Species Description: Alexandrium monilatum is a thecate, chain-forming dinoflagellate (Figure 1 and video). Individual cells are wider than long, thus with anterior-posterior flattening. Plates that make up the theca are rather delicate and not easily differentiated (Figure 2). The Kofoidean plate formula for Alexandrium (Hallegraeff 2003) is Po, 4’, 0a, 6’’, 6c, 9-11s, 5’’’,1p, 1’’’’. Balech (1995) indicated that A. monilatum conforms to this general formula, and has 10s. The 1’ plate does not contact the Po of the apical pore complex (Figure 3), and a small ventral pore is usually present at the junction of 1’, 2’ and 4’ (Figure 3). This ventral pore may be a characteristic confined to Florida specimens.
Cells are joined in chains by exudates from the apical attachment pore (Figure 3, red arrow) and the antapical attachment pore, centrally located and surrounded by several raised radial markings, in the large and concave posterior sulcal plate (Figure 4, red arrow). This attachment is not firm and, though 16-cell chains are quite common, 8- and 4-cell chains are also frequent. In quiet waters, 32-cell chains can be found. In the original description, Howell (1953) found up to 40 individuals per colony. Despite this, when grown in stationary culture, there is a tendency for the colonies to degenerate into unicells.
Alexandrium monilatum is photosynthetic and filled with large number of small brown or yellow-brown chloroplasts that radiate from the centrally located nucleus, which is somewhat lunate. At least some strains are bioluminescent (Latz et al. 2008).
GenBank has limited information on the molecular genetics of this species http://www.ncbi.nlm.nih.gov/.
Habitat & Regional Occurrence: Alexandrium monilatum is a coastal and estuarine planktonic species of warm temperate and tropical environments, and is known from both eastern Pacific and western Atlantic Oceans. It is widespread in the Gulf of Mexico and Caribbean Sea, and has been recorded from the Chesapeake Bay.
Indian River Lagoon Distribution & Abundance: In the IRL, A. monilatum is most often found in summer and early fall, particularly south of Fort Pierce, rarely reaching concentrations of more than a few thousand cells per liter. However, Norris (1983) reported a bloom of 106 cells per liter at Melbourne Beach. It has been rare in the Banana River and Mosquito Lagoon.
Size: The width of individuals is 33-60 µm (Hallegraeff 2003) or 39-67 µm (Balech 1995), with a length of 28-52 µm.
Reproduction: The life cycle has been studied by Walker and Steidinger (1979), and appears to be typical for dinoflagellates (see Figure 9 of Pyrophacus steinii for schematic life cycle stages): fusion of isogametes form a planozygote which then encysts. The cysts are round to ovoid and filed with darkened yellow-brown cytoplasm and lipids. Asexual reproduction is by binary fission.
Toxicity: Although A. monilatum has rarely been implicated in marine mortalities in the IRL since its original description, it was the cause of fish kills due to ichthyotoxins along the Texas coast (Gates & Wilson 1960; Ray & Aldrich 1967) when cell concentrations approached 106 cells per liter. An unusual bloom in a tributary of the Chesapeake Bay had over 107 cells per liter (May et al. 2010). It was earlier reported to produce PSP toxins (saxitoxin and gonyautoxin) with hemolytic and neurotoxic properties, but the primary toxin is apparently goniodomin-A (Hsia et al. 2006). In addition to killing fish, A. monilatum reduces adult feeding activity and increases larval mortality in commercially valuable shellfish, as well as a variety of other invertebrates (May et al. 2010).
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May, SP, Burkholder, JM, Shumway, SE, Hegaret, H, Wikfors, GH & D Frank. 2010. Effects of the toxic dinoflagellate Alexandrium monilatum on survival, grazing and behavioral response of three ecologically important bivalve mollusks. Harmful Algae 9: 281-291.
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