• Category
  • Length Range
    145-261 µm
  • Width Range
    32-37 µm
  • Striae in 10 µm
  • Synonyms
    Navicula iridis Ehrenb. 1843
  • Reported As
    Neidium maximum (Patrick and Reimer 1966, pg. 434, Pl. 37, Fig 1)



Valves are large and lanceolate, with cuneate apices. A longitudinal canal is present on each valve margin. The valve face is relatively flat, however due to the large size, it may be difficult to get an entire specimen into focus.

Areolae are irregular outside of the longitudinal canal. Areolae are smaller near the axial area.

Striae are slightly oblique to the central sternum. Accordingly, the central area tends to be a slightly oblique oval. The angle of the striae differs bilaterally at the apices, with the more strongly converging striae occurring on opposite sides of the raphe at each apex. Voigt discontinuities are typically prominent.

Proximal raphe ends are small and variably hooked in opposite directions. In some specimens the proximal raphe ends are deflected at ~90°. In others, the proximal raphe ends curve ~180°, pointing nearly to the valve apices.

The axial area is constricted near the central area and apices. The axial area is only hyaline on one side of the raphe with the opposite side bearing a series of irregular areolae. Raphe is an oblique slit. Internally, valves bear a rectelevatum. Helictoglossae are robust enough to resolve using LM, and reflex towards the center of the valve.

In SEM, areolae are externally occluded by radiating cribra, and covered internally by hymenes (Hamilton et al. 2019). Distal raphe ends are difficult to resolve using LM, but under SEM, it can be distinguished that this taxon bears lacinia.

This taxon is large in size, comparable to the large species of Pinnularia (Hamilton et al. 2019). Even at low magnification, specimens can be readily identified due to the size and golden-orange iridescence under illumination. Specimens as large as 320 µm have been identified (Hamilton et al. 2019). However, other large Neidium have been confused with N. iridis.

In most assessments in North America, a rather broad species concept has been applied to this taxon. Specimens previously identified as N. iridis should be reevaluated with our modern concept of this species. Those that do not fit this description should be treated as separate taxa, as they are likely new or undescribed species.


This is a rare diatom. It appeared in the greatest abundance in samples recovered from deep areas of the pond which tend to remain submerged even during drought conditions. This taxa was documented alongside Pinnularia and Eunotia taxa, the former of which included taxa of a comparable size to N. iridis. Numerous desmids from the genera Cosmarium, Spirotaenium, Staurastrum, Micrasterias, and Closterium were observed co-occuring with N. iridis.

Credit: Lane Allen
Comparison showing the external valve face (left) and the rectelevatum, which can be esolved at deep levels of focus (right).
Apical Helictoglossa
Credit: Lane Allen
A view of the helictoglossa at a deep level of focus. Scale Bar = 10 µm.
Back Pond Jones Farm
Credit: Lane Allen
Location where N. iridis was collected in Bowie County, TX.

Original Description

N. Iridis, testula magna elongata bacillaris, lateribus planis, apicibus leviter attenuatis obtusis, superficie subtilissime transverse et longitud. lineolata iridis colorem emittente. Icon! An sui generis forma?

  • Basionym
    Navicula iridis
  • Author
    (Ehrenb.) Cleve 1894

Original Images

Niridis Orig Ill
Niridis Orig Descr

Cite This Page

Allen, L. (2021). Neidium iridis. In Diatoms of North America. Retrieved May 28, 2023, from https://diatoms.org/species/neidium-iridis-1


The 15 response plots show an environmental variable (x axis) against the relative abundance (y axis) of Neidium iridis from all the stream reaches where it was present. Note that the relative abundance scale is the same on each plot. Explanation of each environmental variable and units are as follows:

ELEVATION = stream reach elevation (meters)
STRAHLER = distribution plot of the Strahler Stream Order
SLOPE = stream reach gradient (degrees)
W1_HALL = an index that is a measure of streamside (riparian) human activity that ranges from 0 - 10, with a value of 0 indicating of minimal disturbance to a value of 10 indicating severe disturbance.
PHSTVL = pH measured in a sealed syringe sample (pH units)
log_COND = log concentration of specific conductivity (µS/cm)
log_PTL = log concentration of total phosphorus (µg/L)
log_NO3 = log concentration of nitrate (µeq/L)
log_DOC = log concentration of dissolved organic carbon (mg/L)
log_SIO2 = log concentration of silicon (mg/L)
log_NA = log concentration of sodium (µeq/L)
log_HCO3 = log concentration of the bicarbonate ion (µeq/L)
EMBED = percent of the stream substrate that is embedded by sand and fine sediment
log_TURBIDITY = log of turbidity, a measure of cloudiness of water, in nephelometric turbidity units (NTU).
DISTOT = an index of total human disturbance in the watershed that ranges from 1 - 100, with a value of 0 indicating of minimal disturbance to a value of 100 indicating severe disturbance.