The epithallium or epithallus is the outer layer of a crustose

coralline alga Coralline algae are red algae in the order Corallinales. They are characterized by a thallus that is hard because of calcareous deposits contained within the cell walls. The colors of these algae are most typically pink, or some other shade of re ...

, which in some species is periodically shed to prevent organisms from attaching to and overgrowing the alga.

Structure

It is defined as the cells above the

intercalary Intercalation may refer to: *Intercalation (chemistry), insertion of a molecule (or ion) into layered solids such as graphite *Intercalation (timekeeping), insertion of a leap day, week or month into some calendar years to make the calendar follo ...

meristem; these are not involved in photosynthesis. In ''

Phymatolithon ''Phymatolithon'' is a genus of non geniculate coralline red algae, known from the UK, and Australia. It is encrusting, flat, and unbranched; it has tetrasporangia and bisporangia borne in multiporate conceptacles. Some of its cells bear small ...

'', the epithallium is usually one cell thick, whereas in other genera, such as '' Pseudolithophyllum'', multiple cells exist, with the thickness determined by the difference between their rate of production at the intercalary meristem, and the rate of shedding at the surface; thicknesses of 16 cells or more, spanning 100 µm, have been measured in a representative coralline ('' Clathromorphum''). The thickness is variable within species; in ''Lithothamnion'', a single cell thickness is the norm, but three- or four-cell thick regions are also common. The epithallus sometimes overlies the roof of

conceptacle Conceptacles are specialized cavities of marine and freshwater algae that contain the reproductive organs. They are situated in the receptacle and open by a small ostiole.Boney, A.D. (1969). ''A Biology of Marine Algae''. Hutchinson Educational L ...

s, which are exposed only when the overlying epithallus is eventually shed. The epithallium is less strongly calcified than the underlying cells, facilitating its removal. The meristem itself is the least calcified portion; sometimes there is no mineralization at all, which makes it a plane of weakness where breaking often occurs.

Function

Periodic sloughing of this surface is thought to reduce colonization of corallines by kelp (such as ''Laminaria''), epiphytes, and sessile invertebrates. Epithallial cells are covered (in patches) by a cuticle. The deterioration of the outer cells is accelerated in the presence of bacteria.

Comparable structures

A similar mechanism is found in geniculate reds. Epidermal tissue is also shed by unrelated algae: the fleshy reds and browns, (e.g. ''

Chondrus ''Chondrus'' is a genus of red algae containing 11 accepted species: *''Chondrus armatus'' (Harvey) Okamura *'' Chondrus canaliculatus'' (C.Agardh) Greville *''Chondrus crispus'' Stackhouse *''Chondrus elatus ''Chondrus'' is a genus of red a ...

'', '' Ascophyllum''; ''

Halidrys ''Halidrys'' is a genus of brown algae belonging to the family Sargassaceae Sargassaceae is a family of brown algae in the order Fucales The Fucales (fucoids) are an order in the brown algae (class Phaeophyceae). The list of families in th ...

'', ''

Himanthalia ''Himanthalia'' is a genus of brown algae. It is the only genus in the family Himanthaliaceae in the order Fucales. It includes two species: '' Himanthalia durvillei'' and ''Himanthalia elongata ''Himanthalia elongata'' is a brown alga in the ...

'') and the calcaerous greens. Some sea grasses also periodically shed their external cell walls to avoid epiphyte cover.; also In the browns, this is accomplished by shedding cell wall material, without damaging the underlying cells. The epithallus probably originated from cover cells, which are considered to be homologous structures.

External links

For a cross-sectional image in '' Clathromorphum circumscriptum'', see plate 38 (p. 415) in Adey, 1964 (referenced below) Additional images showing the epithallus can be seen in Masaki ''et al.'' (1984).

Refs

NB incomplete citations refer to references in Johnson & Mann (1986). {{reflist, 30em Red algae