The Raphidophyceae, or chloromonads, have chlorophylls a and c, and two membranes of chloroplast endoplasmic reticulum. The anterior flagellum is commonly tinsel, whereas the posterior flagellum is naked (Figs. 18.1, 18.2, 18.3). The freshwater species of the Raphidophyceae are green, whereas the marine forms are yellowish and contain the carotenoid fucoxanthin (Vesk and Moestrup, 1987). The closest relatives of the Raphidophyceae are the Eustigmatophyceae and the Chrysophyceae (Cavalier-Smith and Chao, 1996).
Marine species are euryhaline and eurythermic (tolerant of a wide salinity and temperature range) and occur in temperate and subtropical waters worldwide (Tobin et al., 2013). Marine genera are Chattonella (Fig. 18.3), Fibrocapsa (Fig. 18.1(b)), and Heterosigma (Fig. 18.1(a)). Many of the marine species produce neurotoxic compounds that are similar to brevetoxin (Fig. 18.2). Uptake of the toxin by fish results in depolarization of nerves supplying the heart. This reduces the heart rate, thereby lowering blood pressure, which in turn affects the transfer of oxygen to the gill lamellae, creating hypoxic conditions that lead to fish mortality (Tyrrell et al., 2001).
Toxic red-tide blooms of the marine Chattonella antiqua and Heterosigma carterae (Taylor, 1992) have occurred in the Seto Inland Sea in Japan (Watanabe et al., 1988). These red tides occurred in the summer when a salinity and temperature stratification occurred at a depth of 5–10 m. There was little mixing of waters above and below the stratified layer resulting in the upper layer being deficient in nutrients while the bottom layer was anaerobic. Heterosigma carterae flourishes under these conditions because it has a daily vertical migration of 10–15 m and this allows the alga to move between the stratified layers. This migration enables H. carterae to use the nutrients in the lower layers, and light and oxygen in the upper layer, resulting in the red-tide blooms of the organism. The migration is correlated with the production and degradation of cytoplasmic fat particles (Wada et al., 1987).
Heterosigma carterae has a wide salinity tolerance in culture (3–50%) and loses its motility at temperatures below 10 °C. At 5–10 °C, the alga forms non-motile masses capable of surviving in continuous darkness for up to 15 weeks (Han et al., 2002).