Ericaria selaginoides (Linnaeus) Molinari & Guiry

Other scientific names: Cystoseira tamariscifolia (Stackhouse) Papenfuss
Common names: Bushy Rainbow Wrack

Description: Thalli solitary, up to 1 m long, bushy, with a pronounced greenish-blue, purplish or blue iridescence when submerged (above) which rapidly disappears when taken out of water; attached by a conical disc. Axis cylindrical, up to 60 cm long, usually branched and with an inconspicuous apex. Lateral branch systems arising in spiral sequence, up to 60 cm long, profusely branched in a repeatedly pinnate fashion, showing radial symmetry and bearing simple or bifid spine-like appendages: deciduous, leaving prominent scars or stumps. Cryptostomata present on branches and appendages. Air vesicles often present in axes of ultimate ramuli; ovoid, up to 5 mm long. Receptacles 1-2 cm long, formed from terminal regions of ultimate ramuli and the branches which bear them; irregularly nodose and bearing short simple spine-like appendages; simple or branched, often distal to an aerocyst.
Habitat: Found in large intertidal rock pools and lagoons, where it can be a colourful addition to the flora; also in shallow subtidal.
Why is it blue? The blue colour is the result of opalescent crystals in vesicles of the cells, which improve photosyntheis by modifying light trasmission. The amount of available light drives the intensity of the colour: the darker it is, the more intense the colour. It also seems to be age-dependent in that the younger parts of the fronds have the most intense colour.
Distribution: Southern and western shores of Britain and Ireland, north to the Hebrides in Scotland; Atlantic France and Spain south to North Africa; Western Mediterranean.

Species list

Photographs © M.D. Guiry