Key to the species of Codium
| 1 | Plant without branches | 2 |
| 1 | Plant with branches | 6 |
| 2 | Plant irregularly shaped, flattened and adherent to substrate | 3 |
| 2 | Plant globular, spherical | 5 |
| 3 | Utricle apices with inwardly directed jagged processes | C. phasmaticum |
| 3 | Utricles often pitted but not with distinctly inwardly directed processes | 4 |
| 4 | Plant firm, utricles of various shapes and sizes but mostly less than 1 mm. long, apical wall frequently pitted (alveolate) | C. arabicum |
| 4 | Plant spongy, utricles mostly 1.5 - 3 mm long, 150 - 350 µm diam., apical wall not pitted | C. spongiosum |
| 5 | Plant solid to hollow, but not thin, utricles to several mm long, often more than 1 mm diam (among the largest known in genus) | C. mamillosum |
| 5 | Plant hollow, sack-like, thin, composed of utricles 250 - 280 µm long, mostly about 100 µm diam. | C. saccatum |
| 6 | Plants with trailing, horizontal, divaricately dichotomous, terete branches secondarily attached to each other and to substrate, intertidal or shallow water | C. edule |
| 6 | Plant with erect, dichotomous, terete branches tending to become unilateral as a result of unequal growth from the dichotomies; plant slightly flattened below, markedly flattened above; intertidal to shallow water | C. reediae |