Guide to Guinardia
- Frustules lightly-silicified, delicate
- Cells joined in chain-like colonies
- Valves with shallow valve mantle
- Copulae numerous, ligulate
Frustules are joined in colonies to form straight or curved chains. The connection between frustules is weak and formed by joining of the external process of a rimoportula with a depression on the valve face of the adjacent frustule. Valves are circular, with very shallow mantles and numerous ligulate copulae. The length of the pervalvar axis is often several times the valve diameter.
Diagnostic characteristics for species determination include cingulum structure and the size, shape, and location of the external expression of the rimoportula.
Frustules are very lightly silicified and collapse upon drying. Valve areolae are discernible only with electron microscopy.
Guinardia is a common marine planktonic genus. Six species within the genus are currently recognized (Guiry 2021).