Size and Shape: 'Potato' shaped cells 11-25 X 8-13 µm.  2 sub-apically inserted flagella, one motile one trailing.

Plastids and Pigments: Many greenish-brown or yellow-brown plastids, chla, c1 and c2; ß-carotene and fucoxanthin as accessory pigments.

Distinguishing features: Plastids are truely parietal, cell is only slightly compressed, recurrent flagellum lies in a ventral groove.

Distribution: Worldwide in brackish waters.  Cused a large bloom in Bull's Bay SC in April of 2003.

Impact: Toxic to fish, mechanism of toxicity is very porrly known.

References: 

Lewitus, A.J., Schmidt, L.B., Mason, L.J., Kempton, J.W., Wilde, S.B., Wolny, J.L., Williams, B.J., Hayes, K.C., Hymel, S.N., Keppler, C.J., and Ringwood, A.H. (2003) Harmful algal blooms in South Carolina residential and golf course ponds. Population and Environment. 24: 387-413.

Connell, L. and M. Jacobs 1997. Anatomy of a Bloom: Heterosigma akashiwo in Puget Sound 1997. http://www.nwfsc.noaa.gov/hab/anatomy.htm

Hargraves and Maranda 2002. Potentially toxic or harmful microalgae from the northeast coast. Northeastern Naturalist. 9(1): 81-120. 

Li, Y. and T.D. Smayda. 2000. Heterosigma akashiwo (Raphidophyceae): On prediction of the week of bloom initiation and maximum during the initial pulse of its bimodal bloom cycle in Narragansett Bay. Plankton Biol. Ecol. 47:80-84.