A GUIDE TO ROCK SCALLOP AQUACULTURE

A Publication of the Carlsbad Aquaculture Research Institute

March, 2003

Purple-hinge Rock Scallop, Crassadoma

gigantea, formerly known as Hinnites multirugosus.

Mature rock scallop opened to reveal the large adductor muscle, the portion prepared for human consumption. The individual shown is a gravid female with bright red ovarian tissue packed with eggs.

David L. Leighton, Director

Carlsbad Aquaculture Research Institute

P.O. Box 2600

Carlsbad, CA 92008

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Principal characteristics of the rock scallop

The purple-hinge rock scallop has strong potential to become a substantial figure in controlled production of marine shellfish seafoods. In common with clams, oysters, mussels and other species of scallops, the purple-hinge rock scallop is a filter-feeder, deriving its nutrition from the phytoplankton and thus feeds close to the base of the food chain. Bivalves in general occupy the first level of consumers. Our harvest of marine protein one step above the primary producers is ecologically, energetically and economically sound.

The rock scallop presents advantages to the seafood culturist on several fronts. The broad natural range of Crassadoma from Alaska to southern Baja California, Mexico, and its habitat from coastal bays to deep water offshore reefs indicates its comparably broad tolerance to environmental conditions. As this guide will show, the rock scallop may be cultivated in both protected and not so protected oceanic waters.

Few species of scallops abandon the free-living life style commonly characterizing these shellfish to attach, and in fact cement themselves, to rock or other firm substratum material. Indeed, rock scallops enjoy a free-living juvenile life lasting about six months before cementing by deposition of shell material to an accepted surface.

This behavior allows the rock scallop culturist to regulate the spatial distribution to maximize growth potential and minimize the effects of crowding. To this end, a grow-out panel was developed on which young scallops are held at 1/dm2 (ca. 4/ft2).

Reaching maturity ready for harvest the rock scallops are 10 - 12 cm (4-5 in.) yet are not crowded to compete for food, oxygen, and

living space. Many alternatives exist for scallop grow-out, but frequently crowded scallops cement to one another. Cementation to netting or meshing of containment structures creates difficulties at harvest.

Other approaches to grow-out have included glueing individual juvenile scallops to rearing surface materials, allowing the scallops to cement with growth to secure their positions.

Juvenile rock scallops approximately six months old. During this stage, young rock scallops are able to swim briefly and readily attach to surfaces with byssal fibers. Cementation occurs within the next few months.

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This shellfish continues to be a prime quarry of sport divers and low tide collectors in California. It has not been the subject of commercial fisheries since early in the last century such take has been precluded by California Department of Fish and Game law. Rock scallops, while locally abundant, would be easily overfished since their habitat is restricted largely to rocky promontories and reefs at subtidal depths readily accessible to scuba, hooka, and free divers.

Within the diving community the rock scallop is well-known for its tasty meat. In early studies of rock scallop biology and culture, fresh adductor muscles were compared to commercially available scallop species by a highly objective taste panel evaluation (Leighton and Phleger, 1981). Judged on the basis of flavor, aroma, after-

taste and texture, among other criteria, the rock scallop scored high or highest in these tests. The successful rock scallop aquaculturist would find the product commanding top prices.

Culture of Rock Scallops

In general, rock scallops are cultured by methods similar to those applied in culture of oysters, clams, and other bivalves.

Specifically, rock scallops present differences in their behavior and physiology which require a special set of conditions for spawning, larval culture, metamorphosis and grow-out. The bulk of this guide is devoted to description of the various steps to be followed in culture of this species.

Brood Stock

Unlike many other scallop species, sexes are separate in Crassadoma. Southern California populations exhibit a bimodal annual reproductive cycle, generally with peaks in late spring and mid fall. A deep water population off Point Loma, San Diego, was found to mature about one month later than shallow water populations in the area.

