijimai (Takakura, 1922) comb. nov., ZIHU­3122, female. A, transverse section through the pre­cerebral region; arrowhead indicates innervation from the subepidermal neuroglandular layer to the rhynchodaeum (RD). B, transverse section near the proboscis insertion, showing the inner (IB) and outer (OB) basement membranes, the former is about to disappear, while the latter become thicker, posteriorly. C, transverse section through the foregut region showing the arrangement of the neuro­fibrous layer (NF) against the basement membrane (BM). D, transverse section through the intestinal region showing the arrangement of the neuro­fibrous layer (NF) against the basement membrane (BM). E, transverse section through the pre­cerebral region; arrowhead indicates a connective tissue process from the inner to the outer basement membranes. AG, acidophilic glandular cell; BG, basophilic glandular cell; CL, cephalic blood lacuna; DM, diagonal muscle layer; EP, epidermis; RC, rhynchocoel; UD, upper mid­dorsal nerve.

FIGURE 18. Hubrechtella ijimai (Takakura, 1922) comb. nov., ZIHU­3122, female. A, transverse section through the first region of the proboscis. B, transverse section through the second region of the proboscis; arrowheads indicate spherical bodies. C, enlargement of the muscle cross in B. D, transverse section through the third region of the proboscis. E, transverse section through the fourth region of the proboscis; arrowheads indicate acidophilic spherules. MC, muscle cross in proboscis; PE, proboscis epithelium; PI, proboscis inner circular muscle layer; PL, proboscis longitudinal muscle layer; PN, nervous layer in proboscis; PO, proboscis outer circular muscle layer.

FIGURE 19. Hubrechtella ijimai (Takakura, 1922) comb. nov., ZIHU­3122, female. A, transverse section through the intestinal region; arrowheads indicate the basophilic cells in the intestinal wall around the mid­dorsal vessel. B, transverse section through the cerebral sensory organs (CO). CL, cephalic blood lacuna; DV, mid­dorsal vessel; IN, intestine; LV, lateral blood vessel; OV, ovum; PR, proboscis; RC, rhynchocoel; RM, proboscis retractor muscle; VG, ventral ganglion.

Tables

TABLE 3. Comparison of four characters among Hubrechtella species. Data compiled from Bergendal (1902b), Hylbom (1957), Kirsteuer (1967), Senz (1992, 1993, 2000), Gibson (1979a, b, 1997), Gibson and Sundberg (1999), and Chernyshev (2003).

Taxa	A	B	C	D
H. alba Gibson,1997	0	0	1	1
H. atypica Senz,1992	0	2	1	0
H. combinata Senz, 1993	?	1	1	0
H. ehrenbergi Senz,2000	0	1	1	0
H. dubia Bergendal,1902	0	1	1	1
H. globocystica Senz,1993	1	2	0	0
H. indica Kirsteuer,1967	0	1	0	0
H. juliae Chernyshev,2003	0	1	1	1
H. malabarensis Gibson,1979	0	1	1	1
H. queenslandica Gibson,1979	0	1	0	0
H. sarodravayensis Kirsteuer,1967	0	0	1	0
H. sinimarina Gibson&Sundberg,1999	0	0	0	1
H. ijimai (Takakura, 1922) comb. nov.	1	2	1	0
H. kimuraorum sp. nov.	1	2	1	0