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The Great Barrier Reef of the mussel world

Originally published in the Chronicle Herald (Halifax)

When most of us think of reefs, we think of tropical scenes with brightly coloured fish and bikinis. Maybe some of us also think of the beautiful and fascinating forests of deep sea corals found off Nova Scotia’s shores. It is unlikely, however, that many people think of mussels.

Usually we associate mussels more with garlic butter than with reefs, but in our own Bay of Fundy, mussels are doing some very impressive things. Large groupings of these molluscs have formed extensive fields of reefs on the seafloor, with each reef measuring about 20 metres wide, up to three metres high and 1.2 kilometres long. These are the biggest reefs of their kind known to exist anywhere on the planet. So high tides aren’t the only unusual feature that the Bay of Fundy has to offer!

The reefs in the Bay of Fundy are formed by horse mussels (Modiolus modiolus). Horse mussels are similar to the more familiar blue mussel (the kind that goes well with garlic butter), though horse mussels generally grow to be larger (up to 22 centimetres) with a heavier and more rugged shell.

The horse mussels are able to form reefs because sand and mud that is moved across the seafloor by the strong Fundy tidal currents becomes trapped in the byssal threads that mussels use to attach themselves to hard surfaces. As old mussels die and sediment continues to build, new mussels can build upward on top of the growing mounds of sand and mud and mussel shells, all held together by the byssal threads.

The mussel reefs add structural complexity to the seafloor, providing habitat for communities of other organisms. By filtering food out of the water column and providing nutrients to the seafloor in their waste products, the reef mussels also increase local biological productivity. Because of their important ecological roles and because horse mussel reefs of this size are unique to the Bay of Fundy, the Canadian Parks and Wilderness Society - Nova Scotia Chapter (CPAWS-NS) is working to protect them.

To do this, we are working with government, industry and community members to better understand the reefs, the threats facing them, measures that could be taken to protect them, and how these measures could affect the people living and working in the Bay of Fundy region. Ultimately, CPAWS-NS wants to ensure that these magnificent marine features can be maintained.

Among the more serious threats that need to be addressed, mussel reefs are vulnerable to physical disturbances to the seafloor such as those created by fishing trawls and dredges. Sonograms of the Bay of Fundy seafloor show linear gouges through the reefs in some areas where trawls have already passed through. In the United Kingdom, smaller horse mussel reefs have sustained more widespread damage of this kind and expensive restoration work is now being undertaken in some areas; however, a complete recovery in the near future is unlikely. CPAWS-NS wants to ensure that the Bay of Fundy reefs do not suffer the same fate.

CPAWS-NS also wants to spread the word about this natural marine wonder in our own backyard. So, the next time you’re about to dip a tasty mussel morsel in garlic butter, pause to consider the mighty mussel architects in the Bay of Fundy whose colonies have grown into structures higher than the ceiling in an average room and larger in area than an average city block. Here, on the seafloor in the depths of our Bay of Fundy, the humble horse mussel has created something quite special and worth protecting.

CPAWS-NS wishes to acknowledge support for this work from the Gulf of Maine Council on the Marine Environment, Mountain Equipment Co-op, and the Sage Environmental Program.

Laura Hussey is marine co-ordinator, Canadian Parks and Wilderness Society - Nova Scotia Chapter (CPAWS-NS).