Infection of Coscinodiscus spp. by the parasitoid nanoflagellate Pirsonia diadema: I. Behavioural studies on the infection process

Abstract

The investigation of successive steps involved in the infection process of the marine diatoms Coscinodiscus granii and Coscinodiscus wailesii by the host-specific parasitoid nanoflagellate (PNF) Pitsonia diadema showed that flagellates reacted chemoklinokinetically with changes of swimming pattern to the presence of a host diatom. Chemosensory stimulation appeared to induce readiness for infection, whereas attachment and penetration of the diatom cell wall was induced by a mechanosensory response to morphological features on the diatom frustules. The mean swimming speed of P. diadema flagellates decreased during their infective lifetime of 3 days from an average of 78 μm s-1 to 51 μm s-1, while the frequency of small loops in the swimming pattern increased from 0.8 to 6.3 loops min-1. At high C. granii densities, an epidemic was delayed. It is suggested that this could be caused by overlapping gradients of extracellular material released by the diatoms which impaired the sensing of spatial gradients by PNF and, therefore, the location of hosts.