Mesenchytraeus rhithralis, Healy & Fend, 2002

Healy, Brenda & Fend, Steve, 2002, The occurrence of Mesenchytraeus (Enchytraeidae: Oligochaeta) in ri ‚ e habitats of north-west American rivers, with description of a new species, Journal of Natural History 36 (1), pp. 15-23 : 17-20

Mesenchytraeus rhithralis sp. n.

(®gure 1)

HOLOTYPE: USNM 186799 , stained, whole mounted specimen, collected by S. Fend, 1 March 1988.

PARATYPES: One whole mount from each of the following localities: Yakima River at Thorp WA, USNM 186800 ; Yakima River at Cle Elum WA, USNM 186801 ; Manastash Creek WA, USNM 186802 ; Ruby Creek WA, USNM 186803 ; Cub River ID, USNM 186804 ; Swift Creek WY, USNM 186805 . Serial longitudinal sections from Yakima River at Thorp WA, ROMIZ 19980006 , Teanaway River WA, ROMIZ 19980004 , Swift Creek WY, ROMIZ 19980009 .

Type locality. Manastash Creek, Kittitas Co., Washington, part of the Yakima River drainage. Cobble ri‚e with gravel patches.

Other material examined

Whole-mounted specimens (nine slides), ROMIZ 19980004, 19980005, 19980007, 19980009. About 50 whole mounts from the 19 localities listed above in the collection of B. H., spirit material and many whole mounts in that of S. F.

Etymology. From the Greek `rheithron ’, a current or stream.

Description. Somewhat fragile worms, nearly transparent when alive, length of preserved worms 6±9 mm, width 0.3±0.4 mm in VI, 0.38±0.5 mm at the clitellum, narrowing in terminal segments. Segments 29±41 (exceptionally 45) (mean 35.1, n 5 39). Epidermal nuclei scattered, not in transverse rows. In most specimens, epidermal gland cells conspicuous throughout the body, regularly scattered, situated on small papillae, numerous on the prostomium; glands and papillae absent in specimens from localities 6, 10 and 19. Prostomium an elongate cone (®gure 1A), head pore at or near the tip. Clitellum over XII±XIII with small, irregularly scattered gland cells, absent mid ventrally, up to 30 Mm thick dorsally. Chaetae sigmoid, sharply pointed, with weak nodulus (®gure 1B), in fan-like bundles, four to eight, rarely 10, in ventral bundles, absent ventrally in XII, three to six laterally; length 90±140 Mm (70 Mm in specimens from Hat Creek), more or less equal within a bundle. Foregut widening gradually at 6/7, wall distinctly thicker in VII±VIII, occasionally also in IX, due to taller cells of the gut lining. Gut contents predominantly diatoms. Choragocytes forming a distinct layer from V. Post-pharyngeal bulbs small, not observed in many specimens, other gut diverticula absent. Three pairs of septal glands at 3/4, 4/5 and 5/6, septum at 3/4 often poorly developed, reduced to dorsal and ventral strands of tissue. Septal glands consisting of small dorsal lobes, second and third pairs occasionally united across oesophagus, ventral lobes usually present in V and VI, secondary glands present in mid V and VI, sometimes reduced in V (®gure 1A). Anterior border of brain deeply cleft, posterior border sinuous, length about 100 Mm, maximum width 70±90 Mm (®gure 1A). Dorsal vessel originating in XII or XIII, blood colourless. Coelomocytes oval, 10±28 Mm long, with several small vacuoles (®gure 1C). Nephridia of the Mesenchytraeus type, i.e. lobed and with tubules clearly visible due to poor development of interstitial tissue; three preclitellar pairs at 6/7±8/9, occasionally at 7/8±9/10, anterior pair sometimes absent. Preseptal consisting of nephrostome on a short neck, postseptale with two lobes, eOEerent duct long and partly folded, arising anteriorly between lobes (®gure 1D).

Testes compact, rarely lobed. Seminal vesicle unpaired in XI, extending forward as one or two lobes into X and IX, and back into XII. In a few specimens two lobes extended posteriorly in egg sacs as far as XIV. Sperm funnels about twice as long as wide when extended, but usually bent, of variable size, 120±280 Ö 40±70 Mm, tapering distally. Collar variable, either narrow and equal in diameter to funnel, ¯ared, or apparently absent (®gure 1E). Vasa deferentia stout, thick-walled (®gure 1F), diameter 24±50 Mm, of more or less equal diameter throughout but slightly narrower near funnel and penial bulb, entering penial bulbs centrally; length about equal to diameter of clitellum. Refringent particles sometimes present in vas wall. Bulbs sac-like, compact when retracted, nearly spherical, 80±140 Mm across, consisting of densely staining glandular tissue surrounding a central chamber (®gure 1G, H) which opens to outside by way of an irregular slit. Everted bulbs irregular, appearing thin-walled. Vas enters bulb through a thick pad lined by a glandular epithelium with goblet cells (seen in some sections). Atrial enlargement of vas, atrial glands, and accessory glands attached to bulb absent. Paired egg sacs extending to XIV, occasionally to XV or XVI when fully distended with eggs and ovarian tissue. Spermathecae free, without diverticula, opening ventro-laterally in intersegmental groove 4/5. Ectal ducts stout, thick-walled, 240±360 Mm long, 18± 22 Mm in diameter, leading to large, cylindrical ampullae which are thick-walled when empty but become thin-walled when distended with sperm (®gure 1A). Spermathecae extend ventro-laterall y through several segments, sometimes reaching to X or even XI where they may overlap seminal vesicle. In a few specimens from Ruby Creek, a third spermatheca was present, opening ventro-laterall y at 5/6.

