Torodinium

Torodinium (ˌtɔɹoʊˈdɪniəm) is a genus of unarmored dinoflagellates and comprises two species, Torodinium robustum and the type species Torodinium teredo. The establishment of Torodinium, as well as the characterization of the majority of its morphology, occurred in 1921 and further advances since have been slow. Lack of research is largely due to its extremely fragile and easily deformed nature, which also renders fossil records implausible. The genus was originally characterized by torsion of the sulcus and a posterior cingulum. Since then, new distinctive features have been discovered including an extremely reduced hyposome, a longitudinally ribbed episome, and a canal on the dextro-lateral side. Further investigation into the function of many anatomical features is still necessary for this genus.

The original book in which Torodinium was first named does not explain the meaning of the genus’s name. However, the suffix root “dinium”, commonly used in dinoflagellate naming, is known to be derived from the word “vortex”. The prefix of “toro”, though not confirmed by the original genus identifiers, is likely derived from the Latin root “tort” meaning “wrong” and probably refers to the marked torsion of the sulcus which was first used to identify Torodinium as a distinct and independent genus. Similarly, the etymology of the species T. robustum may be inferred from the Latin word robustum meaning “strong”. This is perhaps referring to the stouter morphology of the species when compared with the longer and more slender type species T. teredo, as this is how Kofoid and Swezy first distinguished the two upon their discovery. The etymology of T. teredo is unknown due to the fact that T. teredo was first named as Gymnodinium teredo in a paper published in 1885.

Torodinium was first characterized as a distinct genus by Charles A. Kofoid (1865–1947), and Olive Swezy in their 1921 book on unarmoured dinoflagellates. Kofoid and Swezy used silk planktonic nets to collect numerous dinoflagellates from the ocean off the coast of La Jolla, California in the summer of 1921. Prior to this study, species of the Torodinium genus were instead considered Gymnodinium, a relative of Torodinium now classified under their common order, Gymnodiniales. The type species of Torodinium, T. teredo, was originally named as Gymnodinium teredo. Later, Schütt found both species, T. robustum and T. teredo, but classified them as the same species, G. teredo. Although Kofoid and Swezy admit in their paper to have only observed T. teredo, their suspicions as to the existence of T. robustum, inferred from Schütt’s drawings, were later confirmed.

Torodinium was deemed its own genus due to its characteristic sulcal torsion and posterior cingulum housing the longitudinal anterior flagellar pore. While no other member of the Gymnodinium genus had ever been found to have a twisted sulcus, T. teredo had a 0.5 turn marked torsion at the anterior end; similarly, while the anterior flagellar pore in Gymnodinium may be located somewhat posteriorly in some species, it never crosses the midline and certainly never exists as close to the antapex as it does in what is now known to be T. teredo. From this, Torodinium teredo was established as the type species for the new genus Torodinium.

Since its original establishment in 1921, there have been no new species added to the Torodinium genus, though changes in knowledge have certainly occurred in regard to the identification and functionality of various anatomical and morphological features, described in more detail later. Research regarding this is lacking largely due to the extremely delicate nature of the athecate genus and subsequent difficulty observing them with microscopy. Fixing the organism from culture often results in deformities while light microscopy has been found to quickly damage live specimen.

Torodinium is a planktonic marine genus of dinoflagellate, occupying mainly warmer water regions due to its lack of protective thecal plates. The genus is free-living, meaning it does not go through a parasitic phase and instead obtains energy independently. Though previously described as exclusively phototrophic, and containing longitudinal chloroplasts filled with chlorophyll a, the question of whether Torodinium is in fact able to feed or not is up for debate.;) The presence of what appears to be a “feeding veil” (present in other dinoflagellates), as well as multiple accumulation bodies resembling food vacuoles give the impression that Torodinium may be able to feed after all, as well as photosynthesize.

Perhaps most important in lending support to this feeding hypothesis was the discovery of an elongated protuberance extending out from the sulcal-cingular region of the small hyposome. This extension was tentatively termed a peduncle, an organelle used for capturing and reeling in prey in feeding dinoflagellates, and has been found to retract in live Torodinium species under stress. Other Gymnodinioid species have been found to have a similar body extension, although they tend to feed by engulfing prey through the sulcal region of the hyposome. It has been theorized that, due to the very reduced size of their hyposome and posterior localization of the sulcus, Torodinium species are not able to perform this same engulfing technique and instead have to employ the use of this body extension to feed, though this is merely conjecture at the present time. Additionally, the discovery of the lateral canal (discussed further later) has led some researchers to believe that it too has a role in ingestion of particulates. If they are confirmed to be the organelles hypothesized, the combination of the tentatively identified feeding veil, food vacuoles, peduncle, and lateral canal would indicate a possible mixotrophic lifestyle in Torodinium.

Both species of Torodinium are pelagic and are distributed globally, mainly in warmer regions of the open ocean. Below is a list of the places where each of the two species has been found individually and where they have been found together.