ELECTRICAL RESPONSES TO WATER-SOLUBLE COMPONENTS OF FISH MUCUS RECORDED FROM THE CNIDOCYTES OF A FISH PREDATOR, PHYSALIA-PHYSALIS

Physalia physalis, the Portuguese man of war, consumes mostly fish and fish larvae. Intracellular recordings from nematocyst-containing cell s (cnidocytes) in small pieces of Physalia tentacle were used to quant ify the electrical responses to diluted and filtered fish skin mucus, 1-100 x 10(-6) M amino acids, monosaccharides, and nucleosides, and se awater, which were delivered upstream of the tissue. Seawater caused r esponses (one pulse only) in about 10% of the applications. Fish mucus extract elicited responses in all applications, producing 1-18 depola rizing pulses (20 mV maximum amplitude). The pulses were characteristi c of post-synaptic potentials (EPSPs). Lucifer yellow and biocytin dye injections showed that the cnidocytes were not electrically coupled. Simultaneous records from two cnidocytes following mucus applications were identical. We propose, therefore, that the chemoreceptors are not on the cnidocytes, but are probably on sensory neurons that innervate clusters of cnidocytes. A < 3,000 MW fraction of mucus elicited respo nses indistinguishable from whole mucus extract. Higher molecular weig ht fractions caused no response. The various test solutions had lower percentages of response (47-92%) and produced significantly fewer puls es than the mucus extract. We conclude that prey capture in Physalia i s facilitated by chemicals present in the mucus covering their fish pr ey. The chemical stimuli probably sensitize the nematocysts to dischar ge upon mechanical stimulation.