H. akashiwo is distinguished from
Olisthodiscus luteus by its only slightly flattened cells versus the very flattened cells of
Olisthodiscus. Also
Heterosigma cells rotate during swimming while
Olisthodiscus does not.
Cells of
H. akashiwo are slightly longer than wide, measuring 11-25 micrometer in length. The newly described species
H. minor is almost round and measures 7-12 micrometer in diameter.
In fixed samples,
H. akashiwo can be difficult to identify (see image below). It may be confused with other species whose cells contain several yellow or yellow-brown chloroplasts such as
Pseudochattonella or naked stages of other silicoflagellates.