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Sacoglossan gastropods on native and introduced hosts in Lough Hyne, Ireland: larval retention and population asynchrony?

  • Cynthia D. Trowbridge (a1), Colin Little (a2), Penny Stirling (a2) and William F. Farnham (a3)


The north-eastern Atlantic sacoglossan gastropod Elysia viridis was studied on littoral and sublittoral shores of the British Isles from 2001 to 2007 to evaluate its potential role in the known decline of the invasive green alga Codium fragile ssp. fragile. Across its European range, this sacoglossan associates with eight genera of algal hosts within three algal orders and two divisions; these hosts include native and introduced macroalgae. The ‘specialist’ herbivore was investigated primarily within the rocky shore community at Lough Hyne Marine Reserve, County Cork, south-west Ireland, with comparative information from surveys of 95 other sites in the British Isles. In Lough Hyne, E. viridis associated with the green algae Codium, Cladophora and Chaetomorpha as well as the red alga Griffithsia. The sacoglossan associated with Codium, Cladophora and Chaetomorpha as well as the red alga Halurus in Devon, Isle of Wight and the Channel Islands. Recruitment of E. viridis to Codium spp. in the lough was substantially higher than in most areas of the British Isles with 100% of the Codium hosts attacked during annual September surveys. The strong autumn pulse of sacoglossan recruitment, coupled with the asynchronous population dynamics compared to other shores, indicates retention of planktotrophic larvae within the lough. The previously reported decline in sacoglossan density with tidal level was hypothesized to be caused by sublittoral predators. Although our experiments failed to demonstrate an effect by large predators, the foraging of the abundant fingerling fish and small crabs could not be tested conclusively. Field experiments were consistent with the interpretation of intense sacoglossan herbivory in certain areas of the lough. However, even if the sacoglossans contribute to the local decline of C. fragile in the lough, they cannot account for the regional decline of the alga on the north-eastern Atlantic shores.


Corresponding author

Correspondence should be addressed to: Cynthia D. Trowbridge Department of Zoology, Oregon State UniversityPO Box 1995, Newport, OR 97365USA email:


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Sacoglossan gastropods on native and introduced hosts in Lough Hyne, Ireland: larval retention and population asynchrony?

  • Cynthia D. Trowbridge (a1), Colin Little (a2), Penny Stirling (a2) and William F. Farnham (a3)


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