Achaeta becki, Schmelz. R. M. & Collado, R., 2005

Schmelz. R. M. & Collado, R., 2005, Achaeta becki sp. nov. (Oligochaeta: Enchytraeidae) from Amazonian forest soils, Zootaxa 1084, pp. 49-57 : 50-56

Achaeta becki sp. nov.

(Figures 1, 2, Table 1)

Holotype. INPA 127 , stained and whole-mounted specimen, sexually mature with clitellum. Brazil, Amazonia, Manaus, 02° 53' 47"S, 59° 59' 45"W, at km 29 on the road from Manaus to Itacoatiara, close to the agroflorestal research station EMBRAPA-CPAA, primary rain forest , terra firme, area flat without elevations, 44-50 m a.s.l., soil type xanthix Ferrasol (FAO typology), pH ca. 4.0. Soil samples collected by J. Römbke and co-workers July 1997, animals extracted from soil by R.M. Schmelz in July 1997.

Paratypes. Five specimens, stained and whole-mounted, sexually submature without fully developed clitellum, from type locality. INPA 128 : two specimens. ZIM OL 14516 : three specimens. All collection data as in holotype.

Etymology. The species is named after Ludwig Beck, former head of the Zoological Department of the "Staatliches Museum für Naturkunde Karlsruhe" (State Museum of Natural History, Karlsruhe), in gratitude for his contributions to Amazonian soil animal ecology, as researcher, promoter, coordinator, mentor, and disseminator.

Description.

Living specimens ca. 6 mm long and 0.2 mm wide. Holotype (preserved) 5 mm long, diameter 0.17 mm in V, 0.23 mm in XII; from XIV on gradually tapering towards rear end, from 0.2 mm in XIV to 0.12 mm. Paratypes smaller. Segment number 34-37.

Body wall (Fig. 1A,D) with thick cuticle, well-developed longitudinal muscle layer, and thin layer of epidermis and ring muscles in between. Dimensions in preserved material: body wall 10-14 µm, up to 25 µm thick in contracted regions; cuticle 3-7 µm, up to 12 µm in contracted regions; layer of epidermis and ring muscles ca. 2 µm; longitudinal muscle layer ca. 5 µm. Ring muscles barely distinguishable. Cuticle thinner on prostomium and in clitellar region. Prostomium short, rounded, wider than long, bent ventrad. Head pore on prostomial tip. At anterior tip of prostomium, between head pore and mouth, a spherical region with very thin epidermis. Septa of 6/7, (7/8), 10/11, 11/12, and 12/13 thin. All other septa thickened, postclitellar septa included. Segment VII (as marked by the septa) shorter than the rest.

No chaetae; no pyriform glands (=setal follicles = bottle-shaped glands). Epidermal gland cells (Fig. 1G) (= lens-shaped epithelial cells) inconspicuous, flat, outline circular, rounded-rectangular in contracted body regions. Cells in 3-4 transverse rows per segment. Segmental number and distribution pattern of cells constant with slight variations. Cells in 3-4 transverse rows per segment; one row in anterior half of a segment, the other 2 or 3 rows in posterior half. Posterior rows close to each other, anterior row isolated. Four to eight cells per row. Cell distribution pattern in bilateral-symmetrical order. More cells dorsally than ventrally. Number of ventral cells increased in foremost and hindmost segments.

Brain (Fig. 1A) about twice as long as wide (one measurement in vivo: 110: 49 µm), posteriorly slightly indented, sides parallel or slightly tapering anteriad. Ventral nerve cord organized in segmental ganglia; cord without perikarya at level of the septa and in anterior third of a segment. Suboesophageal ganglion (segments II-IV) without lateral constrictions; ganglion in V nor larger than in following segments. Post-pharyngeal bulbs inconspicuous.

No oesophageal appendages (= peptonephridia). Pharyngeal (= septal) glands (Fig. 1A, 2A) with unpaired dorsal lobes in IV and V and paired ventral lobes in V and VI, the latter elongate, separate and often extending into VII; no secondary lobes. Chloragocytes larger than coelomocytes, with distinct vesicles; dense but not elevated layer of cells around intestine from VII on down to rear end; cell layer reduced but not absent in XII. Gut widening gradually in VI; widening abrupt in contracted specimens. Ventral intestinal epithelium vesicular and non-staining in XVIII–XXVII, occupying 4-9 consecutive segments in a specimen (N=3). Vesicular cells elevated in 2 or 3 segments. Dorsal bood vessel from XII–XIII; no heart-like expansions observed.

