Epithallus

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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 , 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 ().[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 , which are considered to be homologous structures.[4]

External links[]

For a cross-sectional image in 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. ^ a b 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. ^ a b c Adey, W. H. (1964). "The genus phymatolithon in the Gulf of Maine" (PDF). Hydrobiologia. 24: 377–420. doi:10.1007/BF00170412. hdl:2027.42/42883.
  3. ^ a b Adey, W. H. (1966). "The genus Pseudolithophyllum (Corallinaceae) in the Gulf of Maine". Hydrobiologia. 27 (3–4): 479–497. doi:10.1007/BF00042707.
  4. ^ a b c d Adey, W. H. (1965). "The genus Clathromorphum (Corallinaceae) in the Gulf of Maine". Hydrobiologia. 26 (3–4): 539–573. doi:10.1007/BF00045545.
  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.
  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.
  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.
  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.
  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. ^ a b Russell, G; Veltkamp, CJ (1984). "Epiphyte survival on skin-shedding macrophytes". Marine Ecology Progress Series. 18 (1–2): 149–153. 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. (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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