The epithallium or epithallus is the outer layer of a crustose coralline alga, which in some species is periodically shed to prevent organisms from attaching to and overgrowing the alga.[1]

Structure

It is defined as the cells above the intercalary meristem; these are not involved in photosynthesis.[2] In Phymatolithon, the epithallium is usually one cell thick,[2] 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;[3] thicknesses of 16 cells or more, spanning 100 µm, have been measured in a representative coralline (Clathromorphum).[4] 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.[5] The epithallus sometimes overlies the roof of conceptacles, which are exposed only when the overlying epithallus is eventually shed.[4]

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.[4]

Function

Periodic sloughing of this surface is thought to reduce colonization of corallines by kelp (such as Laminaria),[6] epiphytes,[2][3][7] and sessile invertebrates.[8][9] Epithallial cells are covered (in patches) by a cuticle.[1] The deterioration of the outer cells is accelerated in the presence of bacteria.[10]

Comparable structures

A similar mechanism is found in geniculate reds.[11] Epidermal tissue is also shed by unrelated algae: the fleshy reds and browns,[12] (e.g. Chondrus, Ascophyllum;[13] Halidrys,[14] Himanthalia[15]) and the calcaerous greens.[16] Some sea grasses also periodically shed their external cell walls to avoid epiphyte cover.[17] In the browns, this is accomplished by shedding cell wall material, without damaging the underlying cells.[15]

The epithallus probably originated from cover cells, which are considered to be homologous structures.[4]

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).

  1. 1 2 Johnson, C.; Mann, K. (1986). "The crustose coralline alga, Phymatolithon Foslie, inhibits the overgrowth of seaweeds without relying on herbivores" (PDF). Journal of Experimental Marine Biology and Ecology. 96 (2): 127–146. doi:10.1016/0022-0981(86)90238-8.
  2. 1 2 3 Adey, W. H. (1964). "The genus phymatolithon in the Gulf of Maine" (PDF). Hydrobiologia. 24 (1–2): 377–420. doi:10.1007/BF00170412. hdl:2027.42/42883. S2CID 13005299.
  3. 1 2 Adey, W. H. (1966). "The genus Pseudolithophyllum (Corallinaceae) in the Gulf of Maine". Hydrobiologia. 27 (3–4): 479–497. doi:10.1007/BF00042707. S2CID 31438562.
  4. 1 2 3 4 Adey, W. H. (1965). "The genus Clathromorphum (Corallinaceae) in the Gulf of Maine". Hydrobiologia. 26 (3–4): 539–573. doi:10.1007/BF00045545. S2CID 30943569.
  5. ↑ Adey, W. H. (1966). "The genera Lithothamnium, Leptophytum (nov. gen.) and Phymatolithon in the Gulf of Maine". Hydrobiologia. 28 (3–4): 321–370. doi:10.1007/BF00130389. S2CID 2694963.
  6. ↑ Masaki, T.; Fujita, D.; Hagen, N. T. (1984). "The surface ultrastructure and epithallium shedding of crustose coralline algae in an 'Isoyake' area of southwestern Hokkaido, Japan". Hydrobiologia. 116: 218–223. doi:10.1007/BF00027669. S2CID 40945335.
  7. ↑ Adey, 1973; Johansen, 1981; Littler & Littler, 1984
  8. ↑ Padilla, 1981
  9. ↑ Breitburg, D. L. (1984). "Residual effects of grazing: inhibition of competitor recruitment by encrusting coralline algae". Ecology. 65 (4): 1136–1143. doi:10.2307/1938321. JSTOR 1938321.
  10. ↑ Millson, C.; Moss, B. L. (1985). "Ultrastructure of the vegetative thallus of Phymatolithon lenormandii (Aresch. in J. Ag.) Adey". Botanica Marina. 28 (3): 123. doi:10.1515/botm.1985.28.3.123. S2CID 84471882.
  11. ↑ (Borowitzka & Vesk, 1978)
  12. ↑ Filion-Myklebust, CC.; T.A. Norton (1981). "Epidermis shedding in the brown seaweed Ascophyllum nodosum (L.) Le Jolis". Marine Biology Letters. 2: 45–51.
  13. ↑ Sieburth, J. M.; Tootle, J. L. (1981). "Seasonality of microbial fouling on Ascophyllum nodosum (L.) Lejol., Fucus vesiculosus L., Polysiphonia lanosa (L.) Tandy and Chondrus crispus Stackh". Journal of Phycology. 17: 57–64. doi:10.1111/j.1529-8817.1981.tb00819.x. S2CID 86258108.
  14. ↑ Moss, B L (1982). "The control of epiphytes by Halidrys siliquosa (L.) Lyngb. (Phaeophyta, Cytoseiraceae)". Phycologia. 21 (2): 185–188. doi:10.2216/i0031-8884-21-2-185.1.
  15. 1 2 Russell, G; Veltkamp, CJ (1984). "Epiphyte survival on skin-shedding macrophytes". Marine Ecology Progress Series. 18 (1–2): 149–153. Bibcode:1984MEPS...18..149R. doi:10.3354/meps018149.
  16. ↑ Borowitzka, M. A.; Larkum, A. W. D. (1977). "Calcification in the green alga Halimeda. I. An ultrastructure study of thallus development". Journal of Phycology. 13: 6–16. doi:10.1111/j.1529-8817.1977.tb02879.x. S2CID 86035589. (cited to support this fact in Johnson & Mann 1989, although I missed how the paper supports it)
  17. ↑ Jagels, R. (1973). "Studies of a marine grass, Thalassia testudinum. I. Ultrastructure of the osmoregulatory leaf cells". American Journal of Botany. 60 (10): 1003–1009. doi:10.2307/2441514. JSTOR 2441514.; also Jagels, R. H. (1970). "Cell wall development in a marine monocotyledon". American Journal of Botany. 57 (6): 737–738. JSTOR 2441153.
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