Nested PCR

Abstract

Brachionus variabilis, a facultative epibiont on cladocerans, has a pronounced morphological response to Asplanchna. Adult amictic females of the Asplanchna-induced morph, compared with those of the basic or non-induced morph, had a 13% larger lorica, 150% longer posterior spines and 30–40% longer anterior spines. Similar responses occur in many other brachionid rotifers as an effective defense against Asplanchna predation, but the response of B. variabilis is surprising. The large size and epizoic habit of B. variabilis should provide good protection against this predator. Also, a smaller epizoic congener (Brachionus rubens) readily ingested by Asplanchna has no morphological response to it. Brachionus variabilis also has a pronounced behavioral response to Asplanchna. Without Asplanchna, most individuals are actively swimming, with only a small proportion of them attached to surfaces. With Asplanchna, the proportion of attached individuals is much higher. Significant attachment responses in both young and adult individuals occurred shortly (?5 h) after exposure to live Asplanchna (three experiments) or to its kairomone (five experiments), but not to a Daphnia kairomone. The Asplanchna-enhanced propensity for attachment to surfaces, possibly including cladoceran hosts, may be an additional defense against this predator. This is the first report of a predator kairomone inducing a behavioral shift in a rotifer.