Enteromorpha intestinalis - A refuge for the supralittoral rock-pool harpacticoid copepod Tigriopus brevicornis

Supralittoral rockpools form an extreme environment due to severe fluctuations in salinity, temperature and oxygen concentration. The only seaweed species in supralittoral rockpools around Great Cumbrae, Firth of Clyde was the green alga Enteromorpha intestinalis. Only two animal species were recorded permanently over a three year period in this environment; the orange harpacticoid copepod, Tigriopus brevicornis, and the chironomid larva, Halocladius fucicola. The present study demonstrates the role of the hollow thallus or E. intestinalis in the lifestyles of T. brevicornis (adults and nauplii) and H. fucicola. The thallus provides a moist refuge reducing desiccation when the rockpools completely dry out. Several hundred individuals of T. brevicornis have been observed in a single E. intestinalis thallus. This observation may explain how the species can successfully recolonize completely evaporated supralittoral rockpools after several weeks. The study also raises questions about the reliability of previous population estimates of Tigriopus spp. in light of this constantly overlooked refuge.