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Stephanodiscus

Stephanodiscus are large disc-shaped diatoms known for their size, distinct areolae, and large spines which line the valve margin like eyelashes.  Striae make strong orderly lines from the center of the valve face outwards, and may form grouped rows called fasicles.  There is no separate inner/outer portion of the valve face, but there may seem to be because the face often undulates in a strong concentric pattern that a microscope lens cannot fully focus on.  Stephanodiscus are very common during the summer in Lake Mendota.

S. alpinus

  • Cells are 13.8 - 30.8 µm in diameter. 

  • Fasicles are 9-12 per 10 µm at the valve margin, and are in groups of 1 or 2 striae at the margin.

  • Valve face is strongly undulate.  Areolae are coarse, and disorganized at the center.

  • Spines are less pronounced than on S. niagarae.

For more information, visit diatoms.org here.

(Img. 1-4 same diatom, optically dissected.)

S. niagarae

  • Cells are 21-70 µm in diameter. 

  • Rows of striae are 11-20 per 10 µm at circumference.

  • Pronounced marginal spines are present every 2-3 striae, and curve up to a sharp point.

  • Areolae are distinct and round.

For more information, visit diatoms.org here.

S. parvus

  • Cells are 4.3 - 9.5 µm in diameter. 

  • Rows of striae are 8-12 per 10 µm along the circumference.

  • Valve face is generally flatter than other Stephanodiscus. 

  • Small spines are present in between each fascile.

  • Note the difference in the photo between S. parvus and C. meneghiniana.  Parvus has no inner ring.

For more information, visit diatoms.org here.

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