Distribution, variety and life cycles

Abstract

The known species of cyclostomes are partitioned between the two Hemispheres and separated by a tropical and subtropical zone, where for the most part they are absent. There can be no doubt that the main limiting factor in this antitropical distribution is temperature and their northward or southward range is said to be restricted by the annual 20° isotherm. Only two species of lamprey appear to depart from this pattern. These are the two Mexican freshwater forms, Tetrapleurodon spadiceus and T. geminis, but these occur at high altitudes where water temperatures are unlikely to exceed the levels that lampreys are able to tolerate. At least during their long larval life, a lethal temperature limit of 29–31°C has been established for four different species of ammocoete (Potter and Beamish, 1975). The extreme northward or southward penetration of lampreys in the Northern or Southern Hemispheres cannot be defined with precision since little is known of their oceanic feeding ranges, but in the Northern Hemisphere lampreys certainly occur off the coasts of Northern Scandinavia, Siberia and Alaska, while in the Southern Hemisphere, Geotria australis is known to feed in Antarctic waters (Section 12.4.3).