Phototactic responses of four marine dinoflagellates with different types of eyespot and chloroplast

SUMMARY Great structural variety is seen in the eyespot of dinoflagellates, a structure involved in phototaxis. Although there are several works on the phototactic responses in some species of dinoflagellates, none of the dinoflagellates used in these studies possessed an eyespot and, therefore, we have no knowledge of the relationship between eyespot type and phototactic response. In this study, we determined wavelength dependency curves for phototaxis in four marine dinoflagellates that possess a different type of either eyespot or chloroplast. These include: (i) a dinoflagellate possessing a peridinin‐containing ohioroplast with an eyespot (Scrippsiella hexapraecingula Horiguchi et Chihara); (ii) a dinoflagellate containing a diatom endosymbiont and with the type B eyespot sensu Dodge (1984; (Peridinium foli‐aceum (Stein) Biecheler); (iii) a dinoflagellate with peri‐dinin‐containing chloroplasts, but lacking an eyespot (Atexandrium hiranoi Kita et Fukuyo); and (iv) a dinoflagellate with fucoxanthin, 19′‐hexanoyloxyfucoxanthin and 19′‐butanoyloxyfucoxanthin, but lacking an eyespot (Gymnodinium mikimotoi Miyabe et Kominami ex Oda), Regardless of the eyespot or the chloroplast type, all four dinoflagellates showed similar wavelength dependency curves for phototaxis, with sensitivity between 380 and 520 nm, the highest peak at approximately 440 or 460 nm and smaller peaks or shoulders at 400–420 nm and 480–500 nm. Substantial peaks have also been noted in the ultraviolet range (260–280 nm). The ultrastructural study of the eye‐spot of Scrippsiella hexapraecingula revealed that the eyespot consists of two layers of lipid globules and probably acts as a quarter‐wave stack antenna.