• Category
  • Length Range
    66.7-129.7 µm
  • Width Range
    20.1-24.9 µm
  • Striae in 10 µm
  • Reported As
    Neidium iridis (Patrick and Reimer 1966, pg. 428, Pl. 34, Fig 1)
    Neidium amphigomphus (Metzeltin and Lange-Bertalot 2009, pg. 95, Figs 1–3)



Valves are large and elliptic-lanceolate. Valve margins are convex in most specimens, though the smallest individuals develop parallel valve margins. Apices are subacute. A broad longitudinal canal is present along the valve margin, with additional narrower canals (2-3) extending along the valve mantle. The valve face is flat.

Areolae may be transapically expanded surrounding the central area. Areolae within the longitudinal canal are transapically expanded.

Striae are nearly parallel, becoming slightly convergent at the apices, though 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, occurring approximately halfway between the central area and the apices.

The central area is a transapically expanded ellipse covering approximately 1/3rd to 1/2 of the valve width.

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 hyaline and gently bilanceolate. The raphe is mostly oblique, becoming threadlike as it approaches the terminals.

Internally, valves bear a rectelevatum. Helictoglossae can be resolved using LM, though they are much less robust than those of N. iridis.

In SEM it can be resolved that areolae are flush against the valve face externally, and located in small depressions internally (Lefebvre and Hamilton 2015). Areolae are occluded by cribra. Renilimbia occur surrounding the areolae bordering the axial area and the longitudinal canals. Renilimbia are also scattered across the valve face.


This population was collected from the epipelon of a shallow wetland at high elevation in the Indian Peaks Wilderness, Front Range Mountains of Colorado. This site also held a population of N. amphigomphus and several other Neidium species.

Home of N. amphigomphus
Credit: Lane Allen
Brainard Lake marsh, Colorado, July 2021
N. fossum canals
Credit: Lane Allen
Note the broad innermost canal, and numerous narrow canals.

Original Description

Neidium fossum K.Lefebvre & P.B.Hamilton, sp. nov. (Figs 18, 20, 31–34, 43–47)

Individuals examined for morphological analyses: 27, examined for molecular analyses: 1. Valves are elliptic-lanceolate, with narrowing to subacute apices (Figs 31–34). Valve length 92–142 μm and width 22.5–34.0 μm. Central area is transapically broad and elliptical, covering 1/3 to 1/2 the valve width. The axial area from centre to apex is linear-elliptical. In LM, the raphe appears filiform and linear-elliptical, in SEM the raphe is linear. Proximal ends form hooks in opposite directions and the distal raphe ends form a lacinia (Figs 43–47). Striae are parallel and moderately spaced becoming weakly convergent towards the apices, 17–21 in 10 μm. Voigt faults are present on the secondary side of the valve. Areolae are regularly spaced and irregular in shape, 17–20 in 10 μm. In SEM, externally the areolae are flat to the valve surface sometimes with cribra occlusions and not in deep depressions (Figs 43–44). Internally, the areolae are positioned within small depressions (Fig. 47). One large longitudinal canal is present along each valve margin (Figs 43–47). Additional canals 2–3 or more may extend along the valve mantle. Externally the longitudinal canal is flat with the valve face (Fig. 43), internally the canal forms a bulge along the valve margin (Fig. 46). A single row of linear to linear-elliptical areolae open externally and internally from the canal (Figs 45–46). Internally, areolae are covered by a hymen (Fig. 45). Renilimbia are positioned around areolae along the axial area, and the region of the longitudinal canals (Figs 45–46). Renilimbia can also be found randomly scattered around areolae on the valve face. Areolae form chambers in the valve wall that are highly interconnected transapically (Fig. 47, Supplemental Plate 2, Fig. f). Each proximal helictoglossa forms a double elevated ribbed nodule that is loosely connected to the adjacent helictoglossa by a thin ridge of silica (Figs 45, 47). The terminal helictoglossae form on small thickened nodules close to the apex (Figs 46, 47).

Etymology:—The specific epithet (fossum), Latin meaning ditch, trench or canal, refers to the large longitudinal canal seen in N. fossum

Observations:—Neidium fossum was identified from four of Ehrenberg’s samples, (Andower MA, Bridgewater MA, Pelham MA and Stratford CT), where it had been listed under either N. dilatatum or N. amphigomphus (Figs 18, 20, Supplemental plate 2). From the Ehrenberg samples, the size ranges of N. fossum observed were length: 90–158 μm, width: 22–40 μm and 19–26 striae in 10 μm. Within the modern samples observed in the phylogenetic study, N. fossum (Figs 31–34) has a similar overall shape to N. dilatatum (Figs 23–24), however N. fossum is much smaller than N. dilatatum with a maximum length of 160 μm. This species can be differentiated from N. amphigomphus, by its narrowly rounded ends, and longitudinal canal structure of one large and several small canals (Figs 43–47). It can be differentiated from N. dilatatum (Figs 35–38) with its large longitudinal canal, smaller overall size, more transapically elliptical central area and its straight bifurcating terminal raphe ends (Figs 43, 46). There is taxonomic confusion around large linear-elliptical Neidium taxa with multiple longitudinal canals extending along the valve margins. One clade which is represented by one larger canal with associated smaller canals, can be linked to N. fossum. Specimens identified as Neidium iridis sensu auct nonull.—Camburn & Charles (2000: 21, fig. 2), Metzeltin & Lange-Bertalot (2009: 93, figs 1–4, 94, figs 1–9)—and as Neidium amphigomphus—Metzetlin & Lange-Bertalot (2002: 51, fig. 1), Metzeltin & Lange-Bertalot (2007: 183, fig. 5, 188, fig. 3, 189, fig. 2), Metzeltin & Lange-Bertalot (2009: 95, figs. 1–3)—are N. fossum or closely related undescribed cryptic species. Other similar specimens identified under N. iridis include Patrick & Reimer (1966: 34, fig. 1) and Krammer & Lange-Bertalot (1986: 104, fig. 2).

  • Author
    Lefebvre and P.Ham. 2015
  • Length Range
    92-142 µm
  • Width
    22.5-34.0 µm
  • Striae in 10µm

Original Images

Neidium fossum original LM Scale Bar 20µm with caption
Neidium fossum original SEM
Neidium fossum original description

Cite This Page

Allen, L. (2022). Neidium fossum. In Diatoms of North America. Retrieved June 23, 2024, from https://diatoms.org/species/neidium-fossum


The 15 response plots show an environmental variable (x axis) against the relative abundance (y axis) of Neidium fossum 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.