The genus has thalli of two forms, either erect or prostrate. The erect plants are dichotomously branched to 40 cm long with branches forming a compact spongy structure, not calcareous. The final branches form a surface layer of close palisade cortex of utricles. The non-erect species form either a prostrate or globular thallus with a velvet-like surface, the final branches forming a close cortex of utricles .
Two of these species are very rare in Ireland. Codium adhaerens has been recorded from a few sites on the west coast and from Tory Island on the north coast in County Donegal. In 1837 it was found in Church Bay in County Antrim, but has not been found there since.
Codium adhaerens C.Agardh
Codium bursa (Linnaeus) C.Agardh; has been found in Mulroy Bay in County Donegal in 1977 (specimen in the Ulster Museum) and 1988. There is an old record of it from "near Belfast" between 1793 and 1810.
There are other species of "doubtful validity":- Codium amphibium is included in Harvey's Phycologia Britannica Pl.xxxv. and noted as: "spreading in patches of great extent along the edge of the sea, over the surface of a turf-bog which meets the shore at Roundstone Bay" (Ireland).
Codium cuneatum S. & G. California (U.S.A.)
Codium duthiae Silva Australia, Tasmania and South Africa.
Codium extricatum Silva Cape Agulhas to southern Natal.
Codium fragile (Suringar) Hariot Japan, American Pacific from Alaska to Cape Horn, Australia and New Zealand, British Isles, Norway, Denmark and Netherlands.
Codium fragile (Suringar) Hariot subsp. capense Silva South Africa.
Codium hubbsii Daws. California (U.S.A.)
Codium johnstonei Silva California (U.S.A.)
Codium lucasi Setchell Australia, Tasmania.
Codium geppiorum Australia and Indo-PacificSouth Africa.
Codium spongiosum Australia.
Codium setchellii Gardn. Califirnis (U.S.A.)
Codium intertextum Collins & Hervey
Codium isthmocladum Vickers
Codium tayori Silva
Codium repens (Crouan) Vickers
^ C. van den Hoek, D. G. Mann & M. H. Jahns (1995). Algae: An Introduction to Phycology. Cambridge University Press, Cambridge. ISBN 0-521-30419-9.
^ E. M. Burrows (1991). Seaweeds of the British Isles. Volume 2. Chlorophyta. Natural History Museum, London. ISBN 0-565-00981-8.
Guiry, M.D. & Guiry, G.M. (2007). Genus: Codium taxonomy browser. AlgaeBase version 4.2 World-wide electronic publication, National University of Ireland, Galway. Retrieved on 2007-09-24.
G. Hardy & M. D. Guiry (2006). A Check-list and Atlas of the Seaweeds of Britain and Ireland. British Phycological Society, London. ISBN 3-906166-35-X.
O. Morton (1978). Some interesting records of algae from Ireland. Ir. Nat. J.19: 140–242.
O. Morton (1994). Marine Algae of Northern Ireland. Ulster Museum, Belfast. ISBN 0-900771-28-8.
O. Morton (2003). The marine macroalgae of County Donegal, Ireland. Bull. Ir. biogeog. Soc.27: 3–164.
Hilda M. Parkes (1975). Records of Codium species in Ireland. Proc. R. I. A.75 (B): 123–134.
P. C. Silva (1955). The dichotomous species of Codium in Britain. Journal of the Marine Biological Association of the United Kingdom24: 565–577.
P. C. Silva & D. E. G. Irvine (1960). Codium amphibium: a species of doubtful validity. Journal of the Marine Biological Association of the United Kingdom39: 631–636.