Mesenchytraeus antaeus, a new giant enchytraeid (Annelida, Clitellata) from the temperate rainforest of British Columbia, Canada, with a revised diagnosis of the genus Mesenchytraeus

Mesenchytraeus antaeus sp. n., an unpigmented giant enchytraeid inhabiting Carmanah valley (48 degrees 37'N, 124 degrees 44'W), Vancouver Island, is d escribed. With its 102-127 segments and the size of fixed specimens reachin g 61 x 2.9 tnm (diameter maximal at midbody), this species qualifies as one of the large:st members of the Clitellata family. The body wall comprises: a surprisingly thin cuticle (0.002 mm), the epidermis (0.02-0.04 mm), circ ular muscles (0.02 mm), plus thick longitudinal muscles (eight-layered, tot ally 0.16-0.18 mm) divided into seven unequal fields. The chaetae are group ed in bundles of two to seven but show some unusual traits which suggest in teresting convergences with the lumbricids. For instance, the most lateral chaeta in a bundle is always the largest; the larger chaetae exceed in leng th one-third of a millimetre; the ectal tips of the chaetae are oriented in opposite directions in the anterior and posterior halves of the body. The spermathecal pores are prominent and open laterally in the middle of V, an unprecedented location among enchytraeids. The nature of the spermathecal a mpullae (free, diverticulate and greatly elongate) indicates a close relati onship to a group of species that seems to be confined to the Pacific coast of North America and the Arctic coast of Siberia. This amphi-Beringean lin eage includes other gigantic forms such as M. harrimani Eisen, 1904 and M. maculatus Eisen, 1904, from which, however, M. antaeus is clearly different iated by the lack of pigmentation, the different size distribution of the c haetae within bundles, and the unique arrangement of the male apparatus. Th e latter includes unusually long sperm funnels (extending over 10 segments) and two groups of large accessory glands opening through two circular papi llae at the base of each penial bulb, independently of the penial pores. Al l other congeners known to possess this type of glands (e.g. M. franciscanu s Eisen, 1904) are at most only half as large as M. antaeus. The chaetal mu scles of M. antaeus are heavily infested by encysted nematodes.