Northern rock scallops appear to have a single peak in early summer (Leighton, 1991). In captivity, natural peak periods have been extended (Leighton and Phleger, 1982). In a recent study, adult rock scallops collected from the entrance to Mission Bay, San Diego, have been found sufficiently ripe to release viable gametes upon induction most months of the year (Leighton, 2003).

Spawn Induction

A wide variety of methods to induce spawning were successfully applied to rock scallops (Leighton and Phleger, 1979), including mild heat shock, addition of testis fluid to water containing females, and use of UV-irradiated seawater (Uki and Kikuchi, 1974). Gamete release following injection of the adductor muscle with serotonin (0.2 ml of a 10-3 M solution) was reported by Malakowski (1986) and Rhee and Davis (1998). Our recent two-year study used serotonin injection almost exclusively with good results. During non-peak periods females were less responsive, but males did not fail to produce sperm.

We routinely selected five recently- collected adult scallops, scrubbed to remove algae and other fouling organisms, and injected each using a 3 cm gauge 22 hypodermic needle inserted between the valves at either the anterior or posterior commissure directly into the adductor muscle. Scallops were placed immediately in separate pails containing filtered seawater. Males generally released sperm within 15 -

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30 minutes; females within 1-2 hours.

Fertilization and Incubation of Eggs

Ova should be suspended in filtered seawater and a sperm suspension made in a separate beaker to create a pale white stock. Mixture of gamete solutions must be done with care. Avoid use of too much sperm for fertilization (zygotes and subsequent embryonic stages become abnormal under conditions of polyspermy). Eggs should be fertilized to provide 10-100 sperm/ovum. Under the microscope sperm attaching to the vitelline membrane are seen at the equator. Concentrations are ideal if several sperm are observed in the plane of view.

Following fertilization, ova are allowed to sink. This may take over 15 minutes. At that point superficial water is decanted and the eggs resuspended in newly filtered and UV-treated seawater. This process should be repeated at least twice to ensure excess sperm and other potential contaminants are removed.

We use a series of polyethylene pails (10-Liter volume), distributing approximately 50,000 zygotes to each. Frequently spawning by one or two females provides well over one million eggs.

Fertilized eggs are incubated without aeration for 24 - 48 hours until hatching of trochophore larvae. Hatching rate is temperature-dependent (Leighton, 1979). At hatching, larvae concentrate at and near the surface and may be decanted into freshly filtered, UV-treated seawater. Meshing out to completely separate larvae from the culture medium is done as larvae enter the D shell or straight hinge stage (usually 3-4 days postfertilization). A 55 m nylon mesh mounted on a buoyant wooden frame immersed in filtered seawater is used in our hatchery/laboratory. Once transferred to a new culture pail, smaller algae such as Pavlova lutheri, Thalassiossira pseudonana, and Isochrysis galbana (Tahitian strain) may be supplied as food. We also use Nannochloropsis, but this alga is not utilized. It is, nonetheless, consumed and serves as a marker or indicator for active feeding.

Daily transfers of larvae are required throughout the first two weeks of culture. Thereafter and throughout culture of larvae to the eyed stage (200 - 220 m, 4-5 weeks) larvae must be meshed out each 2-3 days. Temperatures for culture of larvae from southern California rock scallops should be held in the range 14 - 20 o C.

Rock scallop larvae two weeks old. Shell width 135 m.

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As larvae reach a shell width of 180 m and approach metamorphosis, the algae Rhodomonas sp. and Tetraselmis suesica should be included in the diet. Greatest success through the rather critical metamorphosis period was achieved when these foods were supplied (Leighton, 1979).

Metamorphosis

Larvae in the size range 200 - 220

m develop eye spots and an extensible foot. These anatomical advances signal the onset of metamorphosis to the benthic juvenile form. This stage is especially critical to subsequent survival. Seldom have larvae held in pails or beakers succeeded through metamorphosis.

