The Corallinales, coralline algae, are a group of red algae characterised by a calcified thallus and the production of reproductive structures in conceptacles.
Coral: it’s not just an animal thing
Published 1 September 2008
Coral reefs form the world’s most renowned tropical marine habitat, and are some of the most inspiring structures on the planet’s surface. An incredible wealth of biodiversity hides among the reef structure, and reefs are a justly celebrated subject of many popular books. Most of such books will (hopefully) dedicate a reasonable amount of space to the primary constructors of the reef, the corals, describing how these colonial anemone-related cnidarians lay down skeletons of carbonate that, as the layer of coral grows upwards, accumulate to form the reef structure. On this page, I’m going to showcase another significant group of organisms in the formation of coral reefs that tends not to get quite so much press—the coralline red algae of the order Corallinales.
The coralline algae are one of the most distinct groups of the red algae. They receive their name because, like coral, they lay down skeletons of calcium carbonate that both support the organism and dissuade potential grazers. Coralline algae are a very significant component of coral reefs—in at least some areas, coralline algae actually make up a higher percentage of the surface cover of the coral reef than the coral itself. Overgrowth by coralline algae can bind the reef structure, making it more resilient to erosion and wave action. In areas of coral reef restoration, coral larvae settle preferentially on substrate that is already overgrown by coralline algae (Precht 2006). Coralline algae are also able to withstand lower temperatures and higher-energy environments than corals, meaning that algae-dominated reefs also have a much wider distribution than true coral reefs. Coralline algae also include some of the deepest-living photosynthetic organisms in the world, with some species growing at depths of 300 m, in light levels only 0.0005% of surface level that would be imperceptible to humans.
Morphologically, coralline algae are often divided into two groups, geniculate or articulate and non-geniculate or crustose coralline algae. Geniculate coralline algae grow in branching upright thalli formed of a series of segments divided by joints called genicula. Crustose coralline algae generally do just what the label says they do—they usually grow as flat crusts, though sometimes they may grow upright branches which never grow as high as geniculate algae and are never segmented. Some crustose coralline algae actually grow unattached to the substrate and form often rounded, rock-like structures called rhodoliths that in some places litter the sea floor in large rhodolith beds. Rhodoliths grow extremely slowly—Frantz et al. (2000) calculated a growth rate of approximately 0.6 mm per year using carbon-14 mass spectrometry of rhodolith sections, suggesting that the largest rhodoliths with diameters of over ten centimetres could be 100 years old or more. Such slow growth is characteristic of coralline algae as a whole—experimental observations on reef sections where grazers were excluded showed that in their absence the coralline algae quickly became overgrown by fouling macroalgae.
Unfortunately, and despite their ecological significance, systematic studies of coralline algae are relatively few and far between, largely because of the difficulties of working with them taxonomically. Many morphologically informative characters require examination under a compound microscope, a particularly challenging prospect for crustose forms which are, after all, essentially a thin layer of living tissue over a core of solid rock. Despite its superficial convenience, smaller scale morphological features and molecular phylogeny indicate that the division between geniculate and crustose forms is not phylogenetically informative (Harvey et al. 2003). Instead, Harvey et al. (2003) divided the living Corallinales into three families, the Sporolithaceae, Corallinaceae and Hapalidiaceae, based on molecular data as well as the arrangement and morphology of the sporangia. Corallinales have the stupidly complicated life-cycles of the average floridean red alga (though not so stupidly complicated as some), but the important detail is that generations alternate between morphologically identical haploid and diploid generations (Hoek et al. 1995). Corallinales have also not indulged in parasitism to the extent of other red algal groups, though two members of Hapalidiaceae, Choreonema thuretii and Austrolithon intumescens, are endophytic parasites of the Corallinaceae species Jania tenella (Broadwater et al. 2002).
Not surprisingly, coralline algae have a pretty detailed fossil record compared to other algal groups, but, unfortunately, I suspect that a certain degree of scepticism is called for here. I’ve alluded elsewhere to the difficulty of interpreting fossil “algae” in relation to modern taxa—because the type of fine-scale microscopic features used in establishing the relationships of living algal taxa are usually unknown for fossil taxa, the risk that supposed overall resemblances may be the results of convergence rather than true relationship runs pretty high. Fairly unequivocal crown-group coralline algae of all three living families date back to the Cretaceous (Aguirre et al. 2000), but taxa dating right back to near the beginning of the Palaeozoic have also been identified as Corallinales or close relatives. Indeed the Silurian Gracticulaceae have been regarded as being only doubtfully distinct from the living Sporolithaceae (Harvey et al. 2003) though accepting them as such would demand one heck of a ghost lineage between the Silurian and the Cretaceous. The Palaeozoic “Solenoporaceae” also turn up repeatedly in connection with discussions of Corallinales, but the integrity of this group has been regarded as debatable in recent years. Most significantly, the actual type species of the Solenoporaceae, the Ordovician Solenopora spongioides, was recently demonstrated by Riding (2004) to be not an alga at all, but actually a sponge!
