Diatoms are grouped by shape into ten artificial (not strictly evolutionary) classes below to aid identification.

 

Antarctic Freshwater diatoms, in particular species from East Antarctica, are striking for the absence of many morphology types. In the McMurdo Dry Valleys, diatoms are overwhelmingly dominated by naviculoid taxa (Navicula, Diadesmis, Luticola, Stauroneis). Members of other morphology groups are present (achanthoid diatoms, Amphora, Nitzschia), but fewer taxa are present in comparison to the naviculoids.

In contrast to the truncated number of groups and limited diversity in the McMurdo Dry Valley region, a single large temperate lake in the Northern Hemisphere would typically include species from, most, if not all, morphology groups.

See Also: Glossary

Morphology Type Number of Taxa ID
Centric 9 1
Araphid 18 2
Monoraphid 31 3
Eunotioid 4 4
Naviculoid 173 5
Cymbelloid 9 6
Epithemioid 1 7
Amphoroid 3 8
Nitzschioid 28 9
Surirelloid 1 10

 

 

Centric

  • Valve is organized around a point (radial symmetry)
  • Lack significant motility
  • Oogamous sexual reproduction

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Araphid

  • Valve is organized around a line (bilateral symmetry)
  • Lack a raphe system, and therefore lack motility
  • Rimoportulae (labiate process) may be present

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Monoraphid

  • Valve is organized around a line (bilateral symmetry)
  • Raphe system present on one valve
  • Heterovalvar

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Eunotioid

  • Valve is organized around a line (bilateral symmetry)
  • Valves often asymmetrical to the apical axis
  • Raphe system is weak, with raphe located on valve mantle and face
  • Only raphid group with 2 or more rimoportulae (labiate processes)

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Naviculoid

  • Valve is organized around a line (bilateral symmetry)
  • Valves symmetrical to both apical and transapical axis
  • Raphid system well developed, raphe on each valve makes cells highly motile
  • This group has the greatest diversity among the freshwater diatoms

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Cymbelloid

  • Valve is organized around a line (bilateral symmetry)
  • Valves symmetrical to transapical axis, asymmetrical to apical axis
  • Raphid system well developed
  • Valves with apical porefields that secrete mucilaginous stalks or tubes

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Epithemioid

  • Valve is organized around a line (bilateral symmetry)
  • Valves symmetrical to transapical axis, asymmetrical to apical axis
  • Raphid system well developed, and enclosed within a canal

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Amphoroid

  • Valve is organized around a line (bilateral symmetry)
  • Valves symmetrical to transapical axis, asymmetrical to apical axis
  • Raphid system positioned eccentrically, near the valve margin
  • Primarily a marine genus, with a few freshwater representatives

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Nitzschioid

  • Valve is organized around a line (bilateral symmetry)
  • Valves usually symmetrical to both apical and transapical axes
  • Raphid system well developed, and positioned near the valve margin
  • Raphe is enclosed within a canal and raised onto a keel

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Surirelloid

  • Valve is organized around a line (bilateral symmetry)
  • Raphid system extremely well developed, and positioned around the entire valve margin
  • Raphe is enclosed within a canal and raised onto a keel

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