epitheca

Epitheca is a term used in biology to describe one of the two halves of the siliceous cell wall (frustule) of diatoms. A diatom frustule consists of two overlapping halves called the epitheca and the hypotheca. These halves fit together to encase the cell’s cytoplasm and contribute to the distinctive ornamentation of the species.

In diatoms, the epitheca and hypotheca are formed as the cell builds its frustule and can differ in shape, size, and surface features between species, making them useful in taxonomic identification. The two halves are separated during cell division, and the daughter cells inherit one half and form the other anew. Typically, one daughter cell receives the epitheca of its parent, while it constructs a new hypotheca, and after subsequent divisions the roles can alternate.

Size reduction is a common feature of diatom asexual reproduction; with each division, daughter cells tend to be smaller than their parents unless sexual reproduction occurs to restore size. The concept of epitheca is most strongly associated with diatoms, though the term appears in older or broader literature referring to the concept of a two-part thecal shell in silica-shelled algae.

Etymology: from Greek epi-, “upon,” and theca, “case” or “shell.” See also hypotheca, frustule.