Hymen

A hymen is a very delicate, porous membrane of silica occluding an areola or an alveolus. It is visible only with electron microscopy. The pores of the hymen may be round or elongated and range from 5-10 nm (0.005-0.010 µm) in their shortest diameter. The plural is hymenes.

In contrast, a cribrum is an occlusion with larger pores.

Hymenes are found in many raphid genera such as Cocconeis and Neidiopsis. In some raphid diatoms such as Navicula, a hymen forms the only occlusion of an areola, covering the inner opening. In others such as Diploneis, a hymen covers the inner opening of the areola and a cribrum covers the outer opening of the areolae.

See also cribrum and occlusion (a general term for covering).

Alternative Terms

rica

Used in some older literature (e.g., Ross et al., 1979).

Diploneis smithii - hymenes
Image Credit: Shinya Sato and Masahiko Idei
Internal view of Diploneis smithii (TEM) showing areolae in two biseriate striae. Hymenes occlude the openings of the areolae and have pores ~ 9 nm in diameter. Visible through the hymenes are cribra occluding the outside openings of the areolae. Scale bar = 1 µm.
Hymenes  Bahls2014
Image Credit: Loren Bahls
Inside of a valve of Neidiopsis hamiltonii (SEM) showing two adjacent, parallel areolae. Each areola is covered on the inside by a hymen, which has closely spaced pores ~ 8 nm in diameter. Scale bar = 0.5 µm.