PROC. BIOL. SOC. WASH. 95(3), 1982, pp. 509-514 MESONERILLA PROSPERA, A NEW ARCHIANNELID FROM MARINE CAVES IN BERMUDA Wolfgang Sterrer and Thomas M. Iliffe Abstract. — A new species of Archiannelida, Mesonerilla prospera, is described from inland marine caves of Bermuda. This new species is particularly interesting in that it was collected from thin layers of mud on and under stones in totally dark areas of the caves, whereas all other marine Archiannelida (except one deep-water species) are members of the interstitial sand fauna. Recent studies of island marine caves in the Western Atlantic have revealed the presence of rich marine faunas. Comprehensive cave faunal surveys are now underway in Bermuda (Sket and Iliffe 1980) and on San Salvador Island, Bahamas (Carpenter 1981). Additionally, spot collections have been made from a number of sites (Peck 1973). Significant marine troglobites discovered include Remipedia, a new class of Crustacea from Grand Bahama Island (Yager 1981), a variety of caridean shrimps from diverse localities (Hobbs et al. 1977), and a mysid from Jamaica (Bowman 1976). Further collections from anchialine pools, as yet un-classified ecologically, have produced representatives of the amphipod suborder Ingolfiellidea (Stock 1977a), hadziid amphipods (Stock 1977b), an amphipod of the genus Psammogammarus (Stock 1980), the isopod family Microparasellidae (Stock 1977c), and the order Thermosbaenacea (Stock 1976). The Bermuda Islands, site of the current survey, consist of Pleistocene and Recent eolian and marine limestones completely capping a mid-ocean volcanic seamount. The islands' limestone caves were primarily formed during low stands of sea level corresponding to periods of Pleistocene glaciation (Bretz 1960, Palmer et al. 1977, Iliffe 1981). As postglacial sea levels rose, much of the former extent of the caves was flooded by sea water. Approximately 200 inland caves are known from Bermuda, over half of which contain tidal, sea level pools. Bermuda's long-est cave is the 1.8 km, totally underwater Green Bay Cave system (Iliffe 1980). A biological survey of the terrestrial and marine caves of Bermuda was begun in 1978 (Sket and Iliffe 1980). Although most of the animals collected from marine caves were more or less regular immigrants from open littoral habitats, a number of new species including blind and probably subterranean ones were found. Two new species of caridean shrimp, Somersiella sterreri and Typhlatya iliffei (Hart and Manning, 1981); an isopod, Atlantasellus cavernicolus, representing a new family (Sket, 1979); and a new calanoid copepod, Miostephos leamingtonensis (Yeatman, 1980) have so far been described from Bermuda's caves. We here describe a new species of Archiannelida collected from the Walsingham Caves, Bermuda. Mesonerilla prospera, new species Fig. 1 Material. — Walsingham Caves, Hamilton Parish, Bermuda: 8 December 1978, Walsingham Sink Cave, 1 specimen from 1 m water depth collected with long-
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