Diatoms of North AmericaDiatoms of North America

Neidium iridis

(Ehrenb.) Cleve 1894

Neidium iridis2 guide
  • Category
    Symmetric Biraphid
  • Length Range
    145-261 µm
  • Width Range
    32-37 µm
  • Striae in 10 µm
    12-13
  • Reported As
    Navicula columnaris (Schmidt 1886, Tafel 49, Fig 1)
    Neidium schmidtii (Reimer 1959, pg. 30, Pl. 4, Fig 1)
    Neidium maximum (Patrick and Reimer 1966, pg. 434, Pl. 37, Fig 1)

Identification

Neidium iridis LM4 Neidium iridis LM5 Neidium iridis LM2 Neidium iridis LM3 Neidium iridis LM6 Neidium iridis LM1
View image metadata
Neidium iridis SEM3 Neidium iridis SEM1 Neidium iridis SEM1

Description

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.

Autecology

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. (Allen 2022)

A broad taxonomic concept had been applied to this concept prior to the amendment by Hamilton et al. (2019a), (see Foged 1981). Because of this, the taxon has been widely reported but rarely documented. This taxon has a broad and disparate distribution. It was originally described from West Point, NY (Ehrenb. 1843). Extant populations of this taxon have been reported with light micrographs in North America from Bowie Co. TX (this page) and Vancouver (Hamilton et al. 2019). This taxon has also been documented from fossil deposits in Monticello NY, and Maryland (Hamilton et al. 2019a). This taxon is also known to occur at Pilot Knob, IA and FL (pers. obs.)

In the Great Lakes National Parks (Edlund et al., 2011, 2012, 2013), it is exceedingly rare. Though many of the taxa identified as Neidium iridis in counts prior to the revisions by Lefebvre and Hamilton 2015 and Hamilton et al. 2019a belong to other Neidium spp. (N. fossum, N. amphigomphus, N. dilatatum), a trace population of this taxon was observed in Voyageurs at Brown Lake (SMM16226b).

Like many large taxa, it typically only ever occurs in low abundances relative to smaller taxa in the community. Large taxa are typically underestimated using traditional counting techniques (Snoeijs et al. 2002). The importance of taxa with larger cell volumes is usually greater in communities (Snoeijs et al. 2002).

It is known to co-occur with moorland desmids and various Pinnularia (Hamilton et al. 2019a, pers. obs.). Known to occur in ponds, lakes streams and fossil deposits (Hamilton et al. 2019a).

Hamilton et al. (2019b) documented this taxon in a small stream in VanDusen Botanical Garden (CAN) with a pH of 7.86 and a conductivity of 163 µS/cm, elevated nutrient content and low metal content.

The pH of the Jones Farm Pond is 5.7, with a specific conductivity of 30 µS/cm, as measured in early January 2025 following heavy rains.

Rectelevatum
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 Neidium iridis, Eunotia tecta and Stauroneis alabamae 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

Citations & Links

Citations

Links

Updates

Mar 04, 2025 - Addition to Autecology

From 2022 until 2025 the autecological information on this taxon page was limited. As of this update, information on ecology, biogeography, and environmental optima have been added, and nomenclatural synonyms updated. - Lane Allen & Mark Edlund

Cite This Page

Allen, L. (2021). Neidium iridis. In Diatoms of North America. Retrieved March 20, 2025, from https://diatoms.org/species/204408/neidium-iridis-1

Responses

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.