Species Profile Queen Conch, Strombus gigas queen conch, Strombus gigas, is the largest molluscan...

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The queen conch, Strombus gigas, is the largest molluscan gastropod (shell length, SL, of 7 to 9 inches; 18 to 23 cm) of the six conch species found in the shallow sea- grass beds of Florida, the Bahamas, Bermuda, the Caribbean Islands, and the north- ern coasts of Central and South America. The queen conch is found in the territorial waters of at least 36 countries and depen- dent territories. It is known by various names, such as caracol (Mexico, Honduras, Columbia), carrucho (Puerto Rico), cobo (Cuba), and lambi (Hispaniola, French Antilles). Many studies have focused on fisheries management, ecology and culture techniques of S. gigas. In the mid-1970s, cultur- ists began growing queen conch larvae to the juvenile stage for stock enhancement and for grow- out markets as a way to offset fishing pressure. The same cul- ture techniques have been used to study larval ecology and fish- eries oceanography, which con- tribute to the development of regional fisheries management plans. Natural history The adult of this slow-moving mollusc has a heavy shell (5 pounds; 2.3 kg) with spines on each whorl of the spire and a glossy, deep pink, flared aperture (Fig. 1). The orange-yellow man- tle, located around the soft body of the conch, secretes the shell. The hardened tip (the operculum) at the end of the single, black- speckled foot propels the conch forward in a “hopping” motion referred to as a strombid leap. This movement is thought to help the conch make a quick escape from predators and also break up its scent trail. The eyes are highly developed and are at the tips of two protruding stalks. At the base of the eye stalks is a long pro- boscis used for grazing diatoms and detritus from seagrass blades and sand grains. Adult conch have separate sexes (Fig. 2) and are sexually mature at about 4 years, after the lip has fully flared (Fig. 1). Conch are found at a 1:1 sex ratio in unfished populations. Queen conch form large spawning aggre- gations. Fertilization is internal. The typical 6- to 8-month egg-lay- ing season is March to October, with most activity occurring from July to September when water temperatures are the warmest (82 to 86 °F; 28 to 30 °C). The female lays an average of nine egg mass- es per season; each crescent- shaped egg mass contains approxi- mately 400,000 eggs (Fig. 3). The thin, sticky strand of eggs is cov- ered with sand grains to camou- flage it from predators during the 3- to 4-day incubation period. After the eggs hatch, the plank- totrophic veligers (larvae) drift in the water column for 2 to 8 weeks, depending on phytoplank- ton concentration, temperature, and the proximity of settlement habitat. When the veligers are October 2005 SRAC Publication No. 7203 *Harbor Branch Oceanographic Institution, Aquaculture Division, Ft. Pierce, Florida. Species Profile Queen Conch, Strombus gigas Megan Davis* Figure 1. Shell of an adult queen conch, S. gigas (7 to 9 inches; 18 to 23 cm SL). (photograph by Tom Smoyer)

Transcript of Species Profile Queen Conch, Strombus gigas queen conch, Strombus gigas, is the largest molluscan...

The queen conch, Strombus gigas,is the largest molluscan gastropod(shell length, SL, of 7 to 9 inches;18 to 23 cm) of the six conchspecies found in the shallow sea-grass beds of Florida, theBahamas, Bermuda, theCaribbean Islands, and the north-ern coasts of Central and SouthAmerica. The queen conch isfound in the territorial waters ofat least 36 countries and depen-dent territories. It is known byvarious names, such as caracol(Mexico, Honduras, Columbia),carrucho (Puerto Rico), cobo(Cuba), and lambi (Hispaniola,French Antilles). Many studies have focused onfisheries management, ecologyand culture techniques of S.gigas. In the mid-1970s, cultur-ists began growing queen conchlarvae to the juvenile stage forstock enhancement and for grow-out markets as a way to offsetfishing pressure. The same cul-ture techniques have been usedto study larval ecology and fish-eries oceanography, which con-tribute to the development ofregional fisheries managementplans.

Natural historyThe adult of this slow-movingmollusc has a heavy shell (5pounds; 2.3 kg) with spines oneach whorl of the spire and aglossy, deep pink, flared aperture(Fig. 1). The orange-yellow man-tle, located around the soft bodyof the conch, secretes the shell.The hardened tip (the operculum)at the end of the single, black-speckled foot propels the conchforward in a “hopping” motionreferred to as a strombid leap.

This movement is thought to helpthe conch make a quick escapefrom predators and also break upits scent trail. The eyes are highlydeveloped and are at the tips oftwo protruding stalks. At the baseof the eye stalks is a long pro-boscis used for grazing diatomsand detritus from seagrass bladesand sand grains.Adult conch have separate sexes(Fig. 2) and are sexually mature atabout 4 years, after the lip hasfully flared (Fig. 1). Conch arefound at a 1:1 sex ratio inunfished populations. Queenconch form large spawning aggre-gations. Fertilization is internal.The typical 6- to 8-month egg-lay-ing season is March to October,with most activity occurring fromJuly to September when watertemperatures are the warmest (82to 86 °F; 28 to 30 °C). The femalelays an average of nine egg mass-es per season; each crescent-shaped egg mass contains approxi-mately 400,000 eggs (Fig. 3). Thethin, sticky strand of eggs is cov-ered with sand grains to camou-flage it from predators during the3- to 4-day incubation period.After the eggs hatch, the plank-totrophic veligers (larvae) drift inthe water column for 2 to 8weeks, depending on phytoplank-ton concentration, temperature,and the proximity of settlementhabitat. When the veligers are

October 2005

SRAC Publication No. 7203

*Harbor Branch Oceanographic Institution,Aquaculture Division, Ft. Pierce, Florida.

