Following his nose

The Japanese nassa snails that live in my aquarium spend most of their days plowing through the sand at the bottom; I see them only when I clean the tank and sift the sand. But at times, for reasons of their own, they stop digging and head for higher ground. (At the moment, there are four climbing the walls.) Spring fever? Maybe, because they find nothing to eat up there; they're not algae eaters.

Japanese nassa, Nassarius fraterculus, siphon extended, hurrying.

Nassa snails have sturdy sensory siphons. Sometimes they bury themselves in the sand, with only the siphons poking out, waiting for food. Travelling, they follow their nosessiphons, sniffing the water for interesting goodies. Or maybe, for potential mates.

Yesterday, one of the climbers was on top of the moon snail shell, surveying the territory from there.

The eyes are small and inefficient. The sensitive siphon makes up for the lack.

A few seconds after I took this photo, a hermit crab ran up from behind. As soon as she touched the snail's shell, it retracted the siphon, then the body, and clamped itself down on the moon snail.

"Who's that?" 10 seconds later; siphon half-way down. Hermit barely visible in back.

The mud snails, algae eaters, have an oval opening, with a slight notch at both ends. These Nassas, carnivorous scavengers, have to travel longer distances to find food, hence the need for a sensitive water taster. The shell mouth has a separate notch or canal at the front end, to support and protect the siphon, which is the first thing retracted in case of danger; a siphonless Nassa will starve to death quickly.

Parts of a carnivorous marine snail. (From U. of Iowa.)

Comparative shell openings. J. Nassa in the centre. L to R: 3 mud snails, Battillaria attramentaria; unidentified whelk; 3 periwinkles. The whelk and the Nassa have siphon canals and fat lips. And siphons, of course. 2009 photo.