Today I’m at the Swedish Museum of Natural History to listen to some great talks, and maybe promote my species illustrations, at a symposium on fish genetics. Right now I’m listening to a talk about sex determination in fishes and was reminded of the half-smooth tongue sole (Cynoglossus semilaevis), which you can see in this image. An analysis of this species’ whole-genome sequence found that they have the same pair of sex chromosomes found in birds, Z and W (references below). Different sex determination systems have evolved several times in vertebrates, and teleost fishes, like the tongue sole, have remarkably complex and elastic sex determination, with several different sex determination systems. So it’s remarkable that the Z/W sex determination system is so ancient. Many species of teleost fishes can change sex during their lifetimes, depending on different environmental cues, and the environment can also determine the sex of the individual during embryonic development.

This and several other recent additions to my collection of species illustrations are freely available from a shared folder: Illustrations - Shared Copy folder.

Chen, S. et al. (2014). Whole-genome sequence of a flatfish provides insights into ZW sex chromosome evolution and adaptation to a benthic lifestyle Nature Genetics, 46 (3), 253-260 DOI: 10.1038/ng.2890
Graves, J. (2014). The epigenetic sole of sex and dosage compensation Nature Genetics, 46 (3), 215-217 DOI: 10.1038/ng.2903

“The brain is more than an assemblage of autonomous modules, each crucial for a specific mental function. Every one of these functionally specialized areas must interact with dozens or hundreds of others, their total integration creating something like a vastly complicated orchestra with thousands of instruments, an orchestra that conducts itself, with an ever-changing score and repertoire.”
Oliver Sacks, The Mind’s Eye

Burmese python, Python (molurus) bivittatus.
Illustrations - Shared Copy folder

Source image by Flickr user cowyeow. Used with permission.

“I feel weird about eating these days or leaving the house, or existing in a material form at all, because having a body that talks too much and sweats and makes mistakes is exhausting…”

Senegal bichir, Polypterus senegalus.
Illustrations - Shared Copy folder

This is an evolutionary tree of vertebrates highlighting the bichirs, a basal lineage of ray finned fishes (thick blue line above). The Senegal bichir Polypterus senegalus was used as a model for the evolution of terrestrial locomotion in a recent study (reference below). This tree is meant to illustrate the phylogenetic perspective I thought was missing a bit from that study, and especially from a write-up of the study in National Geographic - Evolution’s Baby Steps.

The simple narrative to present this story is that bichirs represent a sort of stop in the transition between aquatic vertebrates, “fish”, and us terrestrial vertebrates. We all know the trope of the fish that crawled up from the sea. On the surface it seems reasonable to assume that bichirs are an evolutionary remnant of that primordial land-living fish; a primitive evolutionary relic somehow stuck in the middle to this day. After all, bichirs can survive out of the water and use their pectoral fins to help propel themselves on land for short distances. They even have simple lungs to help them breathe air. Carl Zimmer’s write-up for National Geographic seems to go straight for this simple narrative, reproducing the trope that “fish” are a primitive stage of vertebrate evolution with the “potential” to one day become us land-living vertebrates.

This is unfortunate. The reality is a bit more complicated and far more exciting. To start with, bichirs are completely removed from the emergence of four-limbed land vertebrates. Bichirs are ray-finned fishes (in blue above) and the transition to land occurred in the lobe-finned fish lineage (in pink above). The time period for the transition to land in the late Devonian, around 370 million years ago, is roughly marked with a grey box in the tree. Bichirs are more closely related to teleost fishes - what we usually think of when we think of a “fish”.

If you locate four-limbed vertebrates or tetrapods in the tree you can see that we are surrounded by “fish” on all sides. In fact, it is entirely possible to argue that we are “fish” as well.

Until we tend to call tetrapods (including ourselves) lobefin fish and accept that all living vertebrates are fish, the term ‘‘fish’’ should be used with caution. - Source

From a phylogenetic perspective, a lungfish would be a better model for the evolution of terrestrial locomotion. However, they don’t use their fleshy fins for terrestrial locomotion, even though they are able to survive and move on land.

So the straightforward narrative of the progression from “fish” to land living vertebrates is, at best, incomplete. It leaves out most of the fascinating diversity of vertebrates, the majority of which are teleost fish, and it ignores all the complex and not at all primitive adaptations found in now living “fish”.

