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A unique strain of the red alga Irish moss (Chondrus crispus) is found solely amongst clumps of blue mussels (Mytilus edulis) in a coastal lagoon in Atlantic Canada. Since about 2000, its bed area has shrunk by >99.9%, coinciding with the arrival of the non-indigenous green crab (Carcinus maenas). This study tested two mechanisms by which green crabs may harm the Irish moss. The hypothesis that green crabs directly consume the alga was tested by exposing fronds and clumps to crabs. Crab interaction with the clumps caused limited fragmentation, consumption was very small, and the condition (visible grazing damage) of fronds did not change significantly. A second hypothesis, that during predation and handling of mussels green crabs indirectly displace the seaweed and remove its attachment substrate, was addressed by placing crabs with Irish moss-mussel clumps containing either large or small mussels. Green crabs removed and ate up to 100% of the small mussels but did not consume or displace large mussels. This study concludes that direct consumption is not a plausible mechanism for green crabs to harm this strain. Instead, green crabs harm could be mediated by mussels, whose removal deprives the giant Irish moss of positional stability. [ABSTRACT FROM AUTHOR]

Irish moss (Chondrus crispus Stackhouse) is a red alga that is common in Atlantic Canada. A unique strain of this species, the giant Irish moss, grows in a single location and is under strict protection. Unlike the common coastal form, the giant Irish moss reproduces solely by fragmentation and is found in gametophyte form. A 99.9% decline in giant Irish moss abundance (1999–2012) prompted this study to address two questions: whether the giant Irish moss remains 100% vegetative and gametophytic, and whether such decline is related to factors leading to a reduction in photosynthetic health. Six populations of the common Irish moss strain were compared with two populations of giant Irish moss, and their life history phases determined using a resorcinol method. The common Irish moss populations exhibited a 65%–86% ratio of gametophytic:tetrasporophytic fronds, while both giant strain populations were 100% gametophytic. Photosynthetic efficiency was measured with Pulse-Amplitude-Modulation (PAM) fluorometry and neither giant moss population had significantly different quantum yield values from the littoral populations. Hence, these analyses provided no evidence of giant Irish moss being exposed to particular sources of stress linked to water or habitat quality and alternative factors explaining the decline of this unique strain are proposed. [ABSTRACT FROM AUTHOR]