Published online by Cambridge University Press: 11 May 2009
Since I first found Trididemnum niveum (Giard) at Salcombe in the autumn of 1951 (Carlisle, 1953) this species has spread and increased greatly in the Plymouth area. I have found it in abundance at Salcombe (Castle rocks and the Salstone), Wembury, and especially at Looe around the Island. The favourite habitat at all these places is the same as that at Roscoff, on the fronds of Cystoseira. It occurs rather less frequently on the holdfasts of Laminaria ochroleuca and occasionally on other species of Laminaria, but very little on any other substrate. It is not truly intertidal; the majority of specimens are to be found just below low-water mark of spring tides, and very rarely are any seen above low-water mark of even the lowest tides of the year.