Young Stomatella sp. ~2mm long.
**Note: For the "Cut to the chase" version, scroll to the bottom of the page**
Lucky is the aquarist that finds this neat little creature in his or her system! Snails in the genus Stomatella commonly hitchhike into our systems via imports from the warm waters of the Indo-Pacific/southwest Pacific region. They also arrive as incidental passengers on mounted coral frags and macroalgae from dealers and fellow hobbyists. These gastropods belong in the Superfamily Trochoidea, which also includes the familiar Turbo, Astraea, and Trochus snails. Although their diminutive, oddly shaped shells bear little resemblance to those of their relatives, the rest is fairly typical.
Basic Description and Behavior
Stomatellids look like a cross between a slug and a snail. The shell appears as little more than an afterthought. That is, a disproportionately small, elongate flattish cap stuck on the back of a slug. The shell’s shape is reminiscent of an abalone, with a posteriorly placed whorl and iridescent mother of pearl on the underneath/inside surface. These snails also lack an operculum (the trap door at the opening of a snail) so there’s no safe retreat from predators. To aid in their survival, they employ a technique shared by many lizards. When threatened, they detach the hind portion of their tail, or in this case the stomatellid’s foot. The detached portion, called the metapodium, writhes and serves as a tasty distraction so that the snail can make a speedy getaway – and I do mean speedy! Although these are most definitely snails, they can move very quickly indeed. The lucky snail zips away and its lost metapodium soon regenerates. Also aiding in stomatellids avoidance of predators is their mostly nocturnal nature. In systems without predators, however, it’s not uncommon to see them out and about during the day on the glass, rockwork, just about anywhere.
Varies from mottled shades of green, tan, brown, cream, rust, gray, pink, even blue, to solid black. The shell itself can be mostly smooth or appear a bit rough due to some short, raised, almost papery areas sticking up from the surface.
The shell maxes out at about 1.25” for the larger species.
The species name most often attributed to these common hitchhikers is Stomatella varia, but there are at least two other possibilities that may be misidentified on occasion. Those two species are Stomatella impertusa and Stomatella planulata. The problem is that they’re all very similar looking, their range overlaps, and complicating things is the fact that they do vary so much within each specie. The species name “varia” in Stomatella varia is indeed a most apt description. Please see the links listed at the bottom for photos and more information on each specie.
Stomatellids are herbivorous grazers that eat microalgae – including diatoms and dinoflagellates, as well as possibly cyanobacteria. They do not eat hair algae.
Fishes and the usual “pickers” such as shrimps (especially Peppermint shrimps/Lysmata wurdemanni), and crabs will eat these little guys. The Stomatellids themselves are peaceful and pose no risk to corals or other livestock. They'll do fine in systems nano-size and up.
Stomatellids reproduce readily in systems which is surprising since they’re “free-spawners”. Reproduction takes place by way of the male climbing up onto a rock, stretching up, and releasing sperm in what looks like trails of smoke into the water. This triggers the female to release a jelly-like mass of eggs. Apparently, after a very short period of time, the embryos develop into free-swimming, non-feeding larvae (aka“veligers”) . After a few days, they descend out of the water column and settle onto the substrate, looking like miniature versions of their parents. I can only guess that their success in reproduction must be tied to the short development from embryo to “snail”. Not surprisingly, these snails mature early as well, reportedly from around a month to 6 weeks so that no doubt also has an impact on their success rate.
The image below is of a Black Stomatella (shell ~3/8") spawning in my tank. Just before this shot, he was standing up on the very tip of his foot, waving in the current, releasing a smoke-like trail of gametes. Wish I'd gotten that shot!
**Cut to the chase!
Classification: Class: Gastropoda, Superfamily: Trochoidea, Genus: Stomatella
Basics: Extremely common hitchhiker, beneficial, harmless herbivorous grazer.
Distinguishing characteristics: Flat cap-like shell on top of a slug, two antennae.
Color: Varies - mottled shades of green, tan, brown, cream, rust, gray, even blue, to solid black.
Size: Shell up to 1.25” long
Habitat: Substrate, rockwork, macroalgae, glass.
Diet: Microalgae – including diatoms and dinoflagellates, possibly cyanobacteria.
Reproduction: Broadcast spawners with a short planktonic/pelagic stage. Reproduce readily in captivity.
Compatibility: Will be eaten by fishes, shrimps, most crabs.
Words of caution: None, these guys are completely reef safe and beneficial.
Photo of a pretty little green stomatellid at WWM (first FAQ on page): Link
Stomatella varia: Link
Stomatella impertusa: Link
Stomatella planulata: Link
Please also enter Stomatella in WWM's search engine for more info/photos: Link
Same black Stomatellid. Small tubes you see around the snail are harmless Vermetid gastropods.