5 October 1982 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 deepwater 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 unclassified ecologically, have produced representative~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 longest 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 lliffe 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- 5 10 PROCEEDINGS OF THE BIOLOGICAL SOCIETY OF WASHINGTON Fig. 1 . Mesonerilla prospera: a, Young female specimen, dorsal view; b, Detail of setae from segments 2-9; c, Detail of setae from segment 1; d, Anterior end, dorsal view; e, Spermatid; f, Sperm. handled dip net; 5 January 1979, Walsingham Cave, 3 specimens from 1 m water depth collected with long-handled dip net; 22 May 1979, Walsingham Cave, 3 specimens collected from 1 m depth with long-handled dip net; 24 July 1979, Walsingham Cave, 3 specimens collected from 1 m depth with slurp gun, 3 specimens collected from 1 m depth with small dip net; 9 October 1981, Cripplegate Cave, 1 specimen collected from surface waters of outflowing tidal spring with plankton net; 28 November 1981, Deep Blue Cave, 1 specimen collected from 12-15 m depth by hand net with scuba; 17 January 1982, Emerald Sink Cave, 1 specimen collected from 11 m depth by hand net with scuba; 26 January 1982, Cherry Pit Cave, 4 juvenile specimens collected by pumping water from 2.5 m depth through a plankton net; 31 January 1982, Myrtle Bank Cave, 1 specimen collected from rock walls and ledges at 5 m depth by hand net with scuba; 23 June 1982, Cherry Pit Cave, 6 specimens collected from 1-5 m depths by hand net. Observations and measurements from 9 specimens were used for the species description. A whole mount of a female specimen collected on 23 June 1982 at Cherry Pit Cave has been deposited as holotype (USNM 73764). Distribution.-Known only from the anchialine habitats of the Walsingham Caves, Bermuda. Habitat.-The Walsingham Caves are located in a 450 m wide strip of land separating the nearly enclosed Harrington Sound from the more open Castle Harbour. At least 40 cave entrances are known from the Walsingham tract, a low, hilly, heavily overgrown area of 0.15 km2. Caves of this area are characterized by fissure entrances and large collapse chambers. Two large underwater caves with a total of 1 km explored length, 20 m depths and 7 known entrances probably represent only segments of a much larger and more complex cave system. Walsingham and Deep Blue Caves have already been connected by cave VOLUME 95, NUMBER 3 511 divers, as have Cripplegate and Myrtle Bank Caves. Both of these groups of caves as well as Cherry Pit and Walsingham Sink Caves are believed to be hydrologically connected as part of the same system. Emerald Sink Cave, located only 300 m away from Cherry Pit Cave, may also be connected. Amplitude and phase of the tides have been measured in Harrington Sound, Castle Harbour and pools of the Walsingham Caves. While there is no significant difference between tides in Castle Harbour and those of the open sea, Harrington Sound tides have an average lag time of 2 hours and 45 minutes and a range of only 30% of the open sea tides. Tides for the Walsingham Caves are intermediate, having an average 1 hour lag and 60% range. This difference in phase between Harrington Sound and Castle Harbour tides produces alternating tidal currents flowing through the caves such that an estimated 50% of the tidal volume of Harrington Sound (half a million m3) passes through caves (Morris et al. 1977). Plankton and organic detritus carried by tidal currents probably provide the primary source of food for the cave animals. Surface salinities in pools of the Walsingham Caves average 27%0,while at l m salinities already reach 34%0. In order to determine where exactly within the cave pools Mesonerilla prospera is living, selective collecting was carried out. Using a slurp gun, material was collected from the steep rock walls of the cave. A small dip net was used to obtain animals from the upper few centimeters of the thick silt on the cave floor. Material from piles of 30-60 cm diameter rocks with only thin layers of silt was collected both with the slurp gun and by moving rocks quickly up and down to generate a flushing action and then sweeping a small net through the disturbed water. Six specimens of Mesonerilla prospera were collected from the piles of rock, three each with the slurp gun and net, while none were obtained from either the cave walls or floor. The discovery of an archiannelid living in inland caves, on and under stones, is exceptional since all other marine members of this group, except one deep-water species (Sterrer 1968), are members of interstitial sand fauna. The collection of four juveniles from open water in Cherry Pit Cave a l ~ d one adult from the Cripplegate tidal spring indicates that subterranean water currents may be significant in determining the distribution of Mesonerilla prospera. Description.