Species ProfileQueen Conch, Strombus gigas

Megan Davis*

Figure 1. Shell of an adult queenconch, S. gigas (7 to 9 inches; 18to 23 cm SL). (photograph by TomSmoyer)

morphologically and physiologi-cally ready, they metamorphoseinto benthic animals in responseto trophic cues from their sea-grass juvenile habitat. Most conch nursery grounds arein seagrass meadows less than 20feet (6 m) deep, where the sea-grass density is 56 to 84 Thalassiashoots per square foot (600 to900/m2). Juveniles have also beenfound in algal flats and on deepbanks. The juveniles remainburied for most of their first year.As herbivorous gastropods, juve-niles and adults feed on a varietyof algae such as the green sea-weed Batophora oerstedi, or ondetritus or diatoms commonlyassociated with the seagrassThalassia testudinum. During thefirst couple of years, the juvenileconch grow longer shells (Fig. 4)and remain in the seagrass beds.Juveniles do not have flared lipsand are sometimes referred to asrollers. As the flaring lip (Fig. 4)begins to form, the sub-adultconch migrate to deeper waternear the reef tract and formspawning aggregations. Queenconch are estimated to have a lifespan of 25 years. Old conch havethick lips and are often calledsamba conch (Fig. 4).

Fisheries and regulations For hundreds of years, queenconch have been harvested as asubsistence food source and theshells used for ship ballast, tribaltools, building materials, jewelryand decoration. Beginning in the1970s, the queen conch commer-cial fishery developed in responseto the rapid growth of tourism inthe Caribbean and the increasedinternational demand for themeat. Dishes such as conch frit-ters, chowder and salad are spe-cialties on menus throughout theregion. Queen conch is consid-ered one of the most importantbenthic fisheries, second only tospiny lobster.Conch are typically fished usingsnorkel gear. Once landed on theboat, the shells are removed anddiscarded onto shell piles called“middens.” The conch meat isabout 7 to 8 percent of the total

weight of the animal, rangingfrom 0.19 to 0.5 pounds (85 to228 g). The fisherman receives$1.00 to $3.00 per conch as awholesale price. The conch meatis processed to remove the thickskin and the white meat is pack-aged and frozen in 5-pound (2.3-kg) boxes. Retail prices for conch

range from $6.00 to $15.00 perpound and continue to increaseas conch stocks become morethreatened. Shells, which are aby-product of the meat market,are sold as curios and souvenirsin the U.S., Europe and theCaribbean for $5.00 to $20.00each.

Figure 2. External sex organs of Strombus costatus (a) male and (b) femalemilk conch, which are similar to those of S. gigas. (photograph by TomSmoyer)

Figure 3. Queen conch, S. gigas, egg mass. (photograph by LeRoy Creswell)

the Turks and Caicos Islands,and USAID in Belize). In 1984,the first commercial conch farmwas established in the Turks andCaicos Islands. Today, queenconch laboratories in Mexico(e.g., CINVESTAV-IPN) andFlorida (e.g., Florida Fish andWildlife Conservation Commis-sion, Harbor Branch Oceano-graphic Institution, and MoteMarine Laboratories) are con-ducting conch research and edu-cation programs.

Culture techniquesBroodstock and egg mass procurementHatcheries must gather egg mass-es from the wild, which can bean unreliable source. Therefore,an enclosed breeding site or “eggfarm” is used to ensure a steadysupply of wild egg masses for thehatchery. A suitable egg farm site • is or was traditionally inhabit-

ed by adult conch,• has a calcareous sand and

coral rubble substrate withlow organic content,

• is located where water move-ment is less than 1 knot,

• is 4 to 8 m deep, and• is within 3 km of the hatchery

facility, for ease of access andsecurity.

To retain the adult conch, the egg farm is enclosed with a rigid,black polyethylene mesh (2 x 2 inches; 5 x 5 cm) fence 1.6 to 3.3 feet (0.5 to 1.0 m) high. Themesh is held perpendicular tothe substrate with reinforcingrods and the ends are attached tomoorings, boulders or the land. Itis not necessary to remove thefence during the nonbreedingseason unless it is in an areawhere winter storms would dam-age it. The egg farm should be stockedannually with sexually matureconch 1 month before the breed-ing season at a sex ratio of 1:1and a density of one conch per100 square feet (one conch/10m2). Conch are transported to the

The United States is the largestimporter of conch from theCaribbean and, in fact, importsapproximately 78 percent of theconch meat in the internationaltrade. About 1 million pounds(458,000 kg) was imported in2004, mostly from the Turks andCaicos Islands (Trade Database:www.cites.org). Imports were ashigh as 4 to 5 million pounds in2002 and 2003 when Hondurasand Dominican Republic con-tributed more than 50 percent ofthe supply to the U.S. In 2003,CITES (Convention for theInternational Trade of EndangeredSpecies of Wild Fauna and Flora)mandated a temporary closure ofthe fisheries in these countriesuntil the stocks increase and asustainable fisheries plan is putinto place. This multilateral envi-ronmental organization protectsand regulates species to ensurethat international commercialtrade does not threaten their sur-vival in the wild.The depletion of conch stocks hasnecessitated management regula-tions such as total fisheries clo-sures (e.g., in Florida since 1986),annual quotas, size regulations,and the prohibition of scuba div-ing. While overfishing is the pri-mary cause of the decline, habitat

