Hemienchytraeus tanjae, Schmelz, R. M. & Römbke, J., 2005

Schmelz, R. M. & Römbke, J., 2005, Three new species of Hemienchytraeus (Enchytraeidae, Oligochaeta) from Amazonian forest soil, Journal of Natural History 39, pp. 2967-2986 : 2976-2984

Hemienchytraeus tanjae sp. nov.

(Figure 3)

Type material

Holotype: INPA 125, stained and whole-mounted specimen, fully mature, coll. December 2000. Paratypes: INPA 126, four specimens, coll. December 1997 (four specimens); ZIM OL 14284, four specimens, coll. December 1997 (one specimen), December 2000 (three specimens). All paratypes stained and whole mounted, fully mature.

Description

Length ca 5 mm (viv) or 4-5 mm (fix), diameter 0.125 mm (viv) or ca 0.12 mm, up to 0.15 mm in XII/XIII, down to 0.1 mm in terminal segments (fix). Segment number 31-34. Segments following clitellum 2-3 x as long as wide when stretched, segments shortening towards posterior end. Two chaetae per bundle throughout, present laterally in XII also in mature specimens. Chaetae without nodulus, pointed distally, blunt proximally, only faintly sigmoid, distal curve rather slight, often inconspicuous (Figure 3C). Chaetae only slightly enlarged in posterior segments; ventrals 28 µm long and 2.5 µm thick in anterior segments, 33 µm long and 3 µm thick in terminal segments. Head pore (Figure 3A) on prostomium in form of a small transverse slit. Epidermal gland cells in three to four rows in preclitellar segments, more cells on dorsal half than on ventral half; in posterior segments one row per segment at chaetal level.

Clitellum (Figure 3D, E) girdle-shaped, i.e. fully developed ventrally, and short, i.e. extending over little more than one segment length, ending at some distance before chaetae of XIII; cellular arrangement reticulate or in indefinite rows on all sides except ventrally; hyalocytes about twice as wide (diameter ca 20 µm, viv) as granulocytes (diameter ca 12 µm, viv); difference more marked in fixed specimens. Hyalocytes largest in specimens with fully developed egg (Figure 3D). Ventrally, in a field as wide as the distance between bursal slits, only granulocytes, arranged in dense transverse rows, clitellum less elevated here.

Body wall usually 4-5 µm thick, up to 10 µm ventrally in preclitellar segments (fix), cuticle thin (<1 µm), preclitellar septa not thickened. Brain (Figure 3A, B) ca twice as long as wide (e.g. 65 µm x 35 µm, fix), deeply incised anteriorly, slightly indented posteriorly, sides parallel or slightly converging anteriad. Post-pharyngeal bulbs conspicuous (Figure 3B). Ventral nerve cord perikarya concentrated in segmental ganglia from V on, i.e. no perikarya in the region of the septa. Oesophageal appendage (Figure 3A, B): unpaired root with moderately large proximal chamber; root and primary branches of about equal length and equal canal width. Two secondary branches on each side, short, with narrow canal lumen. Diameter of appendage decreasing from unpaired root over primary to secondary branches. Pharyngeal glands (Figure 3A, B) all united dorsally in IV, V and VI, connection widest in IV, narrow in VI; primary ventral lobes in V and VI, secondary ventral lobes in V, small, attached to septum 4/5, apparently absent in one specimen. Efferent fascicles (Figure 3B) not conspicuously enlarged. Chloragocytes as a thin layer from V, filled with light brown vesicles (same colour as gut contents). Dorsal blood vessel from XIV. Inflated ventral gut epithelium inconspicuous, from 1/2 XIX-XXI, only slightly higher than epithelium in adjacent anterior and posterior segments, higher than dorsal epithelium in same segments, recognizable mainly by granular texture and less intensive staining in fixed specimens.

Nephridia (Figure 3A, F) in preclitellar segments: four pairs, from 5/6 to 8/9; anteseptale globular, with minute and numerous brownish granules at periphery; funnel orientated obliquely ventrad, with small and narrow anterior projection; postseptale elongate, ca twice (1.7-2.5 x) as long and about as high as anteseptale; length of nephridia 50-85 µm (fix), mostly subterminal rise of efferent duct (also medial and terminal); terminal vesicle small, inconspicuous. Postclitellar segments: first pair at 14/15, nephridia reduced in number in following segments, often unpaired, many segments without nephridia; nephridia in posterior body half larger than in preclitellar segments, longer (up to 140 µm, viv) than body diameter, shape as in preclitellar segments, with mostly terminal rise of efferent duct. Coelomocytes small, length ca 15 µm (viv, fix: 12-16 µm), longer than wide; colour pale, margin wavy, texture with blurred vesicles (viv); cells not numerous, often aggregations postero-dorsally in V, VI and VII and in terminal segments.

