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Dinoflagellate Glossary

Species Name:

Prorocentrum mexicanum

Common Name:      Dinoflagellate



Dinophyta Dinophyceae Prorocentrales Prorocentrum

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Figures of Prorocentrum mexicanum/rhathymum. (1) From IRL, brightfield light microscope. (2) From St. Lucie estuary, brightfield light microscope. (3) From IRL. Radiating trichocysts visible in posterior part of cell; trichocysts lacking in center. Phase contrast light microscope.

Figure 4. Prorocentrum mexicanum, SEM image. Modified from M. Faust, National Museum of Natural History, Smithsonian Institution.

Figure 5. Prorocentrum mexicanum/rhathymum, SEM image. Spine lying across cell is from the diatom Chaetoceros. From the Banana River in Indian River Lagoon system.

Figure 6. Prorocentrum “rhathymum”. SEM image courtesy of Florida Fish & Wildlife Research Institute.


Prorocentrum mexicanum Tafall


Prorocentrum rhathymum Loeblich III, Sherley et Schmidt

Prorocentrum maximum Schiller

Cortés-Altamirano & Sierra-Beltrán (2003) argue that Prorocentrum mexicanum as described in most literature records is, in reality, a misnomer for P. rhathymum. The two names belong to separate species that are distinguished by a slightly different structure of the apical spine, details of poroid and trichocyst number and placement, and habitat (P. rhathymum = benthic; P. mexicanum = planktonic). Other investigators list P. rhathymum as a synonym of P. mexicanum (Steidinger & Tangen 1997; Faust & Gulledge 2002). Given the high degree of morphological variability exhibited by other related species of Prorocentrum, one may question the separation of the two named species. Cortés-Altamirano & Sierra-Beltrán (2003) based their conclusion on a limited number of cells sampled from two locations at the mouth of the Gulf of California (Sea of Cortés). An examination of the LSU-rDNA sequence of P. rhathymum was clustered closely with that of P. mexicanum (Cohen-Fernandez et al. 2010). If one accepts the argument of Cortés-Altamirano & Sierra-Beltrán (2003), cells from the Indian River Lagoon (IRL) resemble P. rhathymum (as described by Taylor et al. 2003). If not, then the acceptable name for the IRL species is P. mexicanum. The primary compilation of algal names, AlgaeBase, currently considers both to be taxonomically accepted names.

Species Description:
Cells are oval or ellipsoidal, with the widest part of the cell either in the middle or the upper half, but not in the lower half. The anterior end is slightly concave in the middle (Figures 1-5). This concavity contains a periflagellar collar that has often been reported as an apical spine. The surface of the large thecal plates is smooth (Figure 5) in younger cells, rugose (Figure 4) in older cells (Faust & Gulledge 2002). Trichocyst pores are present on both plates, numbering up to about 100 per plate, and arranged primarily in radial rows that are best seen in the posterior part of the cell (Figures 3-6). The center of the plate appears to be devoid of pores (Figures 3-6). Living cells are photosynthetic (Figures 1 & 2) and biflagellate, with both flagella emerging from one apical pore.





Habitat & Regional Occurence:
Most records are from subtropical and tropical locations worldwide, but P. mexicanum also occurs in temperate regions during warmer months. Its habitat is exclusively coastal and estuarine, and it is found both in the plankton and on the benthos.

Indian River Lagoon Distribution:
P. mexicanum is found in the IRL in summer and fall, occasionally attaining substantial abundance (Phlips et al. 2011 as P. “rathymum”, and as P. mexicanum in Badylak & Phlips2004). P. rhathymum has been described as a benthic or epiphytic species, and P. mexicanum as a planktonic species. In the shallow IRL, it is difficult to make this distinction.



Cell length is about 30-40 µm and cell width is about 20-25 µm.

Asexual reproduction is by cell division. Since “young” cells have been described as smooth (Faust & Gulledge 2002), it is likely that entirely new thecae are formed when cells divide, rather than (as with some dinoflagellates) the parent cell splitting in two with each theca forming a new companion theca.

This species produces hemolytic substances, and some Florida strains produce okadaic acid, the cause of diarrhetic shellfish poisoning (An et al. 2010). However, in some cases where P. mexicanum and P. rhathymum are considered separate species, it is stated that the latter produces toxins and the former does not.  In the literature, it is not always clear whether some authors separate the two names. There have been several molecular genetic studies of this species, and Genbank contains dozens of available sequences under both names.


An, T, Winshell, J, Scorzetti, G, Fell, JW & KR Rein. 2010. Identification of okadaic production in the marine dinoflagellate Prorocentrum rhathymum from Florida Bay. Toxicon 55: 653-657.

Badylak, S & EJ Phlips. 2004. Spatial and temporal patterns of phytoplankton composition in a subtropical coastal lagoon, the Indian River Lagoon, Florida, USA. J. Plankton Res. 26: 129-1247.

Cohen-Fernández, EJ, Pedroche, FF, Palacios, MR, Hernández, SA & EM del Castillo. 2010. Molecular phylogeny of Prorocentrum (Dinoflagellata) from the Pacific coast of Mexico based on the parsimony analysis of fragments of LSU rDNA and SSU rDNA. Int. J. Plant Physiol. Biochem. 2: 29-37.

Cortés-Altamirano, R & AP Sierra-Beltrán. 2003. Morphology and taxonomy of Prorocentrum mexicanum and reinstatement of Prorocentrum rhathymum (Dinophyceae). J. Phycol. 39: 221-225.

Faust, MA & RA Gulledge. 2002. Identifying harmful marine dinoflagellates. Smithsonian Contrib. U.S. Nat. Herb. 42: 1-144.

Phlips. EJ, Badylak, S, Christman, M, Wolny, J, Garland, J, Hart, J, J Landsberg and six others. 2011. Scales of temporal and spatial variability in the distribution of harmful species in the Indian River Lagoon, Florida, USA. Harmful Algae 10: 277-290.

Steidinger, KA & K Tangen. 1997. Dinoflagellates. 387-584. In: Tomas, C [Ed.]. Identifying Marine Phytoplankton. Academic Press Inc., San Diego, CA.

Taylor, FJR, Fukuyo, Y, Larsen, J & GM Hallegraeff. 2003. Taxonomy of harmful dinoflagellates. 389-432. In: Hallegraeff, GM, Anderson, DM & AC Cembella [Eds.]. Manual on Harmful Marine Microalgae. UNESCO Publishing, Paris.





Unless otherwise noted, all images and text by PE Hargraves
Editing and page maintenance by LH Sweat

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Page last updated: 25 September 2011

Copyright © 2011 Smithsonian Institution

Openings in the theca that can be involved in the extrusion of certain structures from the cell; genetically variable and used for the indentification of species.

One of many dinoflagellates having a cell wall of cellulose plates, which have species designations and symbols according to their location on the cell. See Figure 1 in the Dinoflagellate Glossary.