Mussels are evil.

They are so deceptive, just sitting there, opening their mouth a bit sometimes; otherwise, just sitting. Looking harmless and innocent.

They're not.

Let me backtrack a bit here. My little aquarium is recovering nicely from the summer disaster, becoming the biodiverse environment that the animals are accustomed to. The crabs are as calm, now, as crabs get; the big anemone is feeding more often than not; the snails are plowing through the sand and polishing the walls. I have provided a variety of seaweeds for food, gym equipment, and shelter: sea lettuce, turkish towel, rockweed, eelgrass and kelp. A few tiny greenmark hermits and a pair of big isopods moved in with the last batch of seaweed. There's at least one polychaete worm in the sand. Green amphipods zip busily about.

And then there are the water cleaners. The barnacles (lots of those) and a handful of mussels.

The biggest mussel, Mytilus trossulus, on a scratchy bed.

With his mouth open, filtering the water. One of the tiny hermits watches.

Mussels are filter feeders; they don't need to move about, although they do at first, until they find a place that suits them. Then they lie there, letting the current bring them food. They capture plankton, other microscopic life, and floating leftovers from the crabs' meals or decomposing seaweed. A mussel can scrub up to two and a half litres of water per hour per gram of body weight. That's the contents of my whole small aquarium, given the handful of mussels working there.

The problem - and I never considered it in that light before - is their habit of choosing a spot and tying themselves down. A mussel excretes a liquid that hardens immediately on contact with the water, producing strong hairs, called byssal threads. At the tip of each thread is a dot of glue. Where it touches, it holds.

(Young mussels use these to "walk"; they extend a thread, let it stick and reel it in, pulling themselves up to the contact point. Then they dissolve the connection, let the thread float free, and produce another.)

These threads are incredibly tough. They are made up of a core of collagen, coated with a harder protein bonded with iron ions, (NPR), and may have the tensile strength of five of our Achilles tendons. A mussel will attach itself with 50 or more threads, enabling it to bond to rocks and other mussels. Anchored like this, it can't be swept away even by stormy seas.

The threads have another use, too. Like all creatures, mussels have predators; starfish, birds, crabs and some fish. And whelks, those voracious hole-drillers. Tying themselves down to the rocks and in large groupings helps to protect them from the birds and crabs, but whelks will attach themselves to the shell, drill into the meat and dissolve it. To stop them, a mussel will throw out a few threads, entangle the whelk, and anchor it to the rock, where it will starve to death.

I was looking for hermits, counting them to see if they had all settled in ok, when I noticed a small Nassa snail jabbed into the sand upside-down, point buried. Odd.

When I looked next, it was still there, but had moved slightly. Still alive, then. But why was it in that position?

It was jammed against a stone, still upside-down the next time I looked. In the morning it wasn't moving, so I decided to investigate; I reached in and picked it up. It came along with the stone and the mussel on top, tied down securely. When I broke the threads that held it and left it alone, it woke up and hurried away (at top snail speed). Probably hungry.

I collected all the mussels and examined them. They had glued down another snail and one of the hermits. This would not do!

Well, I can use thread, too. In a double piece of cheesecloth, I wrapped the whole clump, and tied it in a loose net. There! They have water circulation, but the snails and hermits can't get through to them.

Small handful of mussels, with two attached stones and a clamshell. More mussels have been added since, as I found them.

Biological filtration unit. Netted mussels, in the back of the tank.

So far, this has worked. Hermits pick at the cheesecloth, but then walk away, untethered.

Nassa snail, cleaning the glass.

Freed Nassa, still wearing the remains of the byssal threads and a pebble caught in the glue.

Greenmark hermit, now safe from the clutches of the mussels.