Preclitellar nephridia (Fig. 1A,E) at 6/7 and 8/9. Anteseptale with funnel and coils of canal, longer than wide, anteriorly blunt, constricted at septum, often bent upwards or backwards; postseptale about twice as long as anteseptale, gradually tapering distally into short efferent duct. First postclitellar nephridium at 13/14. All nephridia with large terminal vesicle.

Coelomocytes (Fig. 2A) numerous, small, ca. 15-18 µm long in vivo, rounded, ca. 1.5x as long as wide, pale, filled with distinct, globular vesicles; nucleus distinct, cell aggregations opaque but not coloured.

Clitellum (Fig. 1A,B, 2B) in XII and XIII, extending over less than 2 segment lengths (first cell row behind septum of 11/12, last cell row before septum of 13/14), absent midventrally and mid-dorsally. In living specimens, cells on both sides of dorsal body half in ca. 24 conspicuous transverse rows, rows alternatingly narrow and wide. Wide rows composed of large hyalocytes and small granulocytes, narrow rows with granulocytes only. In preserved material, rows indistinct, hyalocytes very prominent. Latero-ventrally only granulocytes, arranged in dense to indistinct rows.

Testes in XI, unpaired, ovaries in XII. Seminal vesicle absent, few cysts free in XI. Spermatozoa (Fig. 1C) ca. 55 µm long, heads ca. 25 µm long in vivo. Sperm funnel (Fig. 1 A,C) small, strongly granulated, about twice as long as wide, tapering distad, with one or two median incisions; collar wider than funnel body. Vas deferens (Fig. 1A,C) very short, about as long as sperm funnel, without loops or coils, leading straight to male pore, distally with a club-shaped thickening, here canal describing a sharp bend immediately before male pore; diameter of vas deferens between 6.5 and 8 µm, distally ca. 12 µm wide or more (preserved material); wall epithelia somewhat irregular, lumen indistinct. Sperm funnel and vas deferens altogether not longer than ca. 170 µm (living and preserved material). Latero-ventral copulatory body muscles (modified ring muscles that traverse the coelom on both sides) strongly developed in XII (Fig. lA). Male pores apparently on body surface (no bursa seen, a small epidermal invagination may be present), surrounded by a small and narrow ring of modified cells, accompanied by two globular, glandular bodies, one in anterior, the other in posterior position (Fig. 1A,C). One mature egg at a time, occupying little more than one segment length.

Spermatheca (Fig. 1A,F) consisting of ectal duct and ampulla, the latter subdivided into ectal dilatation, connecting tube and ental reservoir. Ectal pores lateral, immediately below lateral line, large in fixed specimens (diameter ca. 5 µm). Ectal duct very short, with slight glandular thickening proximally before ampulla; canal lined by cuticle distally, proximally inconspicuous. Ampullar ectal dilatation in V, with smooth walls and distinguishable lumen; sperm arranged in parallel, heads distally, tails proximally. Connecting tube with indistinct canal, traversing segment VI, widening into thin-walled and elongate, sperm-filled reservoir that extends into VII or VIII. No asymmetry or bend in ectal region of spermatheca.

Remarks.

Achaeta becki belongs to a group of species characterized by the origin of the dorsal blood vessel in or near the clitellar region (Table 1). This character, very common in Enchytraeidae in general, is comparatively rare in Achaeta , where the dorsal blood vessel is usually confined to the anterior 6-8 segments. Out of the approximately 36 valid species described so far (the exact number cannot be given, pending a systematic revision of the genus), only three more species have a more posterior dorsal blood vessel origin: A. neotropica Cernosvitov , 1937 (origin in XI-XII), A. iridescens Christoffersen , 1979 (origin in XI-XIII) and A. maorica Benham , 1903 (origin in X). Interestingly, all three are from the southern hemisphere (the bulk of Achaeta species is described from the northern hemisphere); two of them are from South America. They also agree with A. becki in the absence of both pyriform glands, absence of oesophageal appendages, the shape of the pharyngeal glands, and in a thick cuticle (the latter three characters are unknown in A. maorica ); these traits, however, are found in other species of Achaeta as well.

