3.26.2015

TRIPLOID OYSTERS // the local food report

Like humans, most oysters have two sets of chromosomes (diploid). But what happens when instead, they have three?


You might expect disaster. After all, triploidy happens occasionally in all species, and as we know all too well, for people, it's almost always fatal. But for oysters (and amazingly, all kinds of other foods—think bananas and seedless watermelons), something different happens. The animal doesn't die. In fact, it does the opposite. It grows really, really fast. 

The thing is, regular diploid oysters put a lot of energy into reproduction. Starting in their second year, anytime the water temperature climbs above about 60 degrees F (mid-May through mid-September on the Cape), they're working to spawn. They can spawn multiple times over the course of the season, and each spawn requires a tremendous amount of energy. It can also affect their meat, making it watery and somewhat tough. On this traditional schedule, it takes 3 to 4 years for an oyster to grow to market size.

But triploids aren't worried about spawning. Which means that all the energy a diploid uses to spawn, a triploid can put into growing. Andrew Cummings, whose triploid seed you see up there, says his triploids reach market size in 18 months. In addition to being an economic boon, especially during our busy season, this also helps with disease management. Most diseases around here are cumulative, which means they take time to kill an oyster. The less time oysters are on the grant, the less time they have to get sick.


Andrew is one of the first local oystermen to embrace triploids. In other parts of the country—Virginia, for example—as much as 90 percent of hatchery seed sold is triploid. It's popular. But here, our our wild industry is still so robust that very few people are growing triploids. Because they grow so fast, they require extra handling, which means extra work. 

Also, the science of the process is fairly complicated. It took me an hour long conversation with Andrew, nine emails, a phone call to our local hatchery owner, and a phone call to the inventor of the process to understand exactly what goes on. 

To start with the basics, oysters are broadcast spawners. This means they send sperm and eggs out into the water column, these meet, and make a new organism. Sperm and eggs are both haploid (one set of chromosomes), so when they meet, they make a diploid organism.

To get a triploid, you need to spawn a tetraploid (four sets of chromosomes) with a diploid. There are only a few labs in the world producing tetraploid oysters, and they're made by interrupting regular meiosis using a variety of different techniques ranging from heat to cold to chemicals. Labs interrupt meiosis first in a diploid to get a triploid, and then in a triploid to get a tetraploid. 


This sounds confusing, because the whole premise of triploid oysters is supposed to be that they're sterile. It turns out that rarely, a triploid oyster can produce eggs. But they produce maybe 50,000 to a diploid oyster's 20 or so million—not enough to make the oyster "ripe" and trigger spawning. In other words, these animals have eggs, but they're not fertile. They have to be spawned surgically, in a lab, in order to get a tetraploid.

The hatchery on the Cape, ARC (the Aquaculture Research Center in Dennis), keeps about 20 male tetraploid oysters on hand and spawns these with female diploids that growers bring in for brood stock. I talked with the owner, Dick Kraus, and he said only about 10 to 15 percent of seed sold on the Cape right now is triploid. 


If you love oysters, it's likely you've eaten a triploid at some point. Besides the fact that they tend to appear big and exceptionally healthy, they don't look any different from a diploid oyster. I'm curious to hear—what do you think? And if you have questions, fire away. I asked quite a few, so hopefully I have an answer.

Photo credits for this post go to Ralph Alswang. Thank you to Andrew Cummings for sharing.

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All text, photographs, and other original material copyright 2008-2010 by Elspeth Hay unless otherwise noted.