Tuesday, 8 January 2013

Week 5: Native Oyster (Ostrea edulis)

The Native Oyster was @SpeciesofUK from 30th December 2012 to 5th January 2013.

There are hundreds of species of oyster in the world. They are split into two main families – true oysters (which include edible oysters) and pearl oysters. There are also a few other smaller families.

The Native oyster is a species of true oyster.

Oysters are a type of bivalve mollusc. They're a mollusc because they have a mantle (or shell), and they are bivalve because the shell comes in two halves or 'valves.'

The word "oyster" is from Latin "ostreum" via Old French. It is related to Greek "ostrakon" meaning hard shell.1

Oysters in the UK

Around the UK coastline there are two main species of oyster, the Native oyster and, more recently, an introduced species, the Pacific oyster.

The Native oyster, 'Ostrea edulis,' also known as the European Flat oyster, Mud oyster, and Belon oyster, was once a staple food and plentiful around the UK coast, but was overfished in the nineteenth century. Now they're seen as a delicacy

Native Oyster
[Source: H. Zell]

The Pacific oyster, 'Crassostrea giga,' meanwhile, is an introduced species. Although originally from Japan, it is sometimes called the Portuguese oyster, and around the UK is often called the Rock oyster.

The Pacific oyster was introduced to the River Blackwater, Essex, in 1926, and to Conwy, Wales in 1965, to replace the declining stocks of Native oysters.2 It's the most widely produced edible oyster in the world, because it's relatively easy to farm and adapts well to its environment. The Pacific oyster now dominates the oyster industry in the UK.

If you have eaten an oyster in the UK, even one farmed locally, sadly the chances are it was a Pacific Oyster not a Native Oyster.

The best spots to find native oysters in the UK are the Thames Estuary, the west coast of Scotland, the Solent, and the Fal Estuary in Cornwall.

Whitstable in Kent has a famous oyster festival. In Colchester, there is an annual Oyster Feast, held after the Mayor has dredged the first oyster of the season.

Identifying the Native Oyster

Native and Pacific oysters can be distinguished by shell shape. Native oysters are rounder and smoother-edged. Pacific oysters are more elongated with deeply grooved edges.

Native Oyster, 'Ostrea edulis'
[Source: Jan Johan ter Poorten]

Pacific Oyster, 'Crassostrea Gigas'
[Source: Jan Johan ter Poorten]

A native oyster is rough and scaly to the touch, and a yellowish-green in colour. The valves (half-shells) of a native oyster differ - one is concave and attaches to surfaces. The other is flat and has concentric bluish rings.

Inside the oysters the colours range from blue to grey including the famous opalescent ‘mother of pearl.’ The meat inside can vary in color from creamy beige to pale gray.

Native Oyster Shell
[Source: H. Zell]

The Life of an Oyster

Native oysters can live up to 20 years but usually live to around six years.

They fasten themselves to bedrock and need access to plankton so usually are found in water no deeper than 20m, near to the coast.

Oysters eat suspended organic particles and plankton. They feed through gills, pumping water through a filter and sieving out the plankton.

Individual oysters can filter up to 50 gallons of water per day, removing plankton and organic particles, contributing to water quality.3

Females oysters keep their eggs in a gill cavity. They take in sperm as part of their normal filtering process, fertilising the eggs. Female oysters may produce an incredible 2,000,000 eggs.4

Mother of pearl is secreted by oysters around any foreign body that gets inside the shell, like sand or grit. Over time this forms a pearl.

Native oysters are edible oysters not ‘pearl oysters’ and so only rarely produce pearls, which are very poor in quality. Pearl oysters are a different group of oysters altogether, and are not present in UK waters.

A Pearl Oyster
[Source: Manfred Heyde]

Oysters in Literature

The expression “the world's my oyster” is from Shakespeare originally, from The Merry Wives of Windsor:

          “then, the world's mine oyster, which I with sword will open.”

Jonathon Swift said “it was a bold man who first ate an oyster.”