Our best yields of postmetamorphic juveniles have been realized in nylon mesh bottom fiberglass vats or PVC and ABS pipe sections immersed in seawater within a larger

reservoir (Leighton and Phleger, 1981; Leighton, 1991). Moderate aeration keeps food algae in suspension and creates a healthy environment for metamorphosing larvae.

Some success has been achieved by releasing late stage larvae into a large tank supplied coarsely filtered seawater. A "spatting pond" is described by Bourne et al., (1989) in which various substrates (shell, rock and plastic netting) are included. The lack of control of biotic factors inherent in this approach would likely result in varying yields. However, it is expected future study of similar systems will guide development of highly efficient larval setting programs.

Mesh bottom fiberglass pot immersed in a

100 liter acrylic cauldron tank. Rock scallop larvae pass through metamorphosis readily in such systems.

Juvenile rock scallops harvested from the system shown above.

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We have recently explored use of an incubator with controlled photoperiod and temperature for larval culture (Fisher Scientific, Diurnal Growth Chamber). Larvae are contained in capped 1-liter plastic bottles, laid horizontally on motor-driven rollers (see figure below). Controls are set to provide 18 - 20oC, a photoperiod of 14 hr light/ 10 hr dark, and a bottle rotation rate of one turn per minute. Cultures are meshed out once each 3 - 4 days and new food supplied. Larvae placed in the incubator system at an age of one week reach 200 - 220 m within another three weeks. Eyed larvae competent to metamorphose are then transferred to the systems described above.

Bottles containing about 5,000 larvae each are held on rollers within an incubation

chamber described above.

Intermediate Culture

Once juvenile scallops reach 2-3 mm (about three months postfertilization), their nutrition is better satisfied by feeding on natural phytoplankton. Bags constructed of fiberglass window screen serve well to confine early juveniles held in the bay or sea.

As size increases, larger mesh cages are used. We find quarter inch (6 mm) polyethylene mesh is most suitable for containment of young scallops of 1 cm shell height.

Juvenile rock scallops begin to cement to surfaces when 2 - 3 cm in shell height (age ~ six months). Cementation occurs at a larger size in rock scallops in northern regions.

Rock scallops may be reared from 1 cm to market size (10-12 cm) in a two year period. Oyster trays are now available in a variety of configurations, all of which are suitable for containment and grow-out. A difficulty exists when scallops cement either to each other or to the cage itself. This problem is dealt with by removing scallops to newly cleaned trays each few months. This measure is also important to avoid excessive overgrowth by fouling organisms.

Rock scallops will incorporate meshing of lantern nets, pocket nets and related cages into their shells, making harvest difficult and often leading to damage of the containment device.

Another approach to grow-out of rock scallops mentioned earlier was developed by

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the author. A plastic panel formed to provide attachment sites at an optimal spatial distribution receives juveniles prior to cementing. Each scallop cements within a well covered with 6 mm meshing. When cementation within the well is complete, the mesh is removed and the scallop continues to grow at the site until harvest. A stocking density of 4/ft2 was adopted to provide adequate room for scallops to remain separate from each other and reach a shell diameter of 12 cm with minimal competition for food or space. Use of the Leighton and Phleger, Reusable Scallop Rearing Panel is described further at the end of this section.

Juvenile rock scallops may also be glued to substrates. Initial studies by the author (Leighton and Phleger, 1981) used

4 mm asbestos insulating board. New and environmentally friendly materials exist which effectively replace asbestos board in the form of compressed fiber/concrete sheet (Hardiboard and Rhinoboard, to name two brands). Modern adhesives, especially epoxy cements, can be used to secure juvenile scallops by the right or cementing valve. Scallops are tilted to allow the growing margin of the right valve to meet the board surface. Approximately two months should be allowed for natural cementation to progress sufficiently to secure the scallop adequately.

The rearing panels developed by the author are vacuum-formed of Styrene, ABS, or other thermoplastic materials. Wells or depressions 2 inches in diameter (5 cm x 1 cm), perforated to provide points of attachment, are covered by meshing. A fine slit cut into the mesh allows introduction of a single juvenile scallop 1 - 2 cm. Eventual securement is facilitated by a loop of nylon monofilament tied through perforations at the bottom of the well.

