DISCRIMINATION BETWEEN LIVING AND HEAT-KILLED PREY BY A MARINE ZOOFLAGELLATE, PARAPHYSOMONAS-VESTITA (STOKES)

A recombination-deficient strain (EM1035) of Escherichia coli (Migula) was used to test the ability of the marine zooflagellate Paraphysomonas vestita (Stokes) to discriminate between living and heat-killed prey of similar size and morphology. Cell division of EM1035 was prevented by relatively short exposure to UV-irradiation. Fluorescent staining with rhodamine isothiocyanate did not affect the viability of the bacterial cells or the growth and feeding rates of the zooflagellate. P. vestita fed preferentially on living cells when presented with an equal density mixture of heat-killed and UV-irradiated (nondividing) cells. The flagellate discriminated between living and dead cells with a preference ratio of about 20 suggesting that chemosensory cues may be important in the feeding selectivity of some marine protozoans.