Stomatella impertusa is a highly modified snail within the family Trochidae, commonly known as the Top shell snails. It is unique for this family in that it has a low, flat shell and an extensible metapodium that cannot be retracted into its shell. A defining characteristic of the subfamily Stomatellinae, of which S. impertusa belongs, is that it can readily autonomize the metapodium as a defense mechanism and regenerate a fully functioning new metapodium within a few weeks. It is herbivorous, with nocturnal tendencies and resides selectively on hard substrates. It is a relatively small snail, with the shell ranging from 5-25mm. 

S. impertusa is a impressive species that can be found in waters ranging from temperate to tropical from all around Australia to the Indo-Pacific. There are a number of synonyms for S. impertusa, with the most common being Stomatella auricular. 

Individuals of the subfamily Stomatellinae are recognized by their highly modified pedal morphology (Hickman, 1998). They have a large muscular foot that has an enlarged posterior metapodium that extends past the shell and cannot be fully retracted into the shell. The posterior metapodium and anterior propodium can be distinguished both visually and histologically on either side of a line of autonomy. Stomatella are able to autonomize and subsequently regenerate their metapodium in response to disturbance at this line of autonomy. Stomatellinae are the only subfamily of trochids that lack an operculum. Other identifying features are their 2 cephalic tentacles, and 3 epipodial tentacles (Hickman & McLean, 1990).

Stomatella reside on hard substrates in depths ranging from shallow intertidal to subtidal and sublittoral. They are often found on the underside of hard substrates where there is minimal light exposure (Williams et al., 2009, Jansen, 1993). They are known to reside on a variety of hard substrates, including rocks, coral, and coral rubble. (Hickman, 1998, Gofas et al., 2005). They are also known to cluster in large groups on the underside of rocks (Fishelson & Kidron, 2005). It is unknown if this is a group dynamic, or simply due to lack of space. 

Feeding
Not much is known about the specifics of diet, reproduction and life cycle for the subfamily Stomatellinae. They are known to be herbivorous, and often feed on algae. (Peterson & Tollrian, 2001). They have been identified in research as a prey species for coral reef fish (Leray et al., 2012). 

Reproduction
Trochids have separate sexes, however they do not have sexual organs for copulation. Rather they release their sperm and eggs into the water for external fertilization (Hickman, 1998). In a few cases, females lay jelly egg masses in which early development takes place. There is often a very short planktotrophic larval stage prior to morphogenesis (Wilson et al., 1993).

Stomatellinae are defined by their ability to shed the posterior end of their metapodium in response to disturbance. This method is used in hopes that a predator will eat the autonomized metapodium allowing the snail to escape. When pressure is placed onan individual, typically the first response will be for the snail to contractits foot and strongly press down onto the substrate, with only the posterior metapodium being exposed. The animal will also retract their tentacles andepipodial appendages. If the animal feels a great enough pressure it will then autonomize the exposed metapodium. The snail’s autonomized metapodium can continue to contract for hours after it is separated (Fischelson & Kidron, 2005).

Stomatella impertusa is known to be a very fast moving snail. Their locomotion isimpressive, however they are not often seen crawling about during the day. They are known to have a negative photo-axis, or affinity for darkness over light (Fishelson & Kidron, 2005,  Hickman& McLean, 1990). Stomatella are known to be quite active when necessary.They have been observing rapidly crawling to the underside of their substrate when it gets overturned (Jansen, 1993). They have also been observed leaving their hiding places once the sun sets and crawling about in search for food (Fishelson& Kidron, 2005).

I decided to conduct a simple experiment to attempt to learn about their photosensory capabilities and better understand their preference for dark versus light. Although they have two eyes, I observed that their cephalic tentacles are very active during their locomotion. This lead me to hypothesize that they do not simply rely on their eyes to detect light but their tentacles may have photosensory capabilities as well. 

This video shows that locomotive capabilities of S impterusa are impressive for a gastropod. The video also highlights how they depend strongly on their cephalic tentacles while navigating. 

Methods: The experiment consisted of creating a distinct dark and light side to a small dish and placing individuals of S. impertusa into the dish one at a time to record how long it took them to move toward the dark side of the enclosure. 8 individuals of S. impterusa were randomly selected from the aquarium for this experiment. I placed tinfoil completely covering one half of a round clear dish to create a fully shaded and fully illumated region. An individual was taken out of the holding bin and placed at a random location on the light side of the dish. Once they righted themselves, if they were flipped over, I started a timer.  I stopped timing when they were clearly heading for the dark region of the dish. Each individual went through 3 trials, for a total of 24 trials.

After the individuals underwent the first round of trials, I attempted to cut off their cephalic tentacles to repeat the experiment and record the difference in time it takes for them to reach the dark side. Cutting off the tentacles was difficult in that they would retract them as I tried to cut them off. I cut off as much of the tentacle as I could (often no more than half) and cut off at least the tip of each cephalic tentacle for each individual. I then let them alone for 20 minutes to allow them to recover from having their tentacles cut. I then repeated the experiment with the modified individuals to compare the times of the individuals with and without fully intact tentacles.

