Mizuhopecten yessoensis (Jay, 1857)

Common names: Yesso scallop, Ezo scallop, Giant scallop, Japanese scallop, Russian scallop, Primorsky scallop and Common scallop.

Yesso scallop M. yessoensis is a Pacific Asian low-boreal species (Fig. 9) of commercial value, the biggest of all Pectinidae. It occurs along the northern coastline of the Korean peninsula, the coastline of Primorye, near the shores of the Sakhalin islands, South Kuriles and Hokkaido and on the northern coastline of Honshu Island (Scarlato, 1981).

Total populations and biomass.
The most exhaustive studies of Yesso scallop were completed along the coasts of Primorye by Razin (1934), Biryulina and Rodionov (1972) and other authors (Markovskaya, 1951; Bregman, 1979; Kalashnikov, 1986; 1991), who also estimated their population and biomass in specific areas of the region.

In 1932, about 40 million scallops inhabited the area of 16-17 thousand hectares along the coast of Primorye. In 1940's-1960's, the scallop population in most settlements decreased and in some places the mollusk completely disappeared. From 1932 to 1959, the stocks in Peter the Great Bay had reduced thrice. In the following decade, the abundance remained almost the same i.e. about 5'703 million specimens on 906 hectares with biomass of 1'708 tons (Biryulina and Rodionov, 1972).

A significant drop in abundance of the scallops was also observed in the same period in the north areas of Peter the Great Bay (Olga Bay), where the scallop population in 1932 was four times as much as in 1975 (Silina and Bregman, 1986). In addition, according to V. A. Skalkin (1971), the biomass in Aniva Bay (southern Sakhalin) in 1969 was twice as low as compared to 1961-1962. The scallop population became ten times as low in Terpenie Bay (northern Sakhalin) and some areas of the Kuriles. Most investigators believe that intensive industrial fishing mainly caused the overall drop in scallop stocks. At present, commercial reserves of natural Yesso scallop along the coast of Primorye are exhausted because of irrational catching during the latest ten years.

Distribution in Primorye
Yesso scallop was widely spread in southern and middle Primorye occurs in bays and coves and forms aggregations at a depth of 6-30 m. Nevertheless these mollusks were an object of traditional catching in the region till the beginning of 1970's. The map (Fig. 10) one shows locations of existing and promising plantations for bottom cultivation of the scallop. Commercial settlements of this species were well known at Bays of Posjet, Ussuri, Amur, Vladimir, Vostok and Strelok. Average density of settlements at bottom grounds changed from 0.05 up to 1.0 specimen�m^-2 at average 0.1 specimen�m^-2. Average individual biomass was about 0.2-0.4 kg. However at present, average density does not exceed 0.001 specimen�m^-2 at sites described previously by other authors.

Distribution over depths
In shallows the scallop occurs at a depth of 0-5-1.0 m in small inlets inaccessible to wind and waves. Minimum depth corresponds to the winter time water level under the ice cover. Most of the scallops were found in bathymetric range of 4-10 m in closed inlets and additionally at a depth of 20-25 m in open and relatively deep-water sites of bays and inlets. According to Razin (1934) the scallop is found mainly at bathymetric range of 14-30 m at the open coastline of Primorye, however some specimens are at a depth of 48 m.

Most of the mollusks were found along the coastline at a depth of 20-25 m near rugged shores. This is apparently due to the corresponding range of spat subduction horizons in the region i.e. underwater rocks are substrate carriers for attaching scallop larvae. In Peter the Great Bay, the scallop was recorded at a maximum depth of 82 m (Scarlato, 1981).

Age structure of scallop settlements
Biologists believe that this species does not live more than eleven years on the average (Tibilova and Bregman, 1975) and that their real life expectancy ranges from 7 to 9 years (Makarova. 1985). The most extensively studied populations in Primorye have no 1 - 2 year old specimens. Eleven-to-twelve year-old scallops are quite frequent but older specimens are seldom (see Table 4). Some settlements include 70 per cent of specimens with age ranging from 8 to 16 years. Generally, the age structure of the scallop population of various regions reflects the randomness of replenishment and presence of abundant and non-abundant generations (Fig. 11).

