Cystoseire Bleue

Overview

Ericaria selaginoides is a bushy brown seaweed known for its striking, iridescent turquoise-blue appearance when submerged. This perennial species typically grows in rocky intertidal and shallow subtidal zones, where it can form significant marine forests that support diverse local biodiversity.

Distribution & habitat

This species is found across the Northeast Atlantic, ranging from the British Isles south to North Africa, and is also present in the Mediterranean Sea. It thrives in well-lit, rocky environments, commonly inhabiting large intertidal rock pools, lagoons, and shallow subtidal areas on both rocky shores and gravelly flats.

Appearance

Typically reaching 30–60 cm in length, the seaweed features a robust, cylindrical central axis with spirally arranged lateral branches. Its most distinctive feature is the vibrant blue-green iridescence visible underwater, which is caused by opalescent photonic crystals within its cells; this color intensity often depends on the age of the frond and available light. When removed from the water, the plant appears olive-green, eventually turning nearly black as it dries. It attaches to the substrate via a conical, digitated basal disc and often bears small, ovoid air bladders near the tips of its branches.

Biology & behavior

Ericaria selaginoides is a canopy-forming species sensitive to environmental stressors like pollution and eutrophication. Its unique blue coloration is thought to enhance photosynthetic performance by modifying light transmission. The species is monoecious, meaning individual plants contain both male and female reproductive structures, and it reproduces through an oogamic process involving external fertilization.