Chemosensory attraction of zoospores of the estuarine dinoflagellates, Pfiesteria piscicida and P. shumwayae, to finfish mucus and excreta

Abstract Toxic strains of the estuarine dinoflagellates, Pfiesteria piscicida and P . shumwayae , can cause fish death and disease, whereas other estuarine ‘lookalike’ species such as cryptoperidiniopsoids have not been ichthyotoxic under ecologically relevant conditions. Chemosensory attraction of three functional types of these Pfiesteria spp. were separately evaluated for their attraction to fresh fish mucus and excreta. Clonal cultures of actively toxic (TOX-A, engaged in killing fish) and temporarily nontoxic (tested as toxic but without access to live fish for >1 week to 5 months (in most experiments, ≤3 months) as ‘short-duration’ TOX-B; and without access to live fish for ≥1.5 years as ‘long-duration’ TOX-B) functional types of P . piscicida and P . shumwayae were derived from the same clones whereas the non-inducible cultures (NON-IND, tested as incapable of toxic activity in the presence of fish), of necessity, were from different clonal isolates. NON-IND cultures previously had been grown on algal prey for 3–8 months, and had repeatedly been tested as incapable of causing fish distress, disease or death via toxic activity. Attraction to fish materials was based on the number of zoospores that entered microcapillary tubes containing sterile-filtered 15-ppt water (controls), vs. entry into tubes with sterile-filtered mucus and excreta (collected in 15-ppt water) that had been collected from live tilapia, bluegill, hybrid striped bass, and Atlantic menhaden (tested separately within 3 h of removal from live fish). TOX-A zoospores of both Pfiesteria species exhibited the strongest attraction to the fish mucus and excreta, with comparable response to the materials from all four test fish species. Short-duration TOX-B zoospores showed an intermediate response that apparently depended on the duration of mucus separation from the live fish: the shorter the separation period, the stronger the zoospore attraction to the fish materials. In contrast to TOX-A and short-duration TOX-B zoospores, NON-IND and long-duration TOX-B zoospores generally showed little or no response to the fish materials. Zoospores of the cryptoperidiniopsoid demonstrated a moderate attraction that did not appear to depend on the time of isolation from fish. TOX-A zoospores were also tested for attraction to sterile-filtered vs. non-filtered fish mucus (time separated from the live animal, 3–96 h). These zoospores, which initially had been actively attracted, were no longer attracted to the unfiltered fish materials after 48 h, whereas attraction to the sterile-filtered fish mucus and excreta persisted throughout the duration of the experiment. Thus, the attractant signal in the materials was degraded or effectively blocked by the bacterial community within hours of isolation from live fish. This study indicates the importance of functional type or toxicity status, and the importance of the history of exposure to live fish, in the behavioral ecology of Pfiesteria spp. Initial attraction to fish materials strongly depended on the functional type, and on the history of toxic activity. Non-inducible and long-duration TOX-B cultures of Pfiesteria spp., unlike actively toxic and short-duration TOX-B (potentially toxic) strains, initially were virtually unresponsive to fish mucus.