Hypoglossum

Belongs within: Delesseriaceae.

Hypoglossum is a genus of erect red algae with flat thallus blades that are monostromatic except for the midribs (Womersley 2003).

Put out your tongue

Published 4 June 2026

Though they are the most diverse of the major groups of algae by a considerable margin, red algae tend not to draw the greatest attention because many are fairly small in size. Thalli of the red algal genus Hypoglossum, for instance, may grow only a couple of inches in height, at most maybe getting to a foot. The thalli are composed of flat blades lacking lateral veins, being monostromatic (that is, constructed of a single layer of cells) except for along the midrib. Branching derives from the midrib rather than the blade margin (Wynne 1988). The name of the genus refers to the tongue-like appearance of the thallus blades; I have seen the name ‘under tongue weed’ applied to species of this genus as a derivative of the botanical name. As the thallus grows, the lateral wings of the blade may become worn off in the lower sections, leaving the midrib as a tough stipe (Womersley 2003). The base of the thallus grows from a discoid holdfast, usually attached to rock, though some may grow as epiphytes on other algae (particularly on coralline algae, which is arguably much the same as being attached to a rock). Gametophytes are dioecious, with separate male and female individuals. Sexes and sporophytes may differ slightly in the shape of the blades.

Over two dozen species of Hypoglossum are known from temperate and tropical seas around the world. A significant feature in distinguishing species is branching pattern which may be opposite or alternate. Most species exhibit monopodial branching but the southern Australian H. revolutum is regularly sympodial (Womersley 2003). Blades in the northwest Pacific H. minimum are unbranched (Oak et al. 2002). Other features of note include the form of the blade margins, whether entire or denticulate, and the arrangement of cell rows in the blade. Species may also exhibit differences in the development of reproductive organs, such as the location and cell division patterns of tetrasporangia.

Systematics of Hypoglossum

Characters (Womersley 2003): Thallus usually erect, mostly 5–20 cm high, with flat (usually), elongate blades arising from the midrib of older blades, monopodially developed (sympodial in H. revolutum), monostromatic apart form the midrib; lateral veins absent; holdfast discoid, usually epilithic. Apical cells obconical, segmenting to form an axial filament of cells with 2 lateral pericentral cells followed by 2 transverse pericentral cells, the lateral cells forming second-order cell rows with all or some cells forming third-order rows, with all rows reaching the blade margin. Blade margin with or without fourth-order rows, entire or serrate, or with fimbriate uniseriate extensions. New branches arise endogenously from the axial cells. Midrib becoming corticated and forming a subterete stipe below as the blade wings are lost. Reproduction: Gametophytes dioecious. Procarps borne on transverse pericentral (supporting) cells on both sides of the blades, with 2 sterile groups and a 4-celled carpogonial branch. Carposporophyte with a basal fusion cell, much branched gonimoblast, and short chains of ovoid carposporangia maturing sequentially. Cystocarps ovoid to urceolate, sessile to stipitate, ostiolate and sometimes with a conspicuous neck; pericarp formed by erect filaments with 1–3 layers of outer cortical cells. Spermatangial sori on both sides of young blades, usually with sterile midrib and margin, with the primary cells cutting off initials each of which produce several spermatangia. Tetrasporangial sori on smaller blades, with tetrasporangia cut off from several inner second- and third-order cells, occasionally from lateral (or even transverse) pericentral cells, and often from cortical cells, the sorus containing sporangia of different ages and covered by a cortex 1–2 cells thick.

<==Hypoglossum Kützing 1843W03
    |--*H. woodwardii Kützing 1843 [incl. H. hypoglossoides (Stackhouse) Collins & Hervey 1917]W03
    |--H. annae Wynne & de Clerck 2000W03
    |--H. armatum (Agardh) Agardh 1898 [=Delesseria armata Agardh 1894; incl. D. microdonthum Agardh 1898]W03
    |--H. dendroides (Harvey) Agardh 1898 (see below for synonymy)W03
    |--H. harveyanum (Agardh) Womersley & Shepley 1982 (see below for synonymy)W03
    |    |--H. h. var. harveyanumW03
    |    `--H. h. var. fimbriatum Womersley 2003W03
    |--H. heterocystideum (Agardh) Agardh 1898 (see below for synonymy)W03
    |--H. hypoglossum [=Delesseria hypoglossum]G64
    |--H. nucifolium [=Delesseria nucifolia]G64
    |--H. protendens (Agardh) Agardh 1898 [=Delesseria protendens Agardh 1894]W03
    |--H. revolutum (Harvey) Agardh 1898 (see below for synonymy)W03
    `--H. tenuifoliumW03
         |--H. t. var. tenuifoliumW03
         `--H. t. var. carolinianumW03

Hypoglossum dendroides (Harvey) Agardh 1898 [=Delesseria dendroides Harvey 1855, Hydrolapatha dendroides (Harvey) Kuntze 1898, Membranoptera dendroides (Harvey) Kuntze 1891; incl. Hypoglossum dendroides var. lancifolia, Hypoglossum dendroides var. oblongifolia]^W03

Hypoglossum harveyanum (Agardh) Womersley & Shepley 1982 [=Delesseria harveyana Agardh 1872; incl. D. serrulata Harvey 1958 non Harvey 1957, Hypoglossum serrulatum (Harvey) Agardh 1898]^W03

Hypoglossum heterocystideum (Agardh) Agardh 1898 [=Delesseria heterocystidea Agardh 1885; incl. D. hypoglossoides Harvey 1855, Hypoglossum hypoglossoides (Harvey) Womersley & Shepley 1982 non (Stackhouse) Collins & Hervey 1917, Membranoptera hypoglossoides (Harvey) Kuntze 1891, H. marginatum Agardh 1898, Delesseria undulata Agardh 1894, H. undulatum (Agardh) Agardh 1898]^W03

Hypoglossum revolutum (Harvey) Agardh 1898 [=Delesseria revoluta Harvey 1855; incl. Placophora cucullata Agardh 1892, D. denticulata Agardh 1894 nec Montagne 1849 nec Harvey 1855, Hypoglossum denticulatum Agardh 1898]^W03

*Type species of generic name indicated

References

[G64] Gray, J. E. 1864. Handbook of British Water-weeds or Algae. R. Hardwicke: London.

Oak, J. H., M.-R. Park & I. K. Lee. 2002. Taxonomy of Hypoglossum (Delesseriaceae, Rhodophyta) from Korea. Algae 17 (1): 21–31.

[W03] Womersley, H. B. S. 2003. The Marine Benthic Flora of southern Australia. Rhodophyta pt IIID. Ceramiales—Delesseriaceae, Sarcomeniaceae, Rhodomelaceae. Flora of Australia Supplementary Series 18. Australian Biological Resources Study: Canberra, and the State Herbarium of South Australia: Adelaide.

Wynne, M. J. 1988. A reassessment of the Hypoglossum group (Delesseriaceae, Rhodophyta), with a critique of its genera. Helgoländer Meeresuntersuchungen 42: 511–534.