Footprints

One advantage of keeping invertebrates in an aquarium is that you get to see them from a different angle; eye to eye, down at their level. In the case of snails and limpets, as they slide across the glass, you can see their underside, where they hide even their faces.

I've been comparing the different beasties that walk my aquarium walls.

Trophon snail, about an inch long.

These are the snails that eat my mussels, clams and barnacles, and would snack on hermits if they were allowed. As long as there are sufficient barnacles, they behave. Sometimes they sit on the glass for a day or two, digesting the latest meal. Then they head back to the sand and go hunting again. Usually, this is all I see of the underside: the foot, with a bit of speckled skin around the edge.

Trophon on the move.

I caught one travelling, finally. The foot elongates, but stays rounded. If you look closely, you can see the eyes about half-way up the tentacles. These predatory snails may have fairly good eyes, compared to those of the more sedentary types. Limpets, for example, have simple cup-shaped light-sensitive areas; as the cup gets deeper and closes in at the top, it becomes a pinhole eye (like the lens on a pinhole camera; good enough to get a recognizable photo); some marine snails have eyes like this.

Nassa snail.

The Nassas are slow-moving scavengers. They seem to be almost in constant motion, either wandering around on the seaweeds or the walls, or plowing through sand. This one is taking a brief rest.

Nassa, in transit.

When they move, the foot becomes pointed at the rear, and has two sharp "horns" at the front. The eyes are on the tentacles, near the base. The dark spot towards the rear is the shadow of the operculum, the lid that closes the shell when the snail hides.

Mud snail.

This is one of the invasive Japanese mud-flat snails. The foot is more rounded than the Nassa's, and the operculum is redder. The eye is barely visible at the base of the tentacle.

No snail. Anemone (1 1/4 inch across the base), and three limpets. The brown stain is where the anemone parked until last week.

The limpets are hard to see, even though they are usually sitting still. Mostly, they look like a blob of cream flesh underneath the peaked shell.

Moving limpet.

Here's one I caught eating. The mouth is pressed to the glass, scraping off invisible bits of green algae. If you look closely, you can see the two eyes at the base of the tentacles. These eyes are mere cups lined with light-sensitive cells; they show the direction of the light, and not much more. Not a problem; the algae doesn't need to be seen, and the limpet doesn't run from predators, but just seals itself down to the glass (or rock, or shell) so tightly that it can't be pried off. I have watched crabs trying; no use.

They do have one fearful enemy, I discovered:

Trophon capturing a limpet.

Because the trophon just bores through the shell, it doesn't matter if the limpet is glued down. I've been wondering why I keep finding empty limpet shells down on the sand. Now I know.

Anemone with a mind of its own.

This anemone has been a puzzlement from the start. He grew up with a couple of dozen relatives on the far side of the tank, at water level, but soon left home and moved to the front, alone. There, he wandered about, from one spot to another, and occasionally down to settle for a few days on a stone or a shell, but always returning to the centre of the front wall. Finally he decided on a spot and stayed. There he grew, and grew. He is now five or six times the size of his largest cousin, over there on the back wall; tall, fat, and sporting long, hungry tentacles.

A week ago, he moved an inch down, then half an inch up, leaving a brown stain where he'd been attached for so long. And tonight I found him three inches away, wedged in beside the water pump, so close he can't stand straight.

Meanwhile, his rejected family sits along the water line by the window, rarely moving, peacefully catching their small meals, and not growing. I can't figure it out.