Among divers and marine biologists, it's common knowledge that ocean fish lead double lives. Like birds and butterflies, their young often look nothing like the adults, but unlike birds and butterflies, it is the young that are often more beautiful and ornate than their parents. I think this bit of natural history remains largely unknown to the wider world, though.
For instance, here is the larva (for so profoundly different are baby fish, they are given this name) of a previously undescribed species of sea bass. Sadly, it does not appear to have frickin' laserbeams or an ill temper, although it does have seven long spines, the first of which is a kind of frilly:
A photograph of this fish appeared without identification as a "decoration" in a paper on the ecology of marine fish. Smithsonian zoologist Carole Baldwin and curator Dave Johnson could see it was a sea bass larva but didn't recognize the species, and were struck by those seven spines. Almost every larva in this family bears only two spines. That piqued their interest, so they set out to see if they could identify the adult. When the DNA barcode of the larva -- a special section of DNA in the fish's mitochondria that's considered a particularly good indicator of species -- was sequenced, the little fish failed to match any known species.
The solution appeared when Smithsonian scientists separately exploring deep reef off the Caribbean island of Curaçao in a privately owned manned submarine (called, of course, the Curasub) collected some "golden basses" 300 m down (1,000 feet). After a spin through the sequencer, the scientists discovered that hiding among a species of golden bass called Liopropoma aberrans was a cryptic species -- one that physically looks so similar to another they are difficult to distinguish by eye -- that matched the DNA of their little mystery larva. And voilà! A new species was named: Liopropoma olneyi.
When the little be-spined fish grows up, it turned out, it looks like this:
Or sometimes like this:
But if you think this metamorphosis is dramatic, the paper included two even more stunning examples.
Here are the adults of Liopropoma carmabi at top, and L. rubre at botttom,
... and here are their larvae. L. rubre at left, L. carmabi at right.
Here's a closeup of that first number. Note the iridescent green flecks in the bands on the banner.
With these pictures, you can appreciate how extraordinarily hard it was to link larval fish to adults prior to the advent of DNA sequencing technology. Even with the ability to peek at their DNA, you must sometimes still open your eyes and get a little bit lucky to make a match. In fact, even with DNA barcoding, the Smithsonian scientists write, "... species identifications of larvae of most marine fishes remain elusive." Which is kind of mind-blowing, when you think about it. Because it means for most of the fish in the ocean, we still don't know what their young look like. We're not talking about microbes or copepods here. We're talking about the largest, most conspicuous, and "charismatic" creatures in the ocean.
These are but a few examples. But by now you may be wondering: why? Why do baby fish look like members of a different species?
Location, location, location. The habitat these fish swim in as adults is very different from the habitat they occupy as larvae. The larval stage of many ocean fish involves drifting at or near the ocean surface. But as adults, they live somewhere else -- a shallow or deep coral reef, perhaps, or the open ocean, or the ocean deep. Why these larval fish have these ornate streamers in particular I do not know for certain, although I can speculate that they may make them look more like a piece of flotsam, a jellyfish or seaweed than a tasty fish dinner.
In short, shapeshifting enables marine fish to adapt to both environments they live in, although it means that to our eyes, they appear to be two entirely different fish. In a way, they are.
Baldwin C.C. & Johnson G.D. (2014). Connectivity across the Caribbean Sea: DNA Barcoding and Morphology Unite an Enigmatic Fish Larva from the Florida Straits with a New Species of Sea Bass from Deep Reefs off Curaçao., PloS one, PMID: 24825118