Bright green seaweed on the beach: Bright green Gut Weed, Ulva intestinalis, on Whiteford Sands, Gower, South Wales (1) 

An isolated bright green splash of colour provided by Gut Weed, Ulva intestinalis Linnaeus, attached to a rocky outcrop on the otherwise vast sandy expanse that is Whiteford Sands, in Gower, South Wales.

This weed is a green seaweed belonging to the Chlorophycae. Athough it looks from a distance like a uniform blanketing of weed, it is made up of hundreds of thousands of individual fronds or thalli. Each thallus is between 10 – 30 mm long and 6 – 18 mm in diameter. Each frond is ribbon-like, tubular, sometimes crinkly, sometimes swollen and puffed up, often with irregular constrictions along its length. The specific name intestinalis relates to the last feature because the shape is thought to resemble tubes of intestines – although the colour is entirely wrong.

Gutweed on seashore rocks: A rocky outcrop on the beach at Whiteford Sands, Gower, South Wales, showing only partial coverage by Gut Weed at the landward edge (2) 

A rocky outcrop on the beach at Whiteford Sands, Gower, South Wales, showing partial coverage only by Gut Weed at the landward edge nearest to the dunes. I can’t work out why this seaweed is only growing in some areas and not others. Its distribution on the beach is very patchy. The textbooks say that Gut Weed will grow on almost any surface anywhere on the seashore – including rocks, mud, sand, rock pools, and on other seaweeds and seashells – even in the splash zone at the top of beaches and in areas of varying salinity with brackish water or freshwater inputs. Given the ability of Gut Weed to colonise so many places it is difficult to understand why is is restricted to just a couple of patches on this beach.

A second patch is photographed below; a strip of Gut Weed-covered rocks occuring low on the shore. In this picture, taken with the zoom full out, the tide was way, way out and the water actually reduced to a narrow stream flowing down the centre of the Loughor estuary. A heron on the water’s edge was catching fish trapped in the sparkling shallow water.

Gutweed on beach rocks: A strip of Gutweed-covered rocks low on the shore at Whiteford Sands, Gower, South Wales, UK (3) 

Revision of a post first published 9 July 2009


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