degradation may also be a factor,especially the loss of importantnursery habitats close to shore.In 1992, queen conch was listedin CITES Appendix II andbecame the first large-scale fish-eries product to be regulated byCITES. All conch shipmentsmust be accompanied by aCITES permit stating that theexporting country has deter-mined that the specimens havebeen legally acquired and thatthe trade is sustainable.Some locations, including theBahamas, Puerto Rico andFlorida, have established no-fish-ing zones or marine protectedareas (MPA) to help conserve thequeen conch and other ecologi-cally important species. Thesetypes of management efforts,along with regional fisheriesplans and enforcement, will helpthis important species recover.There is considerable interest inthe culture of the queen conch tosupplement dwindling naturalpopulations. Research began inthe 1970s in Los Roques,Venezuela, and in the 1980s sev-eral labs in the region made sig-nificant contributions to queenconch culture (e.g., University ofPuerto Rico, University ofMiami, Foundation for PRIDE in

Figure 4. Juvenile and adult S. gigas shells from 1 month old to 25 years old.(photograph by Kathy Orr)

egg farm under wet burlap bagsand can be held out of the waterfor 5 hours. Conch should beremoved from the egg farm at theend of the breeding season toavoid overgrazing and poor eggproduction the following season. It is difficult to differentiate maleand female conch from theirappearance, although the femalesare usually larger. Size can beused as a general indication ofsex unless fishermen have alteredthis relationship by preferentiallyharvesting the larger females. Amore accurate, but time-consum-ing, method of differentiatingsexes is to place conch on theirsides with their lips pointing up.As they right themselves theirsex can be noted: a verge for amale and an egg groove for afemale (Fig 2). Another way ofdetermining sex is to observeconch in their mating position

(female in front, male behind) orobserve females laying egg masses(Fig. 5). The crescent-shaped, sand-coveredegg mass can be found under theanterior lip of the female conch(Figs. 3 and 5). Egg masses collect-ed directly from underneath thefemales are hardier and easier totransport because embryo devel-opment is minimal. It takesapproximately 24 to 36 hours tolay an egg mass, and the eggshatch in 3 to 5 days. It is not nec-essary to collect a full egg mass;one-half or three-fourths of an eggmass will yield a substantialhatch. Or, small egg masses canbe combined for hatching. Fortransport to the hatchery, the eggmass is placed in a plastic bagfilled with seawater inside a shad-ed bucket of seawater. Conch eggmasses can be shipped within 24hours to other hatcheries or labo-

ratories by placing each egg massin seawater in a beverage-sizedthermos (0.26-gallon; 1-L). Researchers are looking for a reli-able way to induce egg laying incaptivity. In 2000, scientists at theHarbor Branch OceanographicInstitution were the first toobserve egg laying in captivity.One female and three male S.gigas conch were stocked (oneconch per 100 square feet; oneconch/10 m2) and held in a circu-lar breeding tank with an elevatedsand substrate. The breeding tankwas on a recirculating water sys-tem and was kept at 81 to 84 °F(27 to 29 °C), 32 ppt salinity, pH8.0, and ammonia 0.06 to 0.10mg/L (ppm). The breeding tankwas inside a shaded greenhouseand artificial lighting above thetank was set on a 12-hour cycle(8:00 a.m. to 8:00 p.m.). Thebroodstock were fed approximate-

Figure 5. Life cycle stages of the queen conch, S. gigas. (drawing by Bonnie Bower-Dennis).

ly 50 to 70 g per day of prepareddiet—a mixture of koi chow, Ulvaseaweed, seawater and an algi-nate binder. Four egg masseswere laid by the single femaleconch in captivity, and thehatched veligers were culturedsuccessfully to the juvenile stage.

Egg mass handlingIn the hatchery the egg mass isdisinfected to remove bacteriaand predators such as polychaeteworms, nematodes and crus-taceans. The egg mass can be dis-infected by dipping it in freshwater for 5 seconds or in a mildchlorine-seawater solution (0.5%)for 30 seconds, then rinsing it inthree containers of seawater. Theegg mass incubates for 3 to 5days in an upwelling screen (75µm) PVC container (6 inches indiameter and 6 inches high; 15cm x 15 cm) with a flow rate of0.03 gallons per minute per eggmass (0.1 L/min/egg mass) (Fig.6). On the day of hatching, thedeveloping embryos steadilyrotate and the capsulated veligershave velar lobes that are well-defined with cilia and red pig-ment on the foot.

HatcheryIf the feeding regime, tempera-ture, stocking density and waterquality are optimal, the veligercycle is approximately 21 days

Figure 6. Egg mass incubating con-tainers and tank (a). Each egg masscontainer holds an individual eggmass (b) on a screen (c). The waterupwells through the screen (d) andthe water leaves via a small open-ing in the side of the container (e).(drawing by Jackie Aronson)

