British distribution (genus): Widespread, freshwater and marine.
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 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:
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!
Photographed material: from bog pool on Gleniffer Braes, Renfrewshire, April 2000.