Fish Larvae

Lefteye Flounders – One of Nature’s Most Epic Metamorphoses

Today’s larval blog focuses on “flatfishes” belonging to the Family Bothidae (pronounced both-ee-day) (~140 spp.), which are found worldwide in temperate and tropical seas. More than ten species have been recorded in the Caribbean, with eight species present in Trinidad and Tobago waters. These fishes are commonly called “lefteye flounders” because the adults lie on the seafloor on their right sides with both eyes on their left. Yes, bothids have both eyes on one side of their body…well, at least the adults do!

Amazingly, flatfishes do NOT start out flat or with both eyes on the same side of their body. Rather, newly hatched bothids, which are between 1.5 and 3 mm, have one eye on either side of their head and swim vertically – just like normal fishes. During early development however, young flatfishes lose all semblance of normalcy, undergoing one of the most difficult puberties in the animal kingdom, where they transition from cute bilaterally symmetric fishes into total anatomical anomalies.

Picture1
Larval flatfish (Family Bothidae, species unknown) with one eye on either side of its body, which is bilaterally symmetrical and dorso-laterally compressed. Photo Credit: Danté Fenolio

On hatching, flounder eggs give rise to tiny transparent larvae that live in the water column and feed on plankton like other fish larvae. A few days (species dependent) after hatching, the larvae begin to swim with a tilt and one eye starts to force its way over the top of the head to the other side, twisting the skull in the process. This is akin to your left eye migrating over your nose and joining your right eye on the right side of your face! As the eye migrates, the angle of tilt at which the larvae swims increases until the body finally becomes horizontal, swimming like a magic carpet – this is the post-metamorphic juvenile stage. If that’s not enough, the right side also loses color becoming dull white, while the left side changes color to match a sandy seafloor. Eventually, when it is large enough, the transformed flatfish with its two eyes on the left side, sinks and settles on its newly blind side. It will spend the rest of its life mostly buried in the sand, peeping out with its two bulging eyes on stalks. The eyes are able to move independently of each other to locate unsuspecting prey, grabbing it in the blink of an eye. For many species, this unassuming benthic lifestyle is also aided by their amazing powers of camouflage, which allows them to change colors in seconds – a process likely involving both vision and hormones.

Picture2
Larval flatfish (species unknown) midway through metamorphosis. The body is still transparent and the left eye is perched on top the head.  Photo Credit:  NOAA Fisheries West Coast/Flickr (CC BY-NC-ND)

So, next time you are reflecting on the trials and tribulations of your puberty and you think that mood swings, pimples, and hair growth are traumatic, think about these larval flatfishes and their transition into adulthood!

Picture3
Adult Peacock flounder (Bothus lunatus) buried in the sand. Note the two protruding eyes on top of the head. Photo Credit: Ken Clifton

NB: The Family Bothidae is just 1 of 11 families belonging to the Order Pleuronectiformes. All fishes belonging to this order exhibit this metamorphosis (eye migration). In some families the eye migrates to the left side, while in others, it migrates to the right.

Here are the links to 2 YouTube videos to help visualize some of the details in today’s blog:

This 1st video (click here) is a time-lapse of flounder metamorphosis and the 2nd video (click here) focuses on the adult peacock flounder which is commonly found on Caribbean reefs. In the video you can see its general morphology, its swimming behaviour and camouflage skills. Enjoy!

KEY REFERENCES

The Improbable—but True—Evolutionary Tale of Flatfishes by Ferris Jabar. http://www.pbs.org/wgbh/nova/next/evolution/flatfish-evolution/ Accessed online 4th April 2018

Blog Author — Dr. Farahnaz Solomon

 

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