Spine development in the rotifer Keratella cochlearis: induction by cyclopoid copepods and Asplanchna

SUMMARY. 1. Spine development in a clone of unspined Keratella cochlearis was promoted by a water soluble factor released by the copepods Tropocyclops prasinus and Mesocyclops edax and by the predatory rotifer Asplanchna priodonta. Between 9 and 55% of K. cochlearis populations cultured in predator-conditioned media responded to the inducing chemical.
2. The K. cochlearis form possessing a posterior spine of medium length was much less susceptible to predation by small Tropocylops and Asplanchna than the form lacking the posterior spine. These predators consistently selected the unspined form over the spined form when offered equal densities of each. However, both spined and unspined forms were equally susceptible to predation by large Mesocyclops.
3. The phenotypic variation of offspring produced from induced unspined Keratella females encompassed much of the variation reported for the taxon in North America. These morphotypes are similar to the variation of forms reported for the K. cochlearis tecta series known from Europe. We suggest that the North American morphotypes be identified in studies of this species because the presence of the posterior spine can greatly affect predator selectivity.