What Divers Should Know About Seaweed

Often resembling non-woody terrestrial plants underwater, seaweed, or macroalgae, encompasses thousands of macroscopic, multicellular marine species. These include various red, brown, and green macroalgae. Divers commonly observe seaweed inhabiting the littoral zone, primarily anchoring to rocky shores rather than sand or shingle. While many species attach to a substrate several meters deep, some genera like Sargassum and Gracilaria float freely, drifting with currents.

Beyond their distinctive forms, these macroalgae play a vital role in marine ecosystems. Kelps, for instance, provide essential nursery habitats for fisheries and other marine species. Structurally, many feature a lamina or blade, often aided by pneumatocysts or air bladders for flotation, and a holdfast with finger-like haptera to anchor to benthic substrates. The deepest living species are some red algae, while detached fronds, buoyed by pneumatocysts, can drift into the deep ocean, utilized by benthic organisms even at several hundred meters.