Oyster gardening

Ecologists are so enthused by the role of the oyster, that a series of oyster gardening programmes have been devised for hobby farmers with access to the water to grow on their own spat and help restock the reefs. The Maryland programme alone has more than 1000 volunteers. It is not as difficult as it might seem.

The spat are reared in hatcheries and given to the volunteers to grow on in either wire cages dropped by the side of a pontoon or in floating baths which have the advantage of keeping the oysters 24/7 in the richer diet of plankton near the surface. To be a good oyster gardener you have to keep watch that your float or cage does not bang against the sides or the noise frightens the oysters and causes them to close up. You also have to be diligent in winter to make sure they are always under the water because although an oyster can tolerate sub zero temperatures in the water, a frost in the air will kill them if they are exposed.

Instead of digging, an oyster gardener will have to lift their garden out of the water once or twice a week to desiccate the garden, to allow the sunlight and the dryness to kill off invading algae. They also need a good shake each day to make sure any dirt or debris falls away and does not clog up the garden, and once a week a good watering with a hose pipe to keep them clean. Plus the good oyster gardener has to keep a canny watch for oyster predators, especially worms.

In estuarine ecology oyster reefs have particular attractions. They make use of local materials, they are self sustaining, they have a dramatic impact on the marine environment, they protect the banks from erosion, all of which represent a low cost, low energy natural resource to stabilise the rivers.

The plankton cocktail

In the Chesapeake, vast underwater fields of eelgrass, widgeon grass, coontail and many other species flourished and protected sediments from being resuspended and coastlines from eroding. Leaves swaying in the water dissipated wave energy and slowed water currents. Plant seeds go into the plankton cocktail. Losing these submerged grasses, as has happened in Chesapeake, has meant they can no longer act as protector against wave and current damage that tears up smaller seedlings.

These grasses are also home to microscopic plants and animals which if they are not grazed by small fish build up in layers on the surface and starve the grasses of sunlight.

The combined effects of more sediment in the water and the plants encrusted with micro plants is literally to muddy the waters and stop the oxygen from getting through to the estuary floor thereby breaking the cycle of life.

Photosynthesis in an acre of Chesapeake Bay salt marsh can surpass the productivity of the most fertile land farm. These decaying grasses are a food source for the tiny zooplanton in the water that we can only see with a microscope.

This is the organic material at the base of the detritus food pyramid. The marsh grasses and the underwater grasses, usually abbreviated to SAV from the cumbersome Submerged Aquatic Vegetation, are essential to the health of the bay. The submerged grasses create especially valuable habitat for snails, small fish, birds, and larger fish that feed on each other in a hierarchy of life and death.

There was natural band of SAV surrounding the points of land and islands, but little SAV on the bottom of most of the Bay. The depths at which submerged grasses will grow in the tidal waters of Virginia and Maryland is limited by wave action in the shallow end - the roots can't hold the plants in one place where the water is sloshing around - and by light penetration in the deep end. Light can only go through several feet of water naturally before being absorbed. The loss of the oyster reefs has taken away the wave breaks. Pollution has meanwhile poured piles of darkening algae in to the bay where there are now too few oysters left to digest them.

To encourage the plants again, the water has to be cleaned. Enter the oyster, again.

The sway of the evidence has suddenly found ecologists demanding new oyster reefs not for harvesting for food, or to save the local fishing industry, but just for the benign effect the oyster creates for other species. Scientists are not asking for small sanctuaries either, but reefs at least half the depth of the water and demanding that the natural cycle be left untouched by fishermen to allow the estuary to revive. In Barnstaple one third of all the new cultivation paid for out of public money is currently prohibited to fishermen.

Why oyster reefs are good for estuaries

Oysters are good for estuaries. They improve the quality of the water. Healthy oyster beds attract new predators looking to feed on the oysters themselves, but that is just the headline ecology. An oyster will filter as much as 30 gallons of water a day. The benefits are welcomed by the tiny plankton that live both above and below it. If the oysters are not doing their job, then the balance between zoo and plant plankton shifts with the resulting blooms that will darken and snuff out marine life.

