The theca consists of two large thecal plates (Figures 1-4) of various shapes (nearly circular, oval, triangular, heart-shaped) and eight tiny plates in the apical flagellar area (Figure 6 & Monti et al. 2010) around the larger flagellar pores and smaller auxiliary pore. The anterior margin is straight or slightly convex, with a small depression in the middle. The large thecal plates are covered with minute spines (Figures 5 & 6) or papillae (600-700 per plate, according to Faust & Gulledge 2002) and a number of pores, which may serve as exits for trichocyst threads. Cells are 14-22 µm long and 10-15 µm wide. Although in a recent study, Monti et al. (2010) found near total coincidence between sizes of P. minimum and P. balticum.
P. minimum is primarily photosynthetic, with several yellow-brown chloroplasts (Figure 2), although mixotrophy has been reported (Stoecker et al. 1997). A number of interspecific and intraspecific genetic studies have taken place, and several hundred nucleotide and protein sequences associated with P. minimum are available in GenBank:
Potentially Misidentified Species:
When occurring in its round or circular form, P. minimum is very similar to P. balticum. The main differences are the smaller size of P. balticum (about 9-15 µm vs. 14-22 µm in P. minimum) and the presence of two minute apical spines in P. balticum. Both have been recorded in the IRL, and there is some uncertainty as to the details of their distribution here.
Habitat & Regional Occurence:
Prorocentrum minimum is mostly found in brackish water and estuaries of temperate and tropical areas, though it sometimes appears in open coastal waters. Most records are from the northern hemisphere, but that may be an artifact of sampling intensity. It is regarded as an invasive species in some areas (Hajdu et al. 2005). In a review of its biology, Berland & Grzebyk (1991) reported a salinity range of 5-37 PSU and a temperature range of 4-31°C.
Indian River Lagoon Distribution:
This species is common in the IRL, and may form blooms in the summer of >2 mm3 L-1 in biovolume (Badylak & Phlips 2004). Blooms elsewhere can reach a population density above 106 cells per liter.
Asexual reproduction is by binary fission. No record of sexual reproduction has appeared to date, even when the cell cycle was studied in detail by Pan & Cembella (1998).
It is questionable as to whether or not P. minimum produces toxins. A specific toxic compound has not been characterized (Saba et al. 2011), although a number of marine invertebrates have shown inhibition of life processes (allelopathy) or mortality (Grzebyk et al. 1997; Heil et al. 2005; Wikfors 2005) when in the presence of P. minimum. Marine mortalities may also occur when bacterial metabolism de-oxygenates the water during bloom decay. Reports of human illness or death from P. minimum toxin via ingestion of shellfish have not been confirmed (Landsberg 2002). Blooms in excess of 106 cells per liter have occurred in several places, with associated marine mortalities. In the IRL, cell concentrations of 105 cells per liter are not uncommon (pers. obs. & Phlips et al. 2010). Differing environmental conditions during blooms suggest variation in the expression of ‘toxicity’ in this species, although a specific chemical compound has not been identified as ‘the toxin’. Several recent studies have provided strong evidence that variation in toxic effects is dependent upon the environment and its effect on the physiology of P. minimum. Mortalities attributable to P. minimum have not been documented in the IRL.
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