Figure 7. Stages of queen conch, S.gigas, veliger development: (a) newly hatched, 2-lobed veliger(300 µM); (b) four-lobed veliger (4 days old;450 µM); (c) six-lobed veliger (8 to 10 days old;600 µM); (d) metamorphically competentveliger (21 days old; 1.0 to 1.2 mm)with labels for i) eye, ii) tentacle, iii)buccal mass, iv) pigments on foot;and (e) metamorphosed conch (1.2 mm). (photographs a-d by Megan Davisand e by Leroy Creswell)

a b

c d

e

i

ii

iii

iv

long (Fig. 5). The emerged veligeris 300 µm SL and has two velarlobes (Fig. 7a), which divide intofour by the fifth day (Fig. 7b) andsix by the eighth day (Fig. 7c).These lobes continue to elongateuntil the 0.04-inch (1-mm) SLveliger is ready for metamorpho-

sis (Fig. 7d). The lobes are usedfor locomotion, respiration andfeeding. Veligers are cultured in 130- to530-gallon (500- to 2,000-L) coni-cal tanks with gentle aeration(Fig. 8). To keep the veligers in

suspension, a PVC airlift (6 inchesin diameter x 6 inches high; 15cm x 15 cm) with two air lines(without airstones) attached toeither side is positioned 0.2 inches(0.5 cm) from the bottom of thetank (Fig. 8). Conch veligers arelarge and must be stocked at a rel-atively low density (560 to 760per gallon; 150 to 200/L) com-pared to many bivalve species.Density is gradually reduced dur-ing the water changes to a finaldensity of 95 to 150 veligers pergallon (25 to 40 veligers/L) atmetamorphosis. Optimal culturingtemperature is 82 °F (28 °C) and36 ppt salinity. However, veligerscan be grown at temperatures of75 to 90 °F (24 to 32 °C) andsalinities of 26 to 40 ppt. Larval cultures are maintainedeither in a flow-through system(0.3 to 0.6 gallon per minute; 1 to2 L/min) or in static conditions.Higher densities can be main-tained in a flow-through system.Water changes begin the day afterhatching and occur every 48hours. Larvae cannot be dry-sieved; therefore, the PVC sieve(8 inches in diameter x 12 incheshigh; 20 cm x 30 cm) used forwater exchanges is supported in abucket of seawater (Fig. 7). Tohelp cull the dead and slow-grow-

Figure 8. Conical bottom larvaltank for culturing queen conchveligers (a). An airlift is positionedat the bottom of the tank (b).Veligers are siphoned from the tankinto a sieve suspended in a bucketof water (c). (drawing by JackieAronson)

ing veligers, water exchangesstart by using a 100-µm sieve andincrease to a 350-µm sievetoward the end of the larvalcycle.A siphon hose is used to removethe veligers from the tank (Fig.8). This method is less stressfulto the veligers than drainingthem from the bottom of thetank. After the tank is drainedthe veligers are gently hosed offthe sieve into the newly filledtank. The tank is covered and, ina flow-through system, the wateris turned on. A sample of veligers should beobserved daily using a dissectingmicroscope (20X to 40X) to deter-mine their growth, development,and the amount of feed in thegut. A healthy S. gigas veliger has • a dark gut that fills the top

whorls of the shell (indicationof feeding);

• clean velar lobes, fully extend-ed, and cilia moving in awave-like pattern;

• increasing velar lobe and shellwhorl development; and

• growth of 50 to 55 µm SL/day.A veliger is diseased if it • is slow to come out of the

shell, • has bacteria and protozoa in

the shell or feeding on theveliger’s tissue,

• has lobes that are stunted orshow necrosis (Vibrio bacteriainfection),

• moves erratically,• has debris or protozoa on the

outside of the shell, • has a light gut (lack of food),

and • shows no growth and develop-

ment.If there has been a bacterial inva-sion (e.g., Vibrio) during the first7 days, the veligers will shownormal development but all ofthem will die shortly thereafter.If there is a toxicity problem(e.g., water quality, copper, newPVC pipes and glue, insecticides),the veligers will not develop and

all will die on day four. Stressedveligers will excrete mucusstrands that trail behind theswimming larvae and cause themto clump together. This occursfrom overfeeding, high density,and rough water changes. Thecondition is usually temporaryand can be solved by decreasingfood, lowering density, andimproving water exchange meth-ods. A momentary increase inaeration will break the clumps ofveligers apart. Even though veligers hatch withyolk reserves, they will beginfeeding on phytoplankton 6 to 8 hours after hatching. Therefore,to increase survival and vigor theveligers should be fed the morn-ing after hatching. Two species ofphytoplankton or microalgae arecultured to feed conch veligers—flagellated algae, TahitianIsochrysis galbana or CaicosIsochrysis; and the diatomChaetoceros gracilis or Chaetocerosmuelleri. Batch cultures of phyto-plankton are grown in carboys (5-gallon; 20-L) or cylindrical suntubes (65-gallon; 250-L). Thedesirable cell concentration ofIsochrysis in the larval tanks is 1.3x 106 cells per gallon (5 x 103

cells/mL) for newly hatchedveligers and 6.6 x 106 cells pergallon (25 x 103 cells/mL) formetamorphically competentveligers. Chaetoceros is added tothe culture starting on day 15 andthe final concentration in thetank is 0.8 x 106 cells per gallon(3 x 103 cells/mL). In static larvalcultures the phytoplankton isadded as a batch feed, whereas inflow-through larval tanks thealgae cells are diluted with sea-water and drip-fed into the cul-ture water for approximately 12hours.

