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Director’s Debrief #1: Gonads

By Susannah Black
August 31, 2015

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After earning the moniker “No Fun Pete” around the office for my dogged adherence to our high level “science talk” messaging, I’ve decided to add some levity in my first Director’s Debrief by discussing gonads.

Gonads have the ability to both provide a lens deep into the science of our estuary and process of oyster restoration, while still being, you know, the word… gonad.

During the early summer as much as 50% of an oyster’s soft tissue will be gonads.  This is one reason for some fair weather oyster enthusiasts’ predilection for “R” months.  After spawning, oyster meats are typically thin and take some time to recover their meatiness.

Throughout June and July, we studiously observe our oysters’ gonads by periodically dissecting samples from our various growing areas.  While this does not bode well for the dissectees, it provides invaluable insight into the reproductive potential of our various sites.  We like to see well-developed gonads and sometimes take it a step further by creating gonad smears and observing them under the microscope to “sex” our oysters.

You see, it’s impossible to tell the sex of an oyster by looking at it.  Oysters are protandric hermaphrodites, which means they change sex at some point in their life from male to female.  As far as I know, they can change back as well, but generally speaking larger, older oysters are more likely to be female.  For a population to successfully reproduce, it needs a good balance of males and females.

More females than males.

Sperm is cheap and, metabolically speaking, eggs are much more expensive. Females can produce up to 25 million eggs in a single spawning event, males produce billions of sperm cells.  This prolific fertility is necessary because each individual larva’s chance of survival is close to zero.  Even if the eggs are fertilized in the swirling vortex of New York Harbor, where a female oyster might spawn a few hundred yards down a bulkhead from the nearest male, the resultant larva’s outlook is bleak at best.

The tiny creatures, just 5% of a millimeter in diameter, are food for dozens of harbor denizens, swept about by strong currents, contaminated by CSOs (combined sewage overflows), disrupted by vessel traffic,  and in the end (after 10-14 days of development) must find a hard substrate to attach in a harbor that has long since had the bulk of its hard surfaces (oyster shell) removed; harvested by humans, to fill bellies, pave streets and build buildings.

Remarkably, we have seen “wild” juvenile oysters (spat) on our oyster reefs.  After covering just 8 thousandth of one percent of the historic oyster bottom coverage with reef we are seeing some recruits.  That means that in the vast, vast majority of the harbor billions of little oyster larvae settle to the bottom, search for structure and perish in the muck.

I hope, by reading this far, you understand why our system is both larvae and substrate limited.  We must grow and restore enough oysters at a high enough density to allow them to reproduce effectively.  In parallel, we must increase the available substrate in the Harbor, through both our restaurant shell collection and reef construction to create a safe space for the little guys to land.