Special meadows need protection

Some of the diversity of shapes and sizes of bryozoans/hermit crab associations.

A marine reserve off Pukekura/Taiaroa Head would protect some remarkable collaborations.

If we were able to do an underwater bicycle tour on the sea floor, I’d take you on an expedition. We’d leave the calm waters of Otago Harbour and head east for an hour or so, going about 25km offshore. Most of our journey crosses a wide sandy plain, sloping very gently out to sea, with occasional "steps" down, where sea-level once paused on its ancient rise up the shelf.

Then we’d notice the sea floor becoming crunchy, with large gravel and shells under our tyres. These are the deep water meadows of the (proposed) Papanui Marine Reserve. In the shadowy waters, about 60m deep, we have to hold ourselves upright against the current, as cold nutrient-filled waters sweep northwards.

All around us are tufts of life. Some are worm tubes looking like a handful of straws. Some are erect bryozoans - calcareous colonies created by numerous tiny polyps, sometimes called lace corals (though they are not corals) or moss animals (though they are not moss). They look like mini-trees, gnarled and intertwined. Some are bright orange clumps that look like coral heads (but they are also bryozoans). Brittle stars and small fish wander among them. Suddenly, we are surprised as a penguin flaps by, its feathers studded with jewel-like air bubbles. It’s a knee-high forest of life.

I want to show you something special that lives here; we can find them lying on the ground: a purple sort of curving shell, thick as a finger, with a small round hole in it. It’s not a shell, and it’s not a snail - any more. This is what we came to see: the bryozoan-snail-crab association that is especially abundant on the Otago shelf.

The underwater seascape in the Otago bryozoan meadows, revealing orange bryozoans and brownish worm tubes. Photo: Emily Jones

It starts with a snail shell, dead and empty on the seafloor. A hermit crab happens upon it and crawls inside, calls it home. Then a bryozoan larva lands on the shell and starts growing, and that’s where it gets strange. Normally hermit crabs are fastidious house-keepers, cleaning off anything that lands - they don’t want to carry the extra weight. But this hermit crab encourages the bryozoan to grow around the opening of its shell. The bryozoan feeds on scraps from the crab’s food, and grows - meanwhile the crab keeps a hole open so it can still poke its nose out.

Over time, the bryozoan grows into a rough open curved shape, following the original spiral of the snail, effectively extending the size of the shell. The crab, therefore, doesn’t have to move out as it grows - its shell grows too. The bryozoan and the crab grow together for years, sometimes becoming five or even 10 times larger than the original shell.

The weird thing about this relationship is that it’s not especially special. Many different kinds of shells can form the original host, including snails and worm tubes. As many as six different species of hermit crabs can occupy a bryozoan-enhanced shell, and all those species have "regular" individuals who don’t farm bryozoans; they trade-up shells as they grow, sometimes even where there are bryozoans.

Even the bryozoans aren’t special. There are at least 13 species of "hermit bryozoans", and all of them can be found living independently of crabs and shells. Some are white, some are orange, some are purple. It’s just when these three things come together - they make a new thing.

The pink lines show the original shell, now covered in bryozoan, green is the bryozoan, which has grown a long way, and the yellow line is the face and front legs of the hermit crab. Photos: Abby Smith

They say we know less about Earth’s seafloor than we do about the surface of Mars, and that’s probably correct. Unexpected combinations of life are lurking out there, just being quietly amazing. By now, Otago’s super-special bryozoan meadows should have been protected in their own underwater national park, the Papanui Marine Reserve. Instead, fishing and trawling out there continue, while bureaucrats argue. As with Dunedin’s hospital, delay is costly.

Back on the bike and pedalling uphill the whole way, we turn towards land, over the sands of Taiaroa Head and into the calm harbour. We head for home, thinking about how much we need to protect the amazing biodiversity of Otago’s deep waters.

Abby Smith is a professor of marine science at the University of Otago.