Fish Species Classification

There are over 28,000 different species of fish in the world, several hundred of which are present in British waters. There are many different ways of classifying fish, and anyone doing any research into the types of fish which are found around Britain will soon come across numerous terms which are used to divide fish into different groups and categories. most anglers simply divide fish into broad groups such as flatfish, roundfish, skates and ray, shark species etc. However, looking deeper into this subject reveals that the official scientific system of classifying fish (and indeed all living animals) takes a much more complex form. All living life can be places into the following hierarchy of biological classification.

Bony Fish

Monkfish Skeleton

This skeleton of a monkfish shows the number of small bones which are a feature of bony fish.

The vast majority of fish found in British waters fall into the class of Osteichthyes or bony fish, so called because their body is based around a skeleton made up of lots of small bones. Anyone gutting a cod, haddock or similar fish will see how many bones are present in this type of fish. Other characteristics which bony fish possess include one gill slit on either side of the body and one or more sets of paired fins. All bony fish also have scales on their body – even so-called scaleless fish such as fish such as rockling actually have tiny scales which are embedded into the skin and are not visible, but they are present. Bony fish are the most common type of fish not just around Britain and Ireland but around the world – of the 28,000 species of fish found worldwide approximately 27,000 are classed as bony fish. Bony fish are sub-divided into two categories: ray finned fish and lobe finned fish.

Ray Finned Fish: Bony fish which have fins made up of spines (rays) with skin stretched between them fall into the classification of Actinopterygii (ray finned fish). The majority of fish species caught by anglers around the UK fall into this classification. Bony fish can be further divided into flatfish and roundfish – but they are all ray finned bony fishes.

Ray Fin

The dorsal fin of a cod is made up of spines with skin stretched between them.

Examples in British Waters: Cod, haddock, flounder, plaice, rockling, hake, dab, John Dory, sprat, mackerel, king of herrings, pouting, sunfish, coalfish, pollock, bass, mullet and many others.

Lobe Finned Fish: The other type of bony fish are in the class Sarcopterygii and are known as lobe finned fish. These fish do not have fins made up of rays with skin stretched across the, but instead have fins made up of lobes – fleshy appendages which are attached to the body. Lobe finned fish have been around for hundreds of millions of years and are important evidence of evolution as the lobes developed to help fish get out of the sea and move across land – a key step in animal life moving from the sea and to the land. While there were many species of lobe finned fish around millions of years ago the vast majority are now extinct.

Coelacanth

Coelacanth are an example of a living lobe finned fish.

Examples in British Waters: None. Most lobe finned fish are now extinct. The several species of freshwater lungfish are examples of lobe finned fish which survive to this day, as are the two species of coelacanth which live in Indian and African waters.

Cartilaginous Fish – Sharks, Rays and Skate

Cartilage is connective tissue, found in many animals including humans (for example in the nose), it is more flexible and less brittle than bone, while also being strong and durable.  Shark and ray species have a skeleton made out of cartilage, rather than bone (as well as several other physiological differences) which puts them into the Chondrichthyes class. Out of the 28,000 species of fish in the world around 970 are in this class. In British and Irish waters the main cartilaginous fish species are sharks, rays and skates.

Dogfish Fin

The first dorsal fin of a lesser-spotted dogfish, displaying the absence of rays.

Sharks: Shark species have many differences to bony fish. As well as the cartilaginous skeleton all shark species fins which are all one section and are not divided into segmented sections with spiny-rays like in bony fish. Sharks also have multiple gill slits (usually four to six on each side of the body), whereas fish only have one on each side. Another feature of sharks is that they do not have scaly skin like bony fish, but have skin made of dermal denticles (translation: little teeth) which makes all shark species rough to the touch. Sharks also lack a swim bladder, meaning they have to keep swimming or they will sink and have paired nases (nostrils). This way of classifying sharks explains how the humble lesser-spotted dogfish and the great white shark are classed together: both have cartilage skeletons, paired nases, non-ray fins, rough scaleless skin and multiple gill slits. Any angler catching a dogfish really is telling the truth when they say they have caught a shark!

