Monostroma grevillei (Thuret) Wittrock

Cladophora rupestris

Description: Gametophyte thallus sac-like when young, becoming sheet-like, light- to olive-green, delicate, to 100 mm (occasionally much larger), attached by a small disk. Often with trapped oxygen bubbles (above) on sunny days. Spring annual, athough the basal parts and the shell-boring sporophtes persist throughout the year.
Habitat: in rockpools and on rock from mid-tide downwards. Commonly epiphyte on Ceramium, Polysiphonia, and Corallina officinalis in rock pools. Abundant and common January-May.
Distribution: Common and widespread but often under-recorded in Britain and Ireland; Iceland and Norway south to France. Mediterranean records require confirmation.
Key characteristics: saccate plants generally obvious; delicate thallus; abundance in spring. Similar plants in low-slainity situations are likely to be Gayralia oxysperma.

Species list