Marine Invertebrates of Bermuda
Bermuda Sand Scallop/Zigzag Scallop (
Euvola (Pecten) ziczac
Mamie V. Wise
Dr. James B. Wood - Editor
is known by many names. Previously, its scientific name was
, however most current literature lists both
for clarity. Like other scallops, zigzag scallops bear the characteristic two-valved calcium carbonate shells that are rounded along the outer edges and flattened at the bottom near the prominent hinges. On either side of the hinge are projecting “ears” or auricles that contribute to scallops’ distinctive shape (Gosling 2003). In Bermuda zigzag scallops commonly grow to 120 mm but they are generally not as large in the Caribbean (Sterrer 1992, Abbott and Jensen 1967).
Zigzag scallop shells show a wavy, crenulated pattern along their outer edges and have several colored rays varying from white to orange, yellow, or gray (Sterrer 1986). Within this pattern are well-defined annual rings which make determining a scallop’s age relatively easy to the trained eye (Gosling 2003). The zigzag scallop’s lower valve is somewhat cup-shaped whereas its upper valve forms a flat to concave lid (Sterrer 1986). It exhibits a zigzag pattern of stripes on their shell which gives the species its name (Bermuda Biological Station for Research 1998). Interestingly, it also moves in a zigzag pattern when jetting (Sterrer 1992).
Zigzag scallops in particular have a series of bright blue eyes along the edge of their mantle (Sterrer 1992). These eyes, called ocelli, are sensitive to changes in light intensity, and will signal the animal to close its shell if they sense a change in shadows or another nearby disturbance (Gosling 2003). The scallop will also close its shell if it is exposed to the air or mildly threatened (Gosling 2003). Surrounding the ocelli are small sensory tentacles which line the conspicuous inner fold of the mantle. These serve to regulate water flow into and out of the animal (Gosling 2003).
Zigzag scallops have been found historically in several areas around Bermuda, including Harrington Sound, where they were once a popular catch among recreational fishers (Sarkis 2002). Overfishing through the 1970s decimated the population to the point that a series of 1987 SCUBA surveys around the island discovered only two scallops at two different sites (Sarkis 2002). Besides Bermuda, zigzag scallops can be found off Cape Hatteras, North Carolina, throughout the Gulf of Mexico and the Caribbean, and as far south as the state of Santa Catarina, Brazil (Lodeiros et al. 1996).
Zigzag scallops inhabit shallow waters between two and seven meters near the shore and form beds in sandy areas (Dame 1996, Sarkis 2002). Scallops as a group are specially adapted to the temperature and salinity fluctuations that are part of life in the intertidal zone (Dame 1996). Individuals bury themselves with sediment except for their tentacles, which project above the sediment and are usually the only part of the animal visible to observers (Velez et al. 1995). Their concave upper valves hold a thin layer of sediment that is thought to prevent fouling organisms from colonizing (Lodeiros and Himmelman 2000). Given the chance, a variety of fouling organisms will colonize scallop shells, including: algae, barnacles, polychaete tube worms, sponges, hydrozoans, bryozoans, and even other mollusks (Gosling 2003).
Like other bivalve molluscs, Bermuda scallops feed by passively filtering plankton and organic matter from the surrounding water (Jorgensen 1990). They are primarily herbivorous and subsist on phytoplankton, although they have been known to inadvertently consume small planktonic animals as well (Dame 1996). Scallops have even been known to orient themselves in relation to surrounding water currents, however the advantages of such behavior are still being investigated (Jorgensen 1990). Because of their submerged, sand-dwelling lifestyle zigzag scallops in particular extend their tentacles out of the sediment so they can continue to filter feed even when the rest of their bodies are buried in the sand (Velez et al. 1995).
Although they have several adaptations to avoid predation including tightly closing shells and an escape response, scallops remain vulnerable to predation by several different species. Sea stars are the most significant of the scallop’s predators. They employ their numerous tube feet to slowly pry apart the valves of the scallop’s shell (Gosling 2003). Crabs, lobsters, gastropods, sea anemones, and octopuses have also been known to prey on adult and juvenile scallops (Gosling 2003). Larval scallops are more vulnerable to predation during their planktonic stage than adults and juveniles because they can be ingested by a variety of filter feeders including sea squirts, barnacles, and sometimes even adult scallops and other bivalves (Gosling 2003). Other predators of scallop larvae include ctenophores, anemones, and the larvae of other taxa especially fish, crustaceans, and echinoderms (Gosling 2003).
