be basal for the Physarales (Castillo et al. 1998). The tiny sporocarps, only 60 W. Foissner et al. (eds) 0.3–0.8 mm in total height, have been observed thus far almost exclusively in moist cham- ber cultures, where they are rather conspicuous by having a silvery iridescent peridium. All 72 records of which we are aware were obtained from the bark of living trees or (less frequently) shrubs. At least one distribution centre of the species seems to be represented by the arid regions of the western United States (Colorado, New Mexico, Arizona and Utah), although the species is present (albeit with a low frequency) in the less thoroughly studied deserts of Central Asia (southern Russia, Kazakhstan and Mongolia). Fragmentary records from other arid zones of the world (southern Spain, Tunesia and Oman) contribute to the picture of a species apparently restricted to arid steppes and deserts, with a (possibly ques- tionable) record from Myanmar (formerly Burma) as the only noticeable exception (Fig. 3). Since arid habitats are generally understudied for myxomycetes, the distribution pattern presented herein is certainly fragmentary, and more intense studies in shrub deserts or gal- lery forests that occur along rivers in deserts and other arid areas will certainly add to the number of localities from which the species is known. Leocarpus fragilis (Dicks.) Rost. (Physarales), which produces calcareous, orange to red sporocarps that resemble miniature grapes, is one of the most of all distinctive myxomyce- tes. The species is frequently recorded by non-specialists for the group. The large and robust, yellow phaneroplasmodium inhabits ground litter with an acidic pH (most often between 4 and 6). Fruitings can consist of several thousand individual sporocarps. In dry coniferous forests, exceedingly large mass fruitings can develop, and in 1993 one dry pine plantation in eastern Germany (Brandenburg, Eggsdorf near Berlin) had an estimated den- sity of 200–400 fruitings per hectare. With a diameter of 12–14 �m, the spores are rela- tively large for a myxomycete, which should decrease the probability of long-distance dispersal, and local biotypes seem to exist. A form with spores in clusters of two has been described as L. bisporus Nann.-Bremek. and Mitch. (Nannenga-Bremekamp 1989). Some Fig. 2 World distribution map for Barbeyella minutissima Meyl., a myxomycete associated with montane Picea and/or Abies forests. The encircled range indicates the European Alps, with numerous records of the species Protist Diversity and Geographical Distribution 61 specimens from South Africa have almost globose sporocarps, whereas the typical form is egg-shaped (Schnittler, pers. observation of specimens from the Kew Botanical Garden, London). The distribution map (Fig. 4) was developed from 1474 records. Regions with many records were encircled with a line on the map. Two patterns seem evident when the overall distribution is considered. First, L. fragilis rarely occurs in arctic regions but as a litter-inhabiting species it can spread beyond the timberline. As such, its northern distribu- tion may be limited more by climate than microhabitat availability. It is very common in the temperate zone, but seems to be less common in southern temperate zones (e.g., in the Mediterranean region or in the southeastern United States). The species appears to be absent from highly arid regions (records from Israel and Spain are from the less arid parts of these countries) and the humid tropics. However, disjunct occurrences in montane regions are found farther southward (e.g., in the Canary Islands or the African Atlas Moun- tains). Also, in fairly well studied tropical regions (e.g. Central America, Ecuador, Tanzania and Taiwan), L. fragilis has not yet been found, except for a single record from the paramo region of Costa Rica. As shown by a single record from southern Argentina (Tierra del Fuego) and a few from South Africa and southern Australia, it reappears in temperate regions of the southern hemisphere. In contrast to the statement in Martin and Alexopoulos (1969) that L. fragilis is a cosmopolitan species, it seems to display a clear preference for temperate zones. Ceratiomyxa morchella Welden (Ceratiomyxales) was unknown to science until 1954. The fruiting bodies are very evanescent and start to dissipate on the day following develop- ment. However, fruitings of C. morchella are conspicuous, since the stalked, 2.5–4 mm high fruiting bodies can occur in large developments. The stalk is translucent, colourless and crowned by a pure white, globose head that resembles a miniature morel. The ovoid spores, 9–11 £ 6–8 �m in diameter, are formed at the tip of 2–10 �m long stalks that cover the white upper portion of the fruiting body like a covering of fur. According to studies by Fig. 3 Distribution of Protophysarum phloiogenum M. Blackw. & Alexop., a species found almost exclu- sively in arid environments 62 W. Foissner et al. (eds) the authors in Costa Rica and Ecuador, C. morchella is strongly limited to decaying wood with a very acidic pH, a rather rare microhabitat in tropical forests. Without exception, the substrate upon which fruiting occurred for the more than 100 records considered herein consisted of decorticated, moderately to strongly decayed wood of large diameter logs with measured pH values between 3.0 and 4.5. This precludes any confusion with C. sphaerosperma Boedijn, the second tropical species in the genus Ceratiomyxa, since the latter is found exclusively upon substrates with a pH above 6.5 (up to 8), most often por- tions of decaying of hard-shelled fruits. For C. morchella, fruitings with more than 10,000 individual fruiting bodies are not rare, and these typically cover the lower side of large, fallen tree trunks that are kept above the ground by their root disk or other logs. The spores are colourless and thin-walled. Lacking any UV-absorbing pigments, they may not be able to survive the long periods of exposure to light that would be associated with long-distance dispersal in the higher layers of the atmosphere. The preliminary map for C. morchella (Fig. 5) was compiled from only 72 records, reXecting the fact that the slimy fruiting bodies are diYcult to preserve as herbarium specimens. The northernmost records are from sub- tropical Florida and tropical Mexico. The species is fairly common in the Caribbean region, being known from Puerto Rico, Jamaica, St. Vincent and Trinidad. Records for Costa Rica come from all parts of the country except the dry southwest and the highest mountains. It was recorded twice during a survey in western Ecuador (Schnittler et al. 2002). The real distribution centre of the species seems to be the Amazon Basin. In a study carried out in the Yasuni National Park (eastern Ecuador) it was one of the most common species (Schnittler, unpubl. data). Due to the paucity of records, it can not yet be determined if the species commonly occurs only in the Paleotropics. The species is still unknown from tropi- cal Africa, and there is only a single record from eastern Asia (New Guinea) mentioned in the literature. Fig. 4 World distribution map for Leocarpus fragilis (Dicks.) Rost., a temperate-zone myxomycete conWned largely to the northern hemisphere. Encircled ranges in Europe and the United States depict regions with sev- eral hundred records each Protist Diversity and Geographical Distribution 63 Myxomycetes in the context of the current debate on protist distribution On the whole, myxomycetes would seem to be rather opportunistic or “fugitive” organisms (sensu Hutchinson 1951) in that they have a high reproductive potential, seem to possess eVective dispersal mechanisms, and are characterized by rapid development. These proper- ties allow them to exploit successfully habitat islands occurring both temporally and spa- tially in nature. Although a particular habitat within which a species of myxomycete has been established may persist for only a short period of