Systematics of Corallinales
Characters (from Flora of South Australia): Thallus mostly calcified, growing on rock, plants, animals or other hard substrates, or partly to almost entirely endophytic; partly to completely affixed by cell adhesion, envelopment of host axes or a crustose or rhizoidal holdfast, or growing unattached and free-living as rhodoliths; in geniculate taxa, branches composed of alternating uncalcified genicula and calcified intergenicula. Structure pseudoparenchymatous or more rarely partly to largely composed of unconsolidated filaments; organisation usually dorsiventral and/or radial or isobilateral. Construction in non-geniculate taxa monomerous and/or dimerous or diffuse; monomerous portions consisting of a single system of branched, laterally cohering, filaments that collectively contribute to a ventrally or centrally situated core and a peripheral region where portions of core filaments or their derivatives curve outwards towards thallus surface; dimerous portions consisting of two distinct groups of laterally cohering filaments or cells: a unistratose ventral or internal layer in which each filament is composed of palisade cells, non-palisade cells or both, and secondly epithallial cells or multicellular filaments arising more or less perpendicularly from cells of ventral or internal layer of filaments; diffuse portions consisting of branched endophytic or epigenous filaments that are partially or largely non-cohering but usually become partially consolidated in areas of conceptacle production; structure of geniculate taxa usually regarded as multiaxial and differentiated into cortex and medulla; walls of most vegetative cells other than genicula usually impregnated with calcite; cells of adjacent filaments joined by cell-fusions or secondary pit-connections or rarely both, or not interconnected; epithallial cells terminating most filaments at thallus surface, distal walls commonly uncalcified; rhodoplasts discoid and without pyrenoids; pit-plugs with two cap layers including an outer dome-like cap layer. Growth of filaments usually from apical or subapical initials. Vegetative reproduction by thallus fragmentation, apomeiotic bispores or propagules. Gametangial thalli monoecious or dioecious; carpogonia and spermatangia produced in separate uniporate conceptacles or rarely in same conceptacle. Carpogonial filaments mostly 1- to 4-celled, arising from conceptacle chamber floor or rarely also from chamber walls. Carposporophytes developing within female conceptacles after karyogamy; carposporangia commonly terminating short gonimoblast filaments that usually arise from a conspicuous or inconspicuous fusion cell, or developing more or less directly from fertilised carpogonium. Spermatangial filaments unbranched or branched, restricted to conceptacle chamber floor, or borne on floor, walls and sometimes roof. Tetrasporangia formed within uniporate or multiporate conceptacles or within calcified compartments that may be solitary or more commonly aggregated into sori in non-geniculate taxa; each mature sporangium containing four zonately or cruciately arranged spores, occasionally producing apical plugs that individually block a pore of a multiporate conceptacle or a calcified compartment, or collectively block pore of a uniporate conceptacle prior to spore release; meiotic bisporangia sometimes replacing tetrasporangia; 3-celled sporangia exceptionally present. Life history triphasic with more or less isomorphic gametangial and tetrasporangial generations and a heteromorphic carposporangial generation.