-The length of adults (excluding appendages) ranges from 1500 to 2050 pm, with the maximum width of 250-420 pm (to 520 pm including parapodia) somewhat behind midbody (Fig. la). The largest juvenile (i.e., without visible gonads) was 1300 pm; smaller juveniles collected in water pumped from above the bottom measured 300 pm (with 5 segments), 400 pm (7 segments) and 450 pm (8 segments). The prostomium (Fig. Id) is rounded and carries a pair of reddish eyes and 3 tentacles dorsally, and a pair of palps ventrally. The median tentacle can be up to 650 pm long, the lateral ones to 600 pm; in most specimens however, they are much shorter. Palps are up to 230 p m long and 50-60 pm wide. They are gently curved, and are broadest near the base, tapering gradually towards the tip. There are 9 setigerous segments. Whereas each parapodium carries 2 thin and sometimes rather long (to 550 pm) cirri in segments 2-9, the buccal parapodia carry only 1 usually very short cirrus (30 pm). A pair of anal cirri (urites) were seen in only one of the specimens; they were short (120 pm, and 30 pm), and are probably easily lost. Each parapodium carries about 20 setae arranged in 2 bundles. Setae are compound on all segments; they are of fairly equal length 512 PROCEEDINGS OF THE BIOLOGICAL SOCIETY OF WASHINGTON (shaft 170 pm, end piece 45 pm) throughout segments 2-9, but longer (shaft 220 pm, end piece 90 pm) on the first segment. The setae of segment 1 also differ in having a pointed shaft spur whereas it is blunt in the setae of all other segments (Fig. lb, c). The mouth opening is longitudinally slit-shaped and sometimes appears triangular. The internal epithelia of pharynx, esophagus and rectum are ciliated. None of the food particles present in the gut could be identified. Sexes are sepqrate. A female may have up to 8 mature eggs (to 200 pm diameter) arranged on both sides of the gut posterior to segment 4. None of the females observed carried eggs or embryos externally. Of the two males, one had segments 4-9 full of sperm (Fig. If) and spermatids (Fig. le). Sperm is about 22 pm long, with a bullet-shaped head of 2 pm, and a tail of about 20 pm. The unrestrained animal glides slowly over the substratum by means of its ciliation. Like many other Nerillidae, this species shows a rapid escape reaction when disturbed. Etymology.-The name of the species refers to its well-fed appearance, but is also an allusion to Prospero's Cave in Shakespeare's "The Tempest," a play allegedly inspired by the shipwreck of Bermuda's first settlers. Discussion.-The number of segments (9) and the compound setae clearly place the new species in the genus Mesonerilla Remane, 1949. Of 7 species presently ascribed to this genus [M. minuta Swedmark, 1959 has been assigned to another genus (Jouin 1971)], 3 are distinct from M. prospera in being hermaphroditic; a fourth, M. luederitzi Remane, 1949, is known only as juveniles. One of the remaining 3 species, M. biantennata Jouin, 1963, differs from M. prospera in lacking a median tentacle. Another, M. intermedia Wilke, 1953, is characterized by brood protection devices ("elytres") in the female (Jouin 1968) which M. prospera does not have. The most recently described species, M. ecuadoriensis Schmidt and Westheide, 1977, from shallow sandy bottoms in the Galapagos Islands, is also the one that most closely resembles M. prospera; there are differences, however, in the shape of the palps, the number of parapodial cirri, the presence of eyes, and the body size. At 2000 pm, M. prospera is by far the largest of all Mesonerilla species; of the described species M. intermedia reaches 1200 pm, most of the others only measure 1000 pm or less. Finally, the habitat in which M. prospera occurs, sets it apart from all other marine Nerillidae, which live interstitially in sand, with the exception of Paranerilla lirnicola Jouin and Swedmark, 1965, which lives on deep mud bottoms (Sterrer 1968). The only freshwater nerillid, Troglochaetus beranecki Delachaux, 1921, although not closely related morphologically, shares with M. prospera the troglobitic way of existence (Pennak 1971). A biogeographic observation of interest is that M. prospera has its closest relative on another oceanic island, Galapagos, just as Bermuda's cave decapod shrimps are most closely related to species from Caribbean islands (Antigua, Cuba, Caymans, Bahamas), Ascension Island and, again, Galapagos (Hart and Manning 1981). Whether this indicates that oceanic islands, and island cave habitats in particular, are refuges for the descendants of formerly widely distributed but now extinct marine faunas must await further evidence. VOLUME 95, NUMBER 3 Acknowledgments This study was supported by a National Science Foundation Grant (DEB8001836) to Thomas M. Iliffe and Wolfgang Sterrer. Additional support for cave collections was provided by EARTHWATCH and the Center for Field Research, and for cave diving by grants from the Explorers Club and the National Speleological Society. Cave diving equipment and techniques used to conduct this study met standards of the National Speleological Society. 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