Oxytoxum flowing
The dinoflagellates are perhaps one of the better studied groups of microbial eukaryotes but there are still many subgroups of dinoflagellates that remain poorly known. One such subgroup is the genus Oxytoxum. These distinctive looking marine flagellates, though widespread, are rarely collected in large numbers. They are most associated with open waters of warm temperate and tropical regions. Because of their rarity, offshore habitat, and because attempts to culture them in the laboratory have thus far failed, Oxytoxum species have never received the focus accorded to those dinoflagellates that impact humans through their formation of toxic blooms.
Oxytoxum species have an elongate cell shape with the cingular groove closer to the cell’s apex than antapex. Both ends of the cell typically bear a long spine so the overall appearance of the cell is somewhat reminiscent of a spinning top. A preliminary molecular phylogenetic analysis of Oxytoxum by Gómez et al. (2016) supported its separation from the closely related genus Corythodinium, often regarded as synonymous. Both genera share a distinctive arrangement of plates in the theca with five plates in the postcingular ring and a single antapical plate (most dinoflagellates have two antapical plates). However, Corythodinium species are broader with a stronger cingular displacement (in Oxytoxum, the ends of the cingulum form a more or less continuous ring around the cell whereas, in Corythodinium, one end is distinctly offset when it reaches the other). Cyst formation in Oxytoxum and Corythodinium is as yet unknown.
Over fifty species have been assigned to Oxytoxum down the years; even allowing for the separation of Corythodinium, it remains a diverse genus. However, the taxonomic status of many Oxytoxum species remains poorly known (Gómez 2018). Many were described from limited original material and may represent variants of other species. Only a handful have yet been subject to molecular analysis. Of particular note is the potential for confusion due to ontogeny. When Oxytoxum cells divide, one daughter cell ends up with the original apical section of the parent and the other with the original antapex. As a result, young cells initially lack the spine at one end of the cell or the other until their own theca completes development. This can, of course, significantly affect their appearance and different developmental stages may have been mistaken for different ‘species’.
Broader relationships of the Oxytoxum-Corythodinium clade (the Oxytoxaceae) remain uncertain. Gómez et al. (2016) identified them as phylogenetically isolated, without close living relatives, but molecular studies of early branching of dinoflagellates remain poorly resolved. Fensome et al. (1993) suggested similarities between Oxytoxaceae and the Mesozoic fossil families Comparodiniaceae and Nannoceratopsiaceae. If these comparisons carry any weight, the Oxytoxaceae may have been spinning through our seas for hundreds of millions of years.
References
Fensome, R. A., F. J. R. Taylor, G. Norris, W. A. S. Sarjeant, D. I. Wharton & G. L. Williams. 1993. A classification of living and fossil dinoflagellates. Micropaleontology Special Publication 7: i–viii, 1–351.
Gómez, F. 2018. A review on the synonymy of the dinoflagellate genera Oxytoxum and Corythodinium (Oxytoxaceae, Dinophyceae). Nova Hedwigia 108 (1–2): 141–165.
Gómez, F., K. C. Wakeman, A. Yamaguchi & H. Nozaki. 2016. Molecular phylogeny of the marine planktonic dinoflagellate Oxytoxum and Corythodinium (Peridiniales, Dinophyceae). Acta Protozoologica 55: 239–248.