Juvenile and adult scallops are unique among bivalves in that they have a well-developed jumping and jetting response to protect them from predators. When sufficiently disturbed scallops repeatedly clap the valves of their shell together using their well-developed adductor muscles. This shoots strong streams of water out of their shells and allows the scallops to move by jet propulsion (Dame 1996). In fact, several distinct types of jetting motions have been observed in scallops including swimming movements, twisting, and somersaulting end over end (Gosling 2003). Their distinctive shell shape as well as surrounding currents help to make this a rather effective means of escape for scallops including
Zigzag scallops and related species can be afflicted with several problematic parasites including flukes, tapeworms, and thread worms which have been shown to reduce growth rates and fertility among scallop populations (Dame 1996). Perhaps the most harmful of these are polychaete annelids of the genus
(mudworms) that form burrows on the edges of the shells of scallops, as well as those of oysters and mussels (Dame 1996). These annelid worms surround themselves with a layer of mud and their scallop hosts respond to the irritation by secreting a shell layer around them, which protects the worm but greatly taxes the scallop (Dame 1996). Only in extreme cases will a
infestation kill a scallop outright, however such parasites weaken the individual both structurally by damaging its shell and physiologically by using energy which could otherwise be used to sustain the scallop. Together these effects make the scallop more susceptible to predation by other species (Dame 1996).
Zigzag scallops and other bivalves are also vulnerable to a multitude of pathogens, however diseases in scallops are relatively well-understood. It was been claimed that “more is known about the diseases of bivalves than of the diseases of all other marine invertebrates combined” (Dame 1996). A variety of viruses including the herpes virus as well as retrovirus and papillomavirus groups have been known to infect bivalves. Interestingly, each of these infects a particular tissue within the bivalve (Dame 1996). Scallop larvae are especially vulnerable to bacterial infections. Fungi and protozoans have been known to infect bivalves at multiple life stages. Like other species, scallops that live in high concentrations are more susceptible to disease than populations that are more dispersed (Dame 1996). By this standard, zigzag scallops may be slightly less susceptible than other scallop species because they are usually found in relatively low densities (Lodeiros et al. 1996). This could prove to be an important advantage for intertidal ecosystems because epidemic diseases that infect filter feeders can alter the interrelationships of species within the entire ecosystem (Dame 1996).
Zigzag scallops are a hermaphroditic species which reproduces through broadcast spawning (Brokordt et al. 2000). They are known as “dribble spawners” in Bermuda because, while the local population spawns around the same general times of year, there are no synchronous mass spawning events. Instead, individuals spawn at slightly different times within the same season (Owen et al. 2002). Spawning usually occurs twice annually, although exact spawning periods vary by region (Brokordt et al. 2000). Small D-shaped planktonic larvae develop within two days of fertilization and feed on phytoplankton (Bermuda Biological Station for Research 1998). Overall, the larval stage lasts approximately twelve days, after which the larvae settle and become juvenile scallops, which are known as “spat.” Spat have the same general body form as adults, and show a wide variety of colors but will darken as they mature (Bermuda Biological Station for Research 1998).
Considerable research is currently underway or has recently been completed involving zigzag scallops. One recent study has involved energy allocation, specifically the balance between reproduction and locomotion. Along with seasonal reproductive cycles, zigzag scallops undergo a seasonal cycles of energy usage and storage (Brokordt et al. 2000). The adductor muscle serves in locomotion and valve movement, but also it stores glycogen and protein for use in either movement or reproduction. Consequently, the scallop must strike a balance between locomotion and the production of gametes (Brokordt et al. 2000). One study found that the maturation of gonads increased the time that zigzag scallops needed to recover from strenuous exercise (Brokordt et al. 2000). This study could pave the way to a more thorough understanding of bivalve muscle dynamics and energy efficiency.
A related study by J.E. Perez et al. addressed precisely how scallops can display such strong sudden periods of muscle contraction (Perez et al. 2000). Already it is known that they require higher rates of instantaneous ATP synthesis than other bivalves, but the exact mechanisms with which they meet this need are not yet understood (Perez et al. 2000). The role of genetics in these chemical pathways is also being studied more closely (Perez et al. 2000).
Other studies about zigzag scallops have been completed recently at the Bermuda Biological Station for Research. One such study addresses the use of zigzag scallops to measure an environment’s exposure to anthropogenic pesticides. Specifically, it explores the use of hemolymph cholinesterase inhibition to develop a program of periodic environmental sampling that does not require scallop sacrifice (Owen et al. 2002). This study used Rabbit Island in Harrington Sound as well as Agar’s Island in the Great Sound as its Bermuda field sites. The method that was developed could potentially be used to assess environmental impacts of industrial and agricultural activity both in Bermuda and elsewhere, and could possibly be applied to other species of scallops (Owen et al. 2002).
Scallops are very significant commercially as an important part of the global seafood industry. Bermuda alone imports an estimated 50,000 pounds of scallops a year, almost all of which are frozen (Sarkis 1999). Globally, almost one million tons of scallops are cultured every year (Gosling 2003). Until recently, zigzag scallops had not been cultured, mainly because their low densities usually cannot support commercial fisheries (Lodeiros et al. 1996).
In 1987, a doctoral student named Samia Sarkis at the Bermuda Biological Station for Research began work on a scallop aquaculture program involving
Euvola (Pecten) ziczac
as well as the calico scallop
(Bermuda Biological Station for Research 1998). After considerable research about both species and securing private grant funding, Sarkis created a 200 square foot hatchery capable of producing 200,000 spat every two months.