Examples in British Waters: Common shark species such as lesser-spotted dogfish, bull huss, spurdog, smooth-hound, tope and the large shark species which are found around the UK such as blue shark, thresher shark, basking shark and the shortfin mako.

Skates and Rays: Read anything about skates and rays and it will often say that they are related to sharks. This is because they also feature skeletons made out of cartilage and do not have ray-fins. Like sharks rays and skates also lack a swim bladder, meaning they can only move up through the water column with swimming power, and also have paired nases. Despite their very different appearance to sharks both skates, rays and sharks may all come from a common ancestor many hundreds of millions of years ago. However, since cartilage does not remain preserved in the same way bone does it is very difficult for scientists to trace the history of these creatures through fossils in the same way they can with fish.

Examples in British Waters: All ray species (thornback, blonde, small eyed etc.) and the common skate.

Other Cartilaginous Fish: A rare example of another type of cartilaginous fish in the waters of the British Isles is the range of rabbit fish species. This is a species of fish which lives in very deep water (1000 – 3000m) and is never encountered by rod and line anglers. Rabbit fish species have a cartiligious skeleton and non-ray fins which puts them into the same Chondrichthyes class as sharks. However, rabbit fish are placed in the Chimaeriformes order as they are thought to have broke away from sharks in evolutionary terms several hundred million years ago and have remained a separate order ever since. Lampreys also have a skeleton made out of cartilage but defy easy classification (see below).

Other Types of Fish

Eels:

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Eels are in the ray finned bony fish class of Actinopterygii. This may seem strange as they look quite different to fish such as cod and pollock, but physiologically they do have a lot in common. Like bony fish they have a skeleton made up of many small bones and their fins are made up of skin stretched across rays. They also have paired fins, a gill slit on either side of the body and do have scales – although they are very small and buried deep in the skin meaning they are often described as being scaleless. Many species such as ling, hake, tusk, butterfish and rockling are mistaken for eels as they do look superficially similar, but these species are all ray finned bony fish. True eels split away from fish at the order level where they are placed in the Anguilliformes order. The visible differences between eels and fish are as follows: The caudal (tail) fin, dorsal fin and anal fin of eels merges into one single fin and continues all around the body. Eels also typically have smooth bodies which appear scaleless (but are not), and are covered in a layer of slime. They generally lack pelvic fins, but have a small pair of ray-finned pectoral fins. The only two species of true eel which are commonly found in UK coastal waters are the conger eel and silver eel (also known as the common or European eel), although there are a number of very deep sea eels living at depths of 1000m or more such as the cut-throat eel and the slender snipe eel.

Lamprey

Lamprey are a difficult animal to classify.

Lampreys: In the UK the sea lamprey is a rare example of a species which throws the classification system into disarray. They are ancient species which do not seem to have evolved in any way in several hundred million years and do not fit any of the current classifications. Superficially lamprey look like eels but they cannot be classed as an eel species as lampreys have a skeleton made out of cartilage, while eels are bony fish. This skeleton would mean that they are a member of the shark family, but they cannot be as they do not have rough skin made out of dermal denticles, or paired nases. Lamprey also lack jaws (they have a toothed suction plate instead of a mouth) and all shark species are jawed. There is currently no scientific consensus on how to classify lampreys, and some encyclopaedias and websites do not classify them any further than vertebrates – the broadest possible classification as it includes all creatures with a spine. Other sources class them as Agnatha – a class of jawless fish, of which the only extant species are the lampreys and hagfish (Myxine glutinosa).

Summary

The correct scientific classification of fish species is a complicated business. By finding out about the difference between species we can being to understand  how the animal kingdom is understood. For example many anglers will know that sharks ad rays are related, but understanding that both have cartilaginous skeletons and do not have ray fins we can see how they are related. While the scientific classification of fish may not be on angler’s minds while they are fishing it is interesting to develop a deeper understanding of the fish that are present in the waters around the British Isles.

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