An oyster passes two different kinds of faeces, one a nutrient rich excreta which other algae will feed on, and also a psuedofaeces, the material it does not want to eat, like silt, which it has nevertheless helpfully broken down in the sorting.

Much of the richness of the estuary is born just below the surface where the sun can reach the plankton. An oyster kept on a floating reef at this level will fatten almost twice as fast as one moored on the bed.

The reefs are an important artery in the estuarine food chain and a gauge of the health of the habitat for many other life forms. It is not only the oyster spat that will seek to attach to a reef, but so will mussels, barnacles and other small sessile attachers like tiny tunicates and coral like bryozoans that also filter the waters incessantly. They do not seem to compete with each other for food because each prefers a different kind of plankton. Crabs will use the reefs for shelter. Sea anemones for attachment.

This activity encourages the growth of marine fauna. On one north Carolina oyster reef more than 300 types of plantlife were recorded. The critical salt levels that oysters need, also produce the right conditions for a suite of other desirable estuarine organisms.

Projections in the Chesapeake Bay have shown that building up an estuary oyster population again times 25 would result in a 21 percent increase in bay grasses and remove 11 million pounds of nitrogen.

In Maryland they have been videoing the returning oyster reef and recognising the seemingly obvious point that oysters will flourish more in vertical reefs, than on simple river beds or the flat remnants of an old bed. In trials here, the oysters are growing vertically, not horizontally and quickly creating dynamic and vibrant aquaculture around them.

The reason early settlers in America commented again and again at different bays on the abundance of the fishing was the same reason they cursed the dangers of what the oyster reefs might do to the undersides of their boats. They found a natural ready made resource of easy to catch, abundant fish tempted into the shore by the clean waters and the habitat fostered by and around the oyster. No wonder these colonies managed to get a firm grasp so quickly. Without the oyster the tenuous hold those first settlers had on the continent could have been so much more easily dislodged. One might speculate that uncovering America might have taken decades, even centuries longer, as was the case on the west coast.

For ecologists, the oyster is a keystone species and one much valued because of its influence on the larger eco system around it. Across the marshlands of Mississippi and the glades of Florida, the health of the local oyster has been an early warning system that the local ecology is under threat or unbalanced.

By changing the water flows, even quite badly damaged estuaries can be revived relatively quickly. In the beautifully named Caloosahatchee Estuary (christened after the local Caloosa Indians) in Florida, one reclaiming project was hoping to develop 18 acres of oyster beds into 100 acres in 10-15 years, and exponentially beyond that. So encouraged by the potential are different states, that Virginia alone has invested $40 million trying to develop new reef projects.

The reefs themselves provide important protection from erosion for the estuary banks and also for the reeds, grasses and other vegetation that contribute to the nutrient levels in the waters. These grasses were originally valuable to settlers to feed their newly arrived animals, and also as insulation in their new homes and even now are picked as high grade garden mulch. Studies on marshland grasses have shown up the oyster’s true role in the estuary.

Anatomy

Oysters are sensitive. In case of danger, it closes its shell. It reacts to light, to levels of salt in the water, to temperature, to shadow and sound. They have much more complex responses to changes in the environment.

Walk along the beach at low tide near an oyster cove and you can hear a series of short sharp spitting sounds as the oysters react and close their shells, even from quite long distances away.

They are also amazingly powerful. Trials have shown that it would take a pull of more than 9kg to open up a 10 cm mature oyster that has clamped its shell shut.


Its anatomy is encase in the mantle which has three folds or ridges, each of which has a particular function.

The outer ridge secretes the calcareous material to build and repair the shell. In the centre ridge are two rows of fine, dark tentacles or feelers which are the sensors. Any disturbing change in the water or even just a passing shadow stimulates the nerves of the mantle and causes the adductor muscle to close the shell. The inner ridge, the largest of the three, is muscular and mobile. It pumps water in and out of the shell, washing the body and keeping it constantly bathed. So long as the shell stays shut and retains this liquid, it can ignore most predators and survive in polluted waters or exposure at low tide and being transported.

The mouth is by the hinge, the lips surrounded by gills, sometimes called the beard. These are used to breathe and to collect and sort food. They are covered with more tiny, lashing hairs which create an incoming current of water from which food particles are filtered and oxygen absorbed.