Anton Chekov wrote a delightful short story about oysters which you can read here:

          "Papa, what does 'oysters' mean?" I repeated.
          "It is an animal . . . that lives in the sea."
          I instantly pictured to myself this unknown marine animal. . . . I thought it must be something midway between a fish and a crab.

In Lewis Carroll's poem 'The Walrus & the Carpenter,' oysters are deceived into going for a walk along the beach and are then eaten:
          "'I weep for you,' the Walrus said:
          'I deeply sympathize.'
          With sobs and tears he sorted out
          Those of the largest size."

Oysters also appear in Edward Lear's poem 'The Dong with the Luminous Nose.'

          "Landing at eve near the Zemmery Fidd
          Where the Oblong Oysters grow."

Threats to Oysters

Native oysters face threats from a number of other species, whether as prey or through competition. For example, in the UK they are often preyed upon by 'Ocenebra erinacea,' the sting winkle or rough tingle.

Another predator is 'Urosalpinx cinerea,' the American oyster drill, which was accidentally introduced to the British Isles with imports of American oysters. The oyster drill secretes an acidic substance to soften the oyster shell. It then bores a hole through the softened shell and inserts its proboscis to get to the meat.

Oyster Drill
[Source: Manfred Heyde]

Another threat to native oysters is the slipper limpet, which competes for space and secretes ‘mussel mud' which makes it harder for oyster beds to establish.

One animal which, strangely enough, is not a threat to the oyster is the oystercatcher. Oystercatchers actually have nothing to do with oysters, although they do eat enough molluscs such as mussels.

An Oystercatcher
[Source: Andreas Trepte]

Oysters and People

British oysters have been eaten since prehistoric times. They were popular during the Roman invasion and were even exported back to Italy.

In Elizabethan times, oysters were seen a venereal symbol and specifically a metaphor for the vulva.5

Oysters used to be a staple food of the poor in the UK, particularly in London where they were taken from the Thames Estuary.

Although native oysters are severely depleted in the UK, there are still some small companies farming/gathering. In exploited areas, suitable oyster habitat is sometimes created in the form of 'cultch' - broken shells.

In the Blackwater River, Richard Haward still farms Native Oysters. The oysters have firm, creamy, salty flesh.

The Whitstable Native Oyster Store produces local oysters from Kent.

Cornish Native Oysters gathers wild Native Oysters from the Fal Estuary in Cornwall. They are metallic, creamy and sweet.

In Scotland, Native Oysters are still found in West Coast sea lochs. Rossmore Oysters is the only active fishery.

Sadly, some parts of the UK that used to be Native oyster strongholds have seen dramatic depletions in oyster population. In the Solent, once home to Europe’s largest natural oyster fishery, the oyster population has collapsed.6 The once thriving oyster stronghold in Firth of Forth, which at one time produced 30 million oysters a year, is gone.7

Oysters are one of the few foods that are usually eaten alive. Classicly, they are served raw, in the half shell, sometimes with a squeeze of lemon juice.

Native Oysters, Served Simply
[Source: Myrabella]

Oysters can also be cooked in a variety of ways and in a variety of dishes - grilled, fried, poached, steamed or baked.8

Native Oysters aren't harvested between May and August (the months without the letter 'r'). This is the spawning season when the flesh becomes soft and milky.

Some say oysters are an aphrodisiac but there's no scientific proof of this, although they do contain high levels of zinc which is good for male fertility. The supposed aphrodisiac effect may have something to do with oysters often being eaten in social surroundings with alcohol...

Strange but True...

Native oysters are hermaphrodites! They start their lives as male, then after 8-10 months begin to change their sex regularly.

The technical term is 'protandrous alternating hermaphroditism.' Protandrous means they begin as male. Alternating refers to the fact they change sex back and forth several times.

The frequency with which Native oysters change sex depends on temperature. At 16C, they turn female every 3 years; at 20C, every year.9

Oysters aren't the only hermaphrodites in the animal kingdom. Other well known hermaphrodites include snails, slugs, earthworms, and clownfish.


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