MetamorphosisVeligers are metamorphicallycompetent about 21 days afterhatch. At this point their shelllength is about 0.05 inches (1.2mm). The veligers are competentwhen their eyes migrate outward,the tentacles are of equal length,the pigments on the foot changefrom orange to dark green, the

ctendium (i.e., gills) are elongat-ed and functional, and the buccalmass has developed (Fig. 7d ).Several different substances canbe used to induce metamorphosisin queen conch larvae, includingpotassium chloride, hydrogenperoxide, epiphytic diatoms, andred macroalgae. Two of the mostreliable inducers for large-scalecultures are extracts from the redmacroalgae Laurencia poitei andsmall doses of hydrogen perox-ide. At a temperature of 82 to 86°F (28 to 30 °C), these cuesshould induce 75 to 95 percent ofthe conch to metamorphose.The extract of L. poitei is pre-pared from old, thick, red-brownstalks collected from shallow,sandy, grass flats and transportedto shore in mesh bags. Theyounger stalks, which are yellow-orange, are slightly toxic toveligers and will cause only asmall percentage of them tometamorphose. Onshore, themacroalgae fronds are rinsed andsorted to remove coral pieces,sponges and excess sand. Then aratio of 2 g of Laurencia to 1 mLof seawater is blended forapproximately 2 minutes in anindustrial blender. The solutionis frozen for at least 2 days tolyse the cells and release themolecular cues. The frozen solu-tion is thawed overnight, filteredthrough a 200-µm polyethylenescreen, and the resulting extractrefrozen. Four and one-halfpounds (10 kg) of collectedmacroalgae produces 4.2 gallons(16 L) of blended slurry, whichyields approximately 2 gallons(7.5 L) of extract. Before any new Laurencia extractis used, the proper dosage isdetermined by placing 25 compe-tent veligers in three differentextract concentrations—7, 10 and15 mL of Laurencia extract/L ofseawater—for 4 hours. Then theveligers are removed from theextract and the percentage thathas metamorphosed is deter-mined using a dissecting micro-scope (40X). A veliger has meta-morphosed when the velar lobesare lost, it is crawling with itspropodium (foot), and it is

searching for food with its pro-boscis (Fig. 7e). If an extractdosage produces at least 60 per-cent metamorphosis in the sam-ple, it should be effective on thelarge-scale.In recent years, scientists havediscovered a more cost-effectiveprocedure for inducing metamor-phosis. Competent veligers areexposed to 50 µM of 3% pharma-ceutical grade hydrogen peroxide(0.06 mL of H2O2 /1 L of seawa-ter) for 4 hours. The cost perbatch of larvae is about $0.16,compared to $15.00 withLaurencia. Using hydrogen perox-ide reduces the variability foundin natural inducers, and it is asimple, low-cost, safe method forcommercial-scale induction ofmetamorphosis.When a batch of larvae is readyfor metamorphosis, a shallowtray (8 feet long x 3 feet wideand as deep as 6 inches; 2.4 m x0.9 m x 2.4 cm) is filled with sea-water and the inducing substance(Fig. 9). The solution is mixedwell, then eight shallow screen(250-µm) trays, each with an areaof 430 square inches (2,800 cm2),are placed in the trough. The lar-vae are stocked at 325 per squarefoot (3,500 per m2). Veligersshould stop swimming 10 to 30minutes after they are exposed tothe metamorphic cue. If theycontinue to swim, the cue con-

centration should be increasedslightly. During the metamorphicprocess the veligers should beplaced in subdued light. After 4 to5 hours, the system is flushedwith water to eliminate the cue.Newly metamorphosed conch canbe kept in a recirculating systemor in a flow-through system.Water enters each shallow screentray through a small tube (0.15 to0.24 inches in diameter; 4 to 6mm) (Fig. 9), creating a slight vor-tex in the downwelling tray.Conch are kept on the screentrays for 14 to 21 days until theyreach an average size of 0.12 to0.14 inches SL (3 to 4 mm SL).The average growth rate is 0.180mm SL per day or more.Before conch are fed, the traysare gently shaken to removewastes and to dislodge the conchcrawling on the sides. Newlymetamorphosed conch are fedflocculated Chaetoceros gracilis forthe first 2 to 3 weeks. For large-scale operations, this diatom canbe grown in 5,300-gallon (20,000-L) tanks located outside with nocover. For smaller hatcheries, C.gracilis can be cultured in 5-gallon(20-L) carboys or in 65-gallon(250-L) cylinders. Chitosan, amacromolecule from the exoskele-ton of crustaceans, is used in theflocculation procedure. Chitosanis purchased in a powered orflaked form.

Figure 9. Metamorphosis tank (a) with incoming water (b) and screen trays(c). (drawing by Jackie Aronson)

The flocculating procedure beginsby adjusting the microalgae cul-ture to a pH of 6.0 to 7.0 with theaddition of muriatic acid (approxi-mately 0.1 to 1.2 mL of muriaticacid/L of algae). The chitosansolution is well mixed into thealgae culture at a dose of 5 mLper L of algae. A stock solution ofchitosan (10 g chitosan to 1%acetic acid) is prepared inadvance and can be stored forseveral weeks. Sodium hydroxide(5% or 5 g NaOH/100 mL freshwater) is added to raise the pH to8.5 to 9.0 (approximately 1.0 to1.5 mL/L of algae). The algal cellsare stirred again for several min-utes until flocculation isobserved. The flocculated cellssettle to the bottom of the vesseland can be separated from the“clear” water. If small quantitiesof flocculated algae are needed ona daily basis, the settled algae canbe stored in the refrigerator forup to 1 week. On the first day ofmetamorphosis, the conch are fedabout 6 x 107 cells per conch perday. Feeding increases to 20 x 107

cells per conch per day for conchthat are 14 to 21 days old.

