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]
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]
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]
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).
NB incomplete citations refer to references in Johnson & Mann (1986).
Chondrus crispus—commonly called Irish moss or carrageenan moss —is a species of red algae which grows abundantly along the rocky parts of the Atlantic coasts of Europe and North America. In its fresh condition it is soft and cartilaginous, varying in color from a greenish-yellow, through red, to a dark purple or purplish-brown. The principal constituent is a mucilaginous body, made of the polysaccharide carrageenan, which constitutes 55% of its dry weight. The organism also consists of nearly 10% dry weight protein and about 15% dry weight mineral matter, and is rich in iodine and sulfur. When softened in water it has a sea-like odour. Because of the abundant cell wall polysaccharides, it will form a jelly when boiled, containing from 20 to 100 times its weight of water.
Fucus vesiculosus, known by the common names bladderwrack, black tang, rockweed, sea grapes, bladder fucus, sea oak, cut weed, dyers fucus, red fucus and rock wrack, is a seaweed found on the coasts of the North Sea, the western Baltic Sea and the Atlantic and Pacific Oceans. It was the original source of iodine, discovered in 1811, and was used extensively to treat goitre, a swelling of the thyroid gland related to iodine deficiency.
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 red, but some species can be purple, yellow, blue, white, or gray-green. Coralline algae play an important role in the ecology of coral reefs. Sea urchins, parrot fish, and limpets and chitons feed on coralline algae. In the temperate Mediterranean Sea, coralline algae are the main builders of a typical algal reef, the Coralligène ("coralligenous"). Many are typically encrusting and rock-like, found in marine waters all over the world. Only one species lives in freshwater. Unattached specimens may form relatively smooth compact balls to warty or fruticose thalli.
Ascophyllum nodosum is a large, common cold water seaweed or brown alga (Phaeophyceae) in the family Fucaceae. Its common names include knotted wrack, egg wrack, feamainn bhuí, rockweed, knotted kelp and Norwegian kelp. It grows only in the northern Atlantic Ocean, along the north-western coast of Europe including east Greenland and the north-eastern coast of North America. Its range further south of these latitudes is limited by warmer ocean waters. It dominates the intertidal zone. Ascophyllum nodosum has been used numerous times in scientific research and has even been found to benefit humans through consumption.
Dunaliella salina is a type of halophile unicellular green algae especially found in hypersaline environments, such as salt lakes and salt evaporation ponds. Known for its antioxidant activity because of its ability to create a large amount of carotenoids, it is responsible for most of the primary production in hypersaline environments worldwide, and is also used in cosmetics and dietary supplements.
Crustose is a habit of some types of algae and lichens in which the organism grows tightly appressed to a substrate, forming a biological layer. Crustose adheres very closely to the substrates at all points. Crustose is found on rocks and tree bark. Some species of marine algae of the Rhodophyta, in particular members of the order Corallinales, family Corallinaceae, subfamily Melobesioideae with cell walls containing calcium carbonate grow to great depths in the intertidal zone, forming crusts on various substrates. The substrate can be rocks throughout the intertidal zone, or, as in the case of the Corallinales, reef-building corals, and other living organisms including plants, such as mangroves and animals such as shelled molluscs. The coralline red algae are major members of coral reef communities, cementing the corals together with their crusts. Among the brown algae, the order Ralfsiales comprises two families of crustose algae.
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. Conceptacles are present in Corallinaceae, and Hildenbrandiales, as well as the brown Fucales. In the Fucales there is no haploid phase in the reproductive cycle and therefore no alternation of generations. The thallus is a sporophyte. The diploid plants produce male (antheridia) and female (oogonia) gametangia by meiosis. The gametes are released into the surrounding water; after fusion, the zygote settles and begins growth.
In algal anatomy, a pit connection is a hole in the septum between two algal cells, and is found only in multicellular red algae − specifically in the subphylum Eurhodophytina, except haploid Bangiales. They are often stoppered with proteinaceous "pit plugs". By contrast, many fungi contain septal pores − an unrelated phenomenon.
The Corallinaceae are one of the two extant Coralline families of red algae; they are differentiated from the morphologically similar Sporolithaceae by their formation of grouped sporangial chambers, clustered into sori. The Corallinoideae is monophyletic; the other subfamilies form another monophyletic group.
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 holes in the middle; this distinctive thallus texture is termed a "Leptophytum-type" thallus surface, and has been posited as a taxonomically informative character. It periodically sloughs off its epithallus, reducing its overgrowth by algae by as much as 50% compared to bare rock.
Trichocyte in algae are cells which grow on the outside of the thallus, from which hairs grow. In algae, trichocytes grow principally over the summer; their growth is mediated by water temperature and day length.
Calliarthron is a genus containing two species of thalloid intertidal alga. Specimens can reach around 30 cm in size. The thalli take a crustose form. The organisms lack secondary pit connections. Calliarthron reproduces by means of conceptacles; it produces tetraspores, dispores and carpospores. The genus has lignin and contains secondary cell walls, traits which are normally associated with the vascular plants. It is similar to the genus Bossiella.
The extinct Solenoporaceae have traditionally been interpreted as a group of red algae ancestral to the Corallinales.
Phlorotannins are a type of tannins found in brown algae such as kelps and rockweeds or sargassacean species, and in a lower amount also in some red algae. Contrary to hydrolysable or condensed tannins, these compounds are oligomers of phloroglucinol (polyphloroglucinols). As they are called tannins, they have the ability to precipitate proteins. It has been noticed that some phlorotannins have the ability to oxidize and form covalent bonds with some proteins. In contrast, under similar experimental conditions three types of terrestrial tannins apparently did not form covalent complexes with proteins.
Lithophyllum is a genus of thalloid red algae belonging to the family Corallinaceae.
Hildenbrandia is a genus of thalloid red alga comprising about 26 species. The slow-growing, non-mineralized thalli take a crustose form. Hildenbrandia reproduces by means of conceptacles and produces tetraspores.
Amphiroa is a genus of thalloid red algae under the family Corallinaceae.
Spongites yendoi is a species of crustose red seaweed with a hard, calcareous skeleton in the family Corallinaceae. It is found on the lower shore as part of a diverse community in the southeastern Atlantic Ocean and the Indo-Pacific Ocean.
Lithophyllum orbiculatum is a species of thalloid coralline algae, which are a red algae whose cell walls contain calcareous deposits.
Crustaphytum is a genus of red alga first discovered in Taoyuan algal reefs by Taiwanese scientists. The epithet “crusta” refers to crustose thallus and “phytum” refers to plant. Belonging to the family Hapalidiaceae in the order Hapalidiales, Crustaphytum is one kind of crustose coralline algae.