Oxytoxum scolopax, from Taylor (1976).

Belongs within: Peridinoidia.

Oxytoxum flowing
Published 28 January 2023

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.

Cells of Oxytoxum scolopax, from Gómez et al. (2016); scale bar = 20 µm.

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’.

Oxytoxum longiceps, from here.

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.

Systematics of Oxytoxum
Oxytoxum Stein 1883 (see below for synonymy)FT93
|--*O. scolopax Stein 1883SW70
|--O. adriaticum Schiller 1937G18
|--‘Murrayella’ australica Wood 1963S73
|--‘Ceratium biconicum’ non (Kofoid 1907) Dodge & Saunders 1985 (see below for synonymy)S73
|--O. boehmiiG18
|--O. breve Kofoid & Michener 1911G18
|--‘Murrayella’ brianii Rampi 1941 [=Pavillardinium brianii (Rampi) Sournia 1973]S73
|--O. caudatum Schiller 1937 [incl. O. coronatum Schiller 1937]G18
|--O. challengeroides Kofoid 1907G18
|--O. crassum Schiller 1937G18
|--O. cribrosum Stein 1883G18
|--O. curvatum [=Prorocentrum curvatum Kofoid 1907]G18
|--O. depressum Schiller 1937G18
|--O. elongatum Wood 1963S73
|--O. gladiolus Stein 1883G18 (see below for synonymy)
|--O. globosum Schiller 1937G18
|--‘Murrayella’ intermedia [=Pavillardinium intermedium (Pavillard) De-Toni 1936]S73
|--O. laticeps Schiller 1937G18
|--O. longicepsG18
|--O. longum Schiller 1937G18
|--O. mediterraneum Schiller 1937 [incl. O. punctulatum Rampi 1941]G18
|--‘Murrayella’ mimetica Balech 1967 [=Pavillardinium mimeticum (Balech) Loeblich 1970]S73
|--O. minutum Rampi 1941S73
|--O. mitra (Stein) Schiller 1937 [=Pyrgidium mitra Stein 1883]G18
|--O. obliquum Schiller 1937G18
|--‘Murrayella’ ovalis Pavillard 1930 [=Pavillardinium ovale (Pavillard) De-Toni 1936]S73
|--O. ‘ovale’ Schiller 1937 non Murrayella ovalis Pavillard 1930G18
|--O. ovum Gaarder 1954G18
|--O. pachyderme Schiller 1937G18
|--‘Pavillardinium’ pacificum Rampi 1950S73
|--O. parvum Schiller 1937G18
|--O. phalericum Athanassopoulos 1931 (n. d.)S73
|--‘Steiniella’ punctata [=Pavillardinium punctatum (Cleve) De-Toni 1936]S73
|--O. pyramidale Dodge & Saunders 1985G18
|--‘Murrayella’ rotundata [=P. rotundatum (Kofoid) De-Toni 1936]S73
|--O. sceptrum (Stein) Schröder 1906FT93 (see below for synonymy)
|--O. schauinslandii Lemmermann 1905G18
|--O. sphaeroideum Stein 1883G18 (see below for synonymy)
|--‘Murrayella’ spinosa Kofoid 1907 [=Pavillardinium spinosum (Kofoid) Taylor ex Sournia 1973]S73
|--O. ‘spinosum’ Rampi 1941 non Murrayella spinosa Kofoid 1907S73
|--‘Murrayella’ splendida Rampi 1941 [=Pavillardinium splendidum (Rampi) Rampi 1950]S73
|--O. turbo Kofoid 1907 [incl. O. obliquum Schiller 1937]G18
`--O. viride Schiller 1937G18

‘Ceratium biconicum’ non Oxytoxum biconicum (Kofoid 1907) Dodge & Saunders 1985 [=Murrayella biconica (Murray & Whitting) Pavillard 1931, Pavillardinium biconicum (Murray & Whitting) Rampi 1948]S73

Oxytoxum Stein 1883 [incl. Murrayella Kofoid 1907 non Schmitz 1893, Pavillardinium de Toni 1936; Oxytoxaceae, Oxytoxidae, Oxytoxinae]FT93

Oxytoxum gladiolus Stein 1883G18 [incl. O. gracile Schiller 1937G18, O. ligusticum Rampi 1969 non Rampi 1951S73, O. nanum Halldal 1953G18, O. nanusS73, O. rampii Sournia 1973G18, S73, O. variabile Schiller 1937G18]

Oxytoxum sceptrum (Stein) Schröder 1906FT93 [=Pyrgidium sceptrum Stein 1883G18; incl. O. aceratum Rampi 1951G18, S73, O. tenuistriatum Rampi 1941G18, S73]

Oxytoxum sphaeroideum Stein 1883G18 [=O. sphaericum (l. c.)S73, O. spheroides (l. c.)S73; incl. O. sphaeroideum var. conicumG18, O. lativelatum Taylor 1976G18, O. obesum Rampi 1969 (nom. inv.)G18, Pyrgidium pyriforme Haeckel 1899 (nom. inv.)G18, O. semicollatum Taylor 1976G18, O. sphaeroideum var. steiniiG18]

*Type species of generic name indicated


[FT93] 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.

[G18] 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.

[S73] Sournia, A. 1973. Catalogue des espèces et taxons infraspécifiques de Dinoflagellés marins actuels publiés depuis la révision de J. Schiller. I. Dinoflagellés libres. Beihefte zur Nova Hedwigia 48: 1–92.

[SW70] Steidinger, K. A., & J. Williams. 1970. Dinoflagellates. Memoirs of the Hourglass Cruises 2: 1–251.

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