Ascobulla

Ascobulla is a genus of sea snails, a marine gastropod shelled-sacoglossan mollusk in the family Volvatellidae. A more common term for the organism is “bubble snails.” The description of “bubble-shells'' comes from the shape of the head-shield and large external shell.^[2] There are three shelled-sacoglossans: Cylindrobulla, Ascobulla, and Volvatella.^[2] These three oxynoacean are the most primitive living members of the gastropod order Sacoglossa . Much confusion has occurred in the exact classifications between these species, and living animals of Cylindrobulla and Ascobulla have often been considered to be essentially identical,^[2] but more recent research is geared towards distinguishing between the two. Several unambiguous features have been found to clearly distinguish the two: These are the cephalic shield with a shallow furrow found in Cylindrobulla, the broad tooth shape of Cylindrobulla, the number of more than 50 teeth in Cylindrobulla, and the equal length of the radula limbs in Cylindrobulla.^[3] To date, the genus Ascobulla contains 5 species: A. californica, A. fischeri, A. fragilis, A. japonica, and A. ulla.^[3]

• Ascobulla californica (Hamatani, 1971)
• Ascobulla fischeri (A. Adams & Angas, 1864)
• Ascobulla fragilis (Jeffreys, 1856)
• Ascobulla japonica (Hamatani, 1969)
• Ascobulla ulla (Er. Marcus & Ev. Marcus, 1970)

Synonyms

• Ascobulla pusilla (G. Nevill & H. Nevill, 1869): synonym of Ascobulla fischeri (A. Adams & Angas, 1864)
• Ascobulla souverbiei (Montrouzier, 1874): synonym of Cylindrobulla souverbiei (Montrouzier, 1874)

Ascobulla appears to show a global distribution in tropical and subtropical waters. Caribbean (Caribbean coasts of Mexico in mangrove habitats), Mediterranean.^[4] One species within the Ascobulla genus that has received focus in research is Ascobulla fragilis. This snail inhabits the warmer areas of the Mediterranean Sea.^[5] It is also found in the waters near the Cape of Good Hope in South Africa.^[6]

Ascobulla exhibits a cylindrical, often somewhat pear-shaped shell with a concave apex and the protoconch (the embryonic/larval shell) exposed at the center. The shell itself is fairly thin and lightly calcified. It exhibits a sutural slit and a sharp edged sutural keel.^[7] Ascobulla has a cephalic shield, which is a hard external covering.^[7] They have a uniserial radula with dagger-shaped teeth, and the preradular tooth present ^[2] These snails have a foot that is relatively short (never extends beyond the shell), even when extended; the head and foot of the snail may be completely withdrawn back into the shell, and the shell then may be contracted by an anterior adductor muscle.^[8] The eyes of Ascobulla are located laterally, underneath the cephalic lobes . The gill of all shelled Sacoglossa is composed of a series of many parallel, ciliated lamellae, which are folds of the outer tissue layers of the mantle roof and cover the surface of the kidney.^[7]

^[9]

Images from Mikkelsen, 1997:^[2]

1. Ascobulla shell (4.0 mm length)
2. Ascobulla shell, expanded (3.1 mm length)
3. Apex of specimen in Image B
4. Radula (toothed feeding structure) of specimen in Image B

Ascobulla species feed suctorially on species of the siphonalean green alga, Caulerpa.^[2] Ascobulla is often found to be protected by living near  Caulerpa species, and is able to reduce the toxicity that comes from these algae in order to protect itself.^[4] “Suctorial” refers to the adaptation for sucking, or drawing up fluids and nutrients. In this sense, it behaves as a parasitic organism.  Ascobulla, as well as all other sacoglossan species, have a distinctive, lengthened (preradular) tooth that coexists with an ascus (The ascus is a special sac-like structure that is used by slugs to store old teeth).^[3] This tooth is well-adapted for piercing and suckling. This tooth breaks through into the cytoplasmic fluid of the algae, which is suckled out by the organism.^[5] Members of the Ascobulla all have increasing tooth size in their newest teeth, which is another characteristic that distinguishes them from Cylindrobulla.^[3]