Genetic improvement in crustaceans: why prawns lag behind fish breeding

What the authors set out to explain#

Aquaculture production has grown steadily worldwide since 1990, increasing by around 10% per year according to the paper, with a large share of that growth coming from countries including China, Russia, Israel and the United States. That expansion has opened chances to lift output further through breeding, and the paper notes that larger finfish industries such as Atlantic salmon, channel catfish and tilapia, along with mollusc industries such as oysters, have already taken up genetic improvement programs as a route to higher productivity.

Crustacean farming, covering species such as prawns, yabbies, redclaw and marron, has not followed the same path. Jerry, Purvis and Piper, writing for the Association for the Advancement of Animal Breeding and Genetics proceedings, set out to describe why breeding technology has been slow to reach crustaceans, and to outline where genetic gains might still be captured. Their paper is a discussion piece rather than a new experiment, drawing on findings from fish breeding programs and the handful of crustacean selection trials that had been run by 2001.

This kind of stocktake sits alongside other work the same proceedings carried on identifying genetic opportunities in livestock species, including a later paper on novel phenotyping techniques for genetic and genomic predictions that faced a related problem of measuring traits that are hard to record directly on animals.

How the paper builds its case#

The authors work through the conditions that made breeding programs possible in fish, then test each one against the situation in crustacean farming. They point to Atlantic salmon as an example: husbandry and reproductive biology were already well understood when farming of the species began, but a further period passed, according to Gjedrem (1997) as cited in the paper, before enough infrastructure and knowledge existed to justify starting a genetic improvement program. Only after that point were breeding objectives defined and genetic parameters such as heritability estimated for traits including growth rate and age at sexual maturity in Atlantic salmon, citing Gjerde (1986).

Applying that same sequence to crustaceans, the paper identifies five linked obstacles: industry maturity, domestication status, the difficulty of defining breeding objectives, unreliable genetic parameter estimates, and the practical problem of identifying individual animals once they are moulting and growing together. Each of these is treated as a separate barrier rather than a single cause, and the paper argues that all of them need attention before structured programs can become routine in crustacean farming, much as they did for finfish species in earlier decades.

Industry structure and the domestication hurdle#

Some crustacean species, including prawns and yabbies, have been farmed for over twenty years, yet the paper says few industries have reached a stage where genetic improvement is treated as a priority. Most remain small and still developing, and farmers within these industries tend to work independently rather than cooperating at the operational level. That fragmentation, the authors argue, leaves the industry unable to carry the costs of running a proper breeding program.

A more basic problem affects species such as prawns, lobsters and mudcrabs: their lifecycles pass through several larval stages, each needing different rearing conditions, and many will not breed reliably once held in captivity because of unclear environmental triggers or stress. Current practice in these species often depends on wild pre-mated broodstock or wild juveniles stocked straight into ponds. Genetic improvement cannot proceed until the lifecycle is closed enough for progeny to be bred on-farm. The paper mentions that new advances in managing reproduction for several prawn species looked set to overcome this problem, citing personal communication from N. Preston.

Setting breeding objectives without market data#

Deciding what a breeding program should select for is harder in crustaceans than it first appears. The paper cites Henryon et al. (1999), who worked on the freshwater marron Cherax tenuimanus and found that lifting growth rate and the survival of juveniles raised overall profitability, while lifting the growth rate of broodstock, or of the claws, actually reduced it. That kind of economic modelling lets a breeding program be steered by long-term financial outcomes instead of assumption.

The trouble, the authors explain, is that most crustaceans reach the market whole, with price heavily shaped by non-genetic factors such as shell fouling or missing limbs, and there is little information on how buyers would respond to changes such as a crayfish carrying more meat in its claw. Without wide-ranging market research to build reliable economic weightings, most crustacean programs end up working toward plain goals, commonly overall size or the age at which animals become fertile.

A similar tension between production traits and overall profitability has been explored in sheep breeding, where the paper on current flock effects on lifetime reproductive performance examined how selecting for fleece and body traits plays out across a flock's reproductive life.

