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Bulla is a genus of medium to large hermaphrodite sea snails, shelled marine opisthobranch gastropod molluscs. These herbivorous snails are in the suborder Cephalaspidea, headshield slugs, and the order Opisthobranchia.

These snails are popularly known as "bubble snails", and their shells as "bubble shells", because the shell of some of the species is very inflated indeed, almost spherical in shape, and is also very thin and light.

According to some experts, Bulla is currently the only genus in the family Bullidae, which in turn is the only member of the superfamily Bulloidea.

Taxonomy
This family seems to have evolved separately in an early stage of the evolutionary history of the opisthobranchs. For a fuller treatment of the whole group see Cephalaspidea.

Bulla, Haminoea and Smaragdinella form the well-defined monophyletic group Bulloidea, according to the 1996 phylogenetic analysis of Paula M. Mikkelsen (Malacologia, 37(2): 375-442). But, according to Dr. Bill Rudman and others, differences in the alimentary canal and reproductive system, still put Haminoea and Smaragdinella into the separate superfamily Haminoeidea.

Historically, since the 18th century and even in the 20th century, the genus name Bulla has been used for a great number of bubble-shelled species that belonged to the order Cephalapsidea. From the mid-20th century, authors began to restrict species to the genus Bulla in its current meaning. But misidentifications were still numerous through high levels of intraspecific variability in the shell, radula and male genital system. The monograph by Malaquias & Reid (2008) has offered a systematic revision of this genus and has brought order in this genus

Shell description
All Bulla species have large, ovate external shells, which are large enough to accommodate the whole snail when retracted. All species have rather similarly shaped shells. The shells have a deep, narrow umbilicus at the apex. There is no operculum

The smooth shell of Bulla is ovate and expanded, with a deep, sunken involute top. Since there is little difference between the shells and in the morphology of the radular teeth, there is some uncertainty about the exact taxonomy of the species in Bulla.

Anatomy of the soft parts
The gizzard of Bulla is rather different from that of other herbivorous groups. It has three large corneous crushing plates and ancillary corneous spines, instead of just grinding plates. The crawling snails show prominent, frilled or lobed parapodia.

Bulla species have a soft radula.

Circadian Rhythms in Bulla
Bulla, specifically, Bulla goulidana, are model organisms to study circadian rhythms as they are easy to maintain in a laboratory settings, have re-identifiable large cells, and display measurable circadian behaviors. Compared to other mollusks and organisms of study in circadian biology, the retina of the Bulla contains a small number of relatively large neurons. This allows for easier surgical manipulation and distinction between populations of cells and longer intracellular recording time. Since chronobiological studies often require multiple days of recording, the ease of access facilitates data acquisition.

Basal Retinal Neurons
A basal retinal neuron (BRN) is a type of photosensitive neuron believed to be the initiator of compound action potentials (CAPs) in the optic nerve of the Bulla. There are approximately 100 BRNs in each Bulla eye, allowing for easy re-identification of studied cells. BRNs produce CAPs, or an aggregate action potential value of multiple simultaneously firing cells, creating an optic nerve impulse. CAPs are produced by individual excitatory action potentials and lead to the synchronization and amplification of the signal. Each eye in the Bulla is coupled to the other eye through the production of CAPs in the optic nerve. CAPs are an ideal output as they are expressed as large spontaneous impulses, are biphasic, and are recorded extracellularly, giving scientists an easy way to determine whether these cells are “on” or “off. Additionally, CAPs in the Bulla eye demonstrate rhythmic behavior with a near-24 hour period. Together with the ease of re-identification of BRNs, ease of recording, and reliability of circadian CAP production, they are an excellent model to provide insight into the chronobiology, or “time-biology,” of molluscs.

In Bulla, CAPs are most abundant in the late subjective night and decrease nearly twofold in conductance at dawn. Circadian cycles in membrane potential, which are caused by rhythmic changes in membrane conductance, affect the membrane permeability of calcium ions in the BRNs. To test the necessity of increased calcium ion conductance, researchers treated BRNs with EGTA, a chelating agent that “traps” calcium ions and prevents their movement into the cell. Previous experiments showed that light exposure and inducing potassium conductance were sufficient to shift circadian rhythm; however, when EGTA was used in addition to these methods, no circadian shift was observed. This experiment provided support for the necessity of calcium permeability in circadian pacemaker function and ability to match organism activity to environmental light cycles in the Bulla.