Seminal vesicle absent. Few sperm on top of sperm funnel collar, spermatozoa longer than funnel (ca 70 µm, fix), heads short. Sperm funnel (Figure 3E) small, ca twice as long as wide, less than half as long as body diameter (e.g. 53 x 22 µm, viv, 50 x 19 µm, fix), funnel body as wide as or narrower than collar, tapering distad. Vas deferens in XII, usually coiled in small, irregular, consecutive loops ventro-laterally; diameter 3 µm (fix). Male copulatory organ (Figure 3E): glandular body globular, diameter 24-28 µm (fix), bursal slit longitudinal (viv), staple-shaped or broadly U-shaped in fixed specimens, shorter than diameter of glandular body. Accessory copulatory glands absent.

Spermatheca (Figure 3A, B) inconspicuous, often difficult to see (viv, fix), extending into VI or VII, consisting of ectal duct and ampulla, the latter subdivided into ectal dilatation, connecting tube and ental reservoir. Ectal dilatation not seen in all specimens, here ectal duct continuously merging into connecting tube. Ectal duct ca one segment length long, canal and wall epithelia of about the same diameter; ectal pore 2-3 µm wide (fix), up to 4 µm in living specimens, canal lined with cuticle distally, cuticle thinning out proximad. Ectal dilatation of ampulla 10-12 µm wide (fix), connecting tube with inconspicuous canal, ental reservoir thin-walled, 13-15 µm wide (fix). Sperm present in ectal dilatation and ental reservoir; in ectal dilatation, sperm arranged side-by-side in a wisp with nuclei orientated distad and flagella extending straight into proximal tube; in ental reservoir, spermatozoa arranged in a dense and irregular coil. One mature egg at a time, extending over two to three segments when fully developed.

Remarks

The secondary pharyngeal gland lobes are quite small, and apparently absent (i.e. not seen) in one of the paratype specimens. Other characters (especially details of oesophageal appendages, nephridia, and clitellum) are without exception. Two further specimens found at the type locality, one submature, one juvenile, agree with the diagnosis of H. tanjae in the pattern of pharyngeal glands (one pair of secondary lobes in V) and in the distribution of preclitellar nephridia (four pairs, from 5/6 to 8/9). However, the secondary pharyngeal gland lobes are large and conspicuous, terminal chaetae are almost twice as large as preclitellar chaetae and the submature specimen is larger (body diameter 0.225 mm, fix) than mature specimens of H. tanjae . In view of the clearcut differences it would seem unjustified to include these specimens into H. tanjae . Their identity must remain uncertain until more and mature material has been found.

Discussion

Species comparison

Hemienchytraeus siljae , H. patricii , and H. tanjae are clearly separable from each other; Table I lists conspicuous differentiating characters. Likewise, the majority of the 18 presently described Hemienchytraeus species differ in several characters from each of the three new species. This is demonstrated in Table II, where only those differentiating traits are listed that apply to H. siljae , H. patricii , and H. tanjae alike; actually, the number of differences is higher. Only two species require a detailed discussion, H. bifurcatus Nielsen and Christensen, 1959 for its similarity with H. tanjae , and H. stephensoni (Cognetti, 1927) for its similarity with H. siljae and H. patricii .

Hemienchytraeus stephensoni and H. bifurcatus are the only species in the genus with several records after the original description. Both appear to be cosmopolitan. Hemienchytraeus bifurcatus , originally described from Denmark (Nielsen and Christensen 1959), has been recorded from Poland (Makulec 1983), France (Healy 1980), India (Dash 1983, p121), Japan (Nakamura 1984) and Florida (Healy 1996); the latter two papers include morphological notes. Hemienchytraeus stephensoni consists actually of three nominal species: (1) Enchytraeus cavicola Stephenson, 1924 from India (renamed E. stephensoni by Cognetti (1927) because cavicola was preoccupied), (2) E. myrmecophilus Černosvitov, 1930, from Argentina, and (3) E. rangoonensis Stephenson, 1931 from Burma. Černosvitov (1934), recognizing their similarity and at the same time their differences from other previously described enchytraeid species, erected a new genus for them and united them into a single species in the same paper, as Hemienchytraeus stephensoni . Further records of the species are from Lake Titicaca ( Černosvitov 1939), the isle of Madeira (Bell 1962), Brazil (Christoffersen 1979) including Amazonia (Righi 1981), Japan (Nakamura 1984), Ecuador ( Dózsa-Farkas 1989), Florida (Healy 1989, 1996), and China (Xie et al. 1999). All papers mentioned provide more or less detailed morphological notes, those of Bell (1962), Righi (1981) and Healy (1989) excepted.

