LAUSR

Two morphologically distinct Brachionus calyciflorus hatched from Lake Littra sediment: a typical B. calyciflorus Pallas and B. (calyciflorus) “Darling”, a reproductively isolated, globular taxon without posteromedian spines. The Darling taxon is well defended against Asplanchna by its large size (adult lorica length ≥ 310 μm) and short (~50 μm) Asplanchna-induced posterior spines that protect small, vulnerable neonates. The typical B. calyciflorus is smaller (adult lorica length 169–195 μm), and is a mixture of interbreeding clones with or without constitutive posterolateral spines. In one of the former clones, adult posterolateral spine length increased significantly with maternal age, from ~62 to 78 μm. Asplanchna induced increased lorica length and pronounced spine development in all clones, and its effect on females with constitutive posterolateral spines was additive. With Asplanchna, posterolateral spine length was 159 μm for a clone without constitutive posterolateral spines and 207 μm in a clone with 74 μm-long constitutive posterolateral spines. Coexistence of clones with and without constitutive posterolateral spines suggests a trade-off between the benefit of long defensive spines and some cost to spine development. It likely is maintained by fluctuating directional selection in response to changing Asplanchna abundance, and by a sediment bank of diapausing eggs produced by sexual reproduction.