Extreme lifestyle in the deep sea

In a bizarre deep-sea version of foie gras, male whalefish stuff themselves with copepods early in life to fatten their liver, and then stop eating and consume their own liver while pursuing mates. The lifestyle choice is so extreme that these perversely named small fish actually lose their stomach and esophagus while relying on energy stored in their distended liver (see photo, right).

Female whalefish, being more conventional, continue to eat and do the other normal things that fish do. Males and females of the deep sea whalefish are so different that they were thought to be different species until DNA sequencing revealed the strange truth.

The habits of male whalefish rival the extreme adaptations to the deep sea of the bizarre male anglerfish which transforms into a parasitic appendange of the female anglerfish.

An interesting scientific backstory is that male, female, and juvenile whalefish are so different that they were thought to be three different scientific families of fish, only very distantly related. How strange is it that they're really all the same species? According to the Virginia Institute of Marine Science press release:

To put the team’s discovery into perspective, consider that cats, dogs, and walruses also represent three biological families. It’s as if the researchers discovered that dogs are really male walruses, and that kittens aren’t cats but a walrus’s juvenile form.

Their findings, published in the January 22 issue of Biology Letters, represent “the most extreme example of metamorphoses and sexual dimorphism ever documented in vertebrates.”

From the more turgid scientific paper:

We resolve a long-standing biological and taxonomic conundrum by documenting the most extreme example of ontogenetic metamorphoses and sexual dimorphism in vertebrates...Larvae have small, upturned mouths and gorge on copepods. Females have huge gapes with long, horizontal jaws and specialized gill arches allowing them to capture larger prey. Males cease feeding, lose their stomach and oesophagus, and apparently convert the energy from the bolus of copepods found in all transforming males to a massive liver that supports them throughout adult life.

hat tip: Branch

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