Marine Invertebrates of Bermuda Queen Conch (Strombus gigas) By Matthew M. James and James B. Wood (Ed) Taxonomy  Habitat  Ecology  Recent Research  Commercial Importance  Bermuda Laws  Personal Interest  References  Links

Taxonomy


Phylum: Mollusca
  Class: Gastropoda
    Subclass: Prosobranchia
     Order: Neotaenioglossa
       Family: Strombidae


Introduction


Strombus gigas is a marine snail more commonly known as the queen conch. It is also known as the pink conch, lambi, botuto, or guarura (Berg 1976). The queen conch is a strong-shelled species, also having a smooth shell with a row of nodes at the shoulder of a whorl (Sterrer 1986). The shell can be white, tan, or cream colored in appearance (Sterrer 1986). The shell’s aperture is long and narrow, and its coloration is generally rose pink (Sterrer 1986). This coloration is sometimes yellow or light orange in some specimens (Sterrer 1986). It is a large snail, with their shells growing to lengths up to three hundred millimeters (Sterrer 1986). Like other snails it is soft bodied, consisting of a black-speckled foot, a snout-like proboscis, a pair of tentacles, and two eyestalks topped with distinctive, colorful yellow eyes (Randall 1964).

The queen conch is one of six species from the genus Strombus, the others being S. raninus, S. gallus, S. costatus, S. pugilis, and S. goliath (ITIS web site). It is readily distinguished from other species by its deep pink aperture, a feature lacked by all other western Atlantic species (Randall 1964). Queen conchs are also larger than most other species, with few capable of approaching their three hundred millimeter shell length (Sterrer 1986). Only S. goliath is similarly sized, and the queen conch lacks the pronounced spiral grooves on the body whorl and exterior surface of S. goliath’s expanded lip.

Unlike most gastropods, which move through muscular waves of their feet, the queen conch hurls itself in short hops (Randall 1964). The sickle-shaped operculum, located at the posterior end of the foot, is held against the seafloor, and the foot’s backward thrust propels the shell forward, moving the conch by half a body length at a time (Randall 1964). The operculum is shaped differently and smaller sized than those of other snails, so it cannot close like it does in other species (Randall 1964). Instead it is used primarily in a locomotive capacity and to help right the conch when it is overturned (Randall 1964). This is accomplished by extending the body and pushing the substrate with the foot and operculum (Sterrer 1992). The operculum also functions as a defensive weapon against predators (Randall 1964).

Habitat


Queen conchs dwell in warm, shallow seas, ranging from Bermuda in the North down to Brazil (Martin-Mora 1995). They are found throughout the islands of the Caribbean and the Bahamas. They were once common in Bermuda and elsewhere, but overfishing has decimated populations across its range. Today it is rare for someone to encounter a queen conch in Bermuda. Adult conchs are primarily found in seagrass meadows, usually turtle grass or manatee grass (Boettcher and Targett 1996). They are also found in sand flats, and can be found in coral rubble or on reefs (Antigua Barbuda Environment Division 2006). In Bermuda queen conchs seem to prefer turtlegrass beds on the outer reefs as opposed to inshore (Sterrer 1992). Queen conchs have been observed at varying depths, ranging from along the shore during low tide to as deep as two hundred feet (Randall 1964). Generally though, they are found between five and twenty meters (Sterrer 1986). The depth conchs are found may be limited by the ability of seagrass to grow, and is likely tied to the depth of the photic zone (Randall 1964). An herbivorous mollusk that relies on algae as its primary food source, queen conch can only live where it can graze successfully.

Ecology


Various phyla prey upon queen conchs, including reptiles, fish, crustaceans, cephalopods, mammals, and other gastropods (Randall 1964). Twenty two different species were observed either feeding on queen conchs or having remains in their stomach contents, including loggerhead turtles, eagle rays, and spiny lobsters (Randall 1964). This was somewhat surprising to researchers, since the queen conchs are thick shelled and heavily spined, seemingly strong deterrents to predation (Randall 1964). Spiny lobsters introduced to half-grown conchs used their mandibles to break away the conch shells until the inner whorl was exposed, allowing the lobsters access to the conch inside (Delgado 2002).

