Iris snail

The iris snail is small — shell length between eight and twenty-two millimetres — and it is common, and it is, on almost every measure, unremarkable. It grazes kelp blades. It moves slowly. It is not eaten by anything the survey has found particularly interesting. What it has, in compensation, is its foot.

The muscular foot of Iris — the snail has not received a formal binomial in the survey record, which is its own comment on priority — produces a structural iridescence across a wide angular range. This is not pigment. The foot tissue contains ordered microstructures that refract incident light and return it in spectrum-shifted bands. In ambient shelf light, the foot is blue-green. In low-angle light, it becomes gold. A night dive with a focused beam can produce, from a single animal, the full visible spectrum in sequence as the beam angle changes.

Ecology

The species grazes the upper blade surface of shelf kelps, including phosphor-kelp, leaving narrow cleaned tracks that are visible to a diver hovering close. It does not appear to graze below the photic zone. Population density at Karrowin Halt is sufficient that the kelp blades, at certain seasons, carry a dozen animals per square metre without apparent stress to the plant.

In the field

The iris snail is routinely the first animal a new diver learns to name. It is findable, slow, and dramatic at close range. It does not retract from approach; it is not disturbed by a diver's presence. It can be studied, at any recreational depth on the outer shelf, by the simple method of hovering quietly and looking at the kelp.