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).

Balech, E. 1995. The genus Alexandrium Halim (Dinoflagellata). Sherkin Island Marine Station, Cork, Ireland. 151pp.

Gates, JA & WB Wilson. 1960. The toxicity of Gonyaulax monilata Howell to Mugil cephalus. Limnol. Oceanogr. 5: 171-174.

Halim, Y. 1967. Dinoflagellates of the South East Caribbean Sea (East Venezuela). Internationale Revue des gesamten Hidrobiologie 52: 701-755.

Hallegraeff, GM. 2003. Taxonomic Principles. In: Hallegraeff, GM, Anderson, DM & AD Cembella (Eds.). 383-432. Manual on Harmful Marine Microalgae. UNESCO Publishing, Paris. 793pp.

Hsia, MH, Morton, SL, Smith, LL, Beauchesne, KR, Huncik, KM & PDR Moeller. 2006. Production of goniodomin A by the planktonic, chain-forming dinoflagellate Alexandrium monilatum (Howell) Balech isolated from the Gulf coast of the United States. Harmful Algae 5: 290-299.

Howell, JF. 1953. Gonyaulax monilata sp. nov.,the causative dinoflagellate of a red tide in the east coast of Florida in August-September 1951. Trans. Amer. Microsc. Soc. 72: 153-15.

Latz, MI, Bovard, M, VanDelinder, V, Segre, E, Rohr, J & A Groisman. 2008. Bioluminescent response of individual dinoflagellate cells to hydrodynamic stress measured with millisecond resolution in a microfluidic device. J. Exp. Biol. 211: 2865-2875.

Loeblich III, AR. 1970. The amphiesma or dinoflagellate cell covering. In: Yochelson, EL (Ed.). Proceedings of the North American Paleontological Convention, Chicago, September 1969, Part 2. G. Allen Press, Lawrence, KS. 867-929.

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.

Norris, DR. 1983. The occurrence of a toxic dinoflagellate in the Indian River system, Florida. Fla. Sci. 46: 150-153.

Ray, SM & DV Aldrich. 1967. Ecological interactions of toxic dinoflagellates and mollusks in the Gulf of Mexico. 75-83. In: Russell, FE & PR Saunders [Eds.]. Animal Toxins, First International Symposium on Animal Toxins. Pergamon Press, NY.

Taylor, FJR. 1976. Dinoflagellates from the international Indian Ocean Expedition. Bibliothec. Botan. 132: 1-234 + 46 Plates.

Walker, LM & KA Steidinger. 1979. Sexual reproduction in the toxic dinoflagellate Gonyaulax monilata. J. Phycol. 15: 312-315.

Antapex: Posterior-most part of the cell body, excluding spines, lists and similar structures.

Anterior Intercalary Plates: Plates located between the apical and precingular plates.

Apex: Anterior-most part of the cell body.

Apical Groove: Groove located at the anterior part of many dinoflagellate species, extending posteriorly on both the ventral and dorsal surfaces of the cell; also known as the acrobase.

Apical Plates: Plates that surround and touch the cell apex.

Apical Pore Complex: A pore or hole at the cell apex that may have one or more tiny accessory plates; sometimes abbreviated as ‘APC’.

Areolate: Ornamentation of the thecal plates that consists of depressions of variable depth and form.

Cingular Plates: Plates that are located in the cingular, or girdle, groove around the cell.

Cingulum: A furrow encircling the cell that contains the rotatary flagellum.

Cyst: The diploid zygotic dormant stage in the sexual life cycle; usually morphologically dissimilar from the haploid motile stage; also called the ‘dinocyst’ or ‘hypnozygote’.

Dorsal Side: Back side of the cell, opposite of the front ventral side where the sulcus is located.

Epicone: The part of a dinoflagellate cell above the cingulum; usually refers to an ‘unarmored’ (lacking cellulose plates) cell; may also be known as the epitheca or episome.

Epitheca: The part of a dinoflagellate cell above the cingulum; usually refers to a thecate (with cellulose plates) cell; may also be known as the epicone or episome.

Flagellar Area: The vicinity of the origin of the two flagella.

Hypocone: The part of a dinoflagellate cell below the cingulum; usually refers to an ‘unarmored’ (lacking cellulose plates) cell; may also be referred to as the hypotheca or hyposome.

Hypotheca: The part of a dinoflagellate cell below the cingulum; usually refers to a thecate (with cellulose plates) cell; may also be referred to as the hypocone or hyposome.

Lists: Membranous extensions of the cingulum and/or sulcus that extend beyond the cell wall boundary; found in thecate dinoflagellates, especially those from the order Dinophysiales.

Median Cingulum: A cingulum that encircles approximately the midpoint of the cell.

Nematocyst: Ejectile organelle of some dinoflagellate genera; involved in prey capture.

Peduncle: Cytoplasmic appendage near the flagellar pores; organelle associated with phagotrophy.

Pores: Openings in the theca that can be involved in the extrusion of certain structures from the cell; genetically variable and used for identification of species; also known as trichocyst pores.

Postcingular Plates: Plates located immediately below the cingulum.

Posterior Intercalary Plates: Plates located between the postcingular and antapical plates.

Postmedian Cingulum: A cingulum that encircles the cell below the midpoint.

Precingular Plates: Plates located immediately above the cingulum.

Premedian Cingulum: A cingulum that encircles the cell above the midpoint.

Ribs: Supports for sulcal lists.

Sulcal Plates: Plates located in the sulcus.

Sulcus: A longitudinal furrow, often partially enclosing the propulsive flagellum.

Thecate: One of many dinoflagellates having a cell wall of cellulose plates, which have special designations and symbols according to their location on the cell. Number and location of thecal plates is often genus-specific.