Codium tomentosum

Lore & Background

The holdfast of Codium tomentosum is saucer-shaped and has closely woven strands giving it a uniform appearance. The thallus has a dichotomous, much branched structure with thin branches, each with a circular cross section. It grows to 30 cm in length and is spongy, with the texture of felt, and is covered with colourless hairs visible when submerged. The cortex is composed of club-shaped vesicles forming a palisade-like layer.

C. tomentosum is native to the north east Atlantic Ocean from Ireland southwards to the Azores and Cape Verde, and has also been recorded around the coasts of Africa and in various other parts of the world. Its type locality is England. It is found on exposed rocks in deep rock pools on the lower shore.

A study in 2003 in Guernsey investigated whether the related taxon C. fragile subsp. tomentosoides was displacing C. tomentosum, but found that the native species was still thriving on the lower shore while the invasive was restricted to rock pools in the upper mid-littoral region. A similar study in 2000 on exposed shores in western Ireland showed that C. fragile subsp. tomentosoides had decreased while C. fragile subsp. atlanticum had increased, and the native C. tomentosum had maintained its population size.

Reader's Guide

Codium tomentosum is significant as a native green seaweed in the northeast Atlantic, particularly in studies of ecological displacement. Research in Guernsey and western Ireland examined whether the invasive C. fragile subsp. tomentosoides was displacing it, but found that C. tomentosum maintained its population on the lower shore, while the invasive was restricted to upper mid-littoral rock pools. This suggests resilience of the native species in its typical habitat. The sea slug Elysia viridis feeds on C. tomentosum and C. fragile, and has a symbiotic relationship where ingested chloroplasts remain photosynthetically active in the slug's digestive diverticula for several days, providing sugars that the slug metabolises. A lectin named tomentine has been isolated from C. tomentosum by affinity chromatography, showing N-acetylglucosamine-specific activity and being rich in glycine, threonine and valine. These characteristics highlight its ecological and biochemical relevance.

Did You Know?

• Codium tomentosum is also known as velvet horn or spongeweed.
• Its holdfast is saucer-shaped with closely woven strands giving a uniform appearance.
• A lectin called tomentine, rich in glycine, threonine and valine, has been isolated from this species.