Morphological stasis of two species belonging to the L-morphotype in the Brachionus plicatilis species complex

Abstract

Detection and characterization of sibling species complexes in zooplankton are critical to understanding their ecological responses and patterns of evolution. The taxon Brachionus plicatilis is a complex of at least 14 species with three major, deeply diverged clades, which are morphologically distinct. We studied morphometric differences between two species — B. plicatilis sensu stricto and B. ‘Manjavacas’ — which belong to the L-(large) morphotype and often co-occur in ponds or lakes. B. plicatilis s.s. was on average 6% longer than B. ‘Manjavacas’. They differed significantly in the measurements related to lorica spines. A significant discriminant function relating spine measurements was found, however, individuals from each species showed extensive overlap. Our morphometric data provide additional evidence for the species status of B. plicatilis s.s. and B. ‘Manjavacas’. Since these are ancient species, our results support that a morphological stasis occurs in these taxa. We identified COI restriction sites for PvuII and KpnI which are diagnostic for B. ‘Manjavacas’ and B. plicatilis s.s., respectively. We conclude that morphometry is not useful in classifying the two species. At present, this can only be done reliably using molecular methods.