Aquaculture  - Biotechnology topics

Sea scallops (Placopecten magellanicus) have long been an important commercial species in the Atlantic fishery. They are marketed for their large adductor muscle, better known to seafood lovers as ‘the meat’. Because they are such a high-value product, however, some scallop populations have suffered from heavy fishing pressure. This has led to increasing interest in sea scallops aquaculture with commercial hatcheries and grow-out sites now in operation in Nova Scotia, Prince Edward Island, Newfoundland and Quebec.

What is the issue?

The quality of scallop meat varies with the scallop’s reproductive cycle. From April to September, scallops mature and spawn (release their eggs or sperm). The weight of the scallop’s reproductive organs increases up to spawning, but the quality of the scallop meat decreases, and the adductor muscle becomes watery and stringy. This lowers the market value of the meat, as well as the meat count (number of meats per lb./Kg).

Researchers at Fisheries and Oceans Canada (DFO) are working to improve the quality of scallop meat by developing triploid scallops for aquaculture. Triploid scallops have three sets of chromosomes (the threads of DNA that carry genetic information) instead of the two sets found in normal (diploid) scallops. The extra set of chromosomes reduces or prohibits viable development of eggs or sperm.

Scientists believe that sterile scallops should have a better market value for two reasons. First, the energy that would be used for reproduction could be diverted to growth and weight gain. Second, the quality of sexually immature scallops is often higher than that of sexually mature scallops, thus, triploid scallops that do not mature sexually, should also be of higher quality. An additional benefit is that triploid scallops should reach market size in less than two years, rather than the two and a half to three years it currently takes under good growing conditions.

The research plan

DFO researchers will produce triploid scallops, using two general methods. The first method is to expose fertilized scallop eggs to chemical, heat or pressure treatments which makes the egg retain a ‘spare’ copy of the egg’s chromosomes (the ‘polar body’) so they end up with three sets of chromosomes (triploid) instead of the normal two. The second method is to mate diploid scallops with tetraploid scallops. Tetraploid scallops have four sets of chromosomes (produced by suppressing polar body formation in eggs that are occasionally produced by triploid scallops[1]). Tetraploids, unlike triploids, are fully fertile, so mating them with diploids produces triploids.

Once the scallops are produced, researchers will use genetic testing to confirm that they are indeed triploid (see techniques described in ‘Biotechnology to help Protect Wild Salmon Stocks - The Triploid Approach’). The scallops will be sent to industrial aquaculture partners who will place them in grow-out systems alongside diploid scallops to compare their growth performance under field conditions.

The triploid scallops will also be evaluated to see if the way they were produced (through chemical, heat, pressure and tetraploid techniques) has any effect on their larval survival, metamorphosis from the swimming larval stage to the shelled bottom-living stage, and growth performance under field trial conditions.

Benefits of this research

This research should contribute positively towards development of:

a higher-quality year-round scallop that takes less time to reach market weight will improve the competitive position of Canada’s scallop aquaculture sector; and,
better methods of scallop aquaculture will result in less pressure on wild scallop stocks, and therefore, better conservation of those stocks.
 

[1] Although triploidy is close to 100% effective in most animals, some egg-producing cells in molluscs may occasionally revert to the diploid state and produce viable eggs.