Attachment behavior in the rotifer Brachionus rubens: induction by Asplanchna and effect on sexual reproduction

Experiments with two strains of the facultative epibiont, Brachionus rubens, tested the ability of this rotifer to avoid predation by the rotifer Asplanchna, and its propensity for sexual reproduction and consequent diapause at different population densities. Unlike some congeners, B. rubens did not have a morphological response to Asplanchna by developing longer spines or a larger body. However, it responded to this predator, and its kairomone, with a behavioral defense: a higher propensity to transition from free-swimming to attachment, typically in dense aggregations. Attached individuals were less likely to be captured and ingested, so that B. rubens outlived Asplanchna in some mixed-species cultures. Although crowding induces sex in some congeners, it did not do so in B. rubens. Instead, the proportion of sexual (mictic) daughters produced by females cultured in different volumes (0.5-60 ml) was density-independent: similar to 0.2 for the Argentina strain and similar to 0.6 for the Australia strain. Such fixed levels of sex have rarely been detected in rotifers. In B. rubens, they may be a strategy to ensure some sex with diapause at all times, but permit continued population growth via female parthenogenesis at the very high densities that normally occur on hosts and other surfaces.