THE FUNCTION OF THE OCELLOID AND PISTON IN THE DINOFLAGELLATE ERYTHROPSIDINIUM (GYMNODINIALES, DINOPHYCEAE) 1 Fernando G omez 2 Carmen Campos Panisse 3, E-11500 Puerto de Santa Mar  ıa, Spain The marine dinoflagellate Erythropsidinium possesses an ocelloid, the most elaborate photoreceptor organelle known in a unicellular organism, and a piston, a fast contractile appendage unknown in any other organism. The ocelloid is able to rotate, often before the cell swims. The ocelloid contains lenses that function to concentrate light. The flagellar propulsion is atrophied, and the piston is responsible for locomotion through successive extensions and contractions. During the “locomotion mode”, the contraction is ~4 times faster than the extension. The piston attained up to 50 mm  s 1 and the cell jumps backwards at 4 mm  s 1 , while during the piston extension the cell moves forwards. The net speed of ~1 mm  s 1 is faster than other dinoflagellates. The piston usually moved in the “static mode” without significant cell swimming. This study suggests that the piston is also a tactile organelle that scans the surrounding waters for prey. Erythropsidinium feeds on copepod eggs by engulfing. The end of the piston possesses a “suction cup” able to attach the prey and place it into the posterior cavity for engulfing. The cylindrical shape of Erythropsidinium, and the anterior position of the ocelloid and nucleus, are morphological adaptations that leave space for the large vacuole. Observations are provided on morphological development during cell division. Most of the described species of Erythropsidinium apparently correspond to distinct life stages of known species, and the genus Greuetodinium (=Leucopsis) corresponds to an earlier division stage. Key index words: Dinoflagellata; Erythropsis; eyespot; motile behavior; motility; Nematodinium; peduncle feeding; photoreceptor organelle; protist locomo- tion; Warnowia Abbreviations : fps, frames per second; ms, millisec- ond; TEM, transmission electron microscopy The warnowiid dinoflagellates, Erythropsidinium P. C. Silva, Warnowia Er. Lindemann, Nematodinium Kofoid et Swezy and Proterythropsis Kofoid et Swezy, are a group of unarmored heterotrophic species that possess a complex photoreceptor organelle named the ocelloid. Erythropsidinium (=Erythropsis Hertwig) possesses the largest and most elaborate ocelloid among all the unicellular organisms (Colley and Nilsson 2016). Hertwig (1884) described Ery- thropsidinium agile (Hertwig) P. C. Silva (as Erythropsis agilis Hertwig) with an eyespot composed of a pig- ment mass and lens, and a sophisticated body exten- sion capable of extraordinary rapid extension and contraction. This contractile organelle, named pis- ton, dart, prod or tentacle, is unknown in any other organism (Kofoid and Swezy 1921, Greuet 1967). The unusual degree of structural complexity of the ocelloid and the piston generated disbelief among earlier protozoologists. Greuet (1965, 1967, 1968a,b, 1969, 1973, 1977, 1987) carried out numerous observations, mainly ultrastructural studies based on transmission electron microscopy (TEM). The ocel- loid is a highly developed photoreceptor organelle with cornea-, lense-, pigment cup and retina-like structures consisting of stacked membranes (Greuet 1968a, 1987, Gavelis et al. 2015). The ocelloid exhi- bits a parallelism to the structure of metazoan eyes, with all of the major functional components except for the conduction of signals, constructed entirely at the subcellular level. Greuet (1977) considered that the ocelloid derived from a chloroplast and is homologous with the eyespot or stigma. Francis (1967) investigated the optical properties of the lens in Nematodinium. He reported that in the pyriform lens there is no single point of focus but rather a zone of focus, and only objects ~50 lm distant are focused in the retinoid. Francis (1967) suggested that the ocelloid functions as an image-forming eye. Erythropsidinium, together with the other ocelloid- bearing dinoflagellates, belongs to the clade of Gymnodinium F. Stein (G omez et al. 2009). Relatives such as Polykrikos Buetschli are able to shoot ejectile bodies, nematocysts or taeniocysts, to narcotize and immobilize prey such as fast-swimming red-tide dinoflagellates (Matsuoka et al. 2000, Lee et al. 2015). Warnowiid dinoflagellates such as Nemato- dinium also possess nematocyst-taeniocyst complexes (Greuet 1987), and it has been suggested that the ocelloid plays a role in the prey selection and cap- ture, and the cell fires its ejectile bodies, “explosive stingers”, only when the target is at the right direc- tion and distance (Omodeo 1980, Taylor 1980). 1 Received 24 July 2016. Accepted 7 February 2017. 2 Author for correspondence: e-mail fernando.gomez@fitoplanc ton.com. Editorial Responsibility: J. Raven (Associate Editor) J. Phycol. *, ***–*** (2017) © 2017 Phycological Society of America DOI: 10.1111/jpy.12525 1