Pinnularia Ehrenb.    

Kingdom: Chromista
Phylum: Bacillariophyta – diatoms
Order: Pennales

British distribution (genus): Widespread, freshwater and marine.
World distribution (genus): Throughout most of the world.

Pinnularia is a large genus of marine and freshwater diatoms, with freshwater species often abundant in nutrient-poor waters. Photographs shown here are of material from a small bog pool on the edge of birch/sallow carr.

Pinnularia, single cell
Pinnularia, single cell.

Pinnularia, enlarged view
Enlarged view of the striae and rib-like costae characteristic of this genus (see text below)

Pinnularia species are typical pennate diatoms, i.e. elongated and bilaterally symmetrical when viewed from above ('valve view'), as compared with the radial symmetry of the centric diatoms. Many pennate diatoms are, however, curved in various ways and no longer truly symmetrical, but Pinnularia retains its basic, outer symmetry.

The structure is typical of a diatom; the organism is enclosed in a silica 'box', the frustule, with an upper 'lid', the epitheca, closed over the lower part of the 'box', the hypotheca.

The epitheca itself is composed of two parts:

  1. the epivalve - the upper surface, typically ornamented with striae (fine grooves, singular: stria) and/or punctae (tiny holes, singular: puncta). (Striae are often, in fact, lines of punctae.) In Pinnularia, the striae are very prominent, or, rather, the areas between them are thickened, rib-like, and termed costae (singular: costa).
  2. the epicingulum or upper girdle, the side walls of the 'lid'.
Similarly, the hypotheca is composed of the hypovalve, the lower surface, which is also ornamented, and the hypocingulum (or lower girdle).

Running the length of each valve is the raphe, a longitudinal fissure. In Pinnularia the raphe is like a sideways v in cross-section. Raphes are associated with gliding movements; diatoms without raphes move only very slowly, if at all. Pinnularia, in contrast, moves at surprising speed, leastways, fast enough to be annoying when it is being photographed!

Pinnularia cell
Pinnularia (left hand individual) amongst humic material of the bog pool. The raphe is clearly visible.

Pinnularia cell, side view
Girdle view (side view), showing the diatom to be rectangular in median longitudinal plane of focus. This focus does not show the margins of the valves.

Photographed material: from bog pool on Gleniffer Braes, Renfrewshire, April 2000.

© A.J. Silverside
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