Size and Shape: Small (<20 µm long), oval shaped dinoflagellate.  Can be confused with Pfiesteria.

Plastids and Pigments: Yellow-green discoid plastids, number and size vary depending on nutritive mode as this species is mixotrophic.

Distinguishing features: Looks very muck like Pfiesteria, but it will have chloroplasts in both halves of the cell (not just the epitheca as in Pfiesteria).  Blooms turn the water a brownish-red causing what is known as a "mahogany tide".

Distribution: Usually as part of the phytoplankton in estuarine systems, has been found on both coasts of the US, the gulf of Mexico, and Scandanavia.

Impact: Has been known to cause fish kills in S.C. retention ponds and Florida mariculture tanks.  Apparently not highly toxic. 

References: 
Kempton, J.L., Lewitus, A.J., Deeds, J.R., Law, M.H., and Place, A.R. (2002) Toxicity of Karlodinium micrum (Dinophyceae) associated with a fish kill in a South Carolina brackish retention pond. Harmful Algae 2.

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.

Lewitus, A.J., and Holland, A.F. (2003) Initial results from a multi-institutional collaboration to monitor harmful algal blooms in South Carolina. Environmental Monitoring and Assessment. 81: 361-371.

Daugbjerg, Hansen, Larsen and Moestrup, 2000. Phylogeny of some of the major genera of dinoflagellates based on ultrastructure and partial LSU rDNA sequence data. 

Braarud T. 1957. A Red Water Organism From Walvis Bay. Galathea Reports 1:137-138.