What is a fish?

Since a lot of what I’ll be posting will have to do with placoderms and other marine animals I thought it would be a good idea to explore what the word ‘fish’ actually means. What makes something a fish? The question is not as simple as it first appears, we all know in general what we mean when we refer to fish in conversation but this is partially due to the fact that the overwhelming majority of fish today belong to the actinopterygians (ray-finned fishes) which includes everything from gars to goldfish. All these fish share a similar body with paired fins, a tail fin, gills, a bony skeleton, and swim bladders. So let us take the idea that fish are water-dwelling organism with fins, scales, gills, and who are poikilothermic (their body temperature fluctuates with the temperature of their environment). This covers most of the organisms you would refer to as fish in normal conversation but what about eels that don’t have the paired fins or scales? Or the even stranger (and uglier) lampreys and hagfishes that also lack jaws in addition to scales and fins? There are even some fish that can regulate their body temperatures both by more active circulation (Salmon sharks)1 or by producing antifreeze proteins in their blood2. For every rule defining fish there are many exceptions to counter them. The reality is that the group we refer to as ‘fish’ is a grouping of human convenience for swimming organisms in the water, which usually possess scales and gills.

A quick side note I feel obligated by a former professor to pass on! Fish is the proper plural when referring to multiple individuals of the same species while fishes is used when referring to a group of individuals containing more than one species.

Today if we consider only the kinds of fish with scales, paired fins, and gills (teleosts) they are the most diverse group of vertebrates on the planet. If we look at the evolutionary history of marine organisms similar to teleosts there was an even greater diversity in the past. The figure below is from a review of fishes over the last 500 million years and I would highly recommend giving it a read if you have any interest in ichthyology (reference is at the end of the post). The first thing you might notice is that all terrestrial vertebrates (including you dear reader!) are descended from the sarcopterygians (lobe-finned fishes) and thus a technically a fish. Those swim bladders I mentioned earlier are primitively lungs so it is not just land animals that have lungs but the majority of vertebrates. In fact you are more closely related to a goldfish or salmon than either of those is to a shark.


Figure 1. A phylogeny of fishes, extant and extinct from Friedman and Sallan 2012.

It is also probably clear that there a large number of entirely extinct lineages of fishes. Everything from the anapsida to Osteoraci, excluding the eel-like unarmored conodonts, are historically referred to as ostracoderms or agnathans. They are primitive armored fishes that lack any jaws and had their greatest diversity in the late Silurian to early Devonian (~420 mya) hundreds of millions of years before the dinosaurs walked the land and before any vertebrates walked the land for that matter. The many lineages of ostracoderms are a fascinating array of creatures that has only recently received renewed attention for understanding the evolution of all jawed vertebrates. Unfortunately, there is still relatively little known but they more posts about them will probably appear as new publications come out.

Moving into the gnathostomes (vertebrates with jaws) one of the earliest branches were the paraphyletic acanthodians (spiny sharks), which are known from mostly fragmentary remains. The lines including the acanthodians lead on to both chondrichthyes (sharks and rays) as well as teleosts (ray-finned fish, lobe-finned fish, and terrestrial vertebrates). The other branch led to the placoderms (armor skin) which were a highly diverse group of fishes with armor around their heads and part of the trunk. Below are reconstructions of the many forms of placoderms which dominated the Devonian seas before becoming extinct at the end of that period. Placoderms include the earliest known instance of live birth4 and included the largest vertebrates to have ever evolved up to the end of the Devonian likely greater than five meters in length.


Figure 2. The placoderms Coccosteus from Wikipedia.

The next post will be a primer on placoderms before I really start to delve in on specific hypotheses, publications, or theories. I’ll attempt to answer reasonable comments in a timely manner and be glad to answer any questions.


1Goldman, K. J., S. D. Anderson, R. J. Latour, and J. A. Musick. 2004. Homeothermy in adult

salmon sharks, Lamma ditropis. Environmental Biology of Fishes 71:403-411.

2Fletcher, G. L., C. L. Hew, and P. L. Davies. 2001. Antifreeze proteins of teleost fishes. Annual

Review of Physiology 63:359-390.

3Friedman, M. and L. C. Sallan. 2012. Five hundred million years of extinction and recovery: a

Phanerozoic survey of large-scale diversity patterns in fishes. Palaeontology 55:707-742.

4Long, J. A., K. Trinajstic, G. C. Young, and T. Senden. 2008. Live birth in the Devonian Period.

Nature 453:650-652.


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