Scotch bonnet (sea snail)
|Five views of a shell of Semicassis granulata granulata|
The Scotch bonnet (Semicassis granulata), also known as the ridged bonnet, is a medium-sized species of sea snail, a marine gastropod mollusk in the subfamily Cassinae. The common name alludes to the general outline and color of the shell, which vaguely resemble a Tam o' Shanter, a traditional Scottish bonnet.
This species is found primarily in the tropical and subtropical Western Atlantic Ocean, from North Carolina to Uruguay. It is the most common species in this subfamily in North America. A similar-appearing sea snail in the Mediterranean Sea, Semicassis granulata undulata, has been considered a subspecies. These sea snails are predators; they search for their food on sandy stretches of the ocean floor, where they consume echinoderms such as sand dollars, sea biscuits, and other sea urchins.
- Scientific name
The generic name is a combination of the Latin prefix semi, meaning half, and noun cassis, meaning helmet. The specific name of this taxon, granulata, is derived from the Latin noun grana meaning grain. Here it is used in the diminutive form, meaning granulated, or covered in granules, i.e. small grains or pellets, referring to the shell sculpture.
- Common name
The shell of this species was given the common name "Scotch bonnet" because of a vague resemblance to a Tam o' Shanter, a traditional tartan hat that used to be commonly worn in Scotland. The shell has a pattern of square or rectangular brown or tan patches. The shell can sometimes be smooth except for growth lines, but in other individuals it can have a sculpture of incised spiral grooves and even weak axial ribs which, together with the colored patches of the shell, create an effect that is somewhat reminiscent of the patterns of a Scottish plaid.
This species was originally named Buccinum granulatum and described by the Austrian scientist Ignaz von Born in 1778. Since the original description, this taxon was recombined numerous times into different genera and subgenera. Nearly a century after Born's description, in 1877, the Swedish naturalist Otto Andreas Lowson Mörch proposed a new combination and transferred this taxon to the genus Cassis and subgenus Semicassis. In 1944, the American malacologist William James Clench recombined the species as Phalium (Semicassis) granulatum, and five years later the Brazilian naturalist Frederico Lange de Morretes recombined it as Semicassis granulatum. Weaver reallocated it in the subgenus Tylocassis in 1962, though Robert Tucker Abbott would recombine it as Phalium (Tylocassis) granulatum six years later. Miller recombined it as Phalim granulatum in 1983, Kreipl recombined it as Semicassis (Semicassis) granulata in 1997. The currently accepted combination, Semicassis granulata, was proposed by Alan Beu based on paleontological data.
A taxon which in the past was considered to be merely a completely smooth and glossy variety of this species was given the name cicatricosa by Johann Friedrich Gmelin in 1791. This taxon is now recognized as Semicassis cicatricosa, a separate species. The shell form of Semicassis granulata that has nodules on the shoulder was named peristephes by Pilsbry & Mcgintyi, 1939; forms however have no taxonomic significance.  A similar-looking taxon from the Mediterranean Sea was named by Gmelin in 1791. This is now known as the subspecies Semicassis granulata undulata. This subspecies has been mistakenly reported from the Western Atlantic, but does not occur there. Overall there has been some confusion, especially in the popular literature, about which name should be applied to which of these two taxa.
There are published records of Semicassis granulata from several areas of the Western Atlantic Ocean. It is considered the most common species of Cassinae in North America. Regions and countries where this species occurs include the East Coast of the USA, in North Carolina, South Carolina, Georgia and Florida (East Florida, West Florida, and the Florida Keys). It has also been recorded in the Gulf Coast of the USA, including Louisiana and Texas. The species is known to occur in the Caribbean Coast of Central America, including Mexico (Quintana Roo), Nicaragua, Costa Rica, Panama and Colombia, as well as Venezuela (Gulf of Venezuela, Carabobo, Sucre, Isla Margarita and Los Testigos Islands). It is also found in Bermuda, the Greater Antilles, Cuba, Jamaica and Puerto Rico, and further south in the Atlantic coast of South America, Surinam, Brazil (Amapá, Maranhão, Ceará, Rio Grande do Norte, Pernambuco, Alagoas, Bahia, Espírito Santo, Rio de Janeiro, São Paulo, Paraná and Santa Catarina), and also in Uruguay. This species has apparently not yet been reported in the literature as occurring in the Lesser Antilles.
