For the purposes of a visual key, we group diatom shapes into nine artificial (not strictly evolutionary) categories to aid in identification
Why not group by evolutionary relationship?
Category
Centric
- Valves with radial symmetry (symmetric about a point)
- Cells lack a raphe system and lack significant motility
- Cells may possess fultoportulae (strutted processes) and rimoportulae (labiate processes)
- Sexual reproduction is oogamous
Category
Araphid
- Valves with bilateral symmetry (symmetric about a line)
- Cells lack a raphe system and lack significant motility
- Rimoportulae (labiate process) may be present
Category
Eunotioid
- Valves with bilateral symmetry (symmetric about a line)
- Valves often asymmetric to the apical axis
- Raphe system is short and provides weak motility
- Raphe located on valve mantle and face
- Cells may possess 2 or more rimoportulae (labiate processes)
Category
Symmetric Biraphid
- Valves with bilateral symmetry (symmetric about a line)
- Valves symmetric to both apical and transapical axis
- Raphe system well developed and cells may be highly motile
- This group has the greatest diversity among the freshwater diatoms
Category
Monoraphid
- Valves with bilateral symmetry (symmetric about a line)
- Raphe system present on one valve (raphe valve)
- Raphe system absent on one valve (rapheless valve)
- Heterovalvar ornamentation
Category
Asymmetric Biraphid
- Valves asymmetric to apical axis OR asymmetric to the transapical axis, or both
- Raphe system well developed
- Some genera possess apical porefields that secrete mucilaginous stalks
- Other genera secrete mucilagous tubes
Category
Epithemioid
- Valves with bilateral symmetry (symmetric about a line)
- Valves asymmetric to apical axis
- Raphe system well developed and enclosed within a canal
- Raphe system positioned near the valve margin
Category
Nitzschioid
- Valves with bilateral symmetry (symmetric about a line)
- Valves usually symmetric to both apical and transapical axes
- Raphe system well developed and enclosed within a canal
- Raphe system usually positioned near one valve margin
- Raphe sometimes raised within keel
Category
Surirelloid
- Valves with bilateral symmetry (symmetric about a line)
- Raphe system well developed and enclosed within a canal
- Raphe often circumferential on the valve margin and raised onto a keel