Halimeda J.V., 1812, nom. et typ. cons.
Holotype species: Halimeda tuna (J.Ellis & Solander) J.V.Lamouroux
Original publication and holotype designation: Lamouroux, [J.V.F.] (1812). Extrait d'un mémoire sur la classification des Polypiers coralligènes non entièrement pierreux. Nouveaux Bulletin des Sciences, par la Société Philomathique de Paris 3: 181-188.
Taxonomic status: currently recognized as a distinct genus.
Description: Thallus constructed of articulated sequences of flattened calcified segments (plates) of various shapes, alternating with non-calcareous joints (nodes). Growth form erect, pendant, or sprawling, achieving a height (length) of a few centimeters to a meter or more. Attachment to substratum by 1) a single large bulbous holdfast, typically 1 cm. to about 13 cm. long, which can represent up to ca. 40% total biomass and consists of fine siphons with adhering sand particles, 2) by a single small, discrete holdfast of matted filaments, ca. a few mm. to 1 cm. long, or 3) by several diffuse and inconspicuous patches of rhizoids arising from segments or nodes. Growth indeterminate. New segments are added apically, with new ones generally completed in 24-48 hours. Segments constructed of siphons (coenocytic filaments), with walls principally of xxx-1, 3 xylan; cellulose absent, or essentially so. Branching, cohesion and fusion of siphons of segments produces a complex microstructure consisting of 2 main regions a multiaxial or in H. cryptica a uniaxial core of medullary siphons surrounded by a cortex of 2-5, rarely 6 layers of utricles (modified branches). In most species outermost utricles cohere at peripheral edges, producing a continuous segment surface. In multiaxial species medullary siphons 1) remain separate at node (uncorticated region), 2) fuse in pairs or small groups, or 3) fuse into single large unit. Genus heteroplastic, i.e. possessing chloroplasts both with and without starch, and amyloplasts. Plastids may concentrate towards segment's interior at night. In deep-growing thalli (e.g. H. cryptica), their concentration near upper surface produces bilateral coloration greenish on lighted surface and white on umbral surface. Concentric lamellar systems (thylakoid organizing bodies) occur in the plastids. Deposition of aragonitic calcium carbonate begins after about 36 hours of segment development, and occurs first in external fibrous matting of siphon walls; crystals subsequently grow into spaces between siphons. In old segments secondary crystal formation may occur around aragonite needles. Extent of calcification varies with age, taxonomy and environment. Range for mature Halimedas sampled, excluding lightly calcified H. cuneata, is 47-90%. It is highest for oldest (lowermost) segments on thalli and those recently shed. One of the few studies of calcium carbonate content with water depth over <-1 to -57 m. in Jamaica, reported decreased percentages with depth for species simulans and goreauii, little change for tuna, and increases for discoidea, gracilis, opuntia and copiosa. Asexual reproduction is by fragmentation, or by development of new thalli at ends of uncorticated siphons growing out either from segments or from filaments of the holdfast. In sexual reproduction biflagellated gametes are produced in specialized globular to pyriform gametangia on simple or branched uncalcified stalks, which develop as outgrowths from nodes or the surface of segments. Septae probably always absent from bases of gametangia since the report of their presence, based on poor material, has not been verified. Gametes anisogamous in species examined, with standard 9 + 2 arrangement of microtubules. Halimeda is holocarpic, i.e. essentially all contents of its siphons are released into gametangia, with thallus dying thereafter. Zygote development has been followed only for H. tuna. In culture the resultant uninucleate protosphere, ca. 100-150 µm in diameter, with >500 lenticular chloroplasts and starch grains but no amyloplasts produced a juvenile filamentous stage but not the thallus of articulated segments.
Information contributed by: L. Hillis. The most recent alteration to this page was made on 19 Oct 2015 by M.D. Guiry.
