| Semicassis granulata | |
|---|---|
| Apertural view of a faded and slightly discolored shell from North Carolina | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Mollusca |
| Class: | Gastropoda |
| (unranked): | clade Caenogastropoda clade Hypsogastropoda clade Littorinimorpha |
| Superfamily: | Tonnoidea |
| Family: | Cassidae |
| Subfamily: | Cassinae |
| Genus: | Semicassis |
| Species: | S. granulata |
| Binomial name | |
| Semicassis granulata (Born, 1778)[1] |
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| Synonyms[11] | |
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Semicassis granulata, also known as Phalium granulatum, common names the Scotch bonnet or ridged bonnet, is a medium-sized species of sea snail, a marine gastropod mollusk in the subfamily Cassinae.[12]
This species is found in the tropical and subtropical Western Atlantic Ocean, from North Carolina to Uruguay.[11] It is the most common species in this subfamily in North America.[12]
These 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.[13]
In 1965, the shell of this species was named a state symbol of North Carolina, making this the first state to designate an official state shell.[14]
In the past this species has sometimes been confused with Semicassis undulata, a similar-looking Mediterranean species.
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The word semi means half, and cassis means a metal helmet.[15] The specific epithet granulata is derived from the Latin word 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.
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.[13][16]
This species was named and described by the Austrian scientist Ignaz von Born in 1778.
As mentioned above, the shell is variable in surface texture, some shells being more granulated and even having nodules on the shoulders of the whorls, and some other shells being much smoother.
What has often been considered to be merely a smooth variety of this species was given the name: cicatricosa by Johann Friedrich Gmelin in 1791.[17] If this is indeed only a variety, it has no taxonomic validity. However, currently (2010) Dr. Gary Rosenberg of the ANSP is listing Semicassis cicatricosa as a separate species on his database Malacolog 4.1.1.[18]
The shell form that has pronounced nodules on the shoulder of the whorls was named form peristephes Pilsbry & Mcgintyi, 1939. [1]
A similar-looking species from the Mediterranean Semicassis undulata, also named by Gmelin in 1791, has been mistakenly reported from the Western Atlantic but does not occur there.[19] There has been some confusion especially in the popular literature about which name should be applied to which of these two species.
There are published records of Semicassis granulata from the following areas of the Western Atlantic Ocean[11]:
This species has apparently not been reported as occurring in the Lesser Antilles.[11]
It takes approximately six years for a Scotch bonnet to mature.[13] The shell develops the thick outer lip characteristic of the subfamily Cassinae, several times at different growth stages.[16] The snail rests between each stage of growth.[13] When it begins to grow again, the snail usually absorbs the entire outer lip.[13] 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.[13]
The more food a Scotch bonnet consumes during its lifetime, the more elaborate its shell, the glossier its sheen, and the brighter its colors.[13]
The egg-shaped shell of this species usually grows to 2 to 4 inches (5 to 10 cm) in length.[11] The maximum recorded shell length is 121 mm.[20] The shells of the adult females are larger than those of the males.[12][13] The sturdy shell of the adult snail has approximately five whorls with coiled spiral grooves crossed by mostly rather faint growth lines.
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.[13] 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.[13] Shelly sand refers to 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);[21] however, live specimens can be found in depths from 0 to 94 metres (0 to 308 ft).[11] Empty shells have been found in depths of up to 97 metres (318 ft).[11][20]
These snails are often found in association with the offshore Atlantic calico scallop beds, probably attracted by abundant food.[13] Shipwrecks also seem to provide a good habitat for this species.[13]
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.[13] The male fertilizes these eggs. After fertilization, the eggs develop into trochophore larvae. The eggs hatch as shell-less free swimming microscopic veligers (molluscan larvae) that are carried by the ocean currents.[16]
Growth is slow and the veligers are carried by the ocean currents for up to 14 weeks. As the veligers mature and become juvenile snails, they develop a shell and sink to the ocean floor. The mantle secretes the shell, with growth beginning at the apex and typically rotating clockwise. As the shell 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.[13][16]
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.[13] The snail's defense mechanism is to draw its body into the shell. The operculum may provide some protection against smaller predatory species.[13]
After death, the empty shell of this sea snail is often used by hermit crabs.[22] 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.[23]
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.[13][21] With this designation, North Carolina became the first state in the USA to have a state shell.[21]