Aliger gigas
Aliger gigas, originally known as Strombus gigas or more recently as Lobatus gigas, commonly known as the queen conch, is a species of large sea snail, a marine gastropod mollusc in the family of true conches, the Strombidae. This species is one of the largest molluscs native to the Caribbean Sea, and tropical northwestern Atlantic, reaching up to in shell length. A. gigas is closely related to the goliath conch, Titanostrombus goliath, a species endemic to Brazil, as well as the rooster conch, Aliger gallus.
The queen conch is herbivorous. It feeds by browsing for plant and algal material growing in the seagrass beds, and scavenging for decaying plant matter. These large sea snails typically reside in seagrass beds, which are sandy plains covered in swaying sea grass and associated with coral reefs, although the exact habitat of this species varies according to developmental age. The adult animal has a very large, solid and heavy shell, with knob-like spines on the shoulder, a flared, thick outer lip, and a characteristic pink or orange aperture. The outside of the queen conch is sandy colored, helping them blend in with their surroundings. The flared lip is absent in juveniles; it develops once the snail reaches reproductive age. The thicker the shell's flared lip is, the older the conch is. The external anatomy of the soft parts of A. gigas is similar to that of other snails in the family Strombidae; it has a long snout, two eyestalks with well-developed eyes, additional sensory tentacles, a strong foot and a corneous, sickle-shaped operculum.
The shell and soft parts of living A. gigas serve as a home to several different kinds of commensal animals, including slipper snails, porcelain crabs and a specialized species of cardinalfish known as the conchfish Astrapogon stellatus. Its parasites include coccidians. The queen conch's natural predators include several species of large predatory sea snails, octopus, starfish, crustaceans and vertebrates. It is an especially important food source for large predators like sea turtles and nurse sharks. Human capture and consumption date back into prehistory.
Its shell is sold as a souvenir and used as a decorative object. Historically, Native Americans and indigenous Caribbean peoples used parts of the shell to create various tools.
International trade in the Caribbean queen conch is regulated under the Convention on International Trade in Endangered Species of Wild Fauna and Flora agreement, in which it is listed as Strombus gigas. This species is not endangered in the Caribbean as a whole, but is commercially threatened in numerous areas, largely due to extreme overfishing.
Taxonomy and etymology
History
The queen conch was originally described from a shell in 1758 by Swedish naturalist and taxonomist Carl Linnaeus, who originated the system of binomial nomenclature. Linnaeus named the species Strombus gigas, which remained the accepted name for over 200 years. Linnaeus did not mention a specific locality for this species, giving only "America" as the type locality. The specific name is the ancient Greek word gigas, which means "giant", referring to the large size of this snail compared with almost all other gastropod molluscs. Strombus lucifer, which was considered to be a synonym much later, was also described by Linnaeus in Systema Naturae.In the first half of the 20th century, the type material for the species was thought to have been lost; in other words, the shell on which Linnaeus based his original description and which would very likely have been in his own collection, was apparently missing, which created a problem for taxonomists. To remedy this, in 1941 a neotype of this species was designated by the American malacologists William J. Clench and R. Tucker Abbott. In this case, the neotype was not an actual shell or whole specimen, but a figure from a 1684 book Recreatio mentis, et occuli, published 23 years before Linnaeus was born by the Italian Jesuit scholar Filippo Buonanni. This was the first book that was solely about seashells. In 1953 the Swedish malacologist Nils Hjalmar Odhner searched the Linnaean Collection at Uppsala University and discovered the missing type shell, thereby invalidating Clench and Abbott's neotype designation.
Strombidae's taxonomy was extensively revised in the 2000s and a few subgenera, including Eustrombus, were elevated to genus level by some authors. Petuch and Petuch and Roberts recombined this species as Eustrombus gigas, and Landau and collaborators recombined it as Lobatus gigas. In 2020, it was recombined as Aliger gigas by Maxwell and colleagues, which is the current valid name according to the World Register of Marine Species.
Phylogeny
The phylogenetic relationships among the Strombidae were mainly studied by Simone and Latiolais, using two distinct methods. Simone proposed a cladogram based on an extensive morpho-anatomical analysis of representatives of Aporrhaidae, Strombidae, Xenophoridae and Struthiolariidae, which included A. gigas.