Why tracking individuals is so hard in genetic improvement in crustaceans#

A central technical problem for genetic improvement in crustaceans is that these animals shed their exoskeleton as they grow, so they cannot keep the external identification tags routinely used in fish and shellfish. Internal markers exist, including elastomer dyes, numbered tags and passive transponders, but the paper notes these cannot be used in programs that run alongside commercial farming because tagging leaves the animals unfit for sale.

Because individual identity disappears once animals are reared together, researchers need separate holding systems to keep family lines apart, and this brings its own complication: effects of the tank and effects of the mother become tangled together, making animal model estimates or variance figures drawn from half-sib groupings hard to pin down reliably. As a result, published heritability figures for crustaceans tend to rest on comparisons among full-sib families only, and the paper describes these as inflated by the influence of maternal effects. Unless newer tools such as elastomer tags or DNA-based pedigree tracking are applied, the authors say researchers must either accept skewed parameter estimates or wait for results drawn from a live selection program already under way.

The response-to-selection figures reported so far#

Despite these obstacles, the paper reports that the few crustacean selection trials completed by 2001 produced strong gains in growth rate. Jones and colleagues (2000) recorded a growth-rate response equal to 9.5% in each generation working with redclaw, the species Cherax quadricarinatus. Hetzel and colleagues (2000) reported an average direct response equal to 10.7% after a single generation of selection, comparing lines bred for fast and slow growth in the Kuruma prawn. The paper sets these figures beside the growth gains reported across many fish species by Gjedrem (1997), suggesting crustaceans could turn out to be capable of similarly rapid gains.

Three biological features are offered as explanations. Growth shows wide variation between individuals, with male Penaeus japonicus in tank trials showing a coefficient of variation for growth rate near 60%, according to Hetzel et al. (2000); a comparable spread was recorded between separate breeding lines of the freshwater yabby, a species named Cherax destructor, in unpublished data cited by the authors. Generation intervals in prawns, redclaw and yabbies typically run to a year or under, letting yearly gains build up. And strong reproductive output, which can stretch from several hundred young per Australian freshwater crayfish spawning up toward tens of thousands for a prawn, allows breeders to apply very intense selection pressure.

Design choices for breeding programs#

Given the difficulty of tracking pedigree, the paper argues that selecting whole families is usually impractical in crustaceans unless each family is held in a separate cage or DNA-based pedigreeing is applied, both of which call for infrastructure well beyond a typical commercial farm. Selecting on the merit of individual animals, often called mass selection, is therefore presented as the more workable option. The authors note that mass selection is often assumed inferior to family-based selection, but this need not hold where a trait such as growth rate carries moderate heritability, citing Falconer and Mackay (1996).

The main weakness of mass selection, the paper says, is the difficulty of keeping inbreeding under control without pedigree records. In species that produce very large numbers of young, such as prawns, where one female spawning can yield over 100,000 larvae, the best-performing animals chosen as broodstock could end up tracing back to only a handful of parent pairings, sharply cutting the effective breeding population. The authors conclude that programs working with such fecund crustaceans need to deliberately spread selection across enough distinct family groups each generation to hold inbreeding in check.

We think this is the paper's most practically useful point: the headline response-to-selection figures look encouraging, but they rest on a small number of trials, and the inbreeding risk the authors flag suggests early gains could be hard to sustain without deliberate family management as programs grow. The same tension between rapid early gain and long-term population structure has shaped later discussions of opportunities to breed for resistance to breech strike in Merino sheep, where selection intensity has to be balanced against maintaining enough genetic diversity in the flock.

Sources and further reading#

• The paper at AAABG proceedings (aaabg.org), free PDF
• AAABG proceedings online: the association's own site, where its proceedings are free to read
• Animal Genetics and Breeding Unit
• CSIRO research: Australia's national science agency

Questions#

— The Livestock Library team