Scallop growth
The fertilized scallop's eggs (about 60-70 microns in diameter) develop to grow into a larva, which precipitates onto the substrate (shell averages 260-285 microns) in 20-40 days, depending on temperature conditions. Definitive development of scallops terminates on the substrate. The scallop detaches from the substrate in 3-4 months when the shells� height is 10-30 mm. Subsequent scallop growth rate depends on temperature, feeding, water exchange and many other conditions on the sea floor.

The scallop shell grows isometrically to retain its initial form. L. G. Makarova (1985) calculated the general equation for the linear growth of Yesso scallop:

H_t = (160.92 � 18.7) � (1-e^{(-0.378 � 0.04)} � t),

where H_t is the shell height, mm; and t is scallops age, years.

The scallop grows at a temperature ranging from -2�C to +26�C. The optimal growth temperature is 4-6 �C. In Primorye, the temperature optimum is in May-June and in September-October (Silina, Pozdnyakova, 1986). Within the initial three years, the scallop height reaches 90-110 mm, then its growth slows exponentially. The largest specimens occur in settlements on silty-sandy soil in sites with good water exchange conditions and relatively stable temperatures approximately at a depth of 20 m. There they become 190-195 mm height and even longer at the age of 16 and older. In the South Kurile shallows specimens older than 20 with a shell height of 220 mm are met (Skalkin, 1966). In shallow silty inlets, scallops seldom exceed 150 mm and live not more than 10-12 years. In Posjet Bay, we recorded the largest scallop specimen, whose dimensions were as follows: shell height 222 mm, height 202 mm, and width 37 mm. The scallop mass varies proportionally to its linear dimensions (Fig. 12; Silina, Pozdnyakova, 1986).

The share of muscles in the scallop total mass amounts to 10-18 % in various settlements of Peter the Great Bay (Belogrudov, 1981).

Sex structure of settlements
The approximate (male : female) sex ratio in population is 1.0 to 1.0 practically in all the surveyed settlements. Hermaphrodites were observed in not more than 0.3 - 0.4% of all cases (Bregman, 1979). But in the same populations, males dominate in younger generation and females in elder ones. So, at the age of 1-3 sex ratio was approximately 2.0 to 1.0; at the age of 5-6 sex ratio was 1.9 to 1.0 and at the age of 7-8 sex ratio was 1.0 to 1.9. This could be due either to different death rate among both sexes or because of hermaphroditism.

Replenishment
Scallop populations replenish annually owing to spawning, subsequent development of larvae in plankton, their settling on substrate and juveniles� transition to free life on the seabed.

Spawning
Spawning in scallops' population starts at temperature range of 7-9 �C. (Belogrudov, 1981). In the water around Primorye, spawning begins in the middle of May, and spawning ends at the end of June. Spawning begins in shallow waters of southern regions; as the seawater warm up, specimens from the deeper and more northerly settlements begin to spawn. Absolute fertility varies from 25-30 to 180 million eggs (Yamamoto, 1964) depending on age and size.

Larvae morphology
The morphology of larvae and larval shell structure of three widespread scallops: Yesso scallop M. yessoensis, Japanese scallop Ch. farreri and Swift's scallop Ch. swifti were described by V. A. Kulikova with co-authors (1981). All of the larvae are of triangular form and the anterior end is the apex of the triangle. The larvae are inequivalve. The umbos are low, rounded and poorly defined. The taxodonte hinge has several teeth at each side of the hinge line. The central hinge area is undifferentiated. However some specific characteristics were distinguished among the scallop larvae of Peter the Great Bay. Their main differences are in the shell form, umbo form, shell size, number of teeth at each side of the hinge line (Kulikova et al., 1981).

Development in plankton
Duration of larvae growth in plankton lasts from 20 to 40 days (Kas'yanov et. al, 1980). Larvae disseminating current-induced distribution mechanism create new local settlements. Plankton surveys (Belogrudov, 1981) showed that larvae can form dense distributions in Posjet and Peter the Great Bays over several square kilometers. In that case, higher densities are noted at areas with abounding adult mollusks. In bonanza years, larval density reached 200-300 specimens per cubic meter in the closed inlets; at the same time, it did not exceed 20-30 specimens per cubic meter in the adjacent open inlets and bays. Long-term researches in Posjet Bay showed the absence of direct spatial relations between parents and new scallops generations.

