asymmetrical
[èisəmétrikəl, æ̀sə-]
(식물·수학) 비대칭의.
The Pleuronectidae, or Flat-fish, are remarkable for their asymmetrical bodies.
가자미과는 비대칭적인 몸체가 특징이다.
How the flatfish arose in the blink of an evolutionary eye
Flatfish are one of evolution’s more bizarre creations. Yet recent research has suggested their apparently unique morphology may have evolved more than once.
A new study, recently published in BMC Evolutionary Biology, investigates just how unique flatfish are, and discovers that the most remarkable thing about them may be the speed at which they evolved.
A most remarkable face
As one would expect from a group that includes 99% of known fish and almost 1 in 3 vertebrate species, the spiny-rayed fishes are incredibly diverse. However, even amongst these incredibly diverse body-plans and lifestyles, flatfish stand out as being singularly unusual.
Flatfish, which include many familiar food fish like sole, plaice, and halibut, are defined by a bizarre adaptation: a profound cranial asymmetry resulting from one eye migrating to the opposite side of the skull during larval metamorphosis; or, in simpler terms, they have both eyes on one side of their head.
This extreme re-sculpturing of their skulls enables adult flatfish to rest on the seafloor on their eyeless ‘blind’ side, with both eyes pointing upwards (towards potential predators or prey) unobstructed by sediment. This unusual adaptation has clearly been successful for flatfishes, with over 700 species currently in existence.
A not so unique adaptation?
Flatfish can be divided into two groups: the three species of spiny turbot that make up the family Psettodidae, and the much larger suborder Pleuronectoidei.
Unsurprisingly, fish biologists long assumed that both groups of flatfish evolved from a single common ancestor; it is hard to imagine such a bizarre adaptation having evolved multiple times.
Recently however, this common-sense assumption has come under attack. Several studies have found support for the distinct flatfish adaptation having evolved on two separate occasions. Is the flatfish body-plan not as unique as it appears?
Hard to say, has been the honest answer up until now. While there is little doubt that flatfish belong to the wider fish group carangimorpharia (which also includes marlins, swordfish, and remoras), the exact evolutionary relationships among this group are uncertain.
Eight recent phylogenetic studies conducted on carangimorphs have produced eight different evolutionary trees, some of which support a single origin for flat-fish, others of which support multiple origins