Aliger gigas

Aliger gigas

Aliger gigas, Queen conch

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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, from Bermuda to Brazil, reaching up to 35.2 centimetres (13.9 in) in shell length. A. gigas is closely related to the goliath conch, Lobatus goliath, a species endemic to Brazil, as well as the rooster conch, Aliger gallus.

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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 (opening). 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 (fish, sea turtles, nurse sharks). It is an especially important food source for large predators like sea turtles and nurse sharks. Human capture and consumption dates 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 (CITES) 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.

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Animal name origin

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.

Distribution

Geography

Aliger gigas is native to the tropical Western Atlantic coasts of North and Central America in the greater Caribbean tropical zone. Although the species undoubtedly occurs in other places, this species has been recorded within the scientific literature as occurring, in: Aruba, (Netherlands Antilles); Barbados; the Bahamas; Belize; Bermuda; North and northeastern regions of Brazil (though this is contested); Old Providence Island in Colombia; Costa Rica; the Dominican Republic; Panama; Swan Islands in Honduras; Jamaica; Martinique; Alacran Reef, Campeche, Cayos Arcas and Quintana Roo, in Mexico; Puerto Rico; Saint Barthélemy; Mustique and Grenada in the Grenadines; Pinar del Río, North Havana Province, North Matanzas, Villa Clara, Cienfuegos, Holguín, Santiago de Cuba and Guantánamo, in the Turks and Caicos Islands and Cuba; South Carolina, Florida, with the Florida Keys and Flower Garden Banks National Marine Sanctuary, in the United States; Carabobo, Falcon, Gulf of Venezuela, Los Roques archipelago, Los Testigos Islands and Sucre in Venezuela; all islands of the United States Virgin Islands.

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Aliger gigas lives at depths from 0.3–18 m to 25–35 m. Its depth range is limited by the distribution of seagrass and algae cover. In heavily exploited areas, the queen conch is more abundant in the deepest range. The queen conch lives in seagrass meadows and on sandy substrate, usually in association with turtle grass (species of the genus Thalassia, specifically Thalassia testudinum and also Syringodium sp.) and manatee grass (Cymodocea sp.). Juveniles inhabit shallow, inshore seagrass meadows, while adults favor deeper algal plains and seagrass meadows.The critical nursery habitats for juvenile individuals are defined by a series of characteristics, including tidal circulation and macroalgal production, which together enable high rates of recruitment and survival. A. gigas is typically found in distinct aggregates that may contain several thousand individuals.

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Aliger gigas habitat map
Aliger gigas habitat map

Habits and Lifestyle

The species has a large and powerful foot with brown spots and markings towards the edge, but is white nearer to the visceral hump that stays inside the shell and accommodates internal organs. The base of the anterior end of the foot has a distinct groove, which contains the opening of the pedal gland. Attached to the posterior end of the foot for about one third of its length is the dark brown, corneous, sickle-shaped operculum, which is reinforced by a distinct central rib. The base of the posterior two-thirds of the animal's foot is rounded; only the anterior third touches the ground during locomotion. The columella, the central pillar within the shell, serves as the attachment point for the white columellar muscle. Contraction of this strong muscle allows the animal's soft parts to shelter in the shell in response to undesirable stimuli.

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Aliger gigas has an unusual means of locomotion, first described in 1922 by George Howard Parker (1864–1955). The animal first fixes the posterior end of the foot by thrusting the point of the sickle-shaped operculum into the substrate, then it extends the foot in a forward direction, lifting and throwing the shell forward in a so-called leaping motion. This way of moving is considered to resemble that of pole vaulting, making A. gigas a good climber even of vertical concrete surfaces. This leaping locomotion may help prevent predators from following the snail's chemical traces, which would otherwise leave a continuous trail on the substrate.

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Diet and Nutrition

Strombid gastropods were widely accepted as carnivores by several authors in the 19th century, a concept that persisted until the first half of the 20th century. This erroneous idea originated in the writings of Jean-Baptiste Lamarck, who classified strombids with other supposedly carnivorous snails. This idea was subsequently repeated by other authors, but had not been supported by observation. Subsequent studies have refuted the concept, proving beyond doubt that strombid gastropods are herbivorous animals. In common with other Strombidae, Aliger gigas is a specialized herbivore, that feeds on macroalgae (including red algae, such as species of Gracilaria and Hypnea ), seagrass and unicellular algae, intermittently also feeding on algal detritus. The green macroalgae Batophora oerstedii is one of its preferred foods.

Mating Habits

Aliger gigas is gonochoristic, which means each individual snail is either distinctly male or distinctly female. Females are usually larger than males in natural populations, with both sexes existing in similar proportion. After internal fertilization, the females lay eggs in gelatinous strings, which can be as long as 75 feet (23 m). These are layered on patches of bare sand or seagrass. The sticky surface of these long egg strings allows them to coil and agglutinate, mixing with the surrounding sand to form compact egg masses, the shape of which is defined by the anterior portion of the outer lip of the female's shell while they are layered. Each one of the egg masses may have been fertilized by multiple males. The number of eggs per egg mass varies greatly depending on environmental conditions such as food availability and temperature. Commonly, females produce 8–9 egg masses per season, each containing 180,000–460,000 eggs, but numbers can be as high as 750,000 eggs. A. gigas females may spawn multiple times during the reproductive season, which lasts from March to October, with activity peaks occurring from July to September.

