Diatoms are algae that reflect the biotic condition of streams, lakes and estuaries. Diatoms are important indicator organisms because they are sensitive to natural and human impacts, and monitoring their condition provides information about ecosystem health. A scanning electron micrograph (SEM) shows one valve of an unusual species within the genus Aneumastus. This species is not typical of the genus and its relationship to other members of the group is rather poorly known. The raphe slit is prominent, running through the main axis of the valve.