If fish evolved on land, where did they all go? Evolution and Biodiversity in the Ocean

When Carl Sagan described our planet as a “pale blue dot” he was invoking the fact that, despite being called Earth, our world is mostly Ocean. The surface of the Earth is a little more than 70% water and the ocean accounts for 98-99% of our total biosphere–the volume of the planet that can support life. Most contemporary theories point to ocean ecosystems–like deep-sea hydrothermal vents–as the launching point for the emergence and evolution of life. Ocean processes dominate biological interactions, even among unwitting terrestrial actors. A new paper, published in the Proceedings of the Royal Society: Biological Sciences, revisits an old debate about the ocean biodiversity and challenges the notion the ray-finned fishes have a marine origin.

In Why are there so few fish in the sea? the authors begin with the seemingly innocuous question–why are there so many more species in terrestrial environments than in marine environments?  From there, they look at species counts, phylogenetic relationships, and diversification rates to determine the ancestral state of the most recent common ancestor of one fish class, Actinopterygii, the ray-finned fish. What they found was that, despite the vastly smaller habitat available for freshwater fish, the number of actinopterygian species found those ecosystems was roughly equivalent to the number of species found in marine systems. In both systems, the dominant groups are relative newcomers on the evolutionary stage, with superorder-level radiations happening between 111 – 150 million years ago.  Most surprising, the authors discovered that the most recent common ancestor of actinopterygians may have been a freshwater, not marine, fish. Ray-finned fishes may have invaded the ocean from lakes and rivers.

One claim that this paper most emphatically does not make is that fish evolved on land. While the term “terrestrial” is generally synonymous with land, in this paper it includes freshwater aquatic habitats–lakes, rivers, streams–that are (mostly) connected to marine systems. The authors used this framework to compare a group of fishes that occur in both marine and freshwater systems and use that comparison to provide a complementary perspective on the relationship between marine and true land-based ecosystems. This misinterpretation has led several news sources to run with variations on a misleading headline–“Most fish actually evolved on land, not the sea (now corrected)“. To be clear, all fish evolved in the ocean, but one recent class of bony fish may have originally radiated from freshwater ecosystems.

Headlines aside, I have a more fundamental problem with this paper, and that is the assumption that the authors establish as the intellectual motivation for their project. The paper is built around the observation that general biodiversity is greater in terrestrial ecosystems than in marine ecosystems, citing the ocean as containing only 15 – 20% of all species. I have two major problems with that argument. The first is that simple species counts are not necessarily the only, or the best, measure of biodiversity. The second is that it is very likely not true that terrestrial ecosystems have more species than marine ecosystems.

What we talk about when we talk about biodiversity

Biodiversity is one of many metrics we use to measure the health of an ecosystem. In general, it can loosely be defined as the degree of variation among organisms in a given region. One way to measure that variation is to count the number of species in a given region. This is Species Richness. You could also count the number of species, but than weight that by how many individuals of each species occur in the region. Species with more numerous individuals have a greater impact on the region than relatively rare species (though rare species can be important regulators of ecologic function, too). This is Species Abundance. These are relatively simple metrics to calculate, and they provide a simple, easy to compare baseline for biodiversity.

Richness and abundance are not the whole story. Biodiversity is a measure of variation among organisms within a region, and variation is not solely reflected by species-count. Take a look at these two phylogenetic trees. The one on the left shows a group of 10 species that are all fairly closely related to each other. The other shows only two species, but they are much more distantly related to each other. To put it another way, would you find more diversity at your family reunion having cocktails with 50 first cousins or on a 12-person jury in New York City?

Yes, number of species matters, but when discussing biodiversity we need to also consider the degree of variation among species (and, for that matter, the degree of variation within species). Evolution is a continuous, fluid process, and what are not yet species today may become species tomorrow. Conversely, what is a group of closely related species with overlapping ecologic functions today may become a single dominant species that out-competes the others tomorrow.

If we look at global animal biodiversity at the highest taxonomic level, we find that, broadly speaking, there are approximately 35 animal phyla. This number varies with new discoveries and may be significantly higher or lower depending on whether the person defining phyla is a lumper or a splitter, but 35 is commonly recognized as a reasonable current estimate for the total number of animal phyla. Of those, Acoelomorpha, Brachipoda, Bryozoa, Chaetognatha, Cnidaria, Ctenophora, Cycliophora, Echinodermata, Entoprocta, Gastrotricha, Gnathostomulida, Hemichordata, Kinorhyncha, Loricifera, Orthonectida, Phoronida, Placozoa, Porifera, Priapulida, Rhombozoa, and Xenoturbella are either exclusively marine, or mostly marine with some freshwater groups. That’s 21 phyla, or 60% of all known animal evolutionary biodiversity, that calls the oceans its home. In contrast there is only a single phylum (Onychophora) that occurs exclusively on land. Perhaps that better question is “why is there so little evolutionary biodiversity on land?”