Some rare peculiarities that distinguish A. becki are found only in the above-mentioned three species. A. maorica , for example, is the only other Achaeta species with short and straight vasa deferentia. In all other known species of the genus, the vasa deferentia describe loops or coils. The male duct of A. maorica is described as such: "The spermduct, less than twice the length of the funnel, takes a nearly straight course backwards to about the middle of segment xii., when it bends downward almost at right angles; it is dilated after passing through the septum xi./xii., but soon narrwos again, and the external opening is surrounded by a small lens-shaped mass of glandular cells (spermiducal gland), which it perforates at about the center" (Benham 1903: 222). Apart from the apparent congruences with A. becki , this description, together with a figure (Benham 1903, Pl. 25, Fig. 23), gives two characters that distinguish both species: a post-septal dilatation of the vas deferens (unique in the genus), and the presence of a single gland around the male pore (instead of two at some distance from the pore). Further differences of A. maorica from A. becki are: segment number (22), origin of dorsal vessel (X), and details of the spermathecae(extending into IX and X, presence of a ring of gland cells around the ectal pore, possible asymmetry of ampullar ectal dilatation, comp. Benham 1903, Pl. 25 Fig. 25). Further possibly differentiating characters are not or insufficiently known. The original description was based on a single specimen from the profundal of Lake Manapouri in New Zealand. There are no further records of the species, and according to Reynolds & Cook (1976), the type is lost. Coates (1989) considers A. maorica as species dubia.

Another peculiarity in A. becki is the thickening of almost all septa, the postclitellar ones included. This trait is described only for A. iridescens , a species originally described from the state of São Paulo, Brazil. However, most species descriptions do not deal with this trait, so it may be more common in the genus than it presently seems. Apart from the characters already mentioned, A. iridescens further agrees with A. becki in the position of the preclitellar nephridia (at 6/7, 8/9). The position of the spermathecal ectal pores is uncertain: they are described as being lateral—this would be a further character in common with A. becki -but in a figure (Christoffersen 1979, Fig. 11) the position is ventral. A. iridescens differs from A. becki in the presence of intestinal diverticula—a trait so far unique in the genus, but in need of reinvestigation—in body size (body length 13.5-14.5 mm), segment number (55-60), vasa deferentia (long and coiled), larger coelomocytes (up to 40 um) and in the presence of a well-developed glandular bulb around the male pore.

A. neotropica , originally described from northern Argentina, agrees with A. becki in general size and segment number (body 3-4 mm long, 20-34 segments)-apart from the characters already mentioned above. It differs from the new species in the following traits: ventral nerve cord ganglion in V larger than in following segments; first preclitellar nephridia at 7/8 (" Néphridies dépuis le dissépiment VI-VII" ( Černosvitov 1937: 136)), coelomocytes 30 mm long; vas deferens with coils of canal, ca. 4x as long as sperm funnel; male pore surrounded by a conspicuous glandular bulb; spermathecal ectal pores in ventral position. The detailed redescriptions of A. neotropica by Righi (1974) and Christoffersen (1979), based on material collected in the state of Sâo Paulo, Brasil, differ in two important details from the original account: preclitellar nephridia are located at 6/7 and 8/9, and the spermathecal ectal pores are in lateral position. Both traits are the same as in A. becki . The identity of these specimens as A. neotropica cannot be confirmed here. Christoffersen's material is heterogeneous and probably comprises several species. Both original and subsequently collected material should be reinvestigated.

A further peculiarity of A. becki is the segmental number and arrangement of epidermal gland cells. Our illustration (Fig. 1G) follows the scheme introduced by Graefe (1989). The distribution of cells is neither entirely fixed nor completely irregular. There is one transverse row of cells in the anterior half of a segment, contrasted by 2-3 rows in the posterior half. Anterior row and posterior rows are separated by wide interspaces devoid of gland cells. A segmentally repetitive epidermal agglomeration of cell nuclei (Fig. 1G, arrow) is always lined by two cells, one dorsally, one ventrally. In contrast, A. neotropica has 1-3 rows of gland cells per segment; according to Figure 22 in Černosvitov (1937), they are located in the middle of a segment, and the cells are larger and more elongate than in A. becki . Epidermal gland cells in A. iridescens are "absent or little developed" (Christoffersen 1979: 156). In A. maorica , this character is not described.