Latin villosus, hairy, in reference to the hairy rhizomes and sporocarps.
Rhizomes villous with reddish hairs, roots produced at nodes. Stipes in clusters of 2-65 at nodes, 2.5-15 cm long. Fronds 4-leaf cloverlike. Pinnae 2.5-23 x 2-22 mm, fan-shaped with rounded tips, thin, both surfaces with fine, white hairs, these more prominent on abaxial surface and at bases of pinnae, morphology variable with time and amount of moisture, falling off when their habitat dries. Sporocarps single on 1.9 to 6 mm long peduncles at bases of stipes, 4.3-5 x 3-4.1 mm, toothed on proximal superior margin, obscured in a mass of light-colored, wooly hairs (glabrous with age).
Historically at low elevations on Ni 'ihau, O'ahu, and Moloka'i; now a federally listed endangered species known naturally from only two sites on O'ahu and a few on Moloka'i.
Marsilea villosa is related to M. vestita Hook. & Grev., a species widespread in the western half of the United States. It is found in vernal pools and intermittent wet areas. On West Moloka'i it grows in rocky areas that never hold standing water, but get flushed out during winter rains. It may be dormant for long periods during drought, appearing only as a mat of dry rhizomes and stipes with attached sporocarps. With the coming of rain or standing water, usually during the winter months, the rhizomes rapidly produce leaves, the sporocarps open, macrospores are fertilized, and vegetative reproduction begins anew with vigor. When the pools begin to dry up, sporocarps are formed and the pinnae dry and fall off, completing the cycle.
Marsilea villosa may be recognized by its 4-leaf-cloverlike fronds, its habitat in transient pools and intermittent wet areas, and the presence of single sporocarps at the bases of the stipes.