With the exception of Lambis and Terebellum, the remaining taxa were previously allocated in the genus Strombus, including A. gigas. However, according to Simone, only Strombus gracilior, Strombus alatus and Strombus pugilis, the type species, remained within Strombus, as they constituted a distinct group based on at least five synapomorphies. The remaining taxa were previously considered subgenera and were elevated to genus level by Simone. Genus Eustrombus, in this case, included Eustrombus gigas and Eustrombus goliath, which were thus considered closely related.
In a different approach, Latiolais and colleagues proposed another cladogram that attempts to show the phylogenetic relationships of 34 species within the family Strombidae. The authors analysed 31 Strombus species, including Aliger gigas, and three species in the allied genus Lambis. The cladogram was based on DNA sequences of both nuclear histone H3 and mitochondrial cytochrome-c oxidase I protein-coding gene regions. In this proposed phylogeny, Strombus gigas and Strombus gallus are closely related and appear to share a common ancestor.
Common names
Common names include "queen conch" and "pink conch" in English, caracol rosa and caracol rosado in Mexico, caracol de pala, cobo, botuto and guarura in Venezuela, caracol reina, lambí in the Dominican Republic and Grenada, and carrucho in Puerto Rico. I is better known as lambi in the French speaking areas of the Caribbeans.Anatomy
Shell
The mature shell grows to in length in three to five years while the maximum reported size is. However, even though they only grow to be this maximum length, the thickness of the shell is constantly increasing. The shell is very solid and heavy, with 9 to 11 whorls and a widely flaring and thickened outer lip. The thickness is highly important because the thicker the shell, the better protected it is. Additionally, instead of increasing in size once it reaches its maximum, the outside shell thickens as time goes on- an important indicator of how old the queen conch is. Although this notch is not as well developed as elsewhere in the family, the shell feature is nonetheless visible in an adult dextral specimen, as a secondary anterior indentation in the lip, to the right of the siphonal canal. The animal's left eyestalk protrudes through this notch.The spire is a protruding part of the shell that includes all of the whorls except the largest and final whorl. It is usually more elongated than in other strombid snails, such as the closely related and larger goliath conch, Lobatus goliath that is endemic to Brazil. In A. gigas, the glossy finish or glaze around the aperture of the adult shell is primarily in pale shades of pink. It may show a cream, peach or yellow colouration, but it can also sometimes be tinged with a deep magenta, shading almost to red. The periostracum, a layer of protein that is the outermost part of the shell surface, is thin and a pale brown or tan colour.
The overall shell morphology of A. gigas is not solely determined by the animal's genes; environmental conditions such as location, diet, temperature and depth, and biological interactions such as predation, can greatly affect it. Juvenile conches develop heavier shells when exposed to predators. Conches also develop wider and thicker shells with fewer but longer spines in deeper water.
The shells of juvenile queen conches are strikingly different in appearance from those of the adults. Noticeable is the complete absence of a flared outer lip; juvenile shells have a simple sharp lip, which gives the shell a conical or biconic outline. In Florida, juvenile queen conches are known as "rollers", because wave action very easily rolls their shells, whereas it is nearly impossible to roll an adult specimen, due to its shell's weight and asymmetric profile. Subadult shells have a thin flared lip that continues to increase in thickness until death.
Conch shells are about 95% calcium carbonate and 5% organic matter.
Historic illustrations
Index Testarum Conchyliorum contains three illustrations of adult shells from different perspectives. The knobbed spire and the flaring outer lip, with its somewhat wing-like contour expanding out from the last whorl, is a striking feature of these images. The shells are shown as if balancing on the edge of the lip and/or the apex; this was presumably done for artistic reasons as these shells cannot balance like this.One of the most prized shell publications of the 19th century, a series of books titled Illustrations conchyliologiques ou description et figures de toutes les coquilles connues, vivantes et fossiles, contains illustrations of both adult and juvenile A. gigas shells and one uncoloured drawing depicting some of the animal's soft parts. Almost forty years later, a colored illustration from the Manual of Conchology shows a dorsal view of a small juvenile shell with its typical brown and white patterning.