MBLSciShoots: Evolutionary Biology and Cartilaginous Fishes

What do the development of skate gills and human limbs have in common? More than you might think. Join Andrew Gillis, MBL Whitman Scientist and incoming Associate Scientist, in our latest #MBLSciShoots digital learning lesson.

Related Reading

Hard-to-find fish reveals shared developmental toolbox of evolution | University of Cambridge

Modern research might redeem a century-old theory: Our arms and legs started as gills | Washington Post

Adult skates can spontaneously repair cartilage injuries | University of Cambridge

Human teeth traced to fish scales, Cambridge scientists say | BBC News

Questions from the Audience

In the video of the embryo development: Why do they shake left to right? Does it have something to do with the heart beat?

That is an excellent question, and one that we’ve always wanted to answer. The truth is, we don’t know why skate embryos move so vigorously early in development. I don’t think it has to do with the heart beat (as this side-to-side movement happens before they have a heart). At the later stages of development (i.e. from stage 29 onward), there are pores in the egg that open up, so that sea water can flow through the egg case. We can imagine that movement at those later stages can help circulate water (flushing out waste and bring in oxygenated water from outside). But at the earliest stages of development, the egg is still completely sealed, so it is unlikely that embryonic “wiggling” is promoting any sort of fluid exchange between the egg and the environment. Many biological processes are sensitive the mechanical stimulation - in other words, for certain processes to happen, there needs to be some kind of initiation stress, stretch or some other kind of force. So perhaps this movement could be required for the onset of some important developmental process? Or maybe it is part of a “training” period for the developing nervous system, as nerves and muscles begin to form and work together? This is still a mystery.

Do larger skate species take longer to develop, or do they all take about the same time as the little skate?

We don’t actually know how long it takes for many species of skate to development, as most have not been maintained in captivity. Development time is very temperature dependent in cold-blooded vertebrates - so, for example, if you maintain eggs of the little skate at ~15 degrees Celsius, it takes them approximately 5 month to hatch - but if you keep them at colder temperatures, it takes a lot longer (up to a year!). From approximately stage 32 of development (from the video), skate embryos actually have almost all of their organ systems - so from that point until hatching, they are mostly just growing inside the egg. So I would guess that species that hatch at a much larger body size probably would take a lot longer to reach hatching 9as they would spend more time in the egg growing).

What's the coolest species you've ever worked on?

I think that the coolest species we’ve ever worked on is the elephant fish (Callorhinchus milii). It is a holocephalan, which is another group of cartilaginous fishes related to sharks and skates. Most holocephalans spend their entire life in the deep sea, and so are rarely encountered by humans. But in New Zealand and Australia, the elephant fish migrates annually into shallow bays to lay their eggs. Elephant fish have reduce gill arch appendages compared to sharks and skates, and we were curious to learn whether the mechanisms of gill reduction in this group was similar to the mechanisms used by animals with reduced limb skeleton (e.g. lizards that have reduced their limb skeleton). So we had to spend two field seasons diving in bays in Australia and New Zealand to collect eggs of this fascinating fish (I’ve included a link on this work below). In addition to having an interesting gill anatomy, they also have some truly bizarre body features, including an elephant like “trunk” at the front of their head, and a retractable grasping organ (for mating) on the top of their head!

When you started this research, did you expect to find what you did?

Not entirely. We’ve been working on aspects of gill arch development and fin evolution for over ten years now, and as we’ve learned new things, our research plans, projects and interests have shifted. I think that is one of the most exciting things about science: you can have an idea (a hypothesis), but as you study, read and carry out experiments, you are constantly putting your ideas to the test, and then either pursuing them further, adjusting them in light of new evidence, or exploring new directions. So we’ve long though that there was an evolutionary relationship between gills and fins, but our current views on the subject are very different from what they were ten years ago, and are the productive of many years of though, experiment, discussions with colleagues, etc.

Watch the rest of the MBLSciShoot videos on our Youtube channel.