Results: 
The average time it took for a snail with the tentacles intact was 13.4 seconds, with a standard deviation of  5 seconds. 
The average time it took for a snail without the tips of the tentacles was 11.7 seconds with a standard deviation of 4.3 seconds. 

Tables recording the data are included at the end of this section. 

Discussion: 
My initial results showed a very clear negative photoaxis, in that the snails headed to the dark side of the dish quite rapidly every single trial and remained there until I removed them from the dish. My results from the second half of the experiment showed that there was not a significant difference in time it takesf or S. impertusa individuals with and without the tips of their cephalic tentacles to move towards darkness. This goes against my hypothesis that they use their tentacles as a main source of photo-reception. This leads me to wonderif they have other sources of photo-reception or if they are mostly reliant on their eyes to sense light and dark conditions. The extent of what I can conclude from this research is that there are not likely photosensory cells on the tips of the cephalic tentacles. However, there may be photo-receptors present throughout their tentacles, and not just on the tip, because without the tips they still made great use of their tentacles in the same way they did when they were fully intact. This was a very simple experiment conducted with few trials, and a small sample size. If it were repeated with more detail, interesting results about the photosensory capabilities of S. impertusa may be discovered. 

Tentacles: Stomatella possesses two long cephalic tentacles, as well as three pairs epipodial tentacles (Wilson et al., 1993). These extendable tentacles are used to gather tactile information. Trochids also possess epipodial sense organs at the base of these tentacles called macropapillae, and by waving their tentacles frequently, they can allow these organs to have better exposure (Hickman,1998).

Metapodium:The dorsal surface of the Stomatellinae metapodium is pappilate and covered in tubercles. It is also folded, and posseses a deep shell pouch at the anterior (Hickman &McLean, 1990). The metapodium is highly extensible and can expand to be longer than the animal itself (Fishelson & Kidron, 2005).

Eyes: Stomatella posses two eyes that are located on the end of two short eye stalks, which are located near the base of the cephalic tentacles (Hickman, 1998).

Snout: The snout of S. impertusa appears to fit the general description given for the snout of most trochids in that it is broad, tubular and positioned horizontal to the benthos (Hickman, 1998).

Siphon: Stomatella posses left and right neck lobes that have been enrolled to form siphon like troughs that create incurrent and excurrent canals through which water can flow through to the mantle cavity. The neck lobes are made of thin flaps of epipodial tissue. The left trough is the inhalant canal. It is broader and positioned to face anteriorly. The right trough is enrolled more tightly and serves as the exhalant canal, which is oriented posteriorly (Hickman &McLean, 1990,  Hickman, 1998).

Ctenidium & Kidneys
In the superfamily Trochoidea, both kidneys are retained, as well as the two auricles of the heart, however the right ctenidium (gill) is lost (Wilson et al., 1993, Hickman & McLean 1990). This leaves the Trochids with a bipectinate left ctenidium. A defining charactersitic for the trochid family is that the remaining left ctenidium possess a row of bursicles near the ventral axis, which have been discovered to have chemosensory abilities (Hickman & McLean, 1990).

The two kidney of the Trochoideans differ in size as well as function. The right kindey islarger and responsible for disposing of waste, while the smaller left kidney isresponsible for regulating concentrations of water, ions, and substances in theblood. (Hickman, 1998).

Radula
The radula of the sub-family Stomatellinae is very similar to the rhipidoglossate radula of the family Trochidae, and the variation of the radula within Stomatellinae is suspected to be minimal (Hickman & McLean, 1990). The main cahracterisitic that distinguishes the Stomatellinae radula from that of the trochidae is that rachidian tooth is greatly reduced in size, with its cusps being especially small (McLean, 1998). 

It is hard to say when the Stomatellinae evolved because they have the poorest fossil record of any of the main trochid subfamilies. However, It is believed that they have evolved more recently than other trochids for a number of reasons. The oldest fossils that can be confidently identified as Stomatellinae are found in the Pliocene. They are also believed to have a later evolutionary emergence because of the lack of variation of radular morphology within the family, and also due to their center of distribution being so strongly in the Indo-Pacific (Hickman & McLean, 1990).

The Stomatellinae subfamily is considered to be monophyletic (Williams et al., 2009).  It used to be considered it’s own family, however more recent evidence, including similarity of radular morphology to that of the Trochidae family, has allowed for Stomatellinae to be re-classified as a subfamily within the family Trochidae (Hickman, 1998).

There are known cryptic species for S. Impertusa that vary in both color and genetics. These cryptic species have been identified in Australia, Phillipines, and Japan (Williams et al., 2009). There are also a number of synonyms for S. Impertusa including Gena varia ,  Stomatella varia, and Stomatella auricular (Gofas et al., 2005).