Thalli forming small dense entangled tufts of up to 5 cm, reddish-brown color, which blackens by desiccation, with rough touch. Fixed to the substrate by cylindrical, crawling axes from which derive others, also with circular section, irregularly branched. Ramifications with acute apices and behaving small triangular-shaped spines.

Similar species: Gelidium pusillum

Original publication: Kützing, F.T. (1843). Phycologia generalis oder Anatomie, Physiologie und Systemkunde der Tange... Mit 80 farbig gedruckten Tafeln, gezeichnet und gravirt vom Verfasser. pp. [part 1]: [i]-xxxii, [1]-142, [part 2:] 143-458, 1, err.], pls 1-80. Leipzig: F.A. Brockhaus.

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Produces kappa/iota-hybrid carrageenan

I.J. Miller. The chemical structure of galactans from some New Zealand red algae. Botanica Marina 46(6): 572-577, 2003.

Delivopoulos, S.G. & Diannelidis, B.E. (1990). Ultrastructure of carposporogenesis in the red alga Caulacanthus ustulatus (Gigartinales, Caulacanthaceae). Ann.Bot. 66: 387-395.
Delivopoulos, S.G. & Diannelidis, B.E. (1990). Ultrastructure of carposporophyte development in the red alga Caulacanthus ustulatus (Gigartinales: Caulacanthaceae). Marine Biology, Berlin 106: 145-152, 13 figs.
Rio, A. & Cabioch, J. (1988). Apparition du Caulacanthus ustulatus (Rhodophyta, Gigartinales) dans la Manche occidentale. Cryptogamie, Algologie 9: 231-234.
Rueness, J. (1997). A culture study of Caulacanthus ustulatus (Caulacanthaceae, Gigartinales, Rhodophyta) from Europe and Asia. Cryptogamie, Algologie 18: 175-185, 12 figs.
Zuccarello, G.C., West, J. & Rueness, J. (2002). Phylogeography of the cosmopolitan red alga Caulacanthus ustulatus (Caulacanthaceae, Gigartinales). Phycological Research 50: 163-172.

This is the type species (lectotype) of the genus Caulacanthus. 

Basionym: Fucus acicularis var. ustulatus Mertens ex Turner

Lectotype locality: Cádiz, Spain (Silva, Basson & Moe 1996: 282). Type: Clemente; BM-K (Athanasiadis 1996: 60). Notes: This type locality was first recorded by Searles (1968: 46, pl. 10a) who selected Cádiz as the type locality. Turner reported seeing specimens from Clemente (Cádiz) and the Mediterranean (Dr Mohr)