The biogeography of littoral Lecane Rotifera

Abstract

Little is known on the biogeography of Rotifera, particularly regarding the littoral taxa. Here, the biogeography of the most specious group of littoral Rotifera, Lecane, is discussed based on a recent revision of the group, and considering only verified records.

Only 41.3% of all 167 Lecane morphospecies are widely distributed. Of these, 21 (12.6%) are cosmopolitan (sensu strictu), 26 (15.6%) are Tropicopolitan. There are 6 (3.6%) Arctic-temperate and Pantropical taxa. These categories of widely distributed taxa are not clear-cut, as differences in latitudinal distribution are largely gradual. Ten taxa have odd distributions. Most of these are insufficiently known and some may be recent introductions.

All the major zoogeographical regions have their share of endemic taxa, with endemicity rates varying from 6.5% to 21.8%. Thirteen Holarctic, one widespread Eastern hemisphere, eleven Palaearctic, six Palaeotropical, one Australasian and five widespread Western hemisphere taxa have circumscribed ranges encompassing more than one region. The Palaeotropical and Holarctic components are particularly noteworthy, and may result from relatively recent faunal exchange between the Palaearctic and Nearctic, and between the African, Oriental and Australian regions. The different orientation of the major mountain chains of the Eastern and Western hemisphere, and the presence of extensive arid regions in Northern Africa, Arabia and Asia may explain the difference in faunal similarity between the tropical and temperate faunas of the Eastern and Western hemispheres. Ornithochoric dispersal and human introductions may have played a role in the exchange of faunas.

The analysis of pairs or groups of closely related taxa only rarely reveals a causal relation between tectonical events and phylogeny. Most vicariant distributions are readily explained by climatological rather than by tectonical considerations. The ranges of some closely related taxa indicate that they have different capacities of dispersal.

When compared to pelagic Rotifera, littoral Lecane exhibit a relatively wide variety of distribution patterns, and relatively many have restricted distributions. This may be a consequence of, either or both, their lower abundance, or be related to the littoral habitat which, being less predictable, implies more specific adaptations. Lecane is tropic-centred, which may be a consequence of avoidance of competition and/or predation by ‘Cladocera’ and, eventually, Ostracoda.

The most striking feature in rotifer chorology is the large range of many morphospecies. As such, the group's biogeography is more similar to that of freshwater algae than to that of other freshwater zooplankton. The likely explanations for this are the group's great ability for passive dispersal, combined with a short life-cycle and high fecundity, which are consequences of their parthenogenetic reproduction. So, relatively recent long-distance dispersal defines the ranges of the morphospecies. Vicariance plays a role in the distribution of Rotifera, but its importance is generally subordinate to that of dispersal. However, insufficient taxonomic resolution may be responsible for the apparently large ranges of some morphospecies.