Current Research

Triploid and Tetraploid Oysters

Oysters that are naturally occurring in the wild, have two sets of chromosomes in their cells, and are also known as “diploid oysters”. In this aspect, oysters are similar to humans, which are also diploid, because they also have two sets of chromosomes; one set from each parent. In contrast, the triploid oysters have three sets of chromosomes and therefore they are sterile, because meiosis, a type of cell division in reproduction that reduces the number of chromosomes in gametes, cannot occur in animals with odd chromosome sets.

The advantage of sterility is energy that would typically be used for reproduction will be saved and used for growth; therefore they have faster growth and superior meat quality than diploid oysters. In Maryland, more than 90% of growers using cage or bag-based oyster aquaculture use triploid oysters. Additionally, the advantage of faster growing triploids can partially mitigate the impact of low salinity. The triploids (sterile oysters) do not exist naturally, they are produced by mating male tetraploid oysters (four sets of chromosomes) with diploid females and is common practice within the industry. However, tetraploids are more difficult to produce and there fore However, tetraploids are super hard to make, resulting in the tetraploids are very rare and precious in the market. In addition, the triploids have been recognized that more susceptible to mortality than diploids under stressful environment, such as acute low salinity. The triploids’ growth and survival can be further improved by using superior tetraploids and (or) diploids.

Funded by Morgan State University Office of Technology Transfer and Maryland Sea Grant, PEARL aquaculture team are working on developing triploid and tetraploid oysters derived from Maryland native oyster stock, which is supposed to be more adapted to Maryland local environments. The team also created several diploid oyster lines focusing on low-salinity tolerance, fast-growing, and disease resistance traits, and are making continuous genetic improvement on these lines, through which to further enhance the triploid oysters' performance in low salinity waters.

Genomic selection

There are millions of DNA variations on organisms’ genome between individuals even if the siblings. There are some associations between these DNA variations and organisms’ traits. By recognizing the association, people can predict the animals’ parents (also known as broodstock in aquaculture) used for breeding which have the higher possibility to produce the seed with the desirable trait without measuring the broodstock’ traits because sometimes the trait is complex and hard to measure. The selection based on the DNA variations across the whole genome is called genomic selection. It has been widely used in plants and livestock, through which greater genetic gains for a trait can be made within short breeding period.

In a 12-institute collaborative project led by Rutgers University and funded by Atlantic States Marine Fisheries Commission, Dr. Ming Liu and PEARL aquaculture team are working on uncovering the associations between genetic variations and the traits of growth and survival in low salinity waters, developing genomic selection models to assist breeding and evaluating the effectiveness of the models. Once validated, it could shorten the breeding cycles of producing a low salinity oyster line. This is also useful for fast genetic improvement on spat-on-shell that are used in oyster restoration or on-bottom culture, where the spat are usually bred from wild oyster stock to avoid reduced genetic diversity to wild populations.

The Soft-Shell Clam Programs

The soft-shell clam Mya arenaria, native to Northeastern seaboard and known vernacularly by a number of appellations (e.g. steamers, long necks, manoes) is a good candidate to provide diversification for Maryland aquaculture, because it can grow and reproduce in low salinity waters, and historically supported a commercially important wild fishery in Maryland. With the support of Morgan State University Office of Technology Transfer and Maryland Sea Grant, in 2022, the aquaculture team successfully demonstrated large-scale breeding of Maryland wild soft-shell clams within a shellfish hatchery setting, establishing a first milestone toward developing this species as a potential aquaculture product. Several different subtidal culture gears and methods focusing on protecting the clams away from the predators and enabling the growth were developed and are being tested as well. An obstacle that remains unaddressed in the viability of commercial culture is the matter of adapting the species to better cope with heat stress that can lead to excessive mortality; prolonged exposure to seawater over 30°C (86°F) may prove lethal to this animal. Therefore a selective breeding to improve the heat tolerance and growth speed of this species is being taken, which may be helpful to culture Mya to market size within a timeline that minimizes the risk of heat-induced mortality. Additionally, the outcomes of this research could also furnish insights into prospective avenues for rebuilding/restoring diminished wild populations.