The Nature paper describing the study does a better job at justifying why bichirs can be a good model for the evolution of terrestrial locomotion. The authors highlight a series of features that are similar to those of early lobe-finned fishes, like "an elongate body form, rhomboid scales, ventrolaterally positioned pectoral fins and functional lungs", as well as tetrapod-like features like terrestrial locomotion using the pectoral fins. Usually bichirs use their pectoral fins to prop themselves up on the bottom of the lakes and rivers where they live, almost looking as if they were doing pull-ups.

The authors also make a sort-of argument based on the phylogenetic position of bichirs, mentioning that they are the now-living fish closest to the common ancestor of both ray-finned fishes and lobe-finned fishes. This is of course an exciting perspective, but the point is not developed further in the paper. You are left to speculate that perhaps the adaptations that allowed early lobe-finned fish to adapt to terrestrial life were present already in a much earlier ancestor. Although I am not aware of any other line of evidence that would support this. It’s also worth mentioning that the presence of lungs and terrestrial locomotion is not necessarily a primitive feature, as demonstrated by the mudskipper, a teleost fish.

In any case, the aim of the study was to provide experimental evidence for the proposed role of phenotypic plasticity in the evolution of terrestrial locomotion. Both the morphologic and behavioural changes associated with terrestrial locomotion could have arisen relatively quickly through the fine-tuning of the phenotypic variation already found in the population. If it can happen in bichirs today, it could have happened in the late Devonian.

Standen, E., Du, T., & Larsson, H. (2014). Developmental plasticity and the origin of tetrapods Nature, 513 (7516), 54-58 DOI: 10.1038/nature13708

A small image I made to illustrate bioinformatic sequence analysis and phylogenetic tools.

“It is not birth, marriage, or death, but gastrulation which is truly the most important time in your life”

Lewis Wolpert (via icantbelieveitsnotscifi)

I had the great pleasure of sharing a dinner table with Lewis Wolpert after a Darwin bicentennial seminar in 2009. A fantastically entertaining storyteller. I told him about the mites that live on our skin and in our follicles, like Demodex, and he listened with great astonishment and surprise. Goes to show everyone has their knowledge gaps. I don’t remember how we got into that subject though…

(via molecularlifesciences)

I’ve added new chondrichthyan species to my pool of illustrations. Because who isn’t a fan of cartilaginous fishes? They are small-spotted catshark (Scyliorhinus canicula), little skate (Leucoraja erinacea) and elephant shark (Callorhinchus milii). They represent the three major groups of now living cartilaginous fishes: sharks, rays and skates, and chimeras. So despite the name, the elephant shark is in fact not a shark. They are also emerging as important model organisms in the fields of genomics, developmental biology and molecular biology.

You can download and use these illustrations under a Creative Commons license by going to this link: Illustrations - Shared Copy folder.

One in a series of “arrangements” by photographer Emily Blincoe. At once both harshly objective and softly romantic. Wild.

A beautiful photo of a Senegalese bichir (Polypterus senegalus) from Carl Zimmer’s latest blog post for National Geographic. Unfortunately, I think the post and the Nature paper both get totally confused about the term “fish” (there is no such thing) and our common ancestry with early transitional tetrapods and the bichirs. They also completely miss the point about why bichirs are such awesome creatures. I might write a short post about that soon.


Unsung Seafood (for Men’s Health Magazine), sumi ink, pen & digital.


Tiger Beetle, for The Nature Conservancy Magazine

AD Katie Lesser

“But the chief cause of our natural unwillingness to admit that one species has given birth to other and distinct species, is that we are always slow in admitting any great change of which we do not see the intermediate steps.”
— Charles Darwin. "On the Origin of Species…" Chap. XIV. Recapitulation and Conclusion. 1st ed., p. 481. From the beautiful chapter that summarises the whole work and finishes with the more famous "endless forms most beautiful" quote.
“The human understanding resembles not a dry light, but admits a tincture of the will and passions, which generate their own system accordingly; for man always believes more readily that which he prefers. […] in short, his feelings imbue and corrupt his understanding in innumerable and sometimes imperceptible ways.”
— Fracis Bacon. Novum Organum (1620).