But is it NumberWang?

So, what if we assume that species richness is the metric we want to use for global biodiversity? If that is the case, then are there really more species on land than in the ocean? Even within this paper, the authors argue that Actinopterygians account for 96% of all fish and roughly 50% of all vertebrates. Half of all vertebrates are subsumed by this one class of fish! So, without even including sharks and other elasmobranchs, lobe finned fish, jawless fish, chimeras, and a host of other vertebrates, we’ve already accounted for half of all vertebrates. Add those other groups in, which, remarkably, still only consist of 4% of all fishes, and we’re slightly over half of all vertebrates that aren’t terrestrial. But that is only a tiny portion of the diversity of life.

So where does this idea that 75 – 85% of all species are terrestrial come from? For animals, the short answer is insects, perhaps the most specious class of animals, with an estimated 6 to 10 million different species, account for almost half of all known species on Earth. Looking at all eukaryotic organisms, plants hold their own at almost 19% of all known species. Those are some pretty big numbers, but let’s take a step back and look at who’s doing the describing, counting, and cataloging. Terrestrial organisms, that live, work, and play largely in the terrestrial world, are responsible for the catalog of life on earth. This means that, in our exploration of the natural world, there is a profound sampling bias towards terrestrial species.

How profound is this bias? Last year an entire kingdom, Cryptomycota, was discovered that is almost exclusively found in aquatic environments. In general, fungi are almost universally regarded in these diversity estimates as being exclusively terrestrial, but dozens of new papers (including one by me) are finding new fungal species in marine systems. The recently completed Census of Marine Life cataloged 250,000 marine species, and estimates that there are at least another 750,000 to go. This would practically match the current counts for insects and plants, and with a potential for upwards of 10 million marine species, dwarf all other estimates. And that’s without including the estimated 1 billion marine microbes. Those numbers also don’t include the oft-forgotten freshwater ecosystems, which the paper under discussion demonstrated hold half of all ray-finned fish, among many others.

Overall, the land is looking like a pretty lonely place to be.

Let’s get back to the Evolution River Show

Ok, so I didn’t like the framework this paper was built around, but what about the study itself? The findings of Vega and Wiens are pretty cool and, while counter intuitive, make evolutionary and ecologic sense. Freshwater systems, being physically isolated from each other by land and having greater temporal variability should drive more rapid diversification than relatively stable open ocean ocean ecosystems. When connectivity is high, like it is for mobile, pelagic species, you would expect rates of genetic differentiation to be lower. What they found was, despite the total available habitat being much lower in freshwater than marine ecosystems, the number of species and the rate of speciation was roughly equivalent. This means that factors other than freshwater/marine ecosystem are driving speciation.

The more surprising finding was that the basal members of the actinopterygian are all derived from freshwater systems, suggesting that 96% of fish can trace their evolutionary ancestry back to a freshwater ancestor. Only after diversifying did the actinopterygians than radiate back into the open ocean.

It is unfortunate that the authors hung an otherwise very solid study on the framework of “why there are so few marine species” because that first principle shapes their attempts to interpret their data. Assuming that freshwater ecosystems are an appropriate proxy for terrestrial ecosystems when comparing freshwater and marine species, the data indicate not that there are fewer species in marine systems, but that there are roughly equal numbers of marine and terrestrial species, at least when talking about vertebrates. Ecologic factors could drive the original invasion of freshwater systems by proto-actinopterygians. Freshwater ecosystems would have been an unoccupied niche, free of some of the largest and fiercest marine predators to ever live (including that 300-million year old mainstay, the shark). By occupying a niche that effectively excluded the dominant aquatic predators, actinopterygians could have diversified until they filled that niche, before radiating back out into the ocean, but that is entirely speculative on my part.

Overall, Why are there so few fish in the sea? presents a fascinating case study in evolution that suffers from a terra-centric view of biodiversity.

Carrete Vega, G., & Wiens, J. (2012). Why are there so few fish in the sea? Proceedings of the Royal Society B: Biological Sciences DOI: 10.1098/rspb.2012.0075