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Queen conch embryos hatch 3–5 days after spawning. At the moment of hatching, the protoconch (embryonic shell) is translucent and has a creamy, off-white background color with small, pustulate markings. This coloration is different from other Caribbean Lobatus, such as Lobatus raninus and Lobatus costatus, which have unpigmented embryonic shells. Afterwards, the emerging two-lobed veliger (a larval form common to various marine and fresh-water gastropod and bivalve mollusks) spend several days developing in the plankton, feeding primarily on phytoplankton. Metamorphosis occurs some 16–40 days from the hatching, when the fully grown protoconch is about 1.2 mm high. After the metamorphosis, A. gigas individuals spend the rest of their lives in the benthic zone (on or in the sediment surface), usually remaining buried during their first year of life.The queen conch reaches sexual maturity at approximately 3 to 4 years of age, reaching a shell length of nearly 180 mm and weighing up to 5 pounds. Individuals may usually live up to 7 years, though in deeper waters their lifespan may reach 20–30 years and maximum lifetime estimates reach 40 years. It is believed that the mortality rate tends to be lower in matured conchs due to their thickened shell, but it could be substantially higher for juveniles. Estimates have demonstrated that its mortality rate decreases as its size increases and can also vary due to habitat, season and other factors.

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Population

Population threats

Queen conch populations have been rapidly declining throughout the years and have been mostly depleted in some areas in the Caribbean due to the fact that they are highly sought after for their meat and their value. Within the conch fisheries, one of the threats to sustainability stems from the fact that there is almost as much meat in large juveniles as there is in adults, but only adult conchs can reproduce, and thus sustain a population. In many places where adult conchs have become rare due to overfishing, larger juveniles and subadults are taken before they ever mate.

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In the United States (Florida), it is currently illegal to gather or pick the queen conch either recreationally or commercially. In other parts of the world where it is legal, only adult conchs can be fished. The rule is to let each conch have ample time to reproduce before taken out of its habitat, potentially leading to a more stable population. However, this rule has not been followed by countless fishers. On a number of islands, subadults provide the majority of the harvest.The abundance of Aliger gigas is declining throughout its range as a result of overfishing and poaching. Especially because of overfishing, many pockets of conch communities fall below the critical level needed for reproducing. A 2019 study predicted overfishing could lead to the extinction of queen conchs in as little as ten years. Additionally, if the conch fishery collapses, it could potentially leave over 9,000 Bahamian fishers out of work. Trade from many Caribbean countries, such as the Bahamas, Antigua and Barbuda, Honduras, Haiti and the Dominican Republic, is known or thought to be unsustainable. As of 2001, queen conch populations in at least 15 Caribbean countries and states were overfished or overexploited. Illegal harvest, including fishing in foreign waters and subsequent illegal international trade, is a common problem in the region. The Caribbean "International Queen Conch Initiative" is an international attempt at managing this species. On 13 January 2019, the Bahamas' Department of Marine Resources announced it would be making official recommendations to better protect the conch, including ending exports and increasing regulatory staff.

Presently, ocean acidification is another serious threat to the queen conch. Acidity levels are rising and adversely affecting shellfish larvae. Rising atmospheric CO2 levels result in rising levels of carbonic acid in seawater, which is particularly harmful to organisms with calcium carbonate shells and structures. Certain larval stages of shellfish are very sensitive to lower seawater pH.

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Conservation

The queen conch fishery is usually managed under the regulations of individual nations. In the United States all taking of queen conch is prohibited in Florida and in adjacent Federal waters. No international regional fishery management organization exists for the whole Caribbean area, but in places such as Puerto Rico and the Virgin Islands, queen conch is regulated under the auspices of the Caribbean Fishery Management Council (CFMC). In 2014, the Parties to the Convention for the Protection and Development of the Marine Environment of the Wider Caribbean Region (Cartagena Convention) included queen conch in Annex III of its Protocol Concerning Specially Protected Areas and Wildlife (SPAW Protocol). Species included in the Annex III require special measures to be taken to ensure their protection and recovery, and their use is authorised and regulated accordingly.

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This species has been mentioned in the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) since 1985. In 1992 the United States proposed queen conch for listing in CITES Appendix II, making queen conch the first large-scale fisheries product to be regulated by CITES (as Strombus gigas ). In 1995 CITES began reviewing the biological and trade status of the queen conch under its "Significant Trade Review" process. These reviews are undertaken to address concerns about trade levels in an Appendix II species. Based on the 2003 review, CITES recommended that all countries prohibit importation from Honduras, Haiti and the Dominican Republic, according to Standing Committee Recommendations. Queen conch meat continues to be available from other Caribbean countries, including Jamaica and Turks and Caicos, which operate well-managed queen conch fisheries. For conservation reasons, the Government of Colombia currently bans the commercialisation and consumption of the conch between the months of June and October. The Bahamas National Trust is building awareness by educating teachers and students through workshops and an awareness campaign which includes the catchy pop song Conch Gone.

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References

1. Aliger gigas Wikipedia article - https://en.wikipedia.org/wiki/Aliger_gigas

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