Remarks. The species shares the following characters with other members of Mesenchytraeus : nodulated, sigmoid setae and lobed nephridia with little interstitial tissue. Cernosvitoviella Nielsen and Christensen 1959 , which also have nodulated setae, have more compact, unlobed nephridia and a muscular penial bulb is absent. Mesenchytraeus rhithralis is recognized by its relatively small size for the genus, primary septal glands at 3/4, 4/5 and 5/6 and secondaries in V (usually) and VI, a thickening of the gut wall in VII and VIII, intraclitellar origin of the dorsal vessel, a short, stout vas deferens without atrial enlargement or atrial glands, compact penial bulbs without accessory glands, and long, free spermathecae without diverticula. It is recognized at low magni®cation in benthic samples by its long-conical prostomium, many ®ne chaetae per bundle, and generally thin integument that leaves it looking rather wilted compared with other enchytraeids. Dimensions of all parts of the male reproductive system are very variable. The testes can be compact or slightly lobed, the seminal vesicle can be con®ned to two segments or extend from 9/10 to 14/15, whilst size of the sperm funnels, and diameters of vas deferentia and penial bulbs, vary over wide ranges.

Among 25 or so described species of Mesenchytraeus in which the spermathecae are free, i.e. not communicating with the oesophagus, only seven lack diverticula (external or internal) at the junction between the ectal duct and the ampulla. M. magnus Altman, 1936 , recorded from Seattle, Washington, reaches over 70 mm and has short spermathecae; M. altus Welch, 1917 from the Rocky Mountains, Colorado has atrial and penial glands, and a long vas deferens; M. falciformis Eisen, 1878 from the Russian Arctic and Greenland has pigmented coelomocytes, a very small sperm funnel and short spermathecae; M. franzi Nurminen, 1977 from Austria has only two chaetae per bundle, and spermathecae with ectal glands and long ectal ducts; M. tundrus Piper, McLean and Christensen, 1982 from N.E. Russia has yellow-brown coelomocytes, pink blood and short spermathecae; M. diverticulatus Piper, McLean and Christensen, 1982 from N.E. Russia has a gut enlargement with diverticula in VIII, and short spermathecae; and M. sanguineus Nielsen and Christensen, 1959 from Denmark, Britain and Ireland has red blood, sperm funnels with tubular collars, and penial bulbs with long accessory glands. These species diOEer widely from each other and none show particular a nities with M. rhithralis . For nearly all of them, and for most species described prior to 1959, descriptions are incomplete, so it cannot be stated with certainty whether any features of M. rhithralis , or combinations of characters, are unique. In several species listed above, the origin of the dorsal vessel is post-clitellar, but an intra-clitellar origin has been reported for a number of other Mesenchytraeus species.

The presence of septal glands at 3/4, known only for M. kuril Healy and Timm (in press), is unique for enchytraeids and deserves special comment. It is generally accepted that there are no septa anterior to 4/ 5 in enchytraeids, although nephridia, normally associated with septa, have been reported at 2/ 3 in two species of Cognettia ( Bauer, 1993; Christensen and DoÂzsa-Farkas, 1999). In M. rhithralis the septa may be incomplete but are distinctly present, seen for example in longitudinal sections (e.g. ROMIZ 19980004B). The glands are small but appear identical in structure to those at 4/5 and 5/6, and ducts leading from them to the pharynx are clearly distinguishable. The irregular pattern of epidermal cells as shown by nuclei not arranged in transverse rows as in most enchytraeids is also worth noting.

Habitat. In low to sixth order rhithral stream segments with coarse substratum. Highest densities in lotic, erosional habitats, becoming scarce in marginal, depositional habitats where they are generally accompanied by other oligochaetes including enchytraeids (species of Fridericia and Cernosvitoviella ), Nais spp. , Rhyacodrilus spp. , Stylodrilus heringianus ClapareÁde and Rhynchelmis spp. In the Yakima River system (80 sites), the species was present in almost all of the upstream sites draining a montane, forested landscape, and absent from almost all potamal downstream sites within a valley agricultural landscape.

Distribution. Western USA (Washington, Wyoming, Oregon, Idaho, Montana, California). Recorded in this paper from 19 localities, but known to be present in many more throughout the region.