On the eve of scallop cultivation, the number of sexually mature specimens in Minonosok Inlet in 1972 was 70'000. Ten years later, the number has increased up to 650'000 because of sowing culture (Fig. 13). The number of annually collected spat showed instability of larvae settling throughout all period. Nor was there any sign of increase in the number of spat during the year. Apparently, the parent-larvae relationship was indirect because of great dilution of larvae pool by water mass (Kalashnikov, 1986). Water exchange with adjacent bodies of water (about 10 % of waters every day is replaced by tidal) disturbed the parent-larvae relationships.

A comparison of the age composition (Fig. 13) and the results of spat collection in various bays and inlets showed that intensity of replenishment changes asynchronously. So, bonanza generations in one bay do not necessarily correspond to the same in another bay. For instance in 1980, the lowest spat settling (2-20 specimens per collector) was noted in Posjet Bay, but in Vostok Bay it amounted to 400-500 specimens per collector. It is of interest to note that at that year, red tides absent in Vostok Bay were observed in Posjet Bay. However these distinctions were also noted in other bays without red tides, and this showed that intensity of replenishment in some settlements was essentially a local characteristic.

Migration behavior
It is a well-known fact that the Yesso scallops can freely move along the sea floor. The mechanism of reactive movement of freely living scallop is widely known (for example: Dautov and Karpenko, 1983). As a rule, researchers note random movement in natural conditions, or individual scallop behavior in aquarium. Such evidence generally helps to form an idea that the scallop is a migrating species. But the fact that populations are present for many years in specific sites clearly showed that scallops ability to migrate is limited.

Risk factors
Abiotic factors. Survival rates of the pelagic larvae at metamorphosis depends on water temperature (ranges for survival is 5-20 �C, with an optimum of 10-15 �C), salinity (30-40�, optimum of 33�), density (optimum 8 specimens�ml^-1), quantity and composition of food and on abundance of predators (Belogrudov, 1973; Bregman and Guida, 1983; Chan, 1989). At attachment, mortality is caused mainly by the absence of suitable substrate and intolerance to changing environments (Yamamoto, 1964; Belogrudov, 1973). Yu. E. Bregman and G. M. Guida (1983) reported that the number of attached juveniles made only 5-8 % of all fertilized eggs. Settling on the bottom is the next critical period in the life history of the scallop, and the mortality increases sharply. According to G. Yamamoto (1964), only 5-10 % or sometimes none of the settled juveniles survive. A. N. Golikov and O. A. Scarlato (1970) reported that only 4% of the settled juveniles survived as long as 6 months. Only Golikov and Scarlato (1970) have reported on the mortality of older scallops; these researchers conjectured that after 6 months the quantity of survived mollusks decreased gradually. Moreover, they noted that winter was the period during which the highest mortality of scallops occurred off the northwestern coastline of the Sea of Japan. In contrast, in Honshu Island, the most difficult season for scallop survival is summer and early autumn when water temperature increases to above 15 �C (Yamamoto, 1960).

Recent investigation (Silina, 1996) also shows the highest mortality when scallop were less than 2 years while the mortality from 2 to 5 years of age is minimal. And quite the contrary, at 6-7 years of age (probably the beginning of the senile period of scallop development) and upwards to 9-10 (transition to the old-aged stage), scallops mortality increased sharply.

Storms. Storms are another factor, which causes mass deaths of scallops in coastal shallow settlements. Depending on the floor relief, they can increase settlement dispersity over vast flat areas or make them denser at the foothills of cliffs. In either case, a considerable number of mollusks are buried under moving soil. Near shallow coasts, particularly beaches, the majority of scallops die in storm debris. Trivial storms deform settlements; however, the loss is compensated by annual replenishment. While the impact of typhoons (annual frequency about 50 %) are natural calamities for the floor coastline population, including the scallop, for they destroy most of the species around open waters at about 20 m deep. By counting the number of shells in the breaker zone of only one beach following the typhoon "Ellis" in 1983, researchers revealed the simultaneous death of 10,000 specimens of different age. In a similar count in 1986 after the typhoon "Vera", we discovered as many as 72,000 specimens in debris (Kalashnikov, 1984). The joint effect of various factors on the sea floor population showed in perennial changes in density of artificial scallop populations, which regularly declined in all cases during the first cultivation season. The decline was greater in more open and unprotected waters (Fig. 14; Kalashnikov, 1985).