Queen conchs themselves are herbivorous, feeding on algae and algal detritus. They consume several species of algae associated with turtle grass, including Cladophora sp. And Polysiphonia sp., but not the turtle grass itself (Randall 1964). To digest this cellulose laden diet, their digestive system utilizes a crystalline style and a small flexible rod composed of a microprotein gel (Antigua Barbuda Environment Division 2006). The style rotates against the gastric shield in the digestive tract as enzymes are secreted, and in conjunction with salivary glands and esophageal pouches the plant matter is digested (Antigua Barbuda Environment Division 2006). Queen conchs are hardly discriminating eaters; generally whatever algal species is dominant is their primary food source (Randall 1964). They do however go out of their way to avoid consuming benthic organisms like sponges or bryozoans (Randall 1964). Some animal matter was found to be consumed by the conchs, sparking some controversy that they were predators (Randall 1964). However, some small organisms were likely consumed accidentally by the conchs, rather than the victims of outright predation (Randall 1964).

Queen conch reproduction has been much studied for the purposes of aquaculture. Spawning can occur six to eight times during a season, and queen conchs have been observed copulating from mid-March to November, during both day and night. (Randall 1964). Spawning occurs when the male, situated behind the female, inserts a black, spade-like penis called a verge into the female’s siphonal notch (Randall 1964). After receiving sperm from the male, the female retains it for several weeks, releasing it while laying eggs in order to fertilize them (Randall 1964). Mass spawning aggregations have been shown to occur (Sterrer 1992). Eggs are laid in continuous strands with as many as three quarters of a million eggs in one strand (Sterrer 1986). Twelve to fifteen eggs per millimeter of egg strand have been observed, with thirteen eggs most commonly found, and strands were laid at an average rate of one and a half meters per hour (Randall 1964). The whole process of laying eggs occurs over less than a day (Randall 1964). Sandy substrate is a requirement for spawning, as the eggs are deposited in sand (Shawl 2004). Water quality, food supply, and temperature all play a role in the spawning process (Shawl 2004). This last factor is observed in the wild, with a reproductive season characterized by temperature increases throughout the summer months, as well as by a twelve hour photoperiod (Shawl 2004).

The life cycle of queen conchs begins around a week after spawning when larvae exit the egg case (Sterrer 1992). Embryonic development proceeds quickly after fertilization, reaching the gastrula stage after sixteen hours, the trochophore stage after fifty-eight, and becoming veligers, or free floating larvae, after seven days (Randall 1964). These measurements were taken on captive embryos, which traditionally grow slower than those in the wild, so development in a natural setting would proceed at a faster rate (Stoner 1997). The veliger possesses a small transparent shell called a protoconch that will eventually develop into an adult shell (Antigua Barbuda Environment Division 2006). After six days they develop four wing-like lobes, and then they gain two more lobes after twelve days. These larvae can be found in the open water as deep as one hundred meters, but generally occur in the upper ocean layers above the thermocline. After three weeks of floating in the water column, the veligers settle and the lobes turn into feet while the proboscis continues to develop (Sterrer 1992). The veligers settle and metamorphose into their benthic form in response to the presence of specific algae, and settle due to substratum and not waterborne cues (Davis and Stoner 1994). The veligers are metamorphically competent for six days, after which they lose the ability (Davis and Stoner 1994). There appears to be some variability in the response of conchs to different settling cues, such as type of substrate and location (Boettcher and Targett 1996). During this settlement period juvenile mortality is extremely high, up to sixty percent a year. After a month, the conch is shelled and resembles an adult, though it takes three years for the characteristic flaring lip of the shell to develop (Sterrer 1992). Upon reaching a length of eight to ten inches, they become sexually mature (Shawl and Davis 2004). They are known to live for at least six years in the wild (Sterrer 1992).

Recent Research


Over the past few years there have been some interesting work completed in the field of queen conch research. One study found that exposure to predators has an influence on the morphology of conch shells (Delgado et al.2002). It had been observed that queen conch individuals raised in hatcheries had weaker shells and shorter spines than wild conchs (Delgado et al.2002). It was thought that the conchs might, like many other organisms, exhibit some sort of predator related response that the hatchery grown individuals were not experiencing (Delgado et al.2002). In the study, an experimental group of juvenile queen conchs was exposed to caged spiny lobsters, and a control group to empty cages (Delgado et al.2002). At the end of the experiment it was found that the shells of the lobster exposed conchs grew at a slower rate than the control group, but the shells weighed the same, implying that the smaller shells were thicker or denser than their longer counterparts (Delgado et al.2002). In addition, the exposed conchs buried themselves in sand far more often than the control group, showing a behavioral difference between the two groups (Delgado et al.2002).

