Signs of Fall 13: Oysters!

Photo by M. Fischer

(Click here for an audio version of this blog!)

In October, Deborah and I had the pleasure of sitting with our son, Joe, his partner, Marlee, and our daughter, Marian, at an oyster farm on the shore of Puget Sound shucking and eating raw oysters. We had a sense while we sat there of a bond with the ancient, human lineage of shellfish eaters. The movement of people from Siberia down the west coast of North America and into South America probably followed the lines of abundance of shellfish! We looked around at the bubbling water tanks and algae vats of the farm and watched their acres and acres of floating bags rise and fall with the waves and tides. We had so many questions (but mostly we were enjoying the great oysters!).

Let’s do some of the questions now!

What is an oyster?

Pacific oyster,. Photo by Pixabay

Oysters are  mollusks with two shells and an astonishingly simple anatomy. They have no head nor any other body segments, no brain, and no obvious sensory organs. On the inside of their protective shell is a tissue called the mantle that not only secretes the shell but also is full of blood vessels and is able to exchange oxygen and carbon dioxide between the oyster and its surrounding sea water. The oyster also has a prominent set of similarly very vascular, mucous covered gills that it uses to filter the incredible volume of water that it can pass through its open shell. These gills also exchange respiratory gases but even more importantly they gather up algae and other suspended materials from the water and pass them into the oyster’s quite inconspicuous mouth. At the other end of its very short digestive tract there is an even less conspicuous rectum and anus from which it excretes the non-digestible or non-absorbable solid waste. There is also a heart to push the clear blood through the blood vessels, some glands to assist digestion, two tiny kidneys to clean wastes from the blood, some reproductive organs that can change from testes to ovaries and back again with amazing speed, a couple of rudimentary neural ganglia and nerve cords, and a strong adductor muscle that contract and hold the shell tightly closed.

Where do oyster’s live?

Chincoteague Island, USFWS, Public Domain

Oysters live in the shallow shoreline and estuarian waters of most of the oceans of the world. They form large masses of shells in which one generation grows on top of the remains of the previous ones. Each type of oyster seems to be preferentially attracted to and to grow on the shells of its own species, so these great “oyster beds” or “oyster reefs” are often dominated by a single oyster species.

Oysters are a keystone species in their ecosystems. Their reefs generate protective habitats for a large number of vertebrate and invertebrate animals, and their constant intake and filtering of water greatly improves the water quality of their habitats. A single oyster can filter 50 gallons of water each day! Imagine the impact on water quality that one acre of oyster beds (about 750,000 oysters) can have!

How do oyster’s reproduce?

Oyster reef. Photo by Senckenberg Research Institute

When an oyster reaches one year of age its reproductive organs mature into sperm producing testes. Older, larger oysters that have built up significant metabolic reserves may have their reproductive organs transformed into ovaries which produce an astonishingly large number of eggs (up to 100 million) during the seasonal spawning. The production of this many eggs is extremely stressful on the “female” oyster! The trigger for the mass release of gametes in an oyster bed occurs when environmental temperatures reach about 68 degrees F (early summer). The spawning of a few oysters in the bed stimulates more and more oysters to release their sperm and eggs, and the water over the bed becomes cloudy with suspended oyster gametes.

Fusion of the sperm and egg forms a microscopic larva called a “veliger.” The veliger has cilia which it uses to swim about, a mantle which it uses to begin to make a tiny shell, and an eye spot that it uses to find a suitable substrate to attach to.  They also have a muscular foot that they use to crawl about on possible attachment surfaces. After two or three weeks the veliger attaches itself to something solid (often the old shells of oysters of its same species). This attached oyster larva is called a “spat,” and, if undisturbed, it will spend its entire lifetime growing in its attached site and cycling each year in the group spawning.

When an oyster is making eggs, its body changes under the metabolic stress. It becomes quite translucent and takes on an unpleasant, acidic taste. This is one of the reasons that natural oysters are not harvested in the summer (i.e. in the months without “R’s” (May, June, July and August). Oyster farming, though, through a variety of manipulations, has bypassed this summer restriction.

How do you farm oysters?

Oyster farm. Photo by Pixabay

Oyster farming takes the natural oyster life and reproductive cycle and manipulates the various stimuli and life stages to maximize growth rates and yields. Brood oysters have been developed that display specific genetic traits and physical features. The “stud book” for these brood lines is an extensive and complex set of documents! New genotypes from wild oyster strains are regularly incorporated into the breeding gene pool. These brood oysters are placed into large tanks with algae and nutrients and then carefully brought up to 68 degrees F. The warm temperatures simulate the onset of summer (regardless of what the actual month of the year it is!) and stimulate the brood oysters to release sperm and eggs. Fertilization, then, takes place in the water of the brood tank.

The veliger larvae then grow for about six weeks feeding on the algae of the tank. The larvae are constantly sorted and culled via filtration to make sure that a specific larval cohort is made up of individuals of similar sizes and also to get rid of any defective or slow growing individuals. When the larvae reach the appropriate size and development stage they are then given oyster shell materials so that they can attach and become spats.

These oyster shells can be in two forms: 1. Large, intact oyster shells (to which a large number of spats will attach thus forming what are called “shellstock oysters” (which are used in the commercial processing of oyster meats)), or 2. Small, sand-grain sized pieces of oyster shell (to which single spats will attach (this is how the individual oysters typically served in restaurants and bars are generated)).

The spats are then either spread on a suitable intertidal beach (a “Bottom Grow-Out” area) or suspended in bags or cages in inter-tidal zones (an “Off-Bottom Grow-Out” area). The beach “farm” is very simple to set up but may expose the oysters to predators or damage due to storms. The bags and cages are more complex to set up, but may be less impacted by predators and may offer the benefit of smoother and larger volume shells. The oysters stay in their grow-out areas for two or three years until they reach a suitable commercial size.

A very positive aspect of oyster farming in great contrast to most other types of maricuture is that there are very few negative impacts of the artificial oyster beds on the surrounding ecosystems. In fact, the water filtration benefits of the oyster mass actually improves the habitat quality around most oyster farming systems. Recognizing this filtration feature of oysters has led to the use of managed oyster beds in the effluent outflow of other, more potentially polluting fish and shellfish farming systems to the great benefit of system water quality.

Finally, oysters take on specific flavors when grown in specific estuaries or intertidal zones. The term “terroir” has been applied to this site-specific flavor development in wines, and a similar term, “merroir” has been coined for oysters.

“The time has come,” the walrus said, “to speak of many things…”

And , of course, he was talking to the many groups of four, fat oysters that he and the carpenter had taken for a walk on the beach and were about to greedily consume. We might have spent the column today talking about the symbolism of Lewis Carroll’s poem, but I prefer that we talked about oysters!



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