Zoogeographic patterns on very small spatial scales in rock-dwelling Plectostoma snails from Borneo (Gastropoda: Caenogastropoda: Diplommatinidae)

We mapped the fine-scale distribution patterns of Plectostoma microsnails on two small, isolated limestone outcrops in Malaysian Borneo. On both hills, two species were present (Plectostoma simplex and Plectostoma concinnum on Tandu Batu; Plectostoma fraternum and Plectostoma concinnum on Tomanggong Besar), but the patterns we found were differ­ent. On Tomanggong Besar, the two species occupy different parts of the hill and meet along a narrow hybrid zone that is haracterized by a significantly higher rate of predation by Atopos slugs. On Tandu Batu, the two species broadly over­lap and do not form hybrids. The predation rate here is the same in both species, regardless of whether they occur in monospecific localities or in mixed localities. Our results show that even small limestone outcrops of a few hundred metres in diameter cannot be considered to be uniformly populated by limestone dwelling snails, and a detailed analy­sis of intra-hill patterns may reveal information on how dif­ferences among species evolve and are maintained in the face of hybridization.