A more recent study examined the fracture mechanisms of the shell of the queen conch, performing micromechanical analyses on shells to see what was responsible for their resistance to catastrophic fracture (Kamat et al.2004). The shell of the queen conch can resist fracture a hundred to a thousand times better than aragonite, the mineral that makes up ninety-nine percent of its shell (Gorman 2000). The study found that the structure of the shell was designed to allow inherent cracks to reach a certain point, called the ACK limit, without resulting in a catastrophic failure (Kamat et al.2004). The ACK limit, or Aveston-Cooper-Kelly limit, is a point where all ligaments that bridge cracks remain in place as the crack grows, allowing the shell to remain in one piece despite the presence of cracks (Kamat et al.2004). Temperature tests showed that the ductility of the shell’s proteinaceous interphase is the primary factor affecting this limit (Kamat et al.2004). The use of this new knowledge could help guide the creation of tough, lightweight ceramics (Kamat et al., 2000).

Commercial Importance


The queen conch, which is extremely edible, was once part of a thriving fishery, and was utilized for food throughout its range (Randall 1964). In the 1960’s the queen conch was second only to the spiny lobster as a fishery product (Randall 1964). At that time the fishery exceeded two hundred thirty thousand dollars annually in the Bahamas, and nearly four million were being exported from the Caicos Islands to Haiti (Berg 1976). The shells of queen conchs were valued by collectors as well, namely for their pink colored apertures and large size (Randall 1964). Conch nurseries were cultivated to harvest the snails for meat throughout the Atlantic (Berg 1976). Much of the research conducted on the queen conch, especially its breeding behavior, has been in connection to these nurseries.

As with many commercially exploited marine organisms, the queen conch was overfished, and a decline of the fishery began during the 1970’s (Shawl 2004). The populations of many regions, notably Florida, collapsed during this time (Shawl 2004). A moratorium of the fishery began in Florida in 1986, and in 1992 the queen conch was added to the Convention for the International Trade of Endangered Species, or CITES, legal mandate (Shawl 2004). Now countries that export conchs are required to have a CITES permit, and much effort is expended to make sure that both wild populations and nursery bred conchs are managed properly (Shawl 2004). In Bermuda conch populations were especially hard hit, forcing the ban of conch fishing in 1978 (Gascoigne and Elliot). Ten years after the closure of the fishery, seventy-three hectares of conch habitat were surveyed to the depth of twenty meters, and the survey yielded only thirty-nine adult conch and no juveniles (Gascoigne and Elliot). Queen conchs remain protected in Bermuda, indicating that there has been little recovery (Gascoigne and Elliot). Bermuda conch populations are genetically differentiated from populations in the Caribbean, which suggests that the population is self-sustaining and receives little larval input from outside sources (Gascoigne and Elliot). This would explain why the Bermuda populations have been slow to recover, for if such a population neared collapse it would be hard pressed to make any kind of recovery (Gascoigne and Elliot). It seems that the establishment of marine reserves is the best way to allow populations to recover (Stoner 1997). Successful reserves should be large enough that reproductive stock cannot migrate out, and areas that supply larvae into populations must have some level of protection as well (Stoner 1997).

Queen conchs have been utilized throughout its range for aquaculture since the 1960’s, and with the closure or heavy regulation of conch fisheries they now serve as the main source for enhancing stock populations (Shawl 2004). Today much of the work produced on queen conchs is related to aquaculture, since the decline of conch populations has caused researchers to turn to hatcheries for individuals (Stoner 1997). The marriage of commerce and science has produced a great deal of research beneficial to both groups. The study into the developmental plasticity of conch shells was conducted with aquaculture in mind (Delgado 2002). Some work is almost purely of commercial use, such as examining the effectiveness of using hydrogen peroxide to induce larval metamorphosis as opposed to algal extracts (Boettcher et al. 1997). The cooperation between hatcheries and researchers is heartening, and provides hope that populations may one day recover.

Bermuda Laws


As a protected species, the removal of queen conchs is prohibited throughout Bermuda, as stated by the Bermuda Statutory Instrument Fisheries (Protected Species) Order 1978 (Laws of Bermuda). Anyone found in violation of this law, with the exception of those holding a permit for scientific research or conservation, is subject to a fine up to five thousand dollars and imprisonment up to a year (Laws of Bermuda).

