Few animals travel so far to have sex as the European eel. When autumn comes, these eels leave their lakes and rivers and embark on an arduous journey towards the Sargasso sea. Most fish perish in the first leg. Some are crushed in the turbines of hydroelectricity plants, others are caught in basket traps. For those that do manage to exchange sweet for salt water, the journey has only just begun. Their spawning grounds are still 5,000 kilometres (3,000 miles) away. The eels swim for months, without resting or eating. When they arrive in the Sargasso sea, worn-out and exhausted, there is but one task left: to mate, then die.
As one life cycle comes to an end, another one begins. The first major stage in the life of a newborn eel is that of the leptocephalus, meaning 'slender head'. At this stage the eel larvae resemble tiny leaves of glass. Their bodies are completely transparent and flat as a nickel. The leptocephali might look delicate, but make no mistake: these youngsters can fend for themselves. They grow much larger than average fish larvae and are competent swimmers. They have to be, for their first goal in life is to return to the homelands of their parents. Even when they catch the Gulf Stream and other currents in their tails, it can take them up to a year to reach Europe's shores.
When the leptocephali near the coast, they transform into tube-like glass eels. The body plan of a glass eel is much closer to that of a mature eel, although it is still transparent. The miniature eels might hang about in the coastal waters for a while, but eventually they migrate upstream and colonize creaks and streams. Continental waters will be home for the next decade. As they increase in size, the eels climb the rungs of the food chain until they have reached the very top. Before long, the Sargasso calls again.
This might seem like an awfully complicated way to grow up and propagate, and it is. The life cycle of eels and their close relatives (mainly tarpons and bonefish) is one of the most complex known in fish. No other fish go through a leptocephalus stage, for example. These larvae are so different from adult eels that they were seen as a separate species until the end of the 19th century.
Dutch researchers have now found a clue in the eel's genome as to how this complex life cycle arose. Unlike other fish, eels have retained eight complete Hox clusters in their genome. Hox genes are the master controllers of embryonal development. By switching other genes on and off, they shape the lay-out of the future animal. Amongst other things, the location and timing of Hox activity determines where limbs will sprout and how the brain is patterned.
Most vertebrates (including us) have four Hox clusters. The lancelet, one of the closest living vertebrate cousins, gets by with only one. Our extra hox clusters originated during two ancient genome duplications in our distant ancestor. One cluster became two, two became four. The protovertebrates put these additional Hox clusters to good use - they evolved brains, skull and jaws. The lancelets remained brain-, skull- and jawless.
This double duplication wasn't enough for fish. The genes of the ancestor of teleosts went through a third round of duplication, creating 8 Hox clusters. But this time around, the expanded genetic toolkit did not give rise to new organs and bones. On the contrary: most fish got rid of their extra Hox genes and clusters. Zebrafish lost their 8th cluster entirely for example, whereas killifish lost the 6th. Except for the eels, of course. They just kept them all.
The Dutch researchers suggest that this high Hox cluster count allows the eels to unite two vastly different body plans into one life cycle. The transition from leptocephalus to glass eel is certainly dramatic. The gelatinous skeleton of the leptocephalus is absorbed and replaced by bone, the larvae shrink to half their size and the ribbon-shaped body becomes cylindrical. It seems likely that the larvae have to tap into the developmental potential of their extra Hox genes to pull this of, although the researchers haven't tested this yet (they only studied Hox activity in the early embryo).
Of course the mere number of hox genes is not a direct measure of complexity. The slight shifts in timing and location of their activity are what make the difference for a developing larva. Yet there exist a second example of a creature with additional Hox genes and a life cycle that matches that of the European eel in its complexity. The salmon. It might not be a coincidence that the salmon's genome has been duplicated a fourth time.
Charting the long and winded life cycle of the European Eel has taken centuries so far, but time might be running out. Long generation times make the creatures vulnerable to overfishing and habitat destruction. Eel populations have collapsed in recent years, up to the point where the IUCN has redlisted the species. It's a small tragedy that fishermen are still allowed to catch eels young and old. Glass eels looking to return to Europe are harvested in Great Britain, Spain and France. Adult silver eels are caught as they leave for the Sargasso sea in countries like the Netherlands.
Fort Europe. Unreachable, inescapable.
Henkel CV, Burgerhout E, de Wijze DL, Dirks RP, Minegishi Y, Jansen HJ, Spaink HP, Dufour S, Weltzien FA, Tsukamoto K, & van den Thillart GE (2012). Primitive Duplicate Hox Clusters in the European Eel's Genome. PloS one, 7 (2) PMID: 22384188
Eel life cycle from reference.