PEI

Rare Irish moss threatened, and green crab suspected

In the last 40 years more than 99 per cent of a unique strain of Irish moss has disappeared, and researchers suspect an invasive species.

Decline of Irish moss coincides with arrival of green crab

Green crabs are believed to be a major culprit in the decline of giant Irish moss. (Stephanie Kelly/CBC)

In the last 40 years more than 99 per cent of a unique strain of Irish moss has disappeared, and researchers suspect an invasive species.

The green crab has been called fearless, ferocious and aggressive, and its appetite for blue mussels is believed to be a major factor in the decline of giant Irish moss.

The moss grows only in the waters at Basin Head in eastern Prince Edward Island. It is unique both in its size, growing to the size of a dinner plate, and in its lifestyle. It doesn't have a holdfast to anchor itself to a rock or a shell. Instead, it tangles itself up in clumps of mussels.

"[It's] totally dependent on a mussel population to keep it in place," said UPEI PhD candidate Paula Tummon Flynn.

"It would float completely free by itself if it didn't have the mussels to weigh it down."

Researchers prepare to put out an Irish moss/mussel clump to see how green crabs respond to it. (Stephanie Kelly/CBC)

Flynn's research project is to determine just how much of an impact the invasive green crabs are having on the moss, and how that impact might be mitigated.

"It's definitely eating at least some of the mussels," she said.

"The question is whether it's eating enough of the mussel population to cause [Irish moss] to decline like we've seen."

A steep decline

When it was first discovered in the 1970s, giant Irish moss covered 10,000 to 15,000 square metres, said Flynn. Now it's down to about two square metres, but she is not discouraged.

"I think there's hope," she said.

"There's a lot of determined people who care about this Irish moss."

Paula Tummond Flynn is optimistic for the future of Irish moss, despite the green crab invasion. (Stephanie Kelly/CBC)

Flynn is trying to measure the green crab population at Basin Head, and also trying out different configurations of mussel/Irish moss clumps and examining whether green crabs will disturb them. She is trying large clumps versus small clumps, and clumps that have only large mussels in them.

While the green crabs are almost certainly a factor, Flynn said there could be others, such as changes in water temperature, more frequent storm activity, and anoxic events becoming more common.

With files from Stephanie Kelly

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