Sunday, 25 May 2008

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.