Plants bushy or spreading, not tufted. Filaments slightly to profusely branched; plants usually in soft to stiff clumps, often floating; The main axis branches dichotomously and has lateral branches that are shorter, narrower (20 - 40 µm diameter) and arranged alternately, opposite or on one side. Lateral branches near the tip often have shorter branches between longer branches. Attachment is by rhizoids from basal poles of mid to lower cells, or by cluster of basal rhizoids. Pale green to grass green.

Main axes 50-70 µm diameter, cells 4-8x longer than wide containing reticulated choloroplasts.

Cladophora sericea occurs in small amounts with other algae or in abundant loose strands attached to coral, basalt substrate, and other hard surfaces on reef flats near low tide line to 2m deep.

Distribution

Cladophora sericea is a branching green filamentous alga found on most reefs in Hawai‘i. This small green alga is usually a small part of the biomass of the diverse, highly competitive intertidal community. By virtue of its dense and self-shading form, the alga rarely experiences photo inhibition except in the branches on the very top of the tuft, and thus tends to attach to solid substrate areas that are quite exposed. In addition, the alga has demonstrated nitrate storage capabilities which increase its longevity in stressful conditions. All of the Cladophora species are opportunistic settlers that respond to environmental variation. Nutrient loading, water temperature fluctuations, and downwelling illumination all influence the growth rates and longevity of these alga. In areas of eutrophication, opportunistic species like C. sericea may play an important role in coral reel community shifts.

C. sericea has demonstrated invasive characteristics in Hawai‘i, unlike the intertidal C. vagabunda. Approximately 10 years ago the alga became exceedingly abundant on leeward reefs in Maui and large blooms now occur seasonally. During the blooms, large masses of the alga drift in the water column, snagging on coral and rock outcroppings and smothering out the organisms beneath.