Jumat, 06 Mei 2011

SIKLUS HIDUP SIDHAT - Life Circle of Eel - 鰻魚生命周期

SIKLUS HIDUP SIDHAT - Life Circle of Eel - 鰻魚生命周期



SIKLUS HIDUP SIDHAT - Life Circle of Eel - 鰻魚生命周期

卵期(egg-stage)
柳叶鰻(leptocephalus)
玻璃鰻(glass eel)
鰻線(elvers)
黃鰻(yellow eel)
銀鰻(silver eel)
产卵(spawning) 



生活史

鰻魚在陸地的河川中生長,成熟後洄游到海洋中產卵地產卵,一生只產一次卵,產卵後就死亡。這種生活模式,與鮭魚溯河洄游性anadromous)相反,稱為降河洄游性catadroumous)。
其生活史分為6個不同的發育階段,為了適應不同環境,不同階段的體型及體色都有很大的改變:
  • 卵期(egg-stage):位於深海產卵地。
  • 柳葉鰻(leptocephalus):在大洋隨洋流長距離漂游,此時身體扁平透明,薄如柳葉,便於隨波逐流。
  • 玻璃鰻(glass eel):在接近沿岸水域時,身體轉變成流線型,減少阻力,以脫離強勁洋流。
  • 鰻線(elvers):進入河口水域時,開始出現黑色素,卻也形成養殖業鰻苗的補捉來源。
  • 黃鰻(yellow eel):在河川的成長期間,魚腹部呈現黃色。
  • 銀鰻(silver eel):在成熟時,魚身轉變成類似深似深海魚的銀白色,同時眼睛變大,胸鰭加寬,以適應迴游至深海產卵。
鰻魚的性別是後天環境決定的,族群數量少時,雌魚的比例會增加,族群數量多則減少,整體比例有利於族群的增加。
 


Life cycle and breeding grounds

Freshwater eels have a remarkable life cycle, which begins and ends in the ocean. Spawning has never been observed.

Life cycle

Adult eels: spawning at sea

Adult eels probably spawn at some depth in warm seas. New Zealand’s shortfin eels produce 1.5–3 million eggs, and the longfins 1–20 million eggs. Males fertilise the eggs. After spawning, the adults die.

Larvae: drifting to land

Fertilised eggs hatch at the surface and become leaf-shaped larvae, floating on ocean currents towards the coast. They have teeth, but it is not clear for what purpose – they may store calcium for bone development. Their skin may absorb nutrients, as researchers have not found food in the larvae.

Glass eels: migrating into estuaries

Once the larvae reach land, an extraordinary transformation takes place: they become slender, transparent eels, known as glass eels. They arrive at New Zealand’s coast from July to December, with numbers peaking in spring (August–October) – the time of whitebait migration. Glass eels migrate into river mouths or estuaries in astounding numbers.

Up and over

Hydroelectric dams are an obstacle to elvers (young eels) swimming upriver. Some dams have special passes, allowing them to get round the massive concrete walls. But they don’t always need this help. Elvers can climb the 43-metre Arapuni Dam on the Waikato River, and the 75-metre Patea River dam in Taranaki.

 

Elvers: swimming upriver

Glass eels soon turn grey-brown, and in this form they are known as elvers. They migrate upriver, often in swarms and usually at night. Young elvers can climb waterfalls, but lose this skill as they grow.

Adult eels

Elvers become adults, with bigger heads and fatter bodies. After many years in fresh water, eels migrate back down the waterways to the sea. It is thought that males fertilise the eggs once the females spawn out at sea.

Migration to the sea

When they reach breeding size, eels change from ‘yellow-bellies’ to ‘silver-bellies’. The yellow-grey underside becomes grey-white, the head shape changes and the head, back and pectoral fins darken.
Shortfin males migrate in February–March, and longfin males in April. The females soon follow, and both males and females die after spawning. Studies show the species also migrate at different ages:
  • Shortfin males at an average of 14 years (38–58 centimetres), females at 22 years (50–100 centimetres).
  • Longfin males an average of 23 years (48–74 centimetres), females at 34 years (75–180 centimetres).
It is not known how long the journey takes. One female longfin eel that was tagged took 161 days to swim from Canterbury’s Lake Ellesmere (Te Waihora) to a point 160 kilometres north-east of New Caledonia.
Barriers across waterways have hampered their route. One estimate suggests that hydroelectric dams have blocked the longfin eel’s access to the sea in 35% of its habitat.