- Shell description
The egg-shaped shell of this species usually grows to 2 to 4 inches (5 to 10 cm) in length. The maximum recorded shell length is 121 mm. The shells of the adult females are larger than those of the males. The sturdy shell of the adult snail has approximately five whorls with coiled spiral grooves crossed by mostly rather faint growth lines. It takes approximately six years for a Scotch bonnet to mature. Several times at different growth stages, the shell develops a thick outer lip, which is characteristic of the subfamily Cassinae. The snail rests between each of these stages of growth. When it begins to grow again, the snail usually absorbs the entire outer lip. In some cases, especially in deep water, the Scotch bonnet does not absorb the outer lip completely, leaving behind a varix on the whorls of the mature shell.
The surface of the shell varies in the degree of shell sculpture that is present, some shells being more granulated, and even having nodules on the shoulders of the whorls, and other shells being much smoother. The more food a Scotch bonnet consumes during its lifetime, the more developed the shell features are, the glossier the sheen of the shell, and the brighter the color. As is the case in all snails, the final whorl (the body whorl) is the largest and contains most of the animal’s vital organs. The shell has a large aperture, with a thick and toothed outer lip. The inner lip of the aperture has many "pimple-like" bumps on the parietal shield or parietal callus near the siphonal notch.
Scotch bonnets live on shelly sand in moderately shallow water. Shelly sand means there is an abundance of shell fragments mixed in with the sand, and it is typically found where ocean currents are strong. The Atlantic Ocean, especially off the coast of North Carolina, has very strong currents. Divers and local fisherman frequently find Scotch bonnets at depths of about 50 to 150 feet (15 to 46 m); however, live specimens can be found in depths from 0 to 94 metres (0 to 308 ft). Empty shells have been found in depths of up to 97 metres (318 ft). These snails are often found in association with the offshore Atlantic calico scallop beds, probably attracted by the abundant food. Shipwrecks also seem to provide a good habitat for this species.
During the spring, favorable food supplies, adequate light, and optimum water temperature provide conditions for breeding and early growth. At this time, the female deposits hundreds of egg capsules in towers about 4 to 5 inches (10 to 13 cm) high. The male fertilizes these eggs. After fertilization, the eggs develop into trochophore larvae, and the eggs then hatch as free swimming microscopic veliger larvae which can be carried some distance on ocean currents.
Growth is slow, and the veligers are carried by the ocean currents for up to 14 weeks. As the veligers mature, they develop their first shell (the smooth protoconch) and turn into very small juvenile snails, at which point they sink to the ocean floor. As is the case in all shelled mollusks, the mantle is what secretes the shell. Shell growth begins at what will later become the apex of the shell, and typically rotates clockwise. As the animal gradually matures, the mantle continues to secrete shell material. Scotch bonnets complete maturation in one to six years. However, some have lived more than six years.
Crabs like the blue crab and Florida stone crab are predators of this species. They crush the shell, eating the soft internal organs and muscle tissue. The snail's defense mechanism is to draw its body into the shell. The operculum may provide some protection against smaller predatory species.
Use by other invertebrates
After death, the empty shell of this sea snail is often used by hermit crabs. On the coast of the Caraguatatuba region of Brazil, a study of shell use in the hermit crab species Isocheles sawayai, family Diogenidae, was carried out. This study revealed that 11.5% of the population of these hermit crabs were using shells of Semicassis granulata. The selection of shell type was reported as not being random, but was instead described as being influenced by the weight, size, shape and internal volume of the shell, the occurrence of exobionts on the crab, and the degree of resistance the shell offered to predation and desiccation.
In 1965, the state of North Carolina named the Scotch bonnet as its official state shell, in honor of the abundance of Scottish settlers that founded the state. With this designation, North Carolina became the first state in the USA to have a state shell.
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