Body size and developmental response of a rotifer to the predator <i>Asplanchna</i>

ABSTRACT A large and small strain of a new taxon of Brachionus have very different morphological responses to a kairomone produced by the predatory rotifer Asplanchna. The large, less susceptible strain has no posterolateral spines without Asplanchna and only short (46 μm) ones, together with a larger lorica, with it. The small, more readily ingested strain has short (40 μm) posterolateral spines without Asplanchna and extremely long (103 μm) ones, together with a greater increase in lorica length, with it. In this rotifer, large body size provides an effective defense against Asplanchna predation that reduces selection for the possibly costly development of greater spine and lorica lengths in the presence of the predator.