Fish out of water

As a young man, author David Graham worked on a Dannevirke dairy farm where eels slithered through wet grass from one waterway to another. ‘It was quite a common occurrence to leave home in the dark to bring the cows home to be milked and arrive back with several of these Eels, which were easily stunned with a blow on the head, and used for a meal.’

 

Life cycle: a long-standing mystery

For centuries, larval eels were thought to be a separate species: they occur in the ocean and look different from adult, freshwater eels. Then in 1896 the Italian zoologist Giovanni Grassi reported that Leptocephalus brevirostris, known as a saltwater fish, was in fact the larva of the European freshwater eel. But just where at sea they bred was a mystery.

Searching for breeding grounds

In a 1923 paper, Danish biologist Johannes Schmidt stated that American and European eels spawned in the Sargasso Sea, in the Atlantic. In 1926, after sailing his research vessel Dana II to Australia and New Zealand, he concluded that New Zealand eels probably bred somewhere east of New Caledonia. But the exact locations are still not fully known.

Longfin eels

At migration, longfin eels are more ready to reproduce than shortfin eels. Scientists thought this meant their spawning grounds were closer to shore. However, a study showed that the longfin eel had the longest larval stage of any Pacific freshwater eel reported. This suggests that the larvae actually hatch further away from New Zealand, possibly near Tonga. They were also the biggest specimens when they reached coastal waters – so they may have been at sea longer, and travelled further.
 


Life cycle

 

Life cycle

As New Zealand eels have never been seen spawning, this part of their lifecycle is based on what scientists think happens. In autumn, adult eels leave fresh water and enter tropical seas somewhere in the South Pacific, where in deep water females release eggs. Males fertilise them. Adults die after spawning. Eggs hatch into larvae that float to the surface and drift back towards New Zealand. They may take about 17 months to arrive. They then change into glass eels – transparent baby eels. These enter estuaries and turn darker – from which point they are known as elvers. Elvers move upstream and find a suitable place to live, where they grow into adults. Over a decade (or more) later, adult eels head out to sea to spawn and the cycle continues.
 


Glass eels



Glass eels

The young eels that arrive from sea at river mouths are known as glass eels, because they are transparent. Between July and December each year, millions arrive after drifting and swimming thousands of kilometres from the tropics. They are typically 5.5–6.5 centimetres long.
 


Elvers, Karapiro spillway


Elvers, Karapiro spillway

Once glass eels enter fresh water they become darker, and are known as elvers. They swim upriver to find a home in a river, stream, lake or pond. Waterfalls and rapids are no barrier – elvers have a remarkable climbing ability and can even ascend the walls of some hydroelectric dams. In many places ‘eel passes’ have been built to help them get through.
 



Fat head


 

Fat head

The head of a female longfin eel still living in its freshwater home is blunt with a bulbous dome behind the eyes. When the eel migrates to spawn in tropical sea water, the head becomes sleeker as muscle is lost. The eyes and pectoral fins also enlarge. The eel will not feed again, and dies after spawning.
 





The life cycle of the American eel involves several stages. It begins in the ocean when the eel larva, called the leptocephalus, hatches from the egg. The leptocephalus, carried in the Gulf Stream, changes into a glass eel (a more elongated, eel-like shape) near the coast and migrates inland into streams, rivers, and lakes to grow and evolve into the elver (a small version of the adult eel). In fresh water, the elver grows into the larger yellow eel and then finally into the silver eel (almost full-grown). The silver eel then migrates back to the Sargasso Sea and spawns, thereby beginning the cycle once again. (created by Rob Slapkauskas).




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