Dictyostelid Cellular Slime Molds from Caves

Dictyostelid cellular slime molds (dictyostelids) are singlecelled, eukaryotic, phagotrophic bacterivores usually present and often abundant in terrestrial ecosystems (Raper, 1984). These organisms represent a normal component of the microflora in soils and apparently play a role in maintaining the natural balance that exists between bacteria and other microorganisms in the soil environment. For most of their life cycle, dictyostelids exist as independent, amoeboid cells (myxamoebae) that feed upon bacteria, grow, and multiply by binary fission. When the available food supply within a given microsite becomes depleted, numerous myxamoebae aggregate to form a structure called a pseudoplasmodium, within which each cell maintains its individual integrity. The pseudoplasmodium then produces one or more fruiting bodies (sorocarps) bearing spores. Dictyostelid fruiting bodies are microscopic and rarely observed except in laboratory culture. Under favorable conditions, the spores germinate to release myxamoebae, and the life cycle begins anew. Dictyostelids are most abundant in the surface humus layer of forest soils, where populations of bacteria are the highest and microenvironmental conditions appear to be the most suitable for dictyostelid growth and development (Raper, 1984). While the primary habitat for dictyostelid cellular slime molds (or dictyostelids) is the leaf litter decomposition zone of forest soils, these organisms are known to occur in other types of soils. Among these are soils of cultivated regions (Agnihothrudu, 1956), grasslands (Smith and Keeling, 1968), deserts (Benson and Mahoney, 1977), and both alpine (Cavender, 1973) and arctic (Cavender, 1978; Stephenson et al., 1991) tundra. In addition, dictyostelids have been reported from the layer of soil-like material (canopy soil) associated with the epiphytes that occur on the branches and trunks of tropical trees (Stephenson and Landolt, 1998). Dictyostelids also occur on dung and were once thought to be primarily coprophilous (Raper, 1984). However, perhaps the most unusual microhabitat for dictyostelids is the soil material found in caves. Few studies have considered the dictyostelids associated with caves. In what apparently represents the first published report of dictyostelids in caves, Orpurt (1964) reported two species (Dictyostelium mucoroides and Polysphondylium pallidum) from a cave located on Eleuthera Island in the Bahamas. Later, Waddell (1982) reported eight species from Blanchard Springs Cavern in Arkansas. One of these (Dictyostelium caveatum) was new to science. In the most extensive study to date, Landolt et al. (1992) investigated 23 caves in West Virginia. Nine species of dictyostelids were recovered, and three of these were present in at least 10 different caves. One of these three species (Dictyostelium rosarium) was of particular interest, since it had not been recorded from soil samples collected from above-ground sites in an earlier study of the distribution and ecology of dictyostelids in West Virginia (Landolt and Stephenson, 1990). In general, based on available data, the distribution of dictyostelids in caves appears to be rather patchy, but in the microsites where they do occur, these organisms can exhibit surprisingly high levels of abundance and diversity. The objective of the present study was to extend the earlier investigation of dictyostelids in West Virginia caves (Landolt et al., 1992) to caves at a number of other localities, with particular emphasis placed on caves in the Ozark region of Arkansas, Missouri and Oklahoma (Landolt et al., 2005). In addition, these data were supplemented with all known published (Waddell, 1982; Landolt and Stihler, 1998; Reeves et al., 2000; Reeves, 2001; Nieves-Rivera, 2003) and unpublished records of dictyostelids from caves in an effort to summarize what is known about their occurrence in this habitat. DICTYOSTELID CELLULAR SLIME MOLDS FROM CAVES