For almost 10 months straight every year, millions of eels gather at a deep-sea mountain near Australia to have an orgy.
The 2.8 million basketwork eels, which weigh nearly 3.5million kilograms, cram into Patience Seamount just south of Tasmania just to mate.
A study led by Australia’s scientific research agency CSIRO, published today in Royal Society Open Science, counted them for the first time.
The eels, known to scientists as Diastobranchus capensis, are slimy scavengers that look like purple-ish woven noodles.
They’re usually found across the southern coasts of Australia and Africa but have been slithering to a specific ridge at Patience Seamount since the 1980s, peaking from March to May before dispersing in October.
Study lead author Ben Scoulding told the Australian Broadcasting Corporation his team carried out surveys between 2023 and 2025 to count how many eels make the blob.
They used a ‘really expensive scientific fish finder’, as Scoulding put it, called a hydro-acoustic survey, to ping sound waves down below.
The sound would bounce off anything it knocks into, allowing the researchers to see all the wriggling eels.
Dr Scoulding said the ball was about 1,000 metres deep and covered an area about two by three kilometres wide.
‘One of the interesting things is the persistence of the aggregation,’ he said, adding: ‘It is there for the majority of the year.’
CSIRO caught a few eels and found they all had empty stomachs, with most of the females close to laying eggs, called spawning.
This suggests they may stop eating as they prepare to join the writhing eel ball.
The study’s count is far higher than an estimation made in 2015, which was 10s of thousands of eels in April that year.
Dr Scoulding says his team’s findings don’t reveal why the eels have chosen Patience Seamount for their, uh, orgy.
Populations of the eel on the slopes in New Zealand have declined 85% in the past 40 years, though it’s unclear if drops at Patience Seamount have happened before.
And they have good reason to throw these oceanic sex parties – basketwork eels are vulnerable of becoming endangered, according to the International Union for Conservation of Nature’s Red List.
Fisheries are accidentally catching the eels, causing the eel population to slowly but ‘significantly’ decrease, the global scientific authority says.
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