NurseryThe post-larval conch (3 to 4 mmSL) are placed in sand troughs 10feet long x 2 feet wide x 2 feethigh (3m x 0.6m x 0.6 m) at 316conch per square foot (3,400conch/m2) (Fig. 10). These troughshave an elevated sand substratemade up of crushed coral arago-

nite (1 to 3 mm diameter) andconstant water flow and air sup-ply. The water downwellsthrough the sand, is recirculatedthrough a sump and returned tothe trough. This system can alsobe set up as a flow-through sys-tem. Depending on the size ofthe conch, the sand trays aresprayed down every 2 to 4 weeksto remove accumulated wasteproducts. During this process, thesand and the animals are leftintact and the conch lie dormant.Juvenile conch (0.8 to 1.6 inchesSL; 2 to 4 cm SL) can be placedin shallow, circular, fiberglass orcement ponds, or lined raceways,at a stocking density of 14 conchper square foot (150 conch/m2).During this nursery stage, thedensity will need to be reducedto seven conch per square foot(75 conch/m2). Adding sand to thebottom will aid in shell formationand assist with biofiltration.Nursery systems also can beeither recirculating or flow-through. Water should enter theconch ponds or raceways fromone or two spray bars to create acircular flow that exits through acentral standpipe. The optimalflow rate is based on conch den-sity, temperature and feedingstrategy. To enhance growth andsurvival, the nursery ponds orraceways should be covered with100% shade cloth. Juvenile conchprefer shaded areas because inthe wild they are found buried inthe sand. The optimum tempera-

ture for rearing juvenile conch is81 to 84 °F (27 to 29 °C). Growthrates range from 0.180 to 0.250mm per day and survival ratesare as high as 90 to 95 percent. Conch are fed once a day witheither a gel-based diet or a com-mercial conch chow. The gel feedconsists of ground koi or catfishpellets (36 percent), dried Ulvasp. seaweed (16 percent), gelatinor alginate (6 percent), and sea-water (42 percent). The gelatin oralginate allows the feed toremain stable for approximately48 hours. The diet can be manu-factured in a feed mill or madein smaller quantities using astove and stainless steel cookingpot. The approximate cost of theprocessed diet is $3.20 per poundwith catfish pellets and $4.50 perpound when koi pellets are used.The gel feed has a shelf life of 1week if refrigerated and severalmonths if frozen.Juveniles 0.12 to 0.16 inches long(3 to 4 mm SL) are fed approxi-mately 10 mg of feed per conchper day. Larger juveniles (1.2 to1.6 inches SL; 30 to 40 mm SL)are fed approximately 120 mg offeed per conch per day. Smallerconch are fed about 15 percent ofwet-meat weight. As theyincrease in size (2.4 inches SL; 6cm SL) they are fed 2 to 7 per-cent of the wet-meat weight. Thetypical growth rate on the geldiet is 0.18 to 0.20 mm per day,depending on temperature andstocking density. Conch fed thecommercial feed have a feed con-version ratio of 1.5:1 for dryweight of feed to total wet-weightof the conch (shell and meat). The leading edge of the shell of ahealthy juvenile is thin and usu-ally covered with feed, sand andfeces. The conch fills the shell,and the foot and operculum areeasily seen in the aperture. Anundernourished conch recedesinto the shell, making it difficultto see the foot and operculum.When a conch is not growing theshell becomes covered with agreen or blue-green algae.

Figure 10. Juvenile trough system with elevated support (a) for sand substrate(b). The water in the tank (c) drains through the sand and into a sump (d);the pump (e) sends the water into the tank and underneath the substrate (f).(drawing by Jackie Aronson)

Conch are most active at night.During the day they lie dormantwith their apertures toward thesand substrate and tend to be par-tially buried when not feeding.When underfed, conch are activeduring the day searching for food.Juvenile conch in poor health areoften observed on the sand sur-face, positioned with their opercu-la upward and their bodies par-tially extending out of their shells.In this state, their response to astimulus is often sluggish. Thisbehavior could be a result of stag-nant water flow (e.g., low oxy-gen), overfeeding, or stress fromhandling. Remedies include flush-ing the tank, feeding less, and/oradjusting the water flow rate. Thekicking behavior conch some-times exhibit is usually a sign ofstress and has been observedwhen the animals are in low-oxy-gen situations or are fed certaintypes of artificial feed (e.g.,Spirulina).

Grow-outLarger juveniles can be grownfrom 2.8 to 7 inches SL (7 to 18cm SL) in a sea pasture with apredator-exclusion fence or incages. Suspected or known preda-tors of queen conch includebrachyuran crabs, hermit crabs,spiny lobster, octopus, fish,sharks, rays, turtles and other car-nivorous gastropods. The fenceshould extend from the bottom tothe high-tide line. A variety offencing or cage material can beused to exclude predators andcontain conch. Large, circularcages (80 feet in diameter, 5,400square feet; 25 m in diameter,500 m2) with mesh openings (6 to8 inches in diameter; 15 to 20 cmin diameter) will help to excludelarge predators; smaller mesh (2inches in diameter; 5 cm in diam-eter) placed along the bottom willexclude smaller predators andhelp to keep juvenile conch fromescaping. Inexpensive traps suchas empty adult conch shells canbe used to catch octopus; baitedtraps can collect lobsters and gas-tropod (Fasciolaria tulipa andMurex ponum) predators.

The stocking density for juvenileconch should reflect the densityfound in the wild, which is oneto two conch per 10 square feet(one to two/m2) or approximately4,050 conch per acre(10,000/hectare). Cages can bestocked at a higher density (oneconch per square foot; 10/m2)than open sea pasture; however,the natural food supply mayneed to be supplemented with aprepared diet. Stock must be rotated throughthe sea pasture to avoid overgraz-ing. The best grow-out sites arehistorical nursery grounds. Thesesites have a sandy bottom withan intermediate density of sea-grass (56 to 84 Thalassia shootsper square foot; 600 to 900/m2)and high algal productivity. Theyare 7 to 13 feet (2 to 4 m) deepand have strong tidal currents (1to 2 knots) to flush the area andbring in new feed.Grow-out from 2.8 to 7 inches SL(7 to 18 cm SL) will take approxi-mately 1.5 to 2 years at a growthrate of 0.15 to 0.20 mm per day.Conch are harvested from thesea pasture by divers workingfrom small boats. They can beprocessed on-site or sold live torestaurants or wholesalers.Survival in the sea pastureshould be 60 to 90 percent,depending on the effectiveness ofthe cages or fences, the availabil-ity of feed, and the success ofthe predator trapping program.

