One day the Intelligent Designer decided to make a strange creature that looked like a snake but which lived in water like a fish. He used gills like he had used for other fish so they could breathe in water and which He had decided not to use for some other animals which live in water, like seals, whales and turtles and He decided to include some small scales on their skin which don't seem to have any purpose because he designed them with a tough, slimy skin, but obviously these scales weren't there by accident.
But His most brilliant idea was how they were going to breed. He made it so they needed to spend many years living in rivers and lakes and places like paddy fields and even sewers to put on enough weight so they could go on a very long journey across the Atlantic all the way to the Sargasso Sea near America to lay their eggs, so the young eels have to travel all the way back to Europe again. This is obviously much more sensible than just laying their eggs in the rivers where they live, like most other fish do.
To make this journey, He designed eels so that, come the time for breeding, they strip their bodies down to the bare essentials - basically just the equipment for swimming, a large store of fat for the journey and a pair of gonads for reproducing. They have to take up to fifteen years getting fat enough before they do the journey and then they digest their own digestive system to make themselves lighter. This is obviously much better than needing to bother with eating on the journey through an ocean teeming with the sort of food they had been eating in the rivers they grew up in. As any experienced back-packer will tell you, it's obviously much better to be really big and fat before you start a long journey to save you having to bother eating on the way, and then doing away with your digestive system to make you lighter.
In fact, He brilliantly designed them to look like you would expect if they had once been sea-living at a time when their spawning ground was much closer to Europe but now someone had moved it all the way to America as though they were moving the sea bed around. The fact that very many of them don't survive the journey to the Sargasso Sea is all part of the plan obviously because this ensures that only those best at swimming to the Sargasso Sea get rewarded with breeding - and you can't say fairer than that.
He also made sure that they have no hope of ever returning once they've spawned because they can't eat and have used up all their fat, and He had another brilliant plan for them to go to all this trouble so most of their offspring would be eaten by other things on their journey back to Europe just as though their real purpose was to be food for other creatures.
Then, in a brilliant final move, the Intelligent Designer had the brilliant idea of designing a parasitic nematode worm which used to live only in a Japanese relative of the European eel but which He has now changed slightly so it now infects 80-100% of European eels, making it difficult for them to use the swim-bladders he had given them to make swimming easier, so the European eel is now an endangered species, as the number of young reaching Europe from the Sargasso Sea is down to a mere 2% of its former numbers in some places. But then who wants a lot of those ghastly slimy eels around, eh?
You have to hand it to
(Er... no Creationists! That wasn't really a pro-ID blog. It's satire.)