Baltic Eubosmina morphological radiation : Sensitivity to invertebrate predators (induction) and observations on genetic differences

Abstract

Cladocerans in the genus Eubosmina exhibit spectacular and confusing morphological diversification in the Baltic region. The paleolimnological record suggests that lineages in regional lakes diverged recently from morphologically conservative E. longispina/coregoni-like ancestors and developed strong seasonal elongation of antennules, mucrones, or dorsal humps (cyclomorphosis). Allozyme studies on Schleswig-Holstein populations by DE MELO agree with this interpretation by demonstrating: 1) genetic difference between geographically distant E. longispina populations, 2) clustering of E. coregoni forms with local E. longispina lineages, and 3) great genetic similarity among regional E. coregoni forms, despite major morphological differences. Retention of distinguishing traits (shape of dorsal humps, antennule length) in common-garden experiments emphasizes that micro-evolutionary (genetic) differences underlie "form" phenotypes. However, regression of extreme summer forms towards a more conservative morphology in laboratory culture indicates that part of the seasonal variability is attributable to developmental plasticity. Split-clone laboratory experiments demonstrate that longer antennule lengths of an exuberant form, E. coregoni kessleri from the Grosser Ploner See, are increased (induced) in the presence of predatory cyclopoids. Induction responses strengthen the notion that long-term transformations involve evolutionary responses to invertebrate predators (cyclopoid copepods, Leptodora) which colonized large lakes during post-glacial periods, with the particulars of responses showing lineage and lake specificity.