Stock enhancement Reintroducing conch in over-fished habitats has been suggest-ed as a way of re-establishingconch spawning where the natur-al population is not likely torecover. To date there have beenno large-scale reseeding effortswith queen conch juveniles.However, several stock enhance-ment studies conducted inFlorida (Florida Fish and WildlifeConservation Commission), theBahamas (University of Miami,Caribbean Marine ResearchCenter) and Puerto Rico

(University of Puerto Rico) havecontributed valuable data aboutthe feasibility of releasing conchjuveniles into the wild. (Thesestudies released juveniles lessthan 4 inches [10 cm] long.) First,release sites should provide thesame conditions as grow-outenvironments. Released animalsshould be more than 3 incheslong (> 7.5 cm SL) and equiva-lent to wild stocks (e.g, shellmorphology, shell strength, burialpatterns, predator avoidance).Released conch should behatched from several egg massesto improve genetic diversity.Planting should be done in thefall and before the full moon tolessen predation, and severalthousand conch should bereleased at one site. HarborBranch Oceanographic Institutionis researching ways to improvehusbandry techniques so thatjuvenile conch from aquaculturefacilities will be similar to theirwild counterparts. It was deter-mined that conch grown at ahigh density of 18 to 37 conchper square foot (200 to 400/m2) to3 inches SL (7.5 cm SL) on arago-nite sand had the strongest shellsand were able to burrow into thesand.The success of a conch releaseprogram will depend on the costof seed, the quality of the seedstock, and the participation of thegovernment and the local fisher-men. Given the high cost ofconch seed (from $0.20 each for2-cm seed to $0.75 for 7- to 9-cmseed) stock enhancement maynot yet be economically feasible.However, every conch that sur-vives to adulthood can be mea-sured in terms of its reproductiveoutput rather than as a productfor the fisheries. Therefore,releasing juvenile conch andtransplanting adult conch to formspawning aggregations may bethe ideal way to bring backdepleted populations. Grow-outin pastures and pens is the sug-gested method of producingconch for the fisheries market.

MarketsQueen conch has long been af avorite item on the menus ofrestaurants in the Caribbean andFlorida. The tasty, tender meat isprotein-rich (20 percent) and con-tains no saturated fat. Conch meathas a crunchy texture similar tothat of geoduck clams and a tastewith the sweetness of abalone.Because the market is based almostentirely on wild-harvested animals,the demand is for legal size conch(3 to 4 years old) with fully formedl i p s. Retail prices for wild-harve s t-ed conch from Jamaica, the Tu r k sand Caicos Islands, Belize and theBahamas range from $6.00 to$15.00 per pound.N ew markets have been estab-lished for smaller cultured conch,n ow that large numbers of farmed-raised conch are available on a reg-ular basis from the commercialconch farm in the Turks andCaicos Islands. CITES hasa p p r oved these farm-raised conchto be sold live in-the-shell ors h u c ked at a sub-legal size (1.2 to 4inches SL; 3 to 10 cm SL). It isvery difficult to locate large num-bers of conch smaller than 4 inch-es SL (< 10 cm SL) in the wild, sothere is little concern aboutexploiting this size juvenile fromnatural habitats. These hatchery-reared conch are sold live torestaurants in Prov i d e n c i a l e s, theTurks and Caicos Islands, Floridaand New York at a wholesale priceof $0.50 to $0.75 each. These smallconch are served in traditionalescargot dishes, bouillabaisse, pael-las and gumbos; they also can bes e r ved ceviche-style or as tempura,and are boiled or steamed to bes e r ved as medallions. The second market for smallconch is the aquarium trade,where the wholesale price toretailers is $1.75 to $2.25 each.Conch sold for aquaria are 1.2inches (3 cm) SL and graze onalgae and diatoms from the sand,rocks and glass of the aquaria. Before live conch are shipped, theyare sorted by product type (e. g . ,food or aquarium market) andpurged for 12 to 24 hours (no feed)in flow-through or recirculating

s e awater systems. Conch areshipped dry. On the day of ship-ment, the conch are placed on topof layers of moist paper towe l sinside plastic bags that are packa g e dinside insulated boxe s. A smallamount of oxygen is added to theplastic bags before they are closed.Cold packs keep the conch cool dur-ing shipment (72 °F; 22 °C). Conchare shipped by air and will arrivewith 100 percent survival if ship-ment time is less than 36 hours. Some restaurants prefer to buysmall conch shucked or remove dfrom the shell. A specialized shuck-ing tool is used to release the conchmuscle from the columella. Theguts are removed and the conch arep a c kaged in 1-pound bags for ship-ment. There are approximately 96conch per pound at 2.4 inches (6cm) SL and 43 conch per pound at 4inches (10 cm) SL. Bags of conchare refrigerated or frozen and pack-aged in coolers for shipment.Farm-raised conch have been wellreceived by chefs and their clients.Several chefs have stated thatfarm-raised conch is sweeter andmore tender than wild conch. Thetenderness appears to be related totheir small size and the fact thatthey are sold live or fresh, where-as most wild conch are larger andsold frozen.