Once young scallops have cemented within the wells, the overlying mesh is removed. Panels are suspended vertically, usually arranged in arrays of 10 or more.

Portion of a rock scallop rearing panel with juvenile scallops admitted to wells through a slit cut in the mesh.

Harvest and Processing

Mature scallops from cages, panels or trays are allowed to gape, and a sturdy wooden peg placed between the valves. A thin round-ended spatula is then inserted to remove the adductor muscle, viscera and mantle. An alternative procedure may be followed in which the hinge connecting the valves is removed using a tool with a Carborundum blade to cut a V in the shell encompassing the ligament..

Adductor muscle is easily separated from mantle and viscera, rinsed in fresh water and readied for market. While there may exist a market for "trimmings",

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Rearing panel several months after removal of mesh covers. Young scallops at this point have grown beyond the limits of the wells. Panels are 16 x 24 inches.

consumption of the hepatic tissue and gonad should be avoided. The single case of paralytic shellfish poisoning reported for ingestion of rock scallops involved this tissue (Leighton and Phleger, 1981).

Discussion of Strategies for Aquaculture of Rock Scallops

Studies of rock scallop growth rates in southern California show a shell diameter of 10 - 12 cm may be reached in two years of

grow-out from a juvenile size of 1 cm (Leighton and Phleger, 1981). Young scallops held at a series of depths in the ocean off La Jolla from15 feet to 400 feet grew most rapidly at 30 feet, slightly less at 100 feet, and considerably less at depths of 200, 300 and 400 feet (Leighton, 1979).

These results suggest the potential for maximal growth of rock scallops exists at depths of 30 to at least 100 feet in the phytoplankton-rich oceanic water overlying vast areas of shelfland off the southern California coast. Buoyed long lines to suspend rearing cages in mid-water column or bottom-situated strategies could be applied to large-scale rock scallop cultivation programs.

Even a modest program to cultivate shellfish in protected waters of bays and estuaries in southern California is bound to become limited by fishing, boating and other activities. Occasional episodes of pollution, storm runoff salinity reduction, excessive eutrophication (algae blooms) and maintenance problems with fouling are elements which may adversely influence production of rock scallop crops.

Open ocean aquaculture, especially in southern California, would appear highly promising and free of many of the problems foreseen for the bay-based operation. The relatively vast three-dimensional space overlying bottom in the depth range 50 - 300 feet in the Southern California Bight could support major-scale rock scallop grow-out operations.

Considerations applicable to mid- water column long line-suspended stacked tray or panel array rock scallop grow-out
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systems include ease or difficulty of tending and harvest, selection of appropriate depth for primary anchors and level of submerged long lines, and problems with drift kelp entanglement, possible creation of navigation hazards, and competitive use of a portion of the sea in the public domain.

An alternative strategy to employ fields of concrete posts water jet-driven into bottom sand may confront a different set of potential problems (Leighton and Phleger, 1982). In this scheme, diving would be required to plant and harvest scallops. Frame structures could be designed which, stocked with scallops, would rest on the bottom and be raised only for harvest and/or replanting. It goes without further comment that the variety of cultivation strategies for the rock scallop are limited only by the imagination.

Additional Information of Value to Rock Scallop Aquaculture

Juvenile rock scallops need not be cultured in a hatchery. Spat collectors used to accumulate wild set scallops of other species (Leighton, 1991) are also applicable to collection of juvenile rock scallops. Most commonly, fine mesh bags filled with nylon monofilament gill netting are placed in the natural environment for several months (Phleger, Leighton and Cary, 1982) Careful dissection of the monofilament yields young stages of rock scallops together with other species native to the area.

A year-long study of larval recruitment by rock scallops, bay scallops and kelp scallops was conducted in which rock scallops settled in spat collectors all seasons of the year (Leighton, 2003, in prep.)