Discovery of Cell-Autonomous Pacemaker
The eyes of the Bulla exhibit circadian rhythms in the firing of their optic nerves; however, up until the end of the 20th century, the mechanism was unknown. In the 1980s, Gene D. Block conducted extensive research on circadian rhythms in Bulla eyes. In 1982, he ruled out photoreceptor cells as regulators of circadian rhythms, and concluded the pacemaking cells must be in the retina. In 1984, Block and a team of students conducted 74 hour intracellular recordings of Bulla BRNs in constant darkness. This experiment was crucial in determining whether BRNs functioned as a circadian pacemaker as a loss of rhythm in darkness would indicate the absence of an internal clock and that that Bulla are simply following light cues. They found clear circadian rhythms in both membrane potential and frequency of action potential firing in constant darkness, which directly correlated with rhythmic CAP firing in the optic nerve. This indicated that the network that comprised the BRNs and produced CAPs contained a circadian oscillator. In 1993, Block’s lab isolated BRNs in vitro and found that they individually exhibited the same circadian rhythmicity in membrane conductance, thereby generating oscillations in membrane potential. Subsequently, the BRNs were isolated in vitro and proved to be sufficient at photoentrainment, the process of matching an organism’s activity to the light and dark patterns in its environment. The findings that indicated BRNs were both individually intrinsically rhythmic and photo-entrainable led to the conclusion that BRNs function as cell-autonomous pacemakers in the Bulla. This was the first time individual neurons were found to be capable of functioning as circadian pacemakers. The discovery of a cell autonomous pacemaker served as a stepping stone in the field of chronobiology, as there were few circadian pacemakers known to be localized to specific regions, let alone to specific autonomous cells in vitro. It would take another eleven years before cell-autonomous pacemakers comprising the mammalian clock in the SCN were discovered.

Life habits
These snails are mostly nocturnal and can be found on shallow, sandy coasts grazing among sea grasses, feeding primarily on green algae. They bury themselves in mud when the tide is out.

Predators
In the coastal lagoons and bays of California, the colorful nudibranch Navanax inermis is a well-known predator of sea slugs, especially Bulla gouldiana, which it envelopes whole.

