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Diatoms of North AmericaDiatoms of North America
Diatoms of North America
  • Practitioners
  • Morphology
    • Centric
    • Araphid
    • Eunotioid
    • Symmetric Biraphid
    • Monoraphid
    • Asymmetric Biraphid
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  • What are diatoms?
Diatoms of North America

Taxa By Morphology

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?

Centric
Centric
Centric

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
Araphid
Araphid
Araphid

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
Eunotiod
Eunotioid
Eunotiod

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)
Symmetrical Biraphid
Symmetric Biraphid
Symmetrical Biraphid

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
Monoraphid
Monoraphid
Monoraphid

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
Asymmetrical Biraphid
Asymmetric Biraphid
Asymmetrical Biraphid

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
Epithemiod Rhopholoid
Epithemioid
Epithemiod Rhopholoid

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
Nitzschiod
Nitzschioid
Nitzschiod

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
Surirelloid
Surirelloid
Surirelloid

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

Diatoms of North America

The source for diatom identification and ecology

Cite as: Spaulding et al. 2021. Diatoms.org: supporting taxonomists, connecting communities. Diatom Research 36(4): 291-304. https://doi.org/10.1080/0269249X.2021.2006790

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