ABSTRACT

Recent biological discoveries from anchialine caves have produced great surprises and apparent contradictions, with some cavernicolous species appearing to be much older than the caves they inhabit, while others show anomalous and widely separated zoogeographic relationships or even affinities to present deep sea taxa. In addition, many fresh water cavernicoles probably have a marine origin.

Anchialine caves occur as flooded lava tube caves or as solutional, primarily limestone, caves submerged by rising sea levels or by land subsidence. Such caves have several common characteristics, e.g. their occurrence principally on islands, their tidal nature with salinities of at least the deeper waters close to that of the open sea, the isolation of their waters in some inner portions from the open sea resulting in long residence times, and their biological zonation comparable to that of terrestrial caves.

A model for the evolution of fresh water cavernicoles from marine ancestors is here proposed. This model is based upon the observed zonational patterns of submarine and anchialine caves. A suitably preadapted species would first colonize the entrance zone of a marine cave. Subsequent migration farther into the cave could result from the expansive nature of the species. Genetic isolation of the cave population could occur as a result of sea level changes, cave collapse, or temperature variations. Such dispersal and isolation would be accompanied by gradual evolutionary adaptation. In the environmentally stable but food-poor inner reaches of the cave, nutritional strategies would be a prime factor in determining a species' survival. Open inland pools formed by cave collapse would provide plentiful food resources such as photosynthetic algae or surface-derived organic detritus. Upward invasion of these pools would require further adaptation to the brackish or even fresh surface waters typically encountered in the anchialine habitat. A final step would involve dispersal from these inland pools to completely fresh water cave systems. This series of events would be a continuous process not dependent upon sea level movement or position. The great age of limestone platforms such as the Bahamas would provide long
periods for these evolutionary steps to occur. The postulated mobility of cavernicoles is evidenced by their powers of dispersal to colonize comparatively young caves.

Thomas M. Iliffe, Department of Marine Biology, Texas A&M University at Galveston, P.O. Box 1675, Galveston, Texas 77553, USA.
E-mail: iliffet@tamug.ed

Keywords: anchialine, evolution, caves; zonation, dispersal