Know your bones
Looking for a fun challenge? Got some time to kill? Why not dissect a fish and describe every bone of its skeleton?
Performing this task on the two species in the genus Xiphister was the first chapter of my dissertation.
Let me explain.
My dissertation research focused on the comparative anatomy of a family of marine fish called Stichaeidae. The goal of the dissertation was to reconstruct the evolutionary history of the family, and others thought to be its relatives, by comparing the skeletal anatomy of each genus. There are about 37 genera and 80 some odd species of stichaeids. Where does one begin to make these sorts of comparisons?
The primary literature was a good place to start. There is a lot of great fish anatomy work out there, and it’s possible to piece together a pretty good picture of the skeletal anatomy of most stichaeids just by reading previous publications. A sizable portion of my first year as a student went to familiarizing myself with that literature. I went as far as tracking down copies of the original descriptions of the two species of Xiphister, which both were published in 1858, one of which was in German.
The problem was that after months of research, while I was closer to understanding the anatomy of Stichaeidae, I was not quite where I needed to be. There were lots of discrepancies among publications. Terminology changed. Descriptions contradicted themselves. Some publications missed obvious features or ignored whole portions of the skeleton. Using the literature alone left me educated but confused.
What I really needed to do was pick a stichaeid, dissect it from head to tail, and focus on learning all the details of that particular fish. That species then would become the template for comparison of all the other species. I chose Xiphister for a few reasons. First, we had loads of them in our Nunnally Ichthyology Collection at the Virginia Institute of Marine Science, and we could easily go collect more near Friday Harbor Labs. Second, we had a good size series, meaning I could look at the development of their skeletal system. Third, they have some weird characteristics about their mechanosensory lateral-line system (discussed here) that I wanted to look at in more detail.
I used a combination of clearing and staining, whole fish dissections, and x-rays to look at their anatomy. Then, I pulled apart all of the major bits – the neurocranium, suspensorium and jaws, pectoral girdle, axial skeleton, and the like. Then, I illustrated and described every bone. It was time consuming and meticulous work. It was also incredibly effective at helping me learn the anatomy of a stichaeid.
This deep dive into the anatomy of Xiphister was the first chapter and second peer-reviewed publication from my dissertation (if you don’t count the Xiphister locomotion project that wasn’t technically part of my dissertation). In it, I was able to completely describe the anatomy of this genus, clarify points from the primary literature, and discuss their development. Even better, by working for this intimate knowledge of the anatomy of this genus, I had a basis of comparison for all of the other stichaeids and their relatives that was the main part of my dissertation.
Do you like puzzles? Have you ever thought about fish as super complex puzzles and wondered how they are put together? Let me know in the comments below or on Twitter!
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!
Fish Larvae Galore!
Fish larvae are my jam.
I cut my teeth as a newbie ichthyologist by working as a larval fish taxonomist for an environmental consulting firm. I spent 40 hours per week mostly looking through a microscope and identifying fish larvae collected from the northern Gulf of Mexico. I held that job for three years, and I learned a lot about the taxonomy of the early life history stages of marine fishes. But, after three years, I felt I needed to learn more about the anatomy and ecology of larval fishes, more than what the consulting job was offering. I needed my PhD.
A big reason I made the decision to quit my job and to go study at the Virginia Institute of Marine Science (VIMS) was because my PhD advisor, Eric Hilton, was managing a project monitoring larval fishes moving into Chesapeake Bay. Joining his lab allowed me to do three things. First, Eric is the curator of the Nunnally Ichthyology Collection at VIMS, so I got to learn a thing or two about natural history museums, which was pretty good considering my current position as the Collection's Manager for the Department of Ichthyology at the Natural History Museum of Los Angeles County. Second, Eric is an expert on fish anatomy, so my dissertation project focused on comparative anatomy of fishes. And third, I got to have a leadership role on this project and stay in the world of larval fishes.
For this project (https://www.int-res.com/abstracts/meps/v527/p167-180/), we worked in collaboration with colleagues in Delaware who were doing the same sampling we were doing. Once a week, we would go out at night on an incoming tide and collect triplicate sets of larval fish samples. We also would measure a suite of environmental parameters. By doing this, we had a weekly record of environmental conditions and the larval fish communities in both Chesapeake and Delaware Bays for two full years. That’s a really neat data set and made for a cool comparison. Chesapeake and Delaware Bays are only about 175 kilometers apart, and they are two of the largest estuaries along the Mid Atlantic Bight (MAB). Understanding the diversity and the timing/duration of larval fish entering each bay has big implications for the ecology of the whole MAB.
We found that the two bays transitioned differently throughout the year. Chesapeake Bay featured two seasons: one in summer and another in winter. These seasons were defined in Chesapeake Bay by an abundance of gobies and anchovies in summer, and drums, flounder, and menhaden in winter. Delaware Bay, on the other hand, featured four seasons, corresponding to spring, summer, fall, and winter. Even though the bays are separated by only a short distance, structurally they are quite different.
This project wound up being a cool collaborative experience with several members of Eric’s lab and Tim Targett’s lab in Delaware. Filipe, Eric’s postdoc, handled the heavy lifting on the manuscript preparation, which is great because this project was not directly part of my dissertation research (more on that in future blog posts). Filipe did a lot of the data analyses and writing. My major contribution was to coordinate field sampling at our site in Chesapeake Bay and identify our larvae. Those are the skills I was already pretty good at, so I was more than happy to take on those tasks. By being involved in this research, I got to learn a lot more about the ecology of larval fishes along the MAB, so that was awesome.
Have any cool fish larvae stories? Let me know in the comments below or over on Twitter!
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