Comments: Halimeda is widely distributed globally in warm waters, making it a good indicator of tropical environments in the reconstruction of climatic histories. Six to 7 species currently are considered to have a global distribution (Pacific, Atlantic and Indian Oceans) copiosa, discoidea, gracilis, incrassata, opuntia, simulans, tuna, but this interpretation is likely to change with molecular investigation. Halimeda tuna also grows in the Mediterranean; H. cuneata appears to be restricted to the subtropics, although misidentifications (H. discoidea as H. cuneata) cloud its precise distribution. Halimeda is common to very abundant in many coralgal reef systems, where certain species are indicators of within reefal environments. "Rooted" taxa (=holdfast type 1 above), or about 1/4 of the species, grow on sand or mud, the substrata of extensive regions of lagoon and backreef, whereas species with holdfast type 3 can form extensive draperies on hard substrata of the fore-reef, and on coral pinnacles. Vertically, Halimeda extends from the intertidal to at least -150 m. where photon flux densities were calculated as ca. 0.05% surface irradiance; adaptations for low light intensities include accessory pigments such as siphonoxanthin and siphonein, and mobility of plastids within siphons. Carbonate sediments produced by Halimeda make a major contribution to reef mass, estimated as a vertical accumulations of a few to several meters 1000-1 for regions such as the Bahamas and Great Barrier Reef. Five sections are recognized in the taxonomic treatment of Halimeda Rhipsalis (1889) (8 sp.), Opuntia J. Ag. ex De Toni (1889) (8 sp.), Halimeda J. Ag. ex De Toni (1889) (13 sp.), Micronesicae Hillis 1980 (3 sp.), and Crypticae Hillis (1980) (1 sp.). Major morphological taxonomic characters are type of holdfast, cohesion and surface dimensions of peripheral utricles, shape and size of subsurface utricles, and fusion pattern of medullary filaments at mature nodes.
Numbers of names and species: There are 68 species names in the database at present, as well as 67 infraspecific names. Of the species names, 47 have been flagged as accepted taxonomically on the basis of the listed literature under the species name. In some instances, opinions on taxonomic validity differ from author to author and users are encouraged to form their own opinion. AlgaeBase is a work in progress and should not be regarded as a definitive source only as a guide to the literature..
Names: ('C' indicates a name that is accepted taxonomically; 'S' a homotypic or heterotypic synonym; 'U' indicates a name of uncertain taxonomic status, but which has been subjected to some verification nomenclaturally; 'P' indicates a preliminary AlgaeBase entry that has not been subjected to any kind of verification. For more information on a species click on it to activate a link to the Species database):
Click here to also show infraspecific names in the list below.
Verbruggen, H., De Clerck, O., Kooistra, W.H.C.F. & Coppejans, E. (2005). Molecular and morphometric data pinpoint species boundaries in Halimeda section Rhipsalis (Bryopsidales, Chlorophyta). Journal of Phycology 41: 606-621.
Verification of data
Users are responsible for verifying the accuracy of information before use, as noted on the website Content page.
Some of the descriptions included in AlgaeBase were originally from the unpublished Encyclopedia of Algal Genera, organised in the 1990s by Dr Bruce Parker on behalf of the Phycological Society of America (PSA) and intended to be published in CD format. These AlgaeBase descriptions are now being continually updated, and each current contributor is identified above. The PSA and AlgaeBase warmly acknowledge the generosity of all past and present contributors and particularly the work of Dr Parker.
Descriptions of chrysophyte genera were subsequently published in J. Kristiansen & H.R. Preisig (eds.). 2001. Encyclopedia of Chrysophyte Genera. Bibliotheca Phycologica 110: 1-260.
Created: 29 December 2000 by M.D. Guiry
Verified by: 19 October 2015 by M.D. Guiry
Linking to this page: http://www.algaebase.org/search/genus/detail/?genus_id=33699
Please cite this record as:
M.D. Guiry in Guiry, M.D. & Guiry, G.M. 2019. AlgaeBase. World-wide electronic publication, National University of Ireland, Galway. http://www.algaebase.org; searched on 22 January 2019.