Achaeta singularis Schmelz

Schmelz, Rüdiger M., Collado, Rut & Römbke, Jörg, 2008, Mata Atlântica enchytraeids (Paraná, Brazil): The genus Achaeta (Oligochaeta, Enchytraeidae), Zootaxa 1809, pp. 1-35 : 26-29

Achaeta singularis Schmelz , sp. nov.

Figure 6 , Table 4

Holotype: MZUSP #1370, submature specimen with testis and ovary, and developing clitellum, sperm funnels, vasa deferentia, and spermathecae. Cachoeira Natural Reserve, Paraná, Brazil, old-growth forest on Cambisol, Oct. 2004.

Paratypes: MZUSP #1371, six specimens, stained and whole-mounted, one at same stage as holotype, four with testis and ovary, one juvenile; all from type locality, old-growth forest or secondary forest,> 30ys. old (agroforest); May 2003 (1 spcm), March 2004 (2 spms), October 2004 (3 spcms).

Further material investigated: Four juvenile specimens, same locality and dates as type series, investigated in vivo, not preserved.

Etymology: From the Latin " singularis ", meaning 'peculiar', 'unique', referring to the peculiarities of the nervous system, not found in other species of the genus.

Description: Living specimens about 2 mm long. Preserved specimens 1.5–2 mm long and 0.12–0.15 mm wide. Segment number 14–24. Body length and segment number of full-grown individuals unknown.

No pyriform glands, lentiform epidermal gland cells not distinguished.

Body wall thin (3–8 µm), cuticle ca. 2 µm thick; layer of epidermis and ring muscles thicker than longitudinal muscle layer, or all three layers of equal thickness; ring muscles inconspicuous, rings not distinguishable. Septa thin, often barely distinguishable, conspicuous only at 4/5–6/7 ( Fig. 6 A,B).

Head pore on dorsal tip of prostomium. Prostomium ( Fig. 6 A–C) short, blunt; frontal epithelium between head pore and mouth opening thickened to varying degrees; thickness increasing towards mouth opening, abruptly thinner directly dorsally of it, leaving a coelomic recess. A pair of elongate prostomial interior papillae (prostomial ganglia?) dorso-laterally of prostomial nerve, connecting with frontal prostomial surface. All inner proliferations of prostomial epithelium bilateral-symmetrical, rich in nuclei. Lateral thickenings of prostomial epithelium continued posteriad, gradually thinning out and fading into lateral line ( Fig. 6 C). Prostomial musculature well-developed.

Brain ( Fig. 6 A–D) deeply incised anteriorly and posteriorly, about as wide as long. Ventral nerve cord ( Fig. 6 A,B) in anterior body half organized in segmental ganglia. Ganglia of II and III almost separate; ganglion of IV separate, connective short. Following ganglia separated by longer connectives. In posterior segments and towards posterior end, ganglia increasingly fused into a medullar strand. All ganglia rounded like ovoid bodies. Post-phyaryngeal bulbs two pairs, outer pair on afferent fascicles of pharnygeal glands.

Oesophageal appendages ( Fig. 6 A,B) in V, dorso-laterally on oesophagus, paired, pale and finely granular (viv), outline irregular, spongy, stalked, or with short tube-like extensions; no canals seen in IV. Pharyngeal glands ( Fig. 6 A,B) in IV-VI, all united dorsally, all with ventral lobes, widely connected with dorsal lobes; gland in each segment shaped as a horseshoe, positioned obliquely and upside-down on and around oesophagus. Chloragocytes sparse, small, height ca. 4 µm. Dorsal blood vessel from VII. Intestine widening gradually over VI-VIII; no histological peculiarities observed in this region. Pars tumida of posterior midgut circumferal, extending over 1–2 segments, seen in 1/2 XI-1 /2 XVII. Intestine winded in several segments, often corkscrew-like. Pygidium with a medial dorso-ventral notch, i.e. posterior end with two large lateral lobes.

