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Alexandrium monilatum (Howell) Balech, 1995

Species Overview:

Alexandrium monilatum is an armoured, marine, planktonic dinoflagellate. It is a coastal warm water species associated with toxic red tides and massive fish and shellfish kills.

Taxonomical Description:

A very distinctive chain-forming species, A. monilatum typically occurs in long chains of 16 or more cells. Single cells are medium to large, wider than long, and flattened anterio-posteriorly (Figs. 1,2). Epithecal shoulders are occasionally observed. Thecal plates are thin with many delicate pores. Cells range in size between 28-52 µm in length and 33-60 µm in transdiameter width (Balech, 1995, Taylor et al., 1995, Steidinger and Tangen, 1996).

Thecal Plate Description:

The plate formula for A. monilatum is: Po, 4', 6'', 6c, 10s, 5''', 2''''. The large apical pore complex (APC) is broadly triangular and slightly curving posteriorly (Fig. 3). The large apical pore plate (Po) is ovate with a small comma-shaped foramen. The anterior attachment pore (aap) is large and round (Fig. 3). Small pores are present along the margin of the Po. The characteristic first apical plate (1') is not connected to the Po; it is short and broadly pentagonal (Figs. 2,3). The 1' plate is typically without a ventral pore, however, specimens from Florida reveal a pore at the juncture where the 1', 2' and 4' plates meet (Fig. 3) (Balech, 1995, Taylor et al., 1995, Steidinger and Tangen, 1996).

The epitheca and hypotheca are nearly equal. The antapex is slightly concave. The median cingulum is deeply excavated, devoid of lists, and is displaced in a descending fashion one time its width (Figs. 1,2). The sulcus of this species bears a diagnostic feature: a large and rhomboid-shaped posterior sulcal plate (s.p.) (Fig. 4). The s.p. is concave and recessed with radial markings, and contains a large central posterior attachment pore (pap) (Fig. 4) (Balech, 1995, Taylor et al., 1995, Steidinger and Tangen, 1996).

Morphology and Structure:

A. monilatum is a photosynthetic species with central radiating brownish chloroplasts. The quarter-moon shaped nucleus is equatorial (Balech, 1995).


A. monilatum reproduces asexually by binary fission; plane of fission is oblique. This species also has a sexual cycle with armoured isogamous gametes that fuse with cingula at oblique angles (Fig. 5). Gametes range in size from 36 X 36 µm to 47 X 56 µm. After fusion, a planozygote forms which then encysts into a characteristic resting cyst (Figs. 6,7) (Walker and Steidinger, 1979).


A. monilatum is a planktonic estuarine dinoflagellate species associated with toxic red tides and massive fish mortality events in warm coastal waters off Florida, Texas and Venezuela (Howell, 1953, Ray and Aldrich, 1967). Offshore coastal water blooms have also been reported in Florida and Texas (Williams and Ingle, 1972, Wardle et al., 1975). One reported red tide from Texas had cell concentrations ranging from 5 X 10^5 cells/L to 10 X 10^5 cells/L (Gates and Wilson, 1960).

This species produces a dark colored resting cyst as part of its life cycle. The cyst is smooth and round to ovoid. Cysts range in size from 60 to 87 µm in diameter (Figs. 6,7) (Walker and Steidinger, 1979).


Alexandrium monilatum produces a strong ichthyotoxin resulting in a paralyzing effect (Gates and Wilson, 1960, Ray and Aldrich, 1967). From laboratory culture studies, Schmidt and Loeblich (1979) report production of PSP toxins: saxitoxin (STX) and gonyautoxins (GTX); the toxins are hemolytic and neurotoxic (Bass and Kuvshinoff, 1982, Clemons et al., 1980). The toxins produced from this species do not accumulate in shellfish (molluscs do not feed on this species), and it is not toxic to birds (Ray and Aldrich, 1967). Massive fish kills have been reported from Texas bays in the Gulf of Mexico (Gunter, 1942, Connell and Cross, 1950, Ray and Aldrich, 1967) and on the east coast of Florida in the Atlantic Ocean (Howell, 1953).

Habitat and Locality:

Alexandrium monilatum is a warm water species known from subtropical and tropical regions of the Atlantic Ocean: east coast of Florida (Howell, 1953), Venezuela in the Caribbean Sea (Halim, 1967), and Texas in the Gulf of Mexico (Gunter, 1942, Connell and Cross, 1950, Ray and Aldrich, 1967). Populations have also been reported from the tropical Pacific Ocean off Ecuador (Balech, 1995), and surprisingly in the Chesapeake Bay (Morse, 1947).

Alexandrium monilatum