The availability of spat is essential for the culture of commercially important bivalves. In countries where natural spatfall is poor, or entirely lacking in the case of introductions, artificial production of spat is extremely important. Most of the pioneering work on artificial scallop propagation was carried out in Japan, to compensate for the severe depletion of natural stocks.
Due to the natural abundance of Pactinopecten spat in more recent years, the policy towards establishing scallop hatcheries has changed. In the southern provinces of Japan there is, however, an interest in culturing two other species, namely C. nobilis and P. albicans. Both are valuable, but so far trial collectors have not produced enough spat to sustain commercial scale ongrowing.
In North America the trend has been to use hatchery raised animals as sources of spat (Duggan, 1973; Castagna, 1975).
Producing artificial scallop seedlings in hatcheries is a comparatively recent endeavour. A great deal of research is still required, particularly on the breeding behaviour and reproductive physiology of the species.
For example, the spawning period differs not only according to species but also to location, and ambient and environmental conditions at the time. In Japan, the sexually ripe adults of the giant scallop (P. vessoensis) in Mutsu Bay spawn naturally when temperatures are about 8°C (about mid-March); in north Hokkaido, however, gamete release occurs in mid- May.
In captivity, sexually mature adults can be induced artificially to release gonadal products by external stimuli. Kinoshita et al. (1943) first succeeded in inducing spawning by simultaneously increasing the temperature and alkalinity during the spawning period. In addition to thermal shock, it is common to stimulate further the spawning process by adding milt extracted from mature males. Spawning of the Bay scallop, A. irradians is achieved easily by thermal shock and addition of a sperm suspension (Castagna, 1975). Similar results have been reported by Illanes-Bucher (1987) on the Chilean scallop A. purpuratus.
Temperature shock is highly successful with most fully mature bivalves, and functions best when broodstock are correctly conditioned (Loosanoff and Davis, 1963; Costello et al, 1973; Lovatelli, 1985). Conditioning of the broodstock is also critical for production of good quality eggs, larvae, and subsequently spat (Helm et al., 1973). Conditioning bivalve broodstock requires maintainance of adults in an environment where both water temperature and food levels are carefully monitored. By manipulating these parameters, gonad maturation can be controlled. The length of conditioning time depends on the initial gonadal state of the scallops.
Other external stimuli commonly used for spawning are ultra-violet light and introduction of hydrogen peroxide at very low concentrations. Hydrogen peroxide activates the enzymatic reaction of prostaglandin endoperoxide synthesis involved in triggering spawning activity (Morse et al., 1977). The release of ova and sperm from adult scallop is considerable, and a fully mature female of P. vessoensis can produce up to 100 million eggs. In hermaphrodite species, sperm cells are usually shed first followed by the eggs; however, the two kinds of gametes are collected in different containers to avoid polyspermy. When the two gametes are released in the same spawning vessel, the eggs become enclosed in a mass of sperm. There is either no further development of the eggs, or, in a few cases where the amount of sperm present is not so great, development occurs but cleavage is aberrant. This produces clumps of disorganized cells which eventually disintegrate (Comely, 1972). Fertilization is usually initiated by adding 2–6 ml of sperm suspension per litre of egg suspension. Monitoring and management are necessary during this phase, as spawning may continue for several hours.
Fertilized ova of scallop are small (e.g. about 60 um for A. gibbus), and considerable care must be taken in handling. Embryonic development is rapid. Depending on species and ambient conditions, especially temperature, free swimming trochophores will emerge in 8–24 hours. The embryonic stages preceding the straight-hinge stage are most vulnerable to environmental conditions, but with proper management high survival rates can be achieved. Following fertilization, the eggs are gently and repeatedly washed through a suitable size sieve (40–60 um) to remove debris which accompanies spawning. Clean eggs are then transferred to larval culture units filled with filtered and often U.V.-treated seawater at temperature of 20–25°C. Larval rearing temperatures and stocking density are critical parameters at this stage of development. For larvae of the Bay scallop, A. irradians, the optimum temperature for development appears to be 26–28°C, with densities of 40 eggs/ml down to 5 larvae/ml prior to settling (Castagna, 1975); larvae of A. purpuratus are best reared at 23°C (Illanes-Bucher, 1987).
Water in the culture units is gently aerated to maintain adequate oxygen levels. There are no preferred sizes of rearing units, which may be as small as 20–60 l in volume. Larger rearing vessels are used in commercial hatcheries.
Typical phytoplankton cells used for feeding larvae and early spat include Monochrysis lutheri, Chaetoceros calcitrans, Isochrysis galbana, Phaeodactylum tricornutum, Dunaliella tertiolecta, Tetraselmis suecica, Rhodomonas sp. and Gymnodinium splendens. A mixture of two or more algae are usually fed to the developing scallop, as better results are produced than feeding with only a single species (Helm, 1977; Phleger et al., 1981). Water from the culture vessels is changed every 2–3 days to remove metabolites and dead larvae. Larvae normally settle within two weeks, although environmental conditions may affect timing.
The many problems involved in the propagation of spat in hatcheries are gradually being solved. One major problem remaining is finding a suitable large-scale method for rearing the spat once obtained from the hatcheries.