Predators. The first hours and days of life on the sea floor from the moment the mollusks become one year old (shell height about 30 mm) appeared to be the most dangerous ones. During this period, natural death is maximal and young scallops are preyed upon by various starfish species such as Asterias amurensis L�tken, which can grow up to 165 mm in a radius and weigh 450 g. Another species, Distolasterias nipon (D�derlein) is even larger growing up to 250 mm with a weight of 1000 g. These species attack the scallops, age of which is less or the same as that of starfish. One-time ration of one starfish changed from 1 up to 8.5 g of fish and annually amounts 400-450 g (Biryulina, 1972). In view of great abundance of these predators (density can amount up to 15 specimens�m^-2), the damage of population can be considerable. The death rate of young scallop in super dense aggregations (over 100 specimens�m^-2), in which predators temporarily eat only scallop, is especially high.

When storms are so strong that they reach the sea floor, scallops become weaker and readily accessible to predators. Joint effects of storms and starfishes have destroyed an artificial scallop settlement (about two hectares) with a population of over 200'000 specimens for several weeks. In stable conditions, starfish and scallops were noted to co-exist peacefully when they occupied a single habitat for several years in succession. Generally, however, the number of cultivated mollusks declines more in sites where starfish are more abundant.

After the sowing spat are preyed upon by various benthophages fishes such as flounders and bullheads. It is quite possible that at this period scallops are preyed by fishes rather than by starfishes. There are other predators of sown seed and adult scallops, which are of lesser importance but nevertheless, pose a threat to small seed and juveniles. These include the octopuses, some crabs, first of all King crabs Paralithodes camtschatica (Tilesius), and hermit crabs (Kalashnikov, 1986). Crabs predate mainly on seed scallops especially if present in large numbers, for example during seasonal migration, they can greatly denude newly seeded grounds. Some predatory gastropod mollusks pose a threat to both juveniles and adult scallops. E. A. Belogrudov (1973) reported that drilling Muricidae gastropod Boreotrophon candelabrum (Adams et Reeve) and Tritonia japonica (Dunker) could attack and eat the scallops. At the natural populations, about 14-27 % of adult scallops had drilling marks on the shells. D. D. Gabaev and N. K. Kolotukhina (1999) reported that two-year scallops (shell height up to 73 mm) in the cages are preyed upon gastropod Nucella heyseana (Dunker).

Parasites. In comparison to other cultured bivalves, such as ousters and mussels, little is known about the parasites and diseases of scallops. Also epizootic diseases, like those that have devastated the ouster culture industry in parts of the world, have not been encountered by the scallop culture industry. The relative lack of information on parasites and diseases in scallop may be attributed to the shorter of intensive mussel culture, and comparatively fewer investigations. Moreover, insufficient data on scallop parasites indicate the poorness of parasitic fauna and the low vulnerability of the scallop. Infectiousness of scallop by parasites is quite low, as parasitic fauna is scanty and presented only by potentially pathogenic species. Nor was mass scallop death caused by parasites ever recorded. Now it is known about 17 parasites and commensales in scallops (Kurochkin et al., 1986; Kovalenko, 1990; Plyusnin, 1990; Rakov, 1990; Didenko, 1996).

