Tuesday, September 15, 2009

There's always more to learn

I was watching a teeny hermit crab in my aquarium yesterday, and was startled to see this gliding by:

Japanese nassa, Nassarius fraterculus, aka. Hima fratercula

I am used to seeing snails there; mostly periwinkles, and an occasional whelk. They usually show the foot and two thin, pointy tentacles. Sometimes the eyes, at the base of the tentacles, are visible. But this long, thick siphon, half as long as the entire snail? I had not seen that before.

The snail was wandering around, "sniffing" with the siphon, waving it about, poking it into crevices, under shells and stones.

I went through all the entries in Kozloff's index for "sea snail", and finally found this:
Amphissa columbiana ... The almost elliptical aperture is bordered on the inner side by a conspicuous enameled area, and there is a distinct notch -- almost a canal -- through which the snail sticks out its long siphon.
The accompanying photo showed a snail somewhat similar to mine, with a short siphon, protruding barely beyond the tentacles. Still, it was close, and I had been thinking that the shell looked Amphissa-like. I reviewed my photos, and found that "canal" in the aperture:

Foot, tentacles, siphon, one eye. Green and white body. The dark smudge at the back is the operculum on the top of the foot.

I wasn't entirely convinced, though. "My" snail's shell was much more ridged. I leafed through Marine Life of the Pacific Northwest until I found a better match; the Japanese nassa, found at Centennial Beach. Mine came from across the bay at White Rock. Its range is recorded as "Japan, s. BC to n. Washington". I found a photo on the web; the colour,pattern, and relative size of the siphon matched, too.

Why hadn't I noticed that notch in the aperture before? (Maybe because I wasn't looking?) How does it compare to those of other species? I dug out a few shells from local snails to see:

Clockwise, from top: whelk, periwinkles, nassa, Japanese mud snail.

The whelk and the nassa have distinct canals, and a spread lip. The mud snails have a bit of a notch, no lip. And the periwinkles' aperture is a fat teardrop shape, with no provision made for the siphon. The "V" at the other end is the anal notch; they all have it.

A handy diagram of sea snail anatomy is at the bottom of this page. (Translated from the German. Main snail page, with links, untranslated.)

The hermit? He hid while I looked at the snail. I'll find him another day.

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