Anterior nephridia ( Fig. 6 E) at 6/7 and 7/8, ca. 4x as long as wide, no constriction at septum. Anteseptale short, truncate, with brown-yellowish granules, nephrostome embedded in nephridial body, postseptale slightly tapering towards ectal pore; terminal vesicle present. Posterior nephridia ( Fig. 6 F) of similar shape, not larger than preclitellar nephridia.

Coelomocytes ( Fig. 6 G) pale, almost circular, maximum diameter 25 µm, texture finely granular, margins grooved, and with apical thread-like processes; or cells filled with large spherical vesicles, marginal grooves and apical thread-like processes absent. Both types present in one and the same animal, distinguishable in whole mounts. Piles of rod-like, pale, hyaline, refractile crystalline bodies present in some specimens (3 out of 7), all from one site; rods of a pile roughly arranged in parallel, outlines indistinct, as if enclosed in cytoplasma; piles of rods about as large as coelomocytes, present throughout the body interior, not staining, conserved in whole mounts.

Clitellum apparently girdle-shaped. Testis and sperm funnels in XI, ovary in XII (in IX and X, respectively, in 1 paratype specimen); vasa deferentia with few coils, straight in one specimen. Male pores widely separate. Epidermis thickened around male pores. Spermathecal pores laterally at 4/5. Further details of reproductive organs unknown.

Remarks. Species diagnosis: Although only juvenile or not fully mature specimens have been found so far, we do not hesitate to ascribe them to a new species because of four peculiarities of the nervous system, described for the first time in a species of Achaeta , and unknown in combination in other enchytraeids: (1) A short brain with deep anterior and posterior concavities, (2) ganglia of segments II–IV (almost) separate and not fused, (3) anterior ganglia rounded like ovoid bodies, not subdivided into an anterior and a posterior lobe, (4) posterior ganglia fused into a common medullar strand. The usual condition in Achaeta is the same as in the other four species described in this paper: an elongate brain, 1.5– 2 x as long as wide and posteriorly rounded, truncate, or slightly concave at the most, together with a large suboesophageal ganglion composed of the fused ventral ganglia of segments II–IV, while the rest of the ventral nerve cord - from V on down to the rear end - is organized in segmental and slightly lobed ganglia with intervening intersegmental connectives. This set of characters is also present in many European species (Schmelz, pers. obs.), and the available literature evidence supports this picture.

Nevertheless, the poor coverage of the nervous system in many Achaeta species descriptions recommends a comparison using other characters. Some further key characters of Achaeta singularis are: (3) Presumably small body size and low segment number, (4) pyriform glands absent, (5) oesophageal appendages present in V, (6) all pharyngeal glands united dorsally, (7) secondary ventral pharyngeal gland lobes absent, (8) nephridia at 6/7 and 7/8, (9) dorsal blood vessel originating in preclitellar region, (10) spermathecal ectal pores in lateral position; perhaps also (11) the two-lobed pygidium, and (12) piles of rod-shaped crystalline bodies in the coelom, not present in all specimens. The only other Achaeta species that do not disagree with this character set are A. silvatica Nurminen, 1973 from soils near Montreal, Canada, and A. macrocyta Christensen & Dózsa-Farkas, 1999 from soils at the Kolyma Delta, siberian Russia. Both species are described incompletely, and the original description is the only source of information. The only distinguishing character of A. silvatica is in the coelomocytes, broadly spindle-shaped and filled with brown granules. In A. macrocyta the brain is of the common type, twice as long as wide, convex anteriorly and truncate posteriorly ( Christensen & Dózsa-Farkas, 1999, Fig. 20D). We consider A. silvatica as species inquirenda (see above, A. hanagarthi ).

