• Category
  • Diameter
    60-208, usually 80-120 µm
  • Width Range
    10-17 µm
  • Striae in 10 µm
  • Synonyms
    Aulacodiscus radiatus (J.W.Bailey) Brightw.
    Roperia radiata (J.W.Bailey) Kuntze



Valves are circular with hexagonal areolae uniformly distributed on the valve face, 3-4 in 10 µm. The valve surface is flat or slightly concave and the central valve is not distinguished. Four (although sometimes 3-6) prominent ocelli are evenly spaced on the margin of the valve face, along a narrow ridge formed by a slightly scalloped edge. The valve face/mantle transition otherwise appears smoothly rounded. Areolae continue beyond the transition onto the mantle, where they are slightly smaller, usually 5-6 in 10 µm. Areolae are arranged in a radiate pattern from center of the valve, although they are sometimes irregularly organized. Diameter of the ocelli ranges from 3.5-10 µm and is roughly proportional to valve diameter. Rimoportulae are present at the valve/mantle transition and are interspersed between the ocelli, with one or two rimoportulae between each pair of adjacent ocelli. External openings of rimoportulae are flush with the valve face and therefore difficult to detect; on the inside of the valve rimoportulae are visible under SEM at the valve-mantle junction. The rimoportulae are small and in many specimens these are not preserved. Mantle margins are variously ornamented and often form a slight circumferential ridge.


Eupodiscus radiatus was collected in a water sample from Charlotte Harbor, on Florida’s west coast in Charlotte County, Florida. It is also common in sediment cores from Charlotte Harbor and Florida Bay, Monroe County, Florida. It occurs in marine to brackish habitats, but it remains unclear whether the species is epiphytic, planktonic, or benthic as there are few records of living cells. It has previously been identified as a co-dominant species in phytoplankton of the northeastern Gulf of Mexico off Florida’s north coast (Prasad and Nienow 1988) and described as common off the American coast, particularly in southern states (Bailey 1851, Hendey 1964). Prasad and Nienow (1988) found it frequently in plankton samples collected in November, January, and February, but it was absent in April collections. The Charlotte Harbor water sample was collected in June 2010, when salinity was approximately 19.6 ppt. Eupodiscus radiatus was absent from sediment samples taken concurrently.

Other distributional records include the Indian Ocean (van Heurck 1896), the southwestern coast of the British Isles (Hendey 1964), coastal Gambia (Foged 1986), estuaries of Southern Brazil (Fernandes 2003), and elsewhere in the Caribbean (e.g., Navarro 1981).

Ania  Eupodiscus Pic
Credit: Image by Anna Wachnicka
Eupodiscus radiatus from Florida Bay sediment core

Original Description

In form, size, and reticulation resembling the Coscinodiscus radiatus of Ehrenberg but having four (or more?) foot-like projections near the margin. A common form in the Southern States.

  • Author
    J.W.Bailey 1851

Original Images

Eupodiscus radiatus LECTOTYPE
Eupodiscus radiatus orig desc
E Radiatus Orig Des

Citations & Links



Cite This Page

Nodine, E. (2011). Eupodiscus radiatus. In Diatoms of North America. Retrieved April 16, 2024, from https://diatoms.org/species/eupodiscus_radiatus


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