SIKLUS HIDUP SIDHAT - Life Circle of Eel - 鰻魚生命周期
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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，與鮭魚的溯河洄游性（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：
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Life cycle and breeding groundsFreshwater eels have a remarkable life cycle, which begins and ends in the ocean. Spawning has never been observed.Life cycleAdult eels: spawning at seaAdult 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 landFertilised 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 estuariesOnce 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.
Elvers: swimming upriverGlass 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 eelsElvers 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 seaWhen 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:
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.
Life cycle: a long-standing mysteryFor 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 groundsIn 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 eelsAt 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 cycleAs 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 eelsThe 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 spillwayOnce 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 headThe 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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