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The life of an oyster: Spawning

July 16, 2020

The following interview was done earlier this year, and spawning was conducted this spring under strict adherence to social distancing guidelines.

Things are never busier or filled with more expectation than the months approaching spring in an oyster hatchery. Such was the scene at the Horn Point Laboratory, which began removing broodstock (oysters) from the Choptank River in January in preparation for this past spring's spawn.

Of course, this year's spring hasn’t been the typical spring. While the majority of the Horn Point Laboratory campus was devoid of the usual hustle bustle of research, the oyster hatchery staff, practicing a range of measures to ensure everyone’s health and safety, continued to raising oysters. Their responsibility has been to produce the oysters that fuel Maryland’s aquaculture industry. To date, over 700 million larvae have gone to independent oyster farmers and county harvest bars.  

“There’s a lot of work getting oysters ready to spawn,” explains Horn Point Oyster Hatchery Manager Stephanie Tobash Alexander. “When you see (oyster) spat on shell, there’s already three months of hard work and careful planning that has gone into getting to that point.”

Starting in late January oysters are brought into the hatchery’s conditioning lab. The broodstock—adult oysters used in the spawning process—sit in gradually-warmed flowing water, awakening from hibernation, eating, and putting all of their energy into reproductive organs.

“We artificially create a springtime in the hatchery,” explains Alexander. “Broodstock collected in the spring and later in the season will start conditioning naturally in the river as the water temperature have risen, meaning less time required in our conditioning lab.”

Hatchery staff begins opening some of the oysters and checking for reproductive maturity at week six of conditioning. At that time, staff can begin to see the developing eggs and sperm. From there, the hatchery begins to grade the oysters based on a scale from 0 to 4, the latter ranking representing an oyster containing an abundance of gonad material. Most of the time, oysters achieving a ranking of three are removed from the conditioning lab and brought to the spawning table.

“We make it very warm there,” says Alexander. “We want them to think it’s summer at this point. We encourage them to spawn.”

As they sit on the table for several hours, submerged in water that is gradually warmed to 86 degrees F, the oysters will begin to spawn naturally. If not, oyster gametes will be introduced into the water column to create stimulation. Once the oysters filter and detect the gonads in the water, they will start to release their own gametes and kick off the spawning frenzy. This process is called mass stimulation.

Staff patiently watch this process, identifying spawning oysters from the table and separating them by sex. This is done to collect the eggs and the sperm, get a diverse genetic gene pool, and to prevent polyspermy, when too many sperm fertilize one egg. Males will emit a continuous flow of sperm from the dorsal side of their shell, while females clap their valves together to release eggs in a puff from the front of their shells. 

Females can clap every 30 seconds up to an hour, producing anywhere from 2 to 70 million eggs per spawning event. One spawning event can last up to one hour.

Once the spawn is finished, the adults are carefully removed from the spawning tubs and measured. The egg mixture is transferred into a large bucket and diluted with filtered ambient water at the correct salinity and temperature. The eggs are counted using a microscope to tally the number of eggs found in one milliliter of the diluted egg mixture. 

Once the estimated number of eggs is determined, the proper amount of sperm is added to the buckets. The eggs will be allowed to fertilize for ten minutes and will be checked to see how they progress under the microscope. If the eggs are not fertilized more sperm will be added, but if 90-95% of the eggs are starting to undergo cell division, the eggs will be transferred into large larval tanks for grow-out.

And now, the hatchery has oyster larvae, which will be fed and grown for weeks, issuing in another phase of the oyster development process. 

Horn Point Laboratory credits Stephanie Tobash Alexander and her hatchery team for their tireless work through this unprecedented time, as well as the entire Horn Point campus for supporting our community while following the guidelines of the State of Maryland and University System of Maryland. 

Stay tuned for the next installment: The life of an oyster: Larvae.