Spat collector bags of 1 mm2 mesh were approximately 40 x 60 cm. Each was filled with old gill netting (60 m2, filament diameter 0.25 mm). Pairs of bags were immersed at a depth of 5-10 feet in the entrance to Mission Bay, San Diego, for 2-3 months and replaced sequentially through an 18 month period.

Juveniles retrieved from spat collectors generally ranged from 2 to 6 mm; the largest individuals being those entering the mesh bags as late-stage larvae soon after installation. Routinely, small juveniles recovered from spat collectors were placed in intermediate culture until reaching a size of about 2 cm. Thereafter, these scallops were grown to maturity in stacked oyster trays suspended by long-line in Agua Hedionda Lagoon, North San Diego County.

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Acknowledgments

Valuable assistance provided by Steven LePage and Jim DeMattia is greatly appreciated. Thanks are due to John Davis, Carlsbad Aquafarm, for provision of research space. The project was supported by a grant (No. 00-61) from the Resource Conservation District of Greater San Diego County.

References

Bourne, N., C.A. Hodgson and J.N. Whyte. 1989. A Manual for Scallop Culture in British Columbia. Canadian Technical Report of Fisheries and Aquatic Sciences, No. 1694.

Leighton, D.L. and C.F. Phleger. 1977. The purple- hinge rock scallop: a new candidate for marine aquaculture. Proc. World Maricult. Soc. 8:457-469.

Leighton, D.L. and C.F. Phleger. 1981. The suitability of the purple-hinge rock scallop to marine aquaculture. San Diego State Univ., Center for Marine Studies, Sea Grant Technical Rept. No. T-SCSGP001. 85 pp.

Leighton, D.L. and C.F. Phleger. 1982. U.S. Patent 4,317,429. A reusable panel for cultivation of marine bivalves.

Leighton, D.L. 1979. A growth profile for the purple- hinge rock scallop held at several depths off La Jolla, California. Marine Biology 51:229-232.

Leighton, D.L. 1991. Culture of Hinnites and related scallops on the American Pacific Coast. Chapter 7 in: Estuarine and Marine Bivalve Mollusk Culture. W. Menzel, Ed. CRC Press. Boca Raton, Florida.

Leighton, D.L. 2003. Studies to refine aquaculture technology for the purple-hinge rock scalllop, Crassadoma gigantea, and explore potential for its culture in Agua Hedionda Lagoon, Carlsbad, California. Final Report to the Resource Conservation District of Greater San Diego County. Contract No. 00-61.

Malakowski, M., 1986. Gametogenesis, induced spawning and hermaphroditism in rock scallops, Hinnites, from Humboldt Bay, California. Paper presented at the 16th Annual Meeting of the World Mariculture Soc., Reno, Nevada.

Monical, J.B. 1980. Comparative studies on growth of the rock scallop, Hinnites multirugosus (Gale) in marine waters of southern California. Proc. Natl. Shellfish. Assoc. 70: 14

Phleger, C.F., S.C. Cary and D.L. Leighton. 1982. Spat collection of the purple-hinge rock scallop on artificial collectors. Paper presented at the 1982 meeting of the Western Society of Naturalists.

Long Beach, California.

Phleger, C.F. and S.C. Cary. 1983. Settlement of spat of the purple-hinge rock scallop Hinnites multirugosus (Gale) on artificial collectors. Journal of Shellfish Research. Vol. 3, No. 1, 71-73.

Rhee, W.Y. and J.P. Davis. 1998. Larval survival of the rock scallop (Crassadoma gigantea) in the hatchery. World Aquaculture Society (Book of Abstracts), 1998 Meeting, Las Vegas, Nevada, p. 436.

Uki, N, and S. Kikuchi. 1974. On the effect of irradiated seawater with ultraviolet rays on inducing spawning in the scallop, Patinopecten yessoensis (Jay).

Bull. Tohoku Reg. Fish. Res. Lab., 34: 89