The World Is Your Oyster

Small intertidal oyster reef (By Jud McCranie - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=114954020)

Eastern oysters, Crassostrea virginica, are bivalve mollusks related to clams, mussels, and scallops. They inhabit brackish estuaries throughout the Atlantic and Gulf of Mexico coasts. Oysters grow together and form large reefs. In the warmer parts of their range, they tend to be found in the intertidal zone. Freezing temperatures can kill them, so in the colder parts of their range, they are subtidal. These unassuming creatures are vital to healthy estuarine ecosystems but also have a dark side. We'll explore both the good and the bad of oysters.

Seven Reasons Why Oysters Rock

 

1. The oyster is a keystone species. Removing oysters drastically alters the estuarine ecosystem.

A beach on Sapelo Island comprised of oyster shells. Waves and tidal currents remove the shells of dead oysters from the reef and then pile them up on the shore. Without the oysters, this shore would be a marsh bank.

Horseshoe crabs spawning on an oyster shell beach.  The female (larger animal on the left) deposits her eggs in the shell hash and the male (right) fertilizes them. 

2. Oysters are ecosystem engineers. Oysters rock because they make giant, rock-like structures.  They create habitats, known as oyster reefs, that wouldn't otherwise exist.  The three-dimensional structure of an oyster reef is full of nooks and crannies. That means an oyster reef is great habitat for crabs, fishes, snails, worms, and seaweeds.  The oyster pea crab, Zaops ostreum, actually lives within the oyster's mantle cavity, inside its shell.  The oyster is their entire world! 

Oyster larvae are free-swimming plankton. Plankton don't just settle anywhere. They're picky. Oyster larvae prefer to settle and grow on the shells of older oysters.  Once a larval oyster settles, it cements itself in place and spends the rest of its life in that location.  From New Jersey to northern Florida and along the northern coast of the Gulf of Mexico, oysters are the only natural source of hard substrate in estuaries.  As boaters who frequent coastal waterways in this region, we have mixed feelings about all these hard objects in the water!

Intertidal oyster reef in Doboy Sound, GA.  It's a solid, complex structure rising above the soft mudflat.  These reefs provide habitat for over 300 other species and alter the flow of water through the estuary. 

Oysters growing on a piling.  Notice the ribbed mussels (Geukensia demissa) growing among the oysters.

An individual "wild" oyster taken from a reef.  Note the elongate shape.

3. They clarify the water.  Oysters draw water in and filter microscopic phytoplankton with their gills.  By removing phytoplankton and other small particles from the water, they improve water clarity. Clearer water promotes the growth of seagrass by allowing sunlight to penetrate to greater depths. An oyster can filter about 50 gallons of water per day. At one time, the entire volume of the Chesapeake Bay was filtered in just 4 days by the oysters there. Oyster biomass has declined in the Bay because of over-harvesting, habitat destruction, pollution, and disease. The remaining oysters now take over a year to filter the same amount of water. 

4. They take excess nitrogen out of the water.  Nitrogen is an essential nutrient needed by plants and phytoplankton. But too much of a good thing can cause problems, such as harmful algal blooms like red tide. Excess nitrogen often comes from runoff from farms and cities, inadequately treated sewage effluent, and leakage from aging/inadequate septic tanks.  Oysters help to "denitrify" the water when they filter feed. In addition to filtering edible phytoplankton from the water, they also remove other nitrogen-containing particles. But rather than eating them, they pack these inedible particles together and release them as pseudofeces, which sinks to the bottom and becomes part of the sediment (AKA, mud).

Being in the muck means that this nitrogen is no longer available for phytoplankton drifting around in the water.

5. They stabilize the shoreline, reduce erosion, and reduce the amount of suspended sediment in the water.  Large oyster reefs stand firm against the forces of waves and currents. Oyster reefs break the waves before they get to the shore and they help hold sediment in place.  Larval oysters' preference for settling on the shells of other oysters usually results in them settling onto already-existing reefs.  People can help create new reefs by simply placing recycled oyster shells in the estuary. After a few years, newly-recruited young oysters have cemented themselves to the recycled shells and created a new reef. Because they do such a good job of protecting shorelines, engineered oyster reefs are now used as part of "living shorelines" to prevent coastal erosion.

6. They are protandrus hermaphrodites. Oysters all start out life as male but transition to female when they grow large.  Eggs are energetically expensive to produce, so it helps for females to be large.  And, boy, do they produce a lot of eggs: up to 150 million of them. Oysters are broadcast spawners, releasing eggs and sperm into the water column, where fertilization takes place.   