<==Corallinales | i. s.: CuneiphycusBR98 | KatavellaBR98 | TharamaBR98 | LysvaellaBR98 | LasneriaBR98 | StenophycusBR98 | MarinellaR04 | MetasolenoporaR04 | Porolithon sonorenseN10 |--SporolithaceaeHB03 | |--HeydrichiaHB03 | | |--H. homalopastaHB03 | | `--H. woelkerlingiiHB03 | `--Sporolithon Heydrich 1897HL09 | |--S. durumHB03 | |--S. episporumBB05 | |--S. nummuliticum [=Lithothamnium nummuliticum, Archaeolithothamnium nummuliticum]BR98 | |--S. ptychoides Heydrich 1897HL09 | `--S. rude Lemoine 1925EB93 [=Archaeolithothamnium rudeBR98] |--Graticulaceae [Craticulaceae]HB03 | |--PetrophytonBR98 | | |--*P. miyakoenseBR98 | | |--P. florealeBR98 | | `--P. kiaeri Høeg 1932 [incl. Stromatocerium richmondense, Solenopora richmondense]BR98 | `--Graticula Brooke & Riding 2000 [=Craticula Brooke & Riding 1998 non Grunow 1858]BR00 | |--*G. gotlandica (Rothpletz) Brooke & Riding 2000BR00 (see below for synonymy) | |--‘Solenopora’ bohemicaBR98 | |--‘Solenopora’ condensaBR98 | |--‘Solenopora’ garwoodiiBR98 | |--‘Parachaetetes’ improcerusBR98 | |--‘Parachaetetes’ intermediusBR98 | |--‘Parachaetetes’ lamellatusBR98 | |--‘Parachaetetes’ regularisBR98 | |--‘Solenopora’ texanaBR98 | |--‘Parachaetetes’ thomasiiBR98 | `--‘Solenopora’ triasinaBR98 `--+--HapalidiaceaeHB03 `--CorallinaceaeHB03 | i. s.: Solenophyllum Maslov 1935 [incl. Parachaetes subg. Tomilithon Maslov 1962]BB05 | |--*S. paleozoicum Maslov 1935 (see below for synonymy)BB05 | `--S. johnsonii (Maslov) Bassi, Braga et al. 2005 (see below for synonymy)BB05 | Palaeophyllum Maslov 1950BB05 | |--*P. elegans Maslov 1950BB05 | `--P. caucasicum Maslov 1950BB05 | Mesolithon Maslov 1955BB05 | `--*M. lithothamnioides Maslov 1955BB05 | Aethesolithon problematicum Johnson 1964BB05 | PolyporolithonS57 | |--P. conchatum (Setchell & Foslie) Mason 1953 [=Lithothamnium conchatum]S57 | |--P. parcum (Setchell & Foslie) Mason 1953 [=Lithothamnium parcum]S57 | `--P. reclinatum (Foslie) Mason 1953 [=Lithothamnium reclinatum]S57 | Pachyarthron cretaceum (Postels & Ruprecht) Manza 1937S57 | Rhizolamellia Shevejko 1982HL09 | `--*R. collum Shevejko 1982HL09 | Kymalithon belgicum Foslie 1909EB93 | Parakymalithon phylloideum Bucur & Dragastan 1985EB93 | FosliellaHK90 | |--F. cruciataHK90 | `--F. cymodoceaeHK90 | Lithoporella melobesioidesBA05 |--CorallinoideaeHB03 `--+--+--LithophylloideaeHB03 | `--Metagoniolithon [Metagoniolithoideae]HB03 | |--M. charaHB03 | |--M. radiatumHB03 | `--M. stelliferumHB03 `--MastophoroideaeHB03 |--Spongites yendoiHB03 |--Metamastophora flabellataHB03, G05 |--Pneophyllum caulerpaeHB03, HK90 |--NeogoniolithonHB03 | |--N. mamillosumPP64 | `--N. notarisiiPP64 |--Karpathia Maslov 1962BB05 | |--*K. sphaerocellulosa Maslov 1962 [incl. Peyssonnelia antiqua Johnson 1964]BB05 | |--K. nataliae (Maslov) Bassi, Braga et al. 2005 (see below for synonymy)BB05 | |--K. praeantiqua (Moussavian) Bassi, Braga et al. 2005 (see below for synonymy)BB05 | |--K. rara (Moussavian) Bassi, Braga et al. 2005 [=Peyssonnelia rara Moussavian 1988]BB05 | |--K. reposita (Moussavian) Bassi, Braga et al. 2005 [=Peyssonnelia reposita Moussavian 1988]BB05 | `--K. taeniiformis (Moussavian) Bassi, Braga et al. 2005 (see below for synonymy)BB05 `--Mastophora rosea (Agardh) Setchell 1943HS14 Nomen invalidum: Palaeophyllum tesalii Maslov 1950BB05
*Graticula gotlandica (Rothpletz) Brooke & Riding 2000BR00 [=Solenopora gotlandica Rothpletz 1908BR00, Craticula gotlandica (Rothpletz) Brooke & Riding 1998BR00, Parachaetetes gotlandicusBR98; incl. S. gracilis Garwood & Goodyear 1918BR00, P. gracilisBR98]
Karpathia nataliae (Maslov) Bassi, Braga et al. 2005 [=Lithophyllum (Dermatolithon) nataliae Maslov 1956]BB05
Karpathia praeantiqua (Moussavian) Bassi, Braga et al. 2005 [=Peyssonnelia praeantiqua Mousssavian 1988]BB05
Karpathia taeniiformis (Moussavian) Bassi, Braga et al. 2005 [=Peyssonnelia taeniiformis Moussavian 1988]BB05
Solenophyllum johnsonii (Maslov) Bassi, Braga et al. 2005 [=Parachaetetes (Tomilithon) johnsoni Maslov 1962]BB05
*Solenophyllum paleozoicum Maslov 1935 [=Parachaetetes paleozoicus (Maslov) Pia 1937, P. palaeozoicus (l. c.)]BB05
*Type species of generic name indicated
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