1. Sirolpidium zoophthorum Vishniac, 1955 (Lagenidiales).
   This fungus was found in 1972 in juveniles at scallop farm in Posjet Bay.
2. Myxosporidia gen. sp. (Myxosporea)
   Local pestholes, which probably were caused by unknown Myxosporidia, were noted at scallop farm in Minonosok Inlet (Posjet Bay) in April of 1996.
3. Perkinsus sp. (Sporozoa)
   They were unknown until 1979 when a spherical cyst (0.2-03 mm in diameter) of Perkinsus sp. was found at 86 % of discovered scallops (intensity of invasion by 1-2 cysts). Occasionally they can pose serious threat for scallops' spat. Conceivably it was introduced with scallop seed from Aomori prefecture, Japan.
4. Pectenita golikowi Jankowski, 1973 (Ciliophora)
   Approximately all scallops are affected by this endoparasitic infusoria. Intensity of invasion is several tens specimens in scallop intestines. Any kind of pathogenous action on the host is unknown.
5. Trichodina pectenis Stein, 1974 (Ciliophora)
   See below.
6. Trichodina sp. Stein, 1974 (Ciliophora)
   These two species of endoparasitic infusoria were described out of mantle of Yesso scallop. Extensity of invasion is 20-100 % and intensity by several tens specimens. Any kind of pathogenous action on the host is unknown, but probably it can be caused by a secondary parasite.
7. Cliona sp. (Porifera)
   This drilling parasitic sponge affected more often the lower (right) valve and less often upper (left) one. Extensities of invasion are up to 70 % on lower valve and up to 10 % on upper one.
8. Hirudinea gen. sp. (Hirudinea).
   Leeches were found in mantle as isolated instances
9. Podocotype spp. (Trematoda)
   Approximately 1 % of all scallops is affected by trematode metacercaria, which were found in various tissues (including adductor). Intensity of invasion is only one specimen in scallop body.
10. Anisakidae gen. sp. (Nematoda).
    Larvae of these nematodes were found in digestive system as isolated instances
11. Ohridiidae gen. sp. (Nematoda).
    Ohridiidae affects approximately 73 % of scallop. Larvae of these nematodes were found in mantle with intensity of invasion up to 17 specimens.
12. Polydora ciliata (Johnston, 1838) (Polychaeta)
    This widespread and well-known drilling polychaete is responsible for loss of market quality among cultivated and natural Yesso scallops. It affected more often the upper (left) valve. The burrows excavated by Polydora in scallop shell cause unsightly blisters containing compacted mud. Approximately 75 % of scallops have wormholes and blisters. Since 1974 infestation of scallop shells by the boring polychaetes is increasing as siltation of the bottom increases (Silina et al., 2000).
13. Polydora websteri Hartmann, 1943 (Polychaeta)
    Burrows and blisters of this species are indistinguishable from ones of P. ciliata.
14. Dodecaceria concharum Oersted (Polychaeta)
    These drilling polychaete have wormholes similar with ones of Polydora. They can use old holes, created by other polychaetes and sponges.
15. Herrmannella longicaudata G. Avdeev, 1975 (Copepoda)
    These small (maximal length up to 2.2 mm) cyclopoid copepods were found approximately at 73 % of Giant Yesso and Swift scallops. Any kind of pathogenous action on the host is unknown, even in the case of great abundance. Average intensity of invasion by commensales is nine specimens in mantle.
16. Odostomia fujitanii Yokogawa, 1927 (Gastropoda)
17. O. (Evalea) culta Dall et Bartsch, 1906 (Gastropoda)
    These small (maximal shell height up to 5 mm) littoral gastropods often prey on various bivalves as temporary parasite. Gastropods feed with using a long proboscis, which they introduce between shell valves.

Four of 17 pathogens found in scallop are shell drillers. Other twelve species may be true parasites. Only three species of them such as Sirolpidium zoophthorum, Myxosporidia gen. sp. and Perkinsus sp. can pose the serious threat for scallops, however they were found only once. In addition, others caused no noticeable pathology in normal conditions. Moreover, the scallop has no parasites that could endanger humans.

Bacterial contamination. According to recent investigations (Avdeeva and Filipchuk, 1988; Kovalenko, 1989; Plyusnin, 1990; Plyusnin and Cherkashin, 1991; Kovalenko, 1994), except of these 17 pathogens, numerous species of bacterial contaminants have been identified from cultivated scallops (Table 5). Totally 29 species of contaminants were identified on scallop farms. Gram-negative bacteria present the most part of them (22 species). Some of them, as specimens of Aeromonas, Vibrio and Pseudomonas, can be a conditionally pathogenous. These bacteria may be pathogenic in situations where environmental conditions are poor.

Epibioses. Natural and farmed scallops are an excellent substrate for the settlement of many other organisms, which are collectively termed fouling. Sea organisms, which occur on scallops' shells, can be competitors for space and food. Furthermore, epizoans can be an additional barrier that reduced flow and food accessibility.

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