Class BACILLARIOPHYCEAE Diatoms Diatoms are an important component of the food chain in all aquatic environments, yet are frequently over-looked because of their small size. These mostly microscopic algae are easily distinguished from other algae by having a silicified cell wall which is marked with radial or bilateral striations (thin lines) or punctations (small holes). The cell wall is actually a case composed of two parts called valves or frustules; where they join, one overlapping the other, is called the girdle or cingulum. Shape dictates the orientatation (valve or girdle view) that most cells of a given species will be seen in a wet mount, requiring some effort to locate examples in other than the typical view. The diatom valve has a very specialized terminology and a glossary of terms is provided by the California Academy of Sciences. Each diatom cell contains one to many yellowish to brownish chloroplasts. While most diatoms occur as a single cell, some grow attached, either several together or in long chains (filaments). Diatoms often grow on other algae or are prominant in the slime on submerged boulders. Some species of pennate diatoms are capable of a slow, gliding movement. A number of excellent references with photographs are available on the World Wide Web and will be linked on the CPIE algae listing page.
| 65a | (35) | Cells solitary, not attached to others of the same species | [Note D] | ||
| 65b | Cells not solitary, but attached: either several together, or attached at the ends of simple or branched, gelatinous stalks, or attached end to end in long filaments | [66] | |||
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| 66a | (36) & (65) | Cells forming chains, usually attached end on end | [67] | ||
| 66b | Cells attached at tips or attached laterally (on the long side) forming either clusters or fans, but not chains | [71] | |||
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| 67a | (66) | Cells attached at corners, forming zig-zag chains. Valve ornamentation either radially or bilaterally symmetrical | [70] | ||
| 67b | Cells attached end on end in an unbranched filament. Cells at least as long as broad, and typically longer than broad. Circular or complex (undulated) in cross-section. Ornamentation radially symmetrical (but difficult to discern as attached cells are typically in girdle view) ~ Order CENTRALES |
[68] | |||
| 67c | Cells attached end on end in an unbranched filament, with cells in special narrow parts of filament as long as broad, but most cells are 2 to 3 times broader than long. ?Trapezoidal in cross-section. | ??? | |||
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| 68a | (67) |
Cells typically barrel-shaped, attached at rounded ends. Without much ornamentation as typically seen in girdle view: only cingulum (band where the two valves connect) prominent ~ Family COSCINODISCACEAE Melosira spp.
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| 68b | In typical girdle view, barrel-shaped with flat ends, but longitudinal shading indicates an undulated surface. In valve view, triangular, with prominant ridges along each of the three sides. Ornamentation of closely set punctae evident in both views. Common in streams Hydrosera whampoensis Wallich
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| 70a | (67) |
Cells large and quadrangular in girdle view, with undulate sides if seen in valve view. With two to four short, transverse septa inside, perpendicular to valve face, each terminating in a thickened margin, looking somewhat like musical notes ~ Family ANAULACEAE Terpsinoe sp.
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| 70b | Not as above | [71] |
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Note D:
- Your identification is leading to a group of microscopic organisms which, because of their very small size and difficult taxonomy, is not treated further in this key. Two major groups of diatoms are generally recognized: the centric diatoms are radially symmetrical (showing symmetry about a point; usually disc-shaped in valve view) while the pennate diatoms are bilaterally symmetrical (symmetry about a line; most often boat-shaped in valve view).