The susceptiblity of Keratella cochlearis to interference from small cladocerans

SUMMARY. 1. Even at high population densities (300–2000 ind. 1^?1), only one of five small cladoceran species (adult body length <1 mm) significantly suppressed population growth of the rotifer Keratella cochlearis through interference (encounter) competition. At 500 ind. H, adults of D. ambigua (0.96 mm body length) imposed an instantaneous per capita death rate of 0.21 day^?1 on this rotifer. These short-term experiments may have underestimated cladoceran interference because newborn rotifers were rarely present.
2. Newborn rotifers (<12 h old) were much more susceptible than adult rotifers (> 24 h old) to interference from Ceriodaphnia dubia. All of the small cladoceran species tested were very much less likely than large Daphnia (body lengths >1.2 mm) to interfere with K. cochlearis , but perhaps at high population densities they could suppress population growth of susceptible rotifer species by damaging, and possibly eating, relatively small and soft-bodied newborn individuals.
3. K. cochlearis of the tecta form, without a posterior spine, produced offspring of the typica form, with a posterior spine, in the presence of C. dubia. This developmental response is stimulated by at least several, and possibly all, cladocerans and probably reduces the susceptibility of the rotifer to cladoceran interference.