Incredible plants: Codium

Most marine seaweeds can be classified into one of three groups of algae: reds, browns, or greens. The ancestors of today’s green algae are the group from which all modern land plants (mosses, ferns, and seed plants) evolved. Green algae are widespread globally; they are found in freshwater aquatic habitats, terrestrial environments (for example, some form symbioses with lichen-forming fungi), and of course in the oceans.

Codium fragile (upper right) and C. setchellii (throughout
photo) on intertidal rocks at Carmel Point, Monterey Co.,
CA, December 2016.

One fascinating group of green algae is the coenocytic greens. The term ‘coenocytic’ refers to an organism that technically consists of only of a single cell. Plants and algae have cell walls outside their cell membranes that provide structure to tissues. In coenocytic algae, a cross-wall does not fully form between the two daughter cells resulting from cell division. Replication of the nucleus (karyokinesis) occurs normally as with other organisms, but is not followed by complete cell division (cytokinesis). Many of the coenocytic algae are centimeters or decimeters in length, which makes the thought of them being a single cell quite remarkable!

Coenocytic species are common in marine green algae (Class Ulvophyceae, Division Chlorophyta). Examples include Bryopsis and Caulerpa. Many of these species are also called “siphonous” green algae. Other coenocytic macroalgae are calcified (Dasycladales) and grow in tropical coastal oceans.

One globally-distributed coenocytic green is the genus Codium. The distinctive color, texture, and shapes of Codium plants make them pretty easy to recognize in the field. They are true green algae, but in the field they are sometimes such a dark green color that they appear almost black.

Submerged Codium fragile in a tidepool at Davenport Landing, Santa Cruz
Co., CA, November 2009.

Along the Pacific coast of North America, there are several species of Codium. The two that I have encountered while tidepooling are Codium fragile and C. setchellii. C.fragile is a branching species and it has the notorious common name of “dead man’s fingers”. The branches are not in fact wide enough to resemble a human finger, but when the plant lays limp on the side of a rock at low tide, the common name is none-the-less reasonably accurate.

C. setchellii has a very different growth form than C. fragile. It grows like a velvet cushion, tightly adhering to the rock surface. C. setchellii has the common name of “green spongy cushion”, but I think the common name “dead man’s brains” might be a good alternative if we were to keep with the morbid theme.

The Hawaiian flora has 15 species, including both prostrate and branching growth forms.

The plant body of Codium consists of many intertwined filaments, which again are technically a single sell. On the outer surface of the seaweed’s body, the filaments end in a structure called the utricle. Utricles are the site of photosynthesis for the plant. Some utricles are pointed and some are flattened at the tip.

Utricles of Codium sp. under magnification, unknown date.

The next time you are tidepooling on Halloween (or any time of year), be sure to look for dead man’s fingers or dead man’s brains!

References

Abbott IA, Hollenberg GJ. 1976. Marine Algae of California. Stanford University Press, Stanford, CA.

Abbott IA, Huisman JM. 2004. Marine green and brown algae of the Hawaiian Islands. Bishop Museum Bulletin in Botany 4. Bishop Museum Press, Honolulu.

Druehl L 2000. Pacific Seaweeds. Harbour Publishing.

Lewis LA, McCourt RM. 2004. Green algae and the origins of land plants. American Journal of Botany 91:1535-1556.

Lobban CS and Harrison HJ. 1997. Seaweed Ecology and Physiology. Cambridge University Press.

Sze P. 1998. A Biology of the Algae. 3^rd ed. WCB McGraw-Hill. 

Population of C. fragile growing in the intertidal at Botany Beach, Vancouver
Island, BC, summer 2000. The distribution of these plants in a vertical band
is an excellent example of intertidal zonation.