Post-proliferation population of introduced seaweed: decline of a parthenogenetic green seaweed in Irish marine reserve

The potential roles of herbivory and fecundity were investigated in the post-boom population stages of the introduced, green macroalga Codium fragile ssp. fragile on Irish shores. Extensive algal surveys (2001–12) within Lough Hyne Marine Reserve, Co. Cork demonstrated that peak abundances of C. fragile were low (< 2 thalli per m2) and occurred at the upper margins of beds of the native congener C. vermilara. Low fecundity and low recruitment of C. fragile indicated that the lough population may be currently limited by low propagule production and/or survival. A field experiment indicated that limpets (Patella vulgata) did not limit the alga on the western shore of the lough. Recent mass mortality of purple urchins (Paracentrotus lividus) did not influence C. fragile populations, but the long-term influence of urchins is unclear. The decline of this parthenogenetic alga is consistent with theoretical predictions for asexually derived populations.