Love is in the Water

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Local students are not the only ones excited for the summer.

Around the same time our young people start their summer break,

water temperatures begin to rise which signals the Lynnhaven Oyster spawning season.

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Oysters have a remarkable ability as sequential hermaphrodites, changing their sex throughout their lives to spontaneously respond to environmental conditions. Population density, age, temperature, salinity and food availability all are factors that influence this sex change. Sperm is identified by leaving the oyster in a steady stream (on left) whereas eggs are pumped out in clouds.

The oyster's motionless

"mating dance"

Love is in the water, and our native oysters are getting ready to celebrate. When the water temperature exceeds 50ºF, adult oysters cease active feeding and use the energy stored from their late summer and fall feasts to produce sperm and/or eggs. That's right, oysters are sequential hermaphrodites, capable of changing their sex throughout their lives. Once the water reaches 68º-75ºF, oysters begin to display their gender through distinct differences in sperm and egg distribution. (see picture/caption)

For those who like to eat oysters, the spawning season presents a challenge: oysters full of sperm and eggs which turns the oyster creamier to the palate. But after the spawn, all of that creamy material is gone and what is left is a wispy and translucent oyster. Not necessarily bad for you, just unappetizing. This is when wild oysters are officially "out of season".

And people have known this for a long time. The Roman Empire-era conservation measure of never eating oysters in a month without an "R" ensured oysters were allowed to breed and replenish shoreline oyster populations during warmer months and was also a handy reminder that oysters were out of season, and not appetizing. This is the original reason why "one should not eat oysters in a month without an 'R'." To understand this history, and how it evolved in the New World, we recommend our Are Oysters OK in "r"-less months? article. To consider that spawning oysters need not be off-limits, read our A market for creamy oysters? article.

Given that oysters have existed for over 100 million years, it's intriguing to consider whether their remarkable mating abilities play a role in their resilience and evolutionary success by enhancing fertilization chances. Despite each adult generating millions of eggs and sperm, only 1% to 5% reach maturity. This seasonal abundance benefits the estuarial community at the base of the food chain. Humans, too, can be grateful, as oysters have supported our survival throughout history. You can read more about how oysters have supported humankind here.

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The oyster life cycle: After fertilization young oysters are invisible to the naked eye for 2-3 weeks but if you could see them, they would appear to be small clams with a symmetrical bivalve shell appearance. Once they become visible, they appear as spec.s of sand. They are 1-2mm wide which is also about the time they sink, settle and attach to something. This is also the time the oyster metamorphoses, drops its clam-like foot, and starts to develop into its more traditional asymmetrical form.

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Hatcheries help contain, focus and concentrate what is otherwise a broadcast fertilization in nature and a random distribution of oysters over wide areas. Being able to concentrate this bio-mass for the purposes of farming is a time saver for the industry. Graphic courtesy of the Agricultural Research and Extension Center (AREC) of Virginia Tech (report).

Hatcheries do a bit more

than just chaperone

Hatcheries replicate the natural spawning process within a controlled setting to increase the survival rates of young oysters. By maximizing the availability of algae and oxygen in a predator-free environment, they enhance the survivability of these oysters. Additionally, they focus on genetic breeding to produce optimized offspring with high economic value. You can learn more about LOC's partner hatchery process here.

Hatchery practices are designed to maintain maximum biodiversity in the animals they produce each season. For instance, since male oysters generate more sperm than females, a conscientious hatchery will separate males and females during spawning to prevent a single male from fertilizing all the eggs.

Hatcheries are capable of producing both triploid and diploid oysters. Triploids have their reproductive genes suppressed, while diploids are genetically akin to wild reproductive oysters. Why produce both types? The half-shell market requires oysters to be available year-round, with consistent quality and quick delivery. Therefore, the process of creating triploid oysters is designed to meet these market demands. For more information on how and why triploids are bred this way, read here.