They breathe like fish, using both the gills and the mantle. A small, three-chambered heart, lies under the adductor muscle, and pumps colourless blood, with its supply of oxygen, through the body. Also it has two kidneys to purify the blood.

The food is sorted by the lips and passed into its digestive system and then through a coiled intestine to be expelled through the rectal chamber.

They continue to grow through their life, although more slowly as they get older. Some, as can easily be seen in early still lifes can grow to nearly 40cms long and weigh nearly one and half kilos.

Nutrition, better than steak, milk, spinach or liver

Oysters would have been an ideal food for sailors, settlers and early nomadic natives. A dozen oysters can amount to less than 100 calories but are worth as much in protein as 100g of steak and contain as much calcium as a glass of milk. Unusually for any living animal or marine food, they are also a source of vitamin C.

Precise calculations vary from creek to creek, but overall even now they are a superb dietary source of food. Their fat is glycogen and starch. The glycogen, stored glucose, would have been an important source of energy especially useful for people acing long hard days of labour building new settlements.

So too, especially for the sailors, the cocktail of vitamins an oyster provides would have been a viable alternative to fruit and vegetables and help stave off diseases like scurvy.

Vitamin B12 was thought only to originate in fungi and bacteria, but it is the most pronounced of all the vitamins found in an oyster. B12 influences nerve cell activity, the metabolism in general, DNA replication, and affects moods positively. It is often prescribed as a top up for depression.

In lesser amounts in descending order the oyster will have Vitamin B1 (thiamin) which helps convert the glycogen to energy, Vitamin B2 (riboflavin), Vitamin C, niacin, Vitamin A, Vitamin B6 and Vitamin E.

The highest quantity of mineral in an oyster is zinc, which protects the immune system, helps wounds to heal, supports general growth especially in pregnancy and childhood. It also is linked to fertility. Curiously the next largest mineral is copper. Zinc often inhibits the absorption of copper into the body, which in turn allows the body to absorb and use iron. Iron and selenium are the other main minerals among a supporting cast that also include magnesium, phosphorus, manganese and calcium.

Only liver can equal an oyster in terms of the levels of iron and copper it delivers in the diet. Only spinach has as much folic acid.

The shells too have had medicinal and nutritional roles beyond the more obvious role they have played in building roads and footpaths; filling for wharfs, fortifications, and railway embankments. They were raw material for lime, an enricher for arable land They were fed to laying chickens to harden the egg shell. The pharmaceutical industry has also used ground oyster shell, incorporating it in pills to prevent osteoporosis because of its high level of calcium
.

Comb berries and diatoms

An oyster tastes good because of what it it eats. Although the oyster is sessile, its world is far from static. It lives in a universe of constant change. The tidal flow of plankton rushing through the estuary creates myriad changes of environment, diet. It washes them clean as well as delivering the precious algae, which they diligently filter and sort.

Like a good mother surrounded by her brood, the oyster is constantly active.

The plankton on which it survives is an array of micro matter, pushed around by tides and winds, replenished by rainfall and washes off the river banks, a continually evolving living soup held in suspension on the ocean currents, almost a creature in itself, wild, organic, untamed, invisible and highly nutritious.

With some obvious delight, Professor T Rymer Jones recorded in the Aquarian Naturalist in 1858 how active an oyster really is (and at the same time managed to produce that rare thing, a grammatical sentence of more than 100 words):

“Wonderful indeed is the elaborate mechanism employed to effect the double purpose of renewing the respire fluid and feeding the helpless inhabitant of these shells. Every filament of the branchial fringe, when examined under a powerful microscope, is found to be covered with countless cilia in constant vibration, causing by their united efforts powerful and rapid currents, which, sweeping over the entire surface of the gills, hurry towards the mouth whatever floating animalcule or nutritious particles happen to come within the limits of their action, and thus bring streams of nutritive molecules to the very aperture through which they are conveyed into the stomach – the lips and labial fringes acting as sentinels to admit or refuse entrance, as the matter supplied is of wholesome or pernicious character.”