Curiously, H. bifurcatus and H. stephensoni are also the only Hemienchytraeus species in the genus that have been considered as morphologically highly variable [see Černosvitov (1934, 1939) and Healy (1996) for H. stephensoni and Healy (1996) for H. bifurcatus ]. In fact, the range of variation of the species has expanded with almost each redescription and includes characters that in our material are intraspecifically constant and speciesdistinguishing. There is more variation in H. stephensoni than in H. bifurcatus , which coincides with the fact that the former has been redescribed more often. Christoffersen (1979) even distinguished four different morphological variants in Brazilian specimens. If we followed this trend, H. siljae and H. patricii would rank as just two more variants of the polymorphic H. stephensoni , and H. tanjae would be lumped into H. bifurcatus . However, Christoffersen (1979) himself, and also Dózsa-Farkas (1989), already pointed to the possibility that H. stephensoni , as presently conceived, is a group of species rather than a single polymorphic species; the same may apply to H. bifurcatus . Regarding the character distribution in our material, it seems that the morphological variability of at least H. stephensoni has been overestimated in the previous literature.

An evidently needed complete taxonomic revision of H. stephensoni and H. bifurcatus would go beyond the scope of this paper. It is not necessary here, either, our only objective being the demonstration that H. siljae and H. patricii are not junior synonyms of H. stephensoni and that H. tanjae is not a junior synonym of H. bifurcatus . This is achieved by naming taxonomic differences derived from the original descriptions and name-bearing types. Such differences, if present, confirm the validity of the three new species irrespective of the taxonomic status of H. stephensoni or H. bifurcatus . All accounts of the species since the original descriptions will be disregarded in this context. This includes H. stephensoni as conceived by Černosvitov in his revisionary paper ( Černosvitov 1934). Instead, we turn to the original descriptions and types of Enchytraeus cavicola / stephensoni , E. myrmecophilus , and E. rangoonensis . Again, we neither confirm nor reject the synonymies established by Černosvitov (1934).

Hemienchytraeus bifurcatus and H. tanjae differ in (1) size and (2) texture of the coelomocytes; they are described for H. bifurcatus as being ‘‘… slightly smaller than the length of the setae; they contain small, refractile granules’’ (Nielsen and Christensen 1959, p 45, italics added). In H. tanjae , the largest coelomocytes are distinctly smaller than the chaetae (15 µm versus 28-33 µm), and the texture is without refractile granules. Both species differ also in (3) the shape of the brain, being about as wide as long in H. bifurcatus (Nielsen and Christensen 1959, p 140, Figure 27), and in (4) the nephridia; they possess large terminal vesicles in H. bifurcatus (Nielsen and Christensen 1959, p 139; Figures 24, 25). The latter two characters are not dealt with in the text of the description. All four characters are species-constant in our material; the brain is twice as long as wide even in strongly contracted specimens of H. tanjae . [Presence or absence of nephridial terminal vesicles is indeed a good taxonomic character and not a transient adaptation to local edaphic factors or an effect of extraction conditions, as one might possibly assume. In a forthcoming paper, we will describe another new Hemienchytraeus species from South America with constantly large nephridial terminal vesicles. This character is species-constant also in Achaeta (U. Graefe, personal communication)]. Two further differences between H. tanjae and H. bifurcatus can be extracted from the original description of the latter, although they are not fully reliable: (5) Hemienchytraeus bifurcatus is twice as long (10 mm) as H. tanjae ; however, body dimensions in Nielsen and Christensen (1959) are often exaggerated (Schmelz 2003, p 24). (6) The clitellum is ‘‘ strongly elevated’’ (Nielsen and Christensen 1959, p45), but no precise dimensions are given.

The apparent similarity of H. tanjae and H. bifurcatus is probably due to the poor and imprecise original description of the latter. Unknown, for example, are the following characters: (1) size of terminal chaetae; (2) clitellum ventrally; (3) number and distribution of secondary pharyngeal gland lobes; (4) number and distribution of preclitellar nephridia; (5) relative size of nephridia in terminal segments; (6) shape and size of the male copulatory organs. Each of these characters is, from our experience, crucial for a correct identification of Hemienchytraeus species; with respect to the lack of information regarding these characters, H. bifurcatus should rather be considered as species inquirenda. Type material of H. bifurcatus is lost (B. Christensen, personal communication). The type locality - a moist meadow in the grounds of the Mols Laboratory, Denmark - was resampled twice by one of us (R. M. Schmelz, in November 1998 and June 2000), but the species was not found.

