Polykrikos (from Greek “poly” - many, and “krikos” – ring or circle) is one of the genera of family Polykrikaceae that includes athecate pseudocolony-forming dinoflagellates. Polykrikos are characterized by a sophisticated ballistic apparatus, named the nematocyst-taeniocyst complex, which allows species to prey on a variety of organisms. Polykrikos have been found to regulate algal blooms as they feed on toxic dinoflagellates. However, there is also some data available on Polykrikos being toxic to fish.
History of knowledge
Polykrikos was first seen in 1868 by Uljanin and was mistakenly considered as a metazoan larva of a turbellarian flatworms. In 1873 Butschili re-examined the specimen and concluded that the cell was an unusual ciliate, and Bergh later, in 1881, clarified Polykrikos dinoflagellate affinities.
Morphology
Polykrikos is a colony of zooids (units of a colonial organism) that carry out simultaneous functions of a whole cell.
All Polykrikos species have: 1) a slightly curved longitudinal furrow, sulcus, extending to posterior end of the organism 2) a loop-shaped acrobase, which is an anterior extension from the sulcus 3) a transverse furrow, cingulum, with the displacement 4) taeniocyst-nematocyst complexes 5) two or four times less the number of nuclei than of zooids, and 6) ability to disassemble into pseudocolonies with fewer zooids and only one nucleus.
The most distinctive trait of this genus is the formation of multinucleated pseudocolonies that consist of an even number of zooids. Each zooid has a pair of flagella (transverse and longitudinal flagella) and has its own transverse groove, cingulum, but zooid longitudinal furrows, sulci, are fused. Transverse flagellum has the lateral projections, mastigonemes, and striated strand common to other dinoflagellates. Often Polykrikos have half the number of nuclei than zooids, and each pair of zooids shares a nucleus. Within the group there is some variation in which organelles are presented, but trichocysts, nematocysts, taeniocysts, mucocysts and plastids have been observed from different members within the taxon.
Cytoplasm of Polykrikos is characterized by numerous rough endoplasmic reticulum nets, Golgi complexes and vacuoles. Polykrikos are known to produce ejectile organelles, the extrusomes. One of them is a nematocyst formed in zooids. Another extrusome found within the organism is rod-shaped taeniocyst which is distally located to nematocyst and was earlier mistakenly considered as a nematocyst-precursor. Together these organelles are forming taeniocyst-nematocyst complex that is thought to be the best synapomorphy for Polykrikos clade. Golgi-derived vacuoles are shared by both organelles and supply each with molecules needed for its growth along with participating in NTC articulation. Organelles are located in proximity, but lie within different membranes and are separated by a passage, called “chute”. The nematocyst is a larger organelle and lies posterior to taeniocyst. Some recent research have shown that the work of two organelles is coupled, with the taeniocyst adhering to prey, followed by nematocyst discharge leading to prey puncturing and, lastly, retrieving the prey using a tow filament, located on the end of the nematocysts close to posterior vesicle. The tubule, embedded within nematocyst, discharges towards the prey and hypothesized to be used for prey puncturing.
Furthermore, Gavelis et al. deeply examined NTC morphology and ballistic mechanism that were shown to be fundamentally different from cnidarians, demonstrating nematocysts have evolved independently in single-celled dinoflagellates. Encasing coiled tubule capsule, unlike in cnidarians, is sealed, which forces stylet upon firing first to puncture the capsule from within to free the filament, and only later to pierce the prey. As the tubule passes through the nozzle, it opens the operculum and uncoils after. Ballistics in cnidarians nematocysts is driven by synthesis of osmotic propellant poly gamma glutamate synthase, PgsAA, while in Polykrikos it is thought to occur due induced pressure as a result of capsular fibre contraction in the capsule wall. Nucleus is uniquely characterised by a double-layered fibrous cortex that underlines evaginated nuclear envelope; cortex is hypothesized to provide strength and shape to the nucleus, while nuclear evaginations are thought to increase nuclear-cytoplasmic exchange area at cortex perforation sites.
In Polykrikos, well-defined fibrous ribbons are involved in nuclear-flagellar connections, and anchoring to flagellar apparatus might serve in orientation of the nucleus in relation to flagella during processes of movement, mitosis and cell division.