We know what an oyster eats. Scientists have dissected the contents of an adult oyster’s stomach. They find a constellation of tiny plankton that enjoy their own marine biological sub language – a study as early as 1933 reported the following as an oyster’s dinner - algae, dinoflagellates, tintinnids, silico-flagellates, ostracods, eggs and larvae of marine invertebrates, pollen grains from land plants, detritus, sponge spicules, and sand.

It is because of this plankton that the oyster is such a valuable part of an estuary eco system and also why it is such a nutritious food itself. In a sense if it is pure plankton. The richness of this diet allows it to grow, relatively speaking, spectacularly quickly and to create such resilient shell in a short time.

Pause for a moment and consider this micro universe.

Plankton are tiny open-water plants, animals or bacteria. The name, like the word planet, comes from the Greek for wanderer. The generic term can mean anything from invisible microscopic bacteria to larger floaters like jellyfish. The numbers here are awesome, literally hundreds of thousands of separate life forms sharing the space of a few cups of water, all of which perhaps have their own roles and identities beyond the edge of our knowledge or understanding, but in this context just form a mass of food, like an estuarine forest on which other marine predators can graze. Some are similar to leaves and plants others are spawn and young fish, others algae and diatoms.

Typically they have no, or very little, ability to swim for themselves, and exist in continual helpless mutually supportive or predatory flotation. Perhaps they are in mutation on their way to becoming something else and their vulnerability is temporary, but for the moment these creatures or life forms, are almost without predetermination, hostage to the movement of water with the only mathematical certainty that some will survive at the expense of the rest. Some will just happen to pass by an oyster bed.

There are two major types of plankton.

Phytoplankton are akin land to plants and flourish according to the levels of light nearer the surface of the water. They take the energy from the sunlight and transform it into sugar and oxygen. The chemical equation is six molecules of water plus six molecules of carbon dioxide equals one molecule of sugar plus six molecules of oxygen, which is what we all need to survive. Through photosynthesis, phytoplankton convert energy from the sun into organic compounds as food. A byproduct is the oxygen.

The largest concentrations of phytoplankton usually occur in the spring when the rain will wash the bank and spill its own cargo of micro nutrients into the water. When the temperature rises suddenly these plankton can multiply too quickly creating colourful but damaging blooms of algae. Dense blooms block off the light from the bay and kill the creatures lower down and then eventually they die themselves and suffocate everything on the river floor. These are what give the estuary waters their colour. When dinoflagellates dominate, a red-tinted bloom, known sometimes as a mahogany or red tide, may be produced. Mahogany tides typically occur on warm, calm days, often after rain.

Also part of the plankton mass are zooplankton, tiny embryonic fish ranging from microscopic rotifers to macroscopic squids. The smallest recycle the water column, feed off the other plankton while the larger ones are important too because they are a food for larger fish. Much of the zooplankton may be larvae of different fish, including oyster spawn and pediveligers. A gallon of rich estuary water can contain 500,000 different varieties of zooplankton.

Within this mass of life is both dinner and also those looking for dinner. The largest enemy for an oyster will be jelly fish, the smallest might be the colourful comb-jellies, sometimes known as sea-gooseberries. These tiny deep blue creatures have a rainbow coloured spine and look like they were the inspiration for an Apple computer mouse. They grow to about two centimetres and are effective gelatinous predators feeding primarily when they can on the oyster larvae.

The main constituent of plankton are diatoms, tiny silica shelled one-celled creatures. Looked at under the microscope, they often have floral, intricate and beautiful sculpturing. Mostly they are brown or yellow. They carry chlorophylls a and c and the carotenoid fucoxanthin and, like oysters, reproduce asexually, but in their case by cell division. There are upward of 40,000 species of diatoms. The living matter of each diatom is enclosed in a shell of silica that it secretes, again not unlike the oyster. When these creatures die, their shells drop to the ocean floor. Deposits of diatomaceous material, formed underwater in past geologic times are found in all parts of the world. Diatomite is used as an insulating material, in making dynamite and other explosives, and for filters, abrasives, and similar products. Most of the earth's limestone has been deposited by diatoms, and much petrol is of diatomic origin.

That is what an oyster eats. Canned sunshine, as one biologist called it.