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Specimen of the Week 268: The carp – How things got fishy

By Jack Ashby, on 2 December 2016

This week in The Conversation I wrote that there is no biological definition of fish that doesn’t involve humans. However the group that most people recognise as the fishiest are the ray-finned fishes. They have fins supported by a series of fine flexible rods. It is the ray-fins that have gone on to be the dominant vertebrates in the seas, lakes and rivers: there are around 30,000 species. This makes them by far the most diverse vertebrate group, and I’d like to explore how that happened. Among them is this week’s Specimen of the Week:

Common carp skeleton LDUCZ-V543

Common carp skeleton LDUCZ-V543

**The common carp skeleton**

Very fishy fish

Carp, salmon, cod, guppies, goldfish, tuna, sturgeon, plaice, seahorses, perch, bass, and clown fish are all ray-finned fishes. Think of a living “fish” that isn’t a shark, ray, chimera, lungfish or coelacanth and it will be a ray-fin.

The sea is a big place. It’s a lot more three-dimensional than other environments (at its deepest, in the Pacific’s Mariana Trench, it’s 2500km deep); there are a huge range of habitats as one ventures through the depths, across the latitudes, and along the different kinds of shore. Other aquatic environments on land (such as lakes and rivers) also differ depending on the climate and ecosystem of the world around them. This means that fishes have a lot of different niches that they can adapt to, which helps explain some of their diversity. On top of that, ray-finned fish have made some modifications to the way they move and feed which has allowed evolution to get to work on producing many ways to be a fish.

Evolving diversity

A trend in the evolution of ray-fin fishes is that the paired fins (the pelvic and pectoral fins that stick out roughly sideways) tend to become smaller and more manoeuvrable through time. Looking at early fossil members of the group, like Cheirolepis from the Mid Devonian of Scotland, its fin rays are covered in dermal bone and are very immobile. Looking at a more modern species like this carp, the fins are small lack a bony covering. This reflects the fact that earlier jawed vertebrates had to generate lift using their fins as hydrofoils to stop them from sinking (like an aeroplane wing, but in water). While other groups developed other means of counteracting this – like the oily liver of sharks – the ray-finned fishes evolved swim bladders.

Swimming still, swimming fast

These structures allow the fishes to remain motionless in the water without sinking, and so the paired fins were no longer required as hydrofoils and could be reduced and modified. These smaller fins enable many ray-finned fishes to have a great of agility in the water. Combined with speed this can make them very efficient predators.

Exploded cod skull, showing how the bones around the mouth have been detached from the cheek. LDUCZ-V1463

Exploded cod skull, showing how the bones around the mouth have been detached from the cheek. LDUCZ-V1463

Loose lips

By the Late Permian Period (around 250 million years ago) more advanced groups of ray-finned fishes had freed up much of the bone in their skulls. This was probably the most significant event in the evolution of modern fish feeding mechanisms since the development of the jaw. Detaching the bony element of the jaw from the cheek and developing related muscles allowed the jaw to be swung forward when the mouth opened (picture a goldfish pumping its mouth). As the mouth is pushed into a rounded gape, a region of negative pressure is developed inside it and this causes suction, pulling food into the mouth.

The evolutionary consequences of fish lips

The diversity of ray-finned fishes– covering the shallow seas, ocean depths, rainforest rivers and mountain lakes and pretty much every other watery habit across the globe – is arguable a consequence of the evolutionary potential resulting from the way they have modified their skulls and fins.

The important steps in their history include the evolution of jaws by the ancestors; the increased mobility of their fins; the arrival of the swim bladder which enabled more precise movements; the freeing up of bones in the jaw and the development of mechanisms to increase the power and efficiency of the bite. The ray-finned fishes are a huge and global group, their success is no doubt related to their specialised modes of moving and feeding.

Jack Ashby is the Manager of the Grant Museum of Zoology


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