Species



 * Bulla ampulla Linnaeus, 1758 Pacific bulla, ampulle bulla
 * Distribution : on sandy sublittoral bottoms of warmer seas, tropical Indo-Pacific, Pakistan, Sri Lanka, Philippines
 * Length : 60 mm (largest shell of the Cephalaspidea)
 * Description : This is the common Bulla in tropical Indo-Pacific; globose, inflated, moderately solid body whorl. The white aperture is as long as the rest of the shell.The rounded outer lip is extended posteriorly beyond the apex. Columella in a reversed 'S'-shape, smooth and thinly callous. Cream-colored with blotches of dark, purple-brown.
 * Bulla arabica Malaquias & Reid, 2008
 * Bulla bermudae Verrill and Bush, 1900
 * Distribution : Bermudas
 * Length : 3 mm
 * Bulla clausa Dall, 1889 imperforate bubble
 * Distribution : Florida
 * Length : 12 mm
 * Bulla gouldiana Pilsbry, 1895 California bubble, Gould's bubble, cloudy bubble
 * Distribution : Northwest America, California to Ecuador
 * Length : 30–64 mm
 * Description: semi-transparent head, mantle and foot are yellowish-brown with mottled pale-bluish dots; reddish to brown involute (= sunken) apex; the aperture is wide anteriorly, narrow posteriorly; their egg mass is yellow to orange tangled string of jelly, containing oval capsules. Each one contains up to 25 eggs, which develop into veliger larvae.
 * Bulla indolens Dall, 1927
 * Distribution : Georgia
 * Length : 7.5 mm
 * Description : found at depths up to 800 m
 * Bulla japonica T. Habe, 1976
 * Distribution : Japan
 * Bulla krebsii Dall, 1889
 * Distribution : Guadeloupe
 * Length : 8 mm
 * Description : found at depths up to 1400 m
 * Bulla mabillei E. A. A. Locard, 1896 Mabille's bubble
 * Distribution : Turkey, Canaries, Madeira, Cape Verde, West Africa
 * Length : 33–52 mm
 * Description : larger than the other European species; difficult to obtain; color : yellowish-brown with dark bluish dots
 * Bulla morgana Dall, 1908
 * Distribution : West America
 * Bulla occidentalis A. Adams, 1850 (synonym of Bulla striata) common West Indian bubble
 * Distribution : Brazil, North Carolina to Florida, Bahamas, Caribbean.
 * Length : 25 mm
 * Description : thin, rotund, oval shell with a smooth, glazed surface; pale color with brown spots; involute (= sunken) apex; large body whorl; long aperture, wide anteriorly; white columella.
 * Bulla orientalis T. Habe, 1941
 * Distribution : Indo Pacific
 * Description : brown punctuate marks on the shell
 * Bulla peasiana Pilsbry, 1895
 * Bulla punctulata A. Adams In Sowerby, 1850
 * Distribution : Pacific, California, Mexico, Peru
 * Length : 30 mm
 * Description : the shell looks like the one of Bulla ampula, but is smaller and more cylindrical. Its color is cream, with clouding of brown or gray in two to four spiral bands, generally spotted with squarish chocolate dots, bordered to the right by white spots.
 * Bulla quoyii Gray in Dieffenbach, 1843 brown bubble shell
 * Distribution : Southern Australia, northern New Zealand.
 * Length : 44 mm-60 mm
 * Description :The calcified shell has a gray-brown color, with blotches of various shades of brown; the snail has a bright honey-golden color. The hind extremities of the headshield have evolved into tentacles, directing the water over Hancock's organ. The egg-mass is a jelly-like sphere, with the eggs in a spiral string. After the breeding period, there occurs a mass mortality of the animals, just like the sea hares.
 * Bulla solida Gmelin, 1791 solid bubble
 * Distribution : Mexico, Florida, Texas, Cuba, Colombia.
 * Length : 30–52 mm
 * Description : found at depths up to 25 m




 * Bulla striata Bruguière, 1792 common Atlantic bubble, striate bubble
 * Distribution : Mediterranean, Morocco, Canaries, Azores, Atlantic Ocean, Florida
 * Length : 12–30 mm
 * Description : The shell is thin, delicate and rather narrow. The body whorl is oval and convex. The smooth elongated aperture narrows posteriorly, but is wide anteriorly. The columellar callus is rather small; The thin outer lip is incurved and extends a little beyond the apex; The color is brown-gray, with darker, smudged dots and dashes, spread unevenly over the surface. The surface is smooth, with some spiral grooves at the posterior end and at the apical umbilicus. There is no operculum. The foot is well developed. There are no parapodia (fleshy winglike outgrowths). The broadened head has no tentacles. The gills and the osphradium are inside the mantle cavity. The radula has three laterals on each side of the central tooth.
 * Bulla vernicosa Gould, 1859 (most probably a color variant of Bulla ampulla)
 * Distribution : Indo Pacific, Thailand, Indonesia, Philippines, Taiwan, Tonga
 * Length : 27–50 mm
 * Description : white-colored shell with lightbrown spots

Other species
In addition to the above, there are a substantial number of names in Bulla that apply to the species Akera bullata, including Bulla aker a (Gmelin, J.F., 1791), Bulla norwegica (Bruguière, J.G., 1789), Bulla canaliculata (Olivi, 1792), Bulla resiliens (Donovan, E., 1801), Bulla fragilis (Lamarck, J.B.P.A. de, 1822), Bulla hanleyi (Adams A. in Sowerby G.B. II, 1850/1855), Bulla elastica (Sandri & Danilo, 1856), Bulla farrani (Norman, 1890), Bulla globosa (Cantraine, F.J., 1840)

Reference

 * Linnaeus, C. (1758). Systema Naturae per regna tria naturae, secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis. Editio decima, reformata. Laurentius Salvius: Holmiae. ii, 824 pp
 * Powell A. W. B., New Zealand Mollusca, William Collins Publishers Ltd, Auckland, New Zealand 1979 ISBN 0-00-216906-1
 * Glen Pownall, New Zealand Shells and Shellfish, Seven Seas Publishing Pty Ltd, Wellington, New Zealand 1979 ISBN 0-85467-054-8