Dichogaster (Diplothecodrilus), 1996

James, Samuel W. & Divina, Gildas Brice, 2012, Earthworms (Clitellata: Acanthodrilidae, Almidae, Eudrilidae, Glossoscolecidae, Ocnerodrilidae) of the coastal region of Gamba, Ogooué-Maritime Province, southwestern Gabon, Zootaxa 3458, pp. 133-148 : 142-143

Dichogaster (Diplothecodrilus) sp. C

Material. USNM 1180251 , late juveniles, Ivinga , Gamba Complex, Gabon in forested flat on coastal plain at 2.78957°S, 10.04805°E, 15 m asl., 17 May 2008; S. James, G. Divina, G. Moussavou and L. Tchignoumba, colls .

Description. Unpigmented, dimensions 72–78 mm long by 2.5 mm at X, 2.6 mm at XXX; 176 segments, body cylindrical throughout. Setae closely paired throughout; setal formula AA:AB:BC:CD = 5:1:4:1. at X, 9:1:6.5:1 at XXX, DD> 1/2 circumference throughout. Prostomium separated from peristomium dorsally by invaginated ring having the appearance of deep segmental boundary but clearly not a boundary; therefore schizolobous. First dorsal pore 5/ 6, spermathecal pores 7/8/ 9 in AB. Ovipores mid-ventral equatorial in XIV; male pores in XVIII; prostatic pores and penial setae at ends of seminal grooves in AB in XVII–XIX. Seminal grooves slightly convex laterally.

Septa 5/6–12/13 muscular, others membranous; gizzards in V, VI separated by the thick septum 5/6, gizzard of V larger. Intestinal origin XIX, with constriction at 36/37; lateral typhlosoles continuous XXV–XXXVI; main typhlosole XXIV–LV; from XXIV–XXXIII with bifid ventral margin, paired opposite vertical flaps on sides; after XXXIII transition to simple fold. Reniform large calciferous gland paired XV–XVII. Hearts X–XII esophageal, supraesophageal vessel visible X–XII; lateral commissures V,VI to gizzard; VII, VIII, IX to ventral vessel. Nephridia of intestinal segments sac-form, 12 per segment, ventral-most row stomate. Holandric with testes free, small seminal vesicles XI, XII; prostates tubular with long, bowed muscular ducts in XVII, XIX. Penial setae present XVII, XIX; shaft 0.83 mm by 14 µm in ental section, bowed slightly with 5 gentle undulations over 0.24 mm of the ectal portion, ectal-most 0.09 mm tapering to fine point with hooked tip. Ovaries in XIII, free; spermathecae in VIII, IX; immature but showing relatively thick very short duct, long ampulla, with simple diverticulum attached near duct/ampulla junction.

Remarks. This species represented only by juveniles has the complex typhlosolar structure seen in several other species collected at Gamba, and gizzards in V and VI but separated by a muscular septum. Apart from D. tchignoumbai , all others in the collection have membranous septa between the gizzards. Besides the muscular septum 5/6, it also shares with D. tchignoumbai the schizolobous prostomium. The anterior-most projection of the first segment, that which is ordinarily a prostomium either slightly marked off from the peristomium by shallow furrows or grooves, or in the case of zygolobic species, not marked off at all, is here set off from the peristomium by an almost circumferential furrow that reaches posteriorly under the forward edge of the main part of the peristomium. Careful segment counts confirm that this furrow is not an intersegmental boundary.

We have provisionally assigned this species to the subgenus Dichogaster (Diplothecodrilus) based on the location of the first dorsal pores and an estimate that the mature spermathecae will show the characteristic two-parted ampulla of the subgenus. However, it shares the complex typhlosole and penial seta form of gentle undulations leading to a hooked tip, with two taxa having first dorsal pores in 12/13. Penial setal form could well be homoplasious in this comparison, as we see the undulating form in Gamba species assigned to different subgenera, and similar form in many other Dichogaster species. For examples, D. (Diplothecodrilus) mundamensis, Michaelsen, 1897, D. (Dt.) fouoriensis Csuzdi, 1996, D. (Dt.) minutissima Csuzdi, 1996, D. (Dt.) tanganyikae (Beddard, 1902) and D. ( Dichogaster ) greeffi Michaelsen, 1903 have the undulating penial setal form, although ornamented with small scars throughout the undulation ( Csuzdi, 1996a; 2005; 2010), unlike the species reported here on whose penial setal shafts we could detect no ornamentation. The current taxonomy places greater weight on the locations of first dorsal pores and the form of the spermathecal ampulla than on penial setal form, which is probably a good choice.

More problematic than penial setae is the shared typhlosolar structure. It seems unlikely that the identical complex typhlosole structure has been converged upon by members of the two subgenera, or alternatively, retained from an ancestral state at the divergence of the subgenera. Dichogaster tchignoumbai , living in the same conditions as the present species Dichogaster sp. C , has a strongly bifid typhlosole bearing sparse vertical flaps, rather than the densely crowded flaps in the other species with complex typhlosoles. Though it might be reasonable to hypothesize that the soil environment of the coastal plain at Gamba favors the evolution of a complex typhlosole, we have in this small collection two different forms of complex typhlosole. This indicates that the same problem can be solved in different ways. The species with non-bifid typhlosoles bearing densely crowded flaps represent a conflict between one set of characters (dorsal pores, spermatheca form) and another (typhlosoles, maybe penial setae). Worse, Dichogaster (Dt.) sp. C shares the schizolobous prostomium and muscular septum 5/6 with D. (Dt.) tchignoumbai but their typhlosoles and penial setae differ. This casts doubt on the reliability of typhlosole characters. Clearly there is much to be learned about character evolution in Dichogaster , and it is not yet possible to make any firm conclusions about taxonomic boundaries within which character states show homology.