Flatfish and Sharks

Dear Readers,

This post will talk about whether several flatfish observed illustrate convergent evolution or homologous evolution. We will also discuss the difference between sharks and 'ordinary fish' and how they are easily distinguished from one another. 

Flat Fish

Those visible in the large monumental hall were The European Plaice/ Pleuronectes platessa and Sole/ Solea solea. 

European Plaice and Solea Solea

Both flatfish come from the same order being Pleuronectiformes. They are both found in similar habitats like the Mediterranean and enjoy burrowing themselves in the sandy or muddy seabed. Since they come from the same order we can hypothesize that both the Common Sole and European Plaice have a common ancestor. 

Common Stingray with leopard shark directly above

Unlike the two flatfish, Dasyatis pastinaca (Common Stingray) shares the same phylum with the flatfish, however not much else. Diverging into the class Chondrichthyes. Much like the flatfish they can be found in the Mediterranean and in the northeastern Atlantic Ocean around southern Norway. They dwell at the bottom during the day, favoring sandy or muddy bottoms, and become more active during the night. While they do seem to be similar we can see this as convergent evolution. Stingrays have more in common with sharks as they are part of the Chondrichthyes (cartilaginous fish) class while flatfish are part of the Actinopterygii (ray-finned fish) class.

Sharks

While many see sharks as being fish they are actually very easily distinguishable. Unlike Fish, sharks have many gills ranging typically between five to seven on the sides of the head (elasmobranch fish) and a cartilaginous skeleton. They are typically classified within the clade Selachimorpha while fish are seen as Olfactores. Since sharks are able to create powerful movements of the trunk and have the tail propel them forward, the water flows into the shark's mouth and out through the gills (gas exchange occurs here). While this occurs while moving, they do spend a vast amount of time resting, while resting they use muscles of their jaws and pharynx to pump water into themselves. 

Short-tail Nurse Shark/ Pseudoginglymostoma brevicaudatum

Leopard Shark/ Triakis semifasciata

Red-tailed Shark Minow/ Epalzeorhynchos bicolor

Small Spotted Cat Shark/ Scyliorhinus canicula

In the end Sharks are more closely related to rays than sharks as they are within the same Chondrichthyes class, subclass Elasmobranchii. 

Until next time,

Sol