Diatom frustule sculpture is complex, producing a variety of contacts between the
cell membrane and the water. There may be punctae, which are simply holes in the
frustule. There are usually areolae, small boxes embedded in the shell wall with a
large pore in the outer surface and a lacework with many small pores against the cell
membrane. Some openings, such as the labiate process of both centrics and pennates
and the pore plate of the pennates, connect the water outside with specific cell
organelles. Many forms have spines and processes (Fig. 2.8). These can be long, and
they sometimes connect the cells into chains. In centrics this may be a slender
siliceous thread linking the center of one valve to the center of the next (as in
Thalassiosira), or a complete ring of elaborate, interlacing fencing around the valve
edges (as in Skeletonema). In pennates the corners of elongate, nearly rectangular
shells can be joined to form a star (Thalassiothrix), or cells can adhere to each other at
the raphe, forming “rafts” in which the cells slide back and forth along each other’s
length (Bacillaria). It is often assumed that the very long side spines either inhibit
predation or enhance surface area to increase drag and inhibit sinking. However, for
forms with siliceous spines, calculations show that the cost of spines in increased
density exceeds the gain in increased drag. Moreover, Gifford et al. (1981) have
shown that copepods more readily eat a form of Thalassiosira weissflogii with long,
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