Phylogeny: The separate ganglia of segments II–IV and the deep brain concavity in A. singularis are so far unique in the genus, and one may ask whether this is an autapomorphy of the species or a plesiomorphy in the genus. Since pairs of separate segmental ganglia are part of the groundplan of annelids, fusions of ganglia should be considered as secondary or derived. Hence, clefts in the brain and distinctness of the ventral ganglia in II, III and IV as found in A. singularis should be plesiomorphic, representing the ancestral condition where intrasegmental (brain) or intersegmental (segments II–IV) fusions of ganglia are absent or not as far advanced as in the rest of the genus. Under this scenario, A. singularis would represent an early offshoot, perhaps a sister taxon to all other Achaeta species known so far. However, the reverse is possible as well, namely that the traits in question are a true apomorphy of the species. For example, anterior ganglia are intra-segmentally separate in the naidid oligochaete Chaetogaster ( Vejdovský 1884) , and this dissociation is certainly secondary, given the phylogenetic position of the genus in recent molecular phylogenetic studies (e.g., Envall et al. 2006).

Classification: Amongst all genera of enchytraeids, this species can only be ascribed to Achaeta , by virtue of the following character combination: (1) dorsal pore on prostomium, (2) chaetae absent, (3) oesophageal appendages dorsally in V, (4), ganglionated ventral nerve cord. Character No. (3) is only found in Achaeta . Furthermore, the shape of pharyngeal glands and nephridia is similar to other species in the genus. Species without chaetae are also found in Marionina , but not with the character combination as listed above. Marionina is a heterogeneous assemblage of species and allows almost any combination of characters not covered by other genera. The third possibility, to erect a new genus for the species on the grounds of the aberrant nervous system, would seem unwise at present, given the high variation range in many other traits among species of Achaeta , and at the same time the distinctiveness of this genus from all other enchytraeid genera, even with A. singularis included.

Further observations: The apparent intraspecific variability of three further traits cannot be appraised at present and requires more and sexually mature material: (1) The peculiar piles of hyaline, rod-shaped structures were present in all specimens from one site and absent in all specimens from another. They appear to be inert and crystalline, since they do not disappear in the whole mounts like other refractile structures, e.g. the coelomocyte vesicles in A. paranensis . Each rod is itself composed of a number of toothpick-like sticks arranged in parallel. The nature of these crystals is unknown; we suggest a metabolite crystallized for storage, or a defence mechanism against predation. (2) One specimen had testis and ovary shifted 2 segments ahead, i.e. in IX and X, respectively. (3) In another specimen, the vas deferens is short and straight, without coils. The latter may be a transient stage during development or resorption of sexual organs (comp. A. neotropica , remarks).

Figures

FIGURE 6. Achaeta singularis sp. nov. A. Juvenile specimen with primordial testis (2) and ovary (3), dorsal view. Dotted areas mark perikarya in brain and ventral nerve cord. The perikarya of the ganglia in II and III are concentrated on the lateral sides. Note nephridia in VI and VII, the oesophageal appendage in V, the dorsal blood vessel in VI, and the winding intestine. From XII on the ganglia are fused among segments; 4: posterior growth zone. The outer pair of postpharyngeal bulbs was not distinguished in this specimen (comp. B). B. Anterior end, lateral view. Perikarya of ventral nerve cord, brain, and prostomial epithelial thickenings are drawn in. Ganglia of II and III are not completely separate, but there is no uniform suboesophageal ganglion over II – IV as in other Achaeta species (comp. Fig. 3 B). 1: Prostomial papilla. 2: Oesophageal appendage. 3: Dorsal blood vessel origin. C. Prostomium and first segment, dorsal view. Note the brain, deeply incised anteriorly and posteriorly, and the conspicuous prostomial papillae. Here dotted areas mark the perikarya-free areas. Laterally and posteriad, the prostomial epithelial protrusions gradually merge into the lateral line. D. The brain as seen in living specimens. E. Anterior nephridia, lateral view. - F. Posterior nephridia, ventral view. G. Coelomocytes and piles of rod-shaped structures. D and G are freehand sketches from living specimens, the rest is from stained whole mounts.