The resulting aquaculture process began when adult scallops were induced to spawn using a thermal shock method. After fertilization, eggs were distributed into tanks for development into larvae (Bermuda Biological Station for Research 1998). Interestingly, the research team cultured algae in sterile conditions to feed to the growing larvae (Acton 2003). The larvae settled on polyethylene mesh and until they reached a given size. The resulting spat were then transferred into nets suspended in the sea where they were grown until adulthood. The entire growth process took approximately one year from egg to adult (Acton 2003).
Zigzag scallops proved to be an interesting species to culture for several reasons. Most scallops grow more rapidly when cultured in suspended nets rather than nets along the sea floor, although this is not the case for zigzag scallops. Moreover, individuals cultured in cages partially covered with sediment grew even faster than those in cages on the sea floor (Velez et al. 1995). Zigzag scallops’ smooth and convex lower valves make them especially well suited to aquaculture because they allow them to slide easily along the bottom of nets and cages. On the other hand, the edges of their shells are more prone to damage than those of another commonly cultured species
(Freites et al. 1999). In general, cultured zigzag scallops are most successful when grown in relatively low-energy areas like bays and inlets, but with this low energy comes an increased risk of contamination and sudden changes in salinity as a result of freshwater influx (Freites et al. 1999).
In 2001 and 2002, the BBSR program averaged 80,000 scallops each year (Sarkis 2002). In was so successful that it was able to sell scallops to local restaurants, which commonly sold one hundred per day or more (Acton 2003). Not only are the locally grown scallops good sellers, but a recent survey showed that 70% of Bermuda restaurant-owners preferred Bermuda scallops over imports. Moreover, the scallops can go from the net to the table within the same day, which certainly appeals to both vendors and consumers (Sarkis 1999). Even though the BBSR program was a success, it was not designed to be a commercial venture. After fulfilling its goals, the program was concluded in 2003, although it may not have marked the end of scallop aquaculture in Bermuda. One Bermudian fisherman purchased juvenile scallops in hope of continuing zigzag scallop aquaculture on the island (Sarkis 2002).
It is possible that scallop aquaculture may have a future in other areas as well. A private farm in the Turks and Caicos Islands has pursued a culture trial with juvenile scallops from BBSR (Sarkis 2002). In addition, the Food and Agriculture Organization of the United Nations has put forth a proposal for a scallop aquaculture program to be created in Cuba and it is slated to get underway in 2004-2005 (Sarkis 2002). Considering the success of the Bermuda pilot program, it would not be surprising if even more zigzag scallop aquaculture programs were developed in the years to come.
While not a commercial importance in the strictest sense, Wolfgang Sterrer points out that colorful ribbed shells like the zigzag scallop’s are “a familiar sight on certain gas stations,” and thus are recognizable at least because of their shape (Sterrer 1992).
The zigzag scallop is subject to both fisheries and customs laws in Bermuda and is officially categorized as a “protected species.” The taking of any individual from within the Bermuda exclusive economic zone is explicitly prohibited by the “Bermuda Statutory Instrument Fisheries (Protected Species) Order of 1978” which was made under the
Fisheries Act of 1972, Title 25 Item 8 Section 5
(Laws of Bermuda). This law was adopted on April 1, 1978. Any individual found in violation of this law is subject to imprisonment for up to one year, a fine of $5,000 or both. This law does not apply to holders of permits for scientific research or conservation purposes (Laws of Bermuda).
Bermuda scallops also subject to restrictions of the Bermuda Customs Tariff. It is illegal to transport zigzag scallops into or out of the country of Bermuda, either whole or in part, except with a license from the Department of Environmental Protection. This prohibition also applies to zigzag scallop meat in any form (HM Customs Bermuda).
Securing sustainable food sources both from marine and terrestrial sources has been a long-time interest of mine, so I was first interested in the zigzag scallop because of the aquaculture program that was developed in Bermuda. I had been briefly exposed to fish aquaculture but was interested about aquaculture in shellfish. I was especially intrigued by the idea of shellfish aquaculture because of the periodic outbreaks of shellfish poisoning. It seems that if done with ample research and consideration for environmental impacts shellfish aquaculture programs like the one piloted with zigzag scallops could potentially be used to harvest safer yields of shellfish. By implementing closed containment systems while carefully controlling plankton inputs, many of the devastating outbreaks of shellfish poisoning as a result of harmful algal blooms and other events might be avoided at some point in the future. Not only could this bolster shellfish sales, but also help consumers to feel more secure about eating shellfish. While this is certainly just a theoretical application of shellfish aquaculture, in my mind, its industrial potential certainly warrants more investigation. In response to the Bermuda aquaculture program, I was encouraged after reading of the program’s success, especially considering the limited budget and facilities allocated for the project as well as the fact that this particular species had never before been cultured. It attests to the versatility of aquaculture as a whole and its adaptability to a variety of species.
Having grown up in the Tampa Bay area, I was also interested in learning more about a species that is found both in Bermuda and off the coast of Florida. In my area, shellfish are not something you hear much about except during outbreaks of shellfish poisoning or when other problems arise. It was intriguing to read more about how shellfish fit into marine ecosystems during normal conditions.
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BBSR Aquaculture Project Shows Promise
Farming the Sea: Bringing Back Bermuda's Scallops