Studies on the interactions between P.diadema and Cgranii showed that
spreading of infections strongly depended on the internal biological behaviour of
the flagellate, e.g. chemosensory response, swimming patterns and age of flagellate
(Kiihn, 1997b). Light microscope observations and preliminary experiments
indicated that C.wailesii was the preferred host species, whereas C.concinnus was
least attractive as a host (interspecific selectivity). It was not clear whether this
was caused by differences in morphological and/or physiological properties
between the diatom species. Among unialgal cultures of Cgranii and C.wailesii,
certain individual cells were apparently more easily infected than others
(intraspecific selectivity). Studies on parasitoid behaviour are rare, however, and to the author's knowledge both inter- and intraspecific selective infection behaviour
of parasitoid protists has not yet been investigated.