7. They are delicious, whether you eat them raw, roasted over a wood fire, fried and put on a Po' Boy sandwich, or in a stew.  Overharvesting, habitat destruction, water quality degradation, and disease have reduced US oyster biomass by about 88% during the 20th century.  Today, the biomass of oysters in Chesapeake Bay is just 2% of its historical levels.  But thanks to better natural resource management practices, restoration efforts, and aquaculture businesses, oysters are on the rebound.  The largest oyster restoration project in the world is taking place in Chesapeake Bay. Over 1,200 acres of oyster reef have been restored there already.  And oyster aquaculture is on the rise.  Those pretty, cup-like oysters you get at the raw bar are all cultured.  Oyster aquaculture operations provide many of the ecological benefits of natural oyster reefs (as well as jobs and a tasty product).  Oyster aquaculture sites can even be a source of oyster larvae to help restore natural reefs in the local area.  Given all the ecosystem services that oysters provide, this is good news.  If you enjoy eating oysters, it is important to pay it forward.  Go here to find out where you can recycle shells.

A nice selection of locally aquacultured oysters at our favorite fishmonger, Harbor Fish Market, in Portland, Maine.  You can pick & choose oysters from specific growers.  Cultured oysters are grown individually and are manipulated to encourage their shells to grow more rounded and cupped than those of their wild cousins living on the reefs. 

  

Loki thinks they're delicious, too.  He's nibbling the adductor muscles from discarded shells during an oyster roast.

Just when you think you know someone and they seem like responsible members of the community, you then find out that they have a dark side...

Four Big Reasons Why Oysters Suck (or "Shuck," if you prefer) 

1. They are filter feeders, which obviously means they suck a lot of water.  

2. Food Poisoning.  Like all animals, oysters harbor bacteria. The vast majority of these bacteria are harmless, and some are even beneficial.  But there is one particular group of harmful bacteria, called Vibrio, that is found in oysters.  Ingesting Vibrio usually causes mild to moderate "food poisoning" symptoms. We won’t go into the details but trust us, it sucks.  The chances of getting such an infection are greatly minimized, thanks to state and federal shellfish sanitation programs.  If you're harvesting your own oysters, always follow local shellfish harvesting regulations and shellfish handling & preparation guidelines. Remember that pearl of wisdom about eating oysters in months with an "R".

3. Their shells are like Razorblades.  The edges of the shell of a live oyster are extremely sharp and can cause serious wounds.  Receiving a gash from an oyster can ruin your day, or worse (see below...).  In addition to being sharp, the edges of the shell are also delicate.  So when they cut you, they leave behind little shards.  Getting bits of shrapnel in a wound…definitely sucks. 

Just look at those sharp edges.  Some are serrated like steak knives!

4. They harbor flesh-eating bacteria. One particularly nasty species of bacteria found on oysters is Vibrio vulnificus.  This is the little monster that causes flesh-eating infections. It can also cause life-threatening systemic infections.  What normally happens is somebody steps on or falls on an oyster shell, causing a deep laceration wound in which little bits of oyster shrapnel break off.  The Vibrio vulnificus on those shards of oyster shell now find that they rather enjoy human flesh.  So they start eating and multiplying like mad, and you have an infection.  Here is the truly frightening part: according to the CDC, 1 in 5 patients who have Vibrio vulnificus wound infections die, often within 48 hours!  If that doesn’t suck, we don’t know what does.

This is why you should always wear real shoes around oysters.  And this wasn't even a live oyster; it was a shell from a long-deceased oyster worn down by waves on a shell hash beach.  Live oysters have much sharper shells.  (Yes, this is Damon's flip-flop.  Will he ever learn?)

This final point strikes a personal note with us.  Many years ago, Damon fell on a slick ramp while launching a boat at low tide.  (Launching at low tide is never a good idea, btw.)  There were a few oysters growing on the edge of the ramp and they gashed the side of his shin.  He cleaned the wound as best he could but didn’t get it properly treated because...life.  On the following day, we were back out on the boat conducting a fish survey far from civilization. In the afternoon he noticed that his entire leg was red, which he thought was sunburn.  Then we noticed that his leg was swelling around the wound and his skin was hot to the touch.  Instead of heading straight back to shore and going to the hospital, he insisted that we finish the sampling work that we had planned for the day.  When we got back to the lab, he then insisted on changing the lower unit lube oil in the boat's engine before going to the hospital ("Flesh heals, machinery doesn't.").  By the time we finally arrived at the hospital, which was now close to midnight, his entire leg was swollen, from hip to toe.  The triage nurse who initially examined the wound said: "Don’t go home.  Make sure you stick around the ER until you see the doctor.  This infection can kill you.  Next patient, please."  Apparently, she was aware of the CDC's statistics on these infections.  

Take Home Message: If you are cut by an oyster and the wound goes deeper than the outer layers of skin, get medical treatment immediately.  Don't be stubborn. 

These are the reasons why we would warn our undergraduate students that oysters are the most dangerous animals on the coast.  Keep in mind, we supervised students working in the field where sharks, stingrays, alligators, venomous snakes, feral hogs, and wild cattle were all quite common.  And the oysters were what really concerned us. 

Oysters play a crucial role in maintaining the health of estuarine ecological communities, and it is absolutely fantastic that they are on the rebound in many places.  But, like any compelling character in a story, they are complex.  They’re neither pure good nor pure evil.  Treat them with the respect that they deserve.  And if you fall victim to one of these living razorblades, there is nothing like the briny taste of revenge, accompanied by lemon and a dash of cocktail sauce.