Mesenchytraeus pedatus Eisen, 1904

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 : 20-21

Mesenchytraeus pedatus Eisen, 1904

Mesenchytraeus pedatus Eisen, 1904: 55 ±57, ®gures 29, 30, pl. 1, ®gures 13, 14, pl. 9, ®gures 3±6.

Material examined. Three whole mounts from locality 18, USNM 186806; four whole mounts from locality 18, and six from locality 13 in the collection of BH.

Description. The following brief description supplements that of Eisen (1904) and extends the range of variation. Length 14±18 mm, width at VII 0.3±0.5 mm, segments 47±58 (n 510). Clitellar glands small, 12±20 Mm, irregular in shape with spaces of same size. Lateral chaetae ®ve in anterior bundles, two to three from VII; ventral bundles with ®ve to six anteriorly, four to ®ve in postclitellar region, length about 130 Mm throughout. Coelomocytes small for size of worm, 8±10 Mm long, coarsely granular. Dorsal vessel originating in XV or XVI. Seminal vesicle in X±XII, sometimes penetrating into egg sacs. Sperm funnels two or three times as long as wide, 240±340 Mm long. Vas deferens narrow, 5 Mm, widening to form an elongate atrium before entering large penial bulb surrounded by large accessory glands. Egg sacs with ovarian tissue and up to seven eggs extending to XVIII±XX. Spermathecae con®ned to V, consisting of stout ectal ducts with longitudinal striations (probably muscle, see Eisen, 1904, ®gure 29), slightly wider near pore, conical ampullae each with two club-shaped diverticula ending in almost spherical sperm chambers, and ental ducts uniting with gut in V. Some sperm may also be present in ampulla.

Remarks. The species is distinguished by the large glands surrounding the penial bulb, and the form of the spermathecae with two diverticula. Eisen remarked on the exceptional size of the penial papillae which, when fully extended, were as long as the diameter of the body. Present specimens were somewhat larger than Eisen’s (47±58 segments compared with 48). The spermathecal ental ducts could not be traced to their union with the fore-gut, which may indeed be ventral as described by Eisen.

Distribution and habitat. California: Goose Lake, Alturas and other localities in Modoc Co., common in mud of creeks and lakes ( Eisen, 1904); Columbia River, Washington, (site 8), Deschutes River, Oregon (site 13), Sacramento River, California (sites 18, 19). The sites are similar, i.e. fast runs in the larger rivers with coarse sediment dominated by cobble. Mesenchytraeus pedatus seems to inhabit lower gradient, downstream sites compared with M. rhithralis , and also lentic habitats with ®ner sediment. However, it has never been found in warmer, turbid water. It may be accompanied by other oligochaetes and was co-dominant with the lumbriculid Kincaidiana freidris Cook at site 13.