Diploids raised in a hatchery are destined for either a specialty grower or individuals interested in oyster restoration. The "Spat on Shell" method involves placing young diploid larvae into a tank with clean oyster shell. The larvae eventually adhere to the shell, mimicking their natural behavior. However, unlike in the wild, this spat attachment occurs in a protected setting, significantly increasing the percentage of oysters that successfully attach to the shell. This shell, now with spat firmly established, is then introduced into the environment to aid in restoring wild oyster reefs and establishing breeding populations.

Spat catching - the best of both worlds ?

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Triploid technology in aquaculture emerged out of necessity. Due to over-harvesting and disease, wild oyster populations were dwindling. Hatcheries now provide a product that is disease-resistant and can be sold by growers throughout the year, prompting the question of why anyone would return to harvesting wild oysters.

Spat catchers are typically disks or strips like Venetian blinds (below). Once the disks are populated with wild oyster catch, they are sent through a "thrasher" machine that flexes the disks to remove the oysters. In this poorly produced video, you can see the process.

However, some growers certainly continue this practice, utilizing modern materials to catch spat, thereby preserving an ancient tradition. Previously, this was achieved using small sticks that served as sacrificial, semi-permanent homes for the oysters that attached to them.

As the oysters grew and the sticks degraded, the oysters could easily be removed from the wood, or the oysters would eventually fall off the stick and continue growing until they were ready to be harvested. Today, spat catchers are devices placed in the water during spawning to offer maximum surface area for young wild oysters to "set," similar to how they "strike" onto the shells of oysters in a reef.

These devices use flexible materials, allowing young oysters to be removed easily without harming either the oysters or the spat catching equipment. Instead of purchasing seed from a hatchery, farmers can use spat catchers to capture young oysters, enabling them to gather hundreds of thousands of oysters with minimal space for free! These oysters can then be cultivated and grown to maturity using traditional aquaculture equipment.

As you might have guessed however, since these oysters will eventually spawn, their market potential as half-shell table fare is limited to certain times of the year. Perhaps these growers have both diploid and triploid-centric operations such to offer up wild oysters in non-spawning seasons?

This spat catching device is by Zapco Aquaculture. These blades are positioned in close proximity to each other like a Venetian blind.

As we shared in this article, based on data from the Food and Agriculture Organization (FAO) of the United Nations, in 1952, global oyster aquaculture production exceeded wild harvests for the first time, with reported figures of 306,930 and 302,526 metric tons, respectively. Since then, aquaculture production has consistently surpassed wild oyster harvests, reaching 6,125,606 tons in 2019, compared to 133,984 tons from wild harvests. (AGRMC.ORG). It is uncertain whether this data includes wild-spat-catchers within the aquaculture category. Considering the differences between aquaculture and wild harvests, it seems crucial to clarify this distinction, as it is not clear if the differentiation pertains to operational practices or genetic sources.

From a resource conservation standpoint, it can be argued that spat catching utilizes nutrients that would otherwise be wasted, given that only 1-5% of spat reach adulthood. However, this assumes that the rest of the food chain remains unaffected, a claim that is challenging to support. Globally, spat catching might only account for a small portion of the total output from seasonal spawning. While LOC finds spat catching intriguing, it is not a path we are currently entertaining. Despite the seed being free, it requires substantial investments in equipment and labor before any other operations can take place. Additionally, the final product is only available during certain times of the year. Our stance is to leave wild oysters undisturbed, allowing them to recover and enhance water quality by increasing native populations. We do however return used shell after our events to the water nearby natural reefs to assist in their expansion.

Are there health differences between aquaculture and wild-caught oysters? You can do a deep dive here. The short answer is the difference is largely indiscernible. While wild diploid oysters mature slower than hatchery-grown triploids, potentially affecting their health, bio-magnification is more common higher up the food chain. The choice between aquaculture and wild oysters is more about understanding industrial practices and natural process trade-offs.

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We hope this article has been educational. If you have made it this far, congratulations! Let's take a poll. Given everything you have learned about how oysters reproduce and how this impacts the oyster industry, natural populations, your health and the environment....

Which would you prefer: aquaculture oysters or wild oysters ?

Change Image

• Aquaculture Oysters
• Wild Oysters

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