January, 2013 ~ We are rock turners. Ned figures it takes about a 1,000 rocks to score a hit by uncovering some rare or unusual animal. Just offshore small villages or under piers, where we often dive, the hunt involves more trash than rocks but whatever we are turning over, it is exciting to uncover some cool crab or worm or my current favorite, a stomatellid. Our dive guide Denny discovered the 2-inch Papery Stomatella, pictured above, when he picked up an algae covered nylon rice sack that was partially buried in the mucky bottom of Lembeh Strait, Indonesia. The shallow, barren shelf made it easy to have a good look at the scurrying animal before we herded it back to the protection of its bag.
Stomatellids are marine mollusks, in the family Trochidae (top snails). Though they are snails, they do not move at a snail’s pace; we cal them racing snails as you can see in my video at the end of this post. I always thought they moved quickly because we disturbed them from their cozy hiding places, but two months ago, during a night dive off Waigeo, Indonesia, my dive buddy Wendy and I spent a fleeting minute with one already out making turns around the reef.
Now home for the winter and cataloging video, I’m looking for information about these animals. I consulted the old invert zoology textbook, did some digging around the Internet and sent a few emails to see if any info was readily available. It turns out they are quite well known to aquarium keepers because they turn up in live rock. They are considered good because they eat algae and don’t harm other animals. They also spawn readily in captivity, evident by a number of YouTube aquarium videos from which I learned that they are broadcast spawners, releasing sperm and eggs into the water.
They don’t have an operculum, that little calcareous lid used as a protective trapdoor in some other species of snails like the turbans (Click here for a photo of some opercula that I picked up while diving) – I suppose it has no need for one since the long muscular foot cannot be retracted into the shell. I also learned that they practice autotomy; they can drop off the rear end of the foot, leaving it wriggling, supposedly to distract a predator while the animal escapes.
With their frilly mantles, long tentacles and curvy, folding neck lobes these are really pretty animals. The neck lobes are intriguing – I noticed in most of Ned’s photos, the right lobe is rolled up. Not knowing that much about their anatomy I figured this was an inhalant siphon, similar to those of some other snails but then I noticed in Wendy’s photo the left lobe is rolled. Curious. Turns out the left and right neck lobes can be rolled for use as incurrent or excurrent troughs, respectively. Top and turban snails only possess left gills with the ventilating current entering on the left side and exiting on the right (where the anus is also located) so I suppose if I had the opportunity to follow one long enough I might see it roll up one lobe and unfurl the other. Not that exciting for most divers, I’m sure.
So this is our experience so far with stomatellids. If you are a stomatellid aficionado and have other observations to share, I’d love to hear from you. Many thanks to Dr. Richard Willan for answering my questions and steering me to references for additional reading (oh yes, and for the bit about autotomy). ~ Anna DeLoach
Here is my video of the Papery Stomatella from Lembeh, loaded from our BlennyWatcher YouTube channel: