Let’s talk about lampreys!

Lampreys look like eels, but they’re anatomically simpler than other fish. Lacking jaws, lampreys instead have sucker-like mouths lined with concentric rings of teeth. The likelihood that lampreys are the closest surviving descendant of the first vertebrate has motivated numerous studies, including some that involve physics.

My first encounter with lampreys was in 2004. For Physics Today‘s June issue of that year I wrote a news story about the UV-sensing ability of the lamprey’s pineal gland . All vertebrates have pineal glands. Ours is about the size of a lentil and sits deep inside our brain. Its principal function appears to be the secretion of the hormone melatonin, which regulates circadian rhythms.

The lamprey’s pineal lies close to the top of its head—close enough to directly sense light. Biologists knew since the 1960s that the lamprey’s pineal could sense UV. In 2003 Kyoto University’s Mitsumasa Koyanagi and his colleagues set about trying to find out what the lamprey does with its UV-sensing ability.

Using a mix of methods drawn from biochemistry, genetic engineering, and spectroscopy, Koyanagi and his colleagues discovered that the lamprey’s pineal gland contains an unusual bistable pigment. The pigment’s bistability enables the lamprey to sense the ratio of visible to UV light. Because that ratio varies with depth in water, Koyanagi speculated that the pineal helps the lamprey gauge its depth.

Thymus-like organs

I noticed more lamprey-related research earlier this month. Thomas Boehm of the Max Planck Institute of Immunobiology and Epigenetics in Freiburg, Germany, and his colleagues found strong evidence that lampreys have thymus-like organs in their gills.

If confirmed, the finding is significant because it could shed light on the evolution of our immune systems. Unlike other animals, vertebrates have an adaptive immune system that continuously trains pathogen-killing T-cells to respond to new threats. The thymus is where the training takes place. Lampreys have a simple version of the adaptive immune system but before Boehm’s work no one had identified a lamprey organ that resembles the thymus.

My curiosity roused, I looked for the latest research on lamprey ancestry and turned up a paper from last fall. A team led by Philip Donoghue of Dartmouth Medical School in New Hampshire and Kevin Peterson of Bristol University in the UK analyzed the genetic sequences of the microRNAs found in lampreys and in another kind of jawless fish, the hagfish.

Known as phylogenetics, the statistical analysis of DNA and RNA exploits techniques such as Bayesian inference and Markov chain Monte Carlo that are used and in some cases were invented by physicists.

When Donoghue, Peterson, and their collaborators compared the microRNAs of lampreys and hagfish with each other and with other fish, they made two interesting discoveries. First, lampreys and hagfish are branches of the same evolutionary tree, not separate branches as some biologists had argued.

Second, the anatomical simplicity of lampreys and hagfish is not the result of their being more closely related to a primitive ancestor than sharks, cod, and other fish are. Rather, the simplicity results from the jawless fish having lost features and functions over time, just as snakes lost their limbs.

Intriguingly, it now looks as though the closest common ancestor to all today’s vertebrates was more, not less, complex than the lamprey.