Stichaeid lateral lines

One of the coolest adaptations of bony fishes is their mechanosensory lateral-line system. The lateral-line system is a network of sensory organs, called neuromasts, dispersed across the body. When water moves the hair cells that sit on top of the neuromast, a neuron fires that signals the fish’s brain that water has moved. By having a network of neuromasts across the body, fish receive constant information about the way water moves around them and how they move through water.

Neuromasts come in a few different types. One type is known as superficial neuromasts. These are small neuromasts dispersed across the body surface and are in direct contact with the water around the fish. Another type is called canal neuromasts. These are larger neuromasts that are enclosed in a canal just under the surface of the skin. The canals are open to the environment through a series of pores in special scales, called lateral line scales.

These canals, and the neuromasts inside, they interest me.

Fish tend to have canals on the head, known as cephalic canals, and on the body, known as trunk canals. The exact configuration of cephalic and trunk canals can be diagnostic in species identification. The different patterns of cephalic and trunk canals also have implications on mechanosensory lateral line function. Most fishes have a single lateral line that runs down the side of their body.

Most, but not all.

The family Stichaeidae is one of the few fish families in which some species have multiple trunk canals. The condition is variable across the family. Some stichaeids have no trunk lateral line canal, some have a single canal, some have multiple canals with lots of branches, and some have a complex mesh network of canals. Why all that variation? I haven’t a clue. This question is the main reason I selected Stichaeidae as my family of study for my dissertation.

When I came to the Virginia Institute of Marine Science (VIMS) to work on my dissertation, I joined the lab of Eric Hilton, curator of the Nunnally Ichthyology Collection and expert in fish anatomy. This was perfect because I wanted to learn more about fish anatomy. The problem when I arrived at VIMS was that I didn’t know where to start. I had no clear picture of what it meant to study fish anatomy. There are 35,000 species of fish. How do I pick one or one small group to study in excruciating detail, the kind of detail that would see me sink years of study into that species or group? It turns out that I needed to find a group about which I could ask interesting questions.

Why does the stichaeid genus Xiphister have multiple lateral line canals? Do they do anything? Are all of those canals functional? Easy. Now I had my questions and the start of a dissertation.

The first publication of my dissertation (aside from the Xiphister locomotion project) was this one describing the structure of the lateral-line canals in Xiphister. Both species of this genus actually have four canals that run down the body. The question was – do all of these canals contain neuromasts? That is, are all of the canals functional?

To get at this, I used histology to look at the cellular structure of the canals. I examined thin sections of the head, looking for neuromasts inside the cephalic canals, and several sections of the body, looking for neuromasts in the trunk canals. The results were fascinating, and not at all what I was expecting. The cephalic canal had neuromasts pretty much where you would expect them. There was nothing too exciting there, except to say it probably functions normally. The cool part was in those trunk lateral-line canals.

Only two of the four canals in Xiphister actually contained neuromasts. The other canals on the body were just accessory canals, meaning they could not possibly function as part of the mechanosensory system. Ultimately, I don’t know why Xiphister has extra canals that lack neuromasts, but there are a couple of possibilities. One explanation is that all canals in Xiphister, at some point in evolutionary history, contained neuromasts but two of the canals have subsequently lost them. Another possibility is that canal formation in Xiphister operates independently of neuromasts. This would be interesting, because the current idea behind lateral-line canal formation says that neuromasts are required to start the process of canal formation (it’s a complicated mechanism that postulates all neuromasts start on the surface and the canal neuromasts are enveloped by a tube that forms around them). If the latter situation is true, then that means there is another, unexplained mechanism for canal formation.   

This project also allowed me to do some additional cool things. I looked at the support structures of the canals, talked about scale development in Xiphister, and even developed a technique to describe the complexity of lateral-line canal patterns using fractals.

I wish I had more concrete answers about Xiphister and their strange mechanosensory systems, since there are still lots of questions. But, at least we now know more about these weird canals. Some of the things learned from this project will assist broader studies of mechanosensory system evolution and function across all bony fishes, which is pretty cool.

What do you think of fishes' weird mechanosensory system? Let me know in the comments below or on Twitter!