The Vertebrate Family Tree Gains a Slimy New Branch

by Katie Bowell, Curator of Cultural Interpretation

Hagfish

The animals of the world are broadly classified into two groups: invertebrates (those without backbones) and vertebrates (those with backbones). Since Darwin (and others, we can’t forget Wallace) posited the theory of evolution, scientists have been looking for the organism(s) that link those two groups together. Which organisms are the transitional species that bridge the gap between being backboned and not, and show how you get from a complex invertebrate like a tunicate to a simple vertebrate like a lamprey?

Tunicate

Lamprey

For the past thirty years, the hagfish was considered the best living answer to that question. However, recent genetic studies have moved the hagfish’s place on the evolutionary tree.

First things first: what’s a hagfish? Imagine a long, eel-like (though not an eel), marine-dwelling, self-knotting, slime-producing animal that has a skull, but no spinal column. Picturing it?

Back in the days when scientists organized the animals of the world by comparing their physical characteristics, hagfish resembled, and were therefore lumped in with, lampreys. Lampreys are about as low on the vertebrate branch of the tree of life as you could go, so hagfish were plopped on the bottom with them. In the 1970s, genetic analyses found significant differences between the two organisms, and hagfish were moved even further down the evolutionary ranks and presented as a possible “missing link” between the invertebrates and vertebrates. However, new genetic tests have changed that organization again.

Scientists have been looking at microRNA, small, non-coding RNA pieces that regulate whether genes are turned on or off. It turns out microRNA is even more reliable at demonstrating relationships between organisms than genes are, and the microRNA of hagfish and lampreys are showing that the two groups are closely related. Hagfish may be primitive vertebrates, but they’re definitely vertebrates.

And, because it is almost Halloween, I can’t end this post without talking bout the hagfish’s most famous attribute: it’s slime.

Hagfish are able to produce copious amounts of a slimy mucus, which scientists believe is used to help the organisms deter and escape predation. When attacked, hagfish secret skeins of wound-up protein fibers that are finer than spider silk.  When the skeins come in contact with water, they unravel and trap the water within them to form a big, dense, slippery mass of goo. In just a few minutes, an adult hagfish can produce enough slime to completely convert a 5 gallon bucket of water into slime. Now that’s a party trick. And when it’s time to get the slime off, the hagfish simply ties itself into a knot that works its way down the organisms body, pushing the slime off.