A literature comparison of H. stephensoni with H. siljae and H. patricii is even more difficult. All original descriptions (i.e. those of E. cavicola / stephensoni , E. myrmecophilus , and E. rangoonensis ) lack many details that we consider as taxonomically important. Hemienchytraeus patricii is actually indistinguishable from E. cavicola / stephensoni , and H. siljae differs only in body size and segment number. Fortunately, type material of all three nominal species ( E. cavicola / stephensoni , E. myrmecophilus , and E. rangoonensis ) is extant and available. A complete description of this material together with a critical discussion of the taxonomic history of H. stephensoni and its degree of morphological variation will be provided elsewhere. Here we only highlight some characters that the name-bearing types of all three nominal species have in common, in order to facilitate the taxonomic discussion.

The investigated syntype specimens of E. cavicola / stephensoni , E. myrmecophilus , and E. rangoonensis share the following characters: (1) clitellum girdle-shaped, ventrally complete; (2) clitellum strongly developed, cells 2-4 x as high as wide (30-45 µm high), more granulocytes than hyalocytes; (3) two pairs of secondary ventral pharyngeal gland lobes, in V and VI; (4) four pairs of preclitellar nephridia, from 6/7-9/10; (5) several preclitellar septa strongly thickened. These characters are not dealt with in the original descriptions, except characters (2) and (5) in the account of E. myrmecophilus . A further common character can be derived from the original descriptions (Stephenson 1924; Černosvitov 1930; Stephenson 1931): (6) body length below 1cm, segment number below 40. These six characters are sufficient to establish H. siljae and H. patricii as species of their own: H. siljae differs in characters (1), (2), (3), (5), and (6); H. patricii differs in characters (1), (2), (4), and (5).

Concluding remarks

The descriptions of H. siljae , H. patricii , and H. tanjae have been carried out to the greatest possible detail, and they combine observations on living and preserved material. It has been demonstrated in other enchytraeid genera, especially in Fridericia , that intraspecific character variability had been overestimated in previous studies (Rota 1995; Schmelz 2003). Higher intraspecific constancy of characters allows and necessitates the elaboration of species descriptions in much finer detail; furthermore, the combined investigation of living and preserved material has proved to be advantageous if not necessary for unequivocal species delineations (Rota and Healy 1999; Schmelz 2003). These findings established in Fridericia are, from our experience with the Amazonian material, directly applicable to Hemienchytraeus . Character deviations that occur in single specimens must nonetheless be assessed carefully in order to distinguish between intraspecific variants and interspecific differences. For example, deviations in the shape of the pharyngeal glands were observed in some specimens of all three species; their peculiarity suggested some sort of malformation, partly induced by parasites. In two specimens initially identified as H. tanjae , however, a deviation in the size of the secondary pharyngeal gland lobes coincided with two other character differences (large size differences between anterior and posterior chaetae and a generally larger body), suggesting a different species rather than intraspecific variation.

The three new species described here are distinguishable from each other in at least 16 different traits. Most notable are: (1) size difference of chaetae in terminal and preclitellar segments; (2) branching pattern of the oesophageal appendage; (3) number and location of secondary pharyngeal gland lobes; (4) number and location of preclitellar nephridia; (5) distribution pattern of clitellar gland cells, especially on the ventral side. It should be noted that the first four characters (and seven of the 16 characters listed in Table I) acquire their species-specific idiosyncrasy already in juvenile specimens, hatchlings perhaps excepted. This is good news for ecological studies. Using these non-sexual characters, and provided that the species composition of a given site is more or less known, it should be possible to identify correctly a large number of juveniles to species level, in addition to the sexually mature specimens, which usually make up only a small percentage of all individuals.

On the other hand, only one of the above-mentioned characters (no.2) has been dealt with accurately in the majority of previous species descriptions; all other characters were often considered as intraspecifically variable, or they were not dealt with at all, especially in the older literature. Consequently, a much lower number of characters was available here for comparisons with previously described species. The differences are slight in the case of H. tanjae and H. bifurcatus . However, we question the validity of the latter, because type material that could help to complement the poorly detailed original account is lost. Regarding H. siljae and H. patricii , the reinvestigation of the types of H. stephensoni was necessary in order to establish species-separating differences. The situation encountered here with H. bifurcatus and H. stephensoni is not uncommon in Enchytraeidae taxonomy, where the still largely unrevised older literature is almost as great an obstacle for an accurate species identification as is the animals’ difficult anatomy itself. We hope that this contribution, together with forthcoming revisions, will facilitate more extensive taxonomic research in the tropics on this important group of soil animals.