Abstract
The long history of agriculture is basically a history of plant and animal domestication. Agriculture would not be possible without domestication and genetic improvement of crops and livestock. Shellfish aquaculture is relatively new and has relied on wild seed for much of its short history. Wild seed are unreliable and lack characteristics desired for aquaculture such as adaptation to high-density culture conditions, fast growth, disease resistance, and uniformity. The use of wild seed precludes genetic improvement, which is a major contributor to increases in agriculture production since 1960. Further and more rapid improvements of agricultural crops and animals are being made through advanced genetics and genomics. Shellfish species are cultured in open waters and face more environmental stress than terrestrial animals. Climate changes such as warming and acidification of world oceans present new challenges to shellfish and shellfish culture. More and more diseases and unexplained mortalities are affecting cultured molluscs. As cultured molluscs are either wild or have a short history of domestication, the need for genetic improvement is great. It is inconceivable that shellfish aquaculture can be sustained without serious efforts in genetic improvement. The development of hatchery production has made genetic improvement of molluscan shellfish possible. Currently, over 30 species of molluscan shellfish are produced in hatcheries and subjected to some degrees of genetic improvement or domestication. They include many species of oysters, scallops, mussels, clams and abalone. Once the life cycle is closed, hatchery production will inevitably lead to genetic changes with or without intentional selection. Genetics is the study of genes, genetic variation, and heredity of characters in living organisms. Extensive studies have been conducted on genetics of molluscan shellfish including simple mendelian inheritance of allozymes, inheritance of shell color and sex, population genetics, estimation of heritability, development of genetic markers and maps, identification of quantitative trait loci (QTL) and functional genes. Genomes and transcriptomes of many molluscan shellfish have been sequenced, yielding rich information about genes and molecular mechanisms underlie molluscan development, function, immunity and physiology. While these studies have improved our understanding of molluscan genetics and biology, most of them have not led to actual genetic improvements for shellfish aquaculture. Academic studies on molluscan genetics are not the focus of this chapter. Rather this chapter focuses on practical molluscan breeding and related studies with the goal of providing some guidance to shellfish breeders and hatchery managers.