The Bermuda Customs Tariff also imposes restrictions involving queen conch. The importation or exportation of queen conchs, whether live, dead, in whole or in part (HM Customs 2005). This applies to the meat of queen conchs as well (HM Customs 2005)). The only exception to the rule is if a license from the Department of Environmental Protection is held (HM Customs 2005).

Personal Interest


Like many others, I have eaten and enjoyed conch meat in a variety of forms. Conch fritters and conch chowder were particularly notable delicacies, and I ate them with relish when taking trips to the Florida Keys. It was during those trips that I also encountered conchs in the wild along a sandbar, and the large snails made an impression on me as quite an interesting organism as they moved across the sand. I also discovered that their populations were not faring well, and while I was not surprised that human impacts had affected yet another organism for the worse, I grew concerned over their future. Over the course of this project I have been heartened by the efforts to sustain the populations of the queen conchs and others, even if most of this effort is being expended for the purpose of harvesting the conch. Wild populations of queen conch have not recovered by any stretch of the imagination, but with time and continued management and research recovery may occur at some point in the future.

References

Berg, C.J. (1976). Growth of the Queen Conch Strombus gigas, with a Discussion of the Practicality of Its Mariculture. Marine Biology 34: 191-199

Boettcher, A., C. Dyer, J. Casey, and N. Targett. (1997). Hydrogen peroxide induced metamorphosis of the queen conch, Strombus gigas: Tests at the commercial scale. Aquaculture 148: 247-258

Boettcher, A. and N. Targett. (1996). Induction of metamorphosis in queen conch, Strombus gigas Linnaeus, larvae by cues associated with red algae from their nursery grounds. Journal of Experimental Marine Biology and Ecology 196: 29-52

Davis, M. and A. Stoner. (1994). Trophic cues induce metamorphosis of queen conch larvae (Strombus gigas Linnaeus). Journal of Experimental Marine Biology and Ecology 180: 83-102

Delgado, G., C. Bartels, R. Glazer, N. Brown-Peterson, and K. McCarthy. (2004). Translocation as a strategy to rehabilitate the queen conch population in the Florida Keys. Fishery Bulletin 102: 278-288

Delgado, G, R. Glazer, and N. Stewart. (2002). Predator-Induced Behavioral and Morphological Plasticity in the Tropical Marine Gastropod Strombus gigas. Biological Bulletin 203: 112-120

Gascoigne, J. and M. Elliott. Nassau Grouper and Queen Conch in the Bahamas Status and Management Options. New Providence: B.R.E.E.F, 2002

Gorman, J. (2000). Conch yields clues for future materials. Science News 158: 6

HM Customs Bermuda, Bermuda Customs Tariff 2005, (2005).

Kamat, S., H. Kessler, R. Ballarini, M. Nassirou and A. Heuer. (2004). Fracture mechanisms of the Strombus gigas conch shell: II-micromechanics analyses of multiple cracking and large-scale crack bridging. Acta Materialia 52: 2395-2406

Kamat, S., X. Su, R. Ballarini and A. Heuer. (2000). Structural basis for the fracture toughness of the shell of the conch Strombus gigas. Nature 405: 1036-1040

Laws of Bermuda, Fisheries (Protected Species Order 1978), Bermuda Statutory Instrument BR 8/1978, (1978)

Martin-Mora, E. (1995). Developmental plasticity in the shell of the queen conch Strombus gigas. Ecology 76: 981-994

Queen Conch. Antigua Barbuda Environment Division. Online. Internet. http://www.environmentdivision.info/features/queen_conch.pdf. Accessed 23 October 2006

Randall, J. (1964). Contributions to the Biology of the queen conch, Strombus gigas. Bulletin of Marine Science of the Gulf and Caribbean 14: 246-295

Shawl, A. and M. Davis. (2004). Captive breeding behavior of four strombidae conch. Journal of Shellfisheries Research 23: 157-164
Sterrer, W. (1992). Bermuda’s Marine Life. Flatts, Bermuda: Bermuda Zoological Society.

Sterrer, W., Ed. (1986). Marine Fauna and Flora of Bermuda: A Systematic Guide to the Identification of Marine Organisms. New York: John Wiley & Sons.

Stoner, A. (1997). The status of the queen conch, Strombus gigas, research in the Caribbean. Marine Fisheries Review 59: 14-22

Links

Antigua Barbuda Environment Division Queen Conch Page
International Queen Conch Initiative
NOAA Queen Conch Page
Bermuda Department of Environmental Conservation