EconomicsNow that queen conch are beingcultured successfully and wildstocks have dwindled below sus-tainable levels in several countries,private and government groupsfrom Caribbean countries haveexpressed interest in setting uphatcheries for seed production,stock enhancement and commer-cial grow-out. To be economically successful, acommercial queen conch farmneeds to produce at least 500,000j u venile conch per year either tosupply the specialty markets forsmall conch or to stock into grow -out pastures. If the farm sells500,000 small conch (2.4 to 4 inch-es SL; 6 to 10 cm SL) to the foodm a r ke t s, the gross yearly reve n u ewould be about $375,000 ($0.75each for 4-inch SL conch). It is esti-

mated that another 3,000 or soconch per month could be sold tothe wholesale aquarium market at$0.75 each, for an additional ye a r l ygross revenue of $27,000. The maina d vantages of selling smaller conchare that the product is ready form a r ket in 6 to 12 months and thereis no need for the sea pasture. Another market for conch is othernursery and grow-out farms. Theestimated value of conch seed is$0.01 per mm of shell length, or$0.10 for 0.4-inch SL (1-cm SL)conch and $0.70 for 3-inch SL (7-cm SL) conch. There is an ever-increasing need for countries tohave centralized hatcheries thatcan supply nursery and grow-outfarms with seed conch.If a farmer planned to harvest500,000 legal-size conch per yearfrom sea pasture cages, the farmerwould have to stock 50 acres (20hectares) of sea pasture at 1 conchper square foot (10 conch/m2) andallow for crop rotation. Becausethe grow-out time is 1.5 to 2 years,the farmer should have two cropsin the sea pasture at the sametime. Therefore, a total of 100acres (40 hectares) are needed. Ifoptimal survival is 90 percent,557,000 juvenile conch would haveto be planted in the sea cages toyield the 500,000 legal-size conch.These farmed-raised conch willproduce 100,000 pounds (45,350kg) of meat at five conch perpound or 0.18 pounds of meat perconch (85 g of meat per conch).The farmer should expect to get apremium price for farm-raisedconch based on niche-marketprices established by the CaicosConch Farm (up to $20.00 perpound wholesale). At this price theannual gross revenue would beapproximately $2,000,000 per year.The infrastructure required to pro-duce 500,000 conch per yearincludes an egg farm, a hatchery, apost-larval and nursery facility,and a grow-out pasture (Table 1). Ifconch will be processed on site, aprocessing facility is also required.It is estimated that 12 staff mem-bers would be needed to operatethe onshore and offshore facilitiesof a farm this size.

SummaryThe queen conch, S. gigas, is aprized delicacy throughout theCaribbean and Florida and haslong been harvested both for itsmeat and its beautiful shell.Commercial fishing has depletedwild populations. Culturing queenconch could relieve fishing pres-sure and provide animals forrestocking overfished habitats.Methods of culturing juvenilequeen conch are well established.Egg masses are collected from thewild and the hatched veligers arecultured for 21 days on phyto-plankton. After metamorphosis,the post-larval conch are grown onsand in shallow tanks until theyare almost 1 inch (2 cm) SL. Theyare fed a combination of flocculat-ed phytoplankton and preparedfeed. Conch are kept in nurseryponds onshore until they are 2.8inches (7 cm) SL. Then they areplaced in a sea pasture for grow-out. Small juvenile conch are mar-keted as specialty food. Legal-sizeconch are used in more traditionaldishes such as conch fritters, salador chowder. To be economically

sustainable, a commercial conchfarm should produce 500,000 juve-nile conch per year.

AcknowledgementsSpecial thanks to Amber Shawl,Ashley Spring, Peter Van Wyk andNancy Daves, who contributedinformation for this publication.This is Harbor BranchOceanographic Institution, Inc.contribution # 1605.

Suggested readings Davis, M. 2000. Queen conch

(Strombus gigas) culture tech-niques for research, stockenhancement and growoutmarkets. Pages 127-159 in M.Fingermna and R.Nagabhushanam, editors.Marine Biotechnology. SciencePublishers, Inc., USA.

Davis, M., B. A. Mitchell and J. L.Brown. 1984. Breeding behav-ior of the queen conchStrombus gigas Linne held in anatural enclosed habitat.Journal of Shellfish Research4:17-21.

Delgado, G. A., C. T. Bartels, R.A.Glazer, N. J. Brown-Petersonand K. J. McCarthy. 2004.Translocation as a strategy torehabilitate the queen conch(Strombus gigas) population inthe Florida Keys. FisheriesBulletin 120:278-288.

Randall, J. E. 1964. Contributionsto the biology of the “queenconch” Strombus gigas. Bulletinof Marine Science of the Gulfand Caribbean 14:246-295.

Shawl, A. and M. Davis. 2004.Captive breeding behavior offour Strombidae conch.Journal of Shellfish Research23:157-164.

Stoner, A. W. 1994. Significance ofhabitat and stock pre-testingfor enhancement of naturalfisheries: experimental analy-ses with queen conch. Journalof the World AquacultureSociety 25:402-424.

The work reported in this publication was supported in part by the Southern Regional Aquaculture Centerthrough Grant No. 2003-38500-12997 from the United States Department of Agriculture, Cooperative StateResearch, Education, and Extension Service.

SRAC fact sheets are reviewed annually by the Publications, Videos and Computer Software SteeringCommittee. Fact sheets are revised as new knowledge becomes available. Fact sheets that have notbeen revised are considered to reflect the current state of knowledge.