Meg Vandenberg is another example of a student who came to UW Friday Harbor Labs to take a course, uncertain about her future, and ended up finding her life niche! When people talk about “transformative” experiences at marine stations, this is a fairly common kind of transformation. Now and then, of course, students come to take a course and find out that a given topic is not their cup of tea – but of course that is critical information too. As we gear up for busy spring and summer quarters at FHL, we look forward to all of the positive change and discovery that lies ahead for our students!

How Our Environment Shapes Us: Armor’s Got a Point!

by Megan L. Vandenberg

A lot of benthic organisms have the same goal: to stay put in the complex fluid dynamics of the intertidal or resist the ocean’s currents. Many do this by the power of suction or strong attachments – think mussels or limpets, or even the suction cups of lumpsuckers. Poachers (family Agonidae) are a group of benthic fish that have an unusual approach to resisting flow: being completely covered in heavy, boney armor plates. These fishes have a wide array of armor morphologies featuring large spines or bumpy ridges (Figure 1). Despite being completely covered in armor, these fish are still swallowed whole by predators that have large enough gapes. We believe that for these poachers, armor has alternative functions such as increasing passive bending stiffness or decreasing their drag. Understanding the multifunctionality of such a quirky, intricate and bizarre feature has become the focus of my research. Much like these organisms that try to stay in one place despite pushes and pulls in their environment, I too have had a desire to remain at Friday Harbor Labs ever since I first came as an undergraduate.

My first foray into biological research began with morphology. As an undergraduate, I took the Bio Imaging research apprenticeship at FHL with Adam Summers and became enamored with all the different techniques you can use to gain a new perspective on the form of an organism. In that class we learned CT-scanning, scanning electron microscopy, histology, macro-photography, and clearing and staining, each allowing us to unlock a different piece of the morphology puzzle. For my project I aimed to understand the morphology of the baleen of five species of whales, using my newfound knowledge and techniques to explore the structures at the micron level (Figure 2). I found that the keratinous plates of the baleen break away to reveal the hair-like fringe inside, which forms the water filter for capturing planktonic food (Vandenberg and Cohen et al., 2023). The drive of the other students to submit their best creations for our class critiques pushed me to better understand the tools I was using. I loved the close-knit community formed during the class, and leaving FHL the first time I wasn’t sure I would ever find such a community in the academic realm again…so I had to come back.

I came back again the next summer for the Functional Morphology and Ecology of Marine Fishes course, also known as Fish class. My love for form and function, bred at FHL with the unparalleled access to morphological techniques, led me to my current curiosity: fish armor.

Poachers are some of the craziest looking fish I have ever seen, and their evolution of such extraordinary full-body armor creates so many interesting form/function questions. In this class I wanted to understand the effects of different parameters on drag as water passes over them: spine size, plate angle, and percent of overlap of plates. Using 3D-printed models I made of poacher armor, I quantified drag of the different shapes using a force transducer on models placed in a flume. My preliminary results showed that large spines decreased drag, and that spine size was more important than other variables. However, 5 weeks was simply not enough time to even scratch the surface of a complex feature such as armor. The Labs community, the access to investigative techniques, the natural inquisitive nature that comes from simply being at FHL…all led me to decide to continue researching armor at the University of Washington (and FHL) for my PhD. I began my research here in the Fall of 2023, and that close-knit community I craved as an undergrad has since become a family.

Armor’s Got a Point

Fishes use biological armor for an array of purposes, from protection to hydrodynamics. The diverse shapes and arrangements found in nature leads to a huge diversity of function; for example, the shape of the armor cages of seahorse tails enables their prehensile nature, allowing them to grip objects in their environment (Lees et al., 2012; Porter et al., 2013). The poacher family includes 47 species of fish, and they all have 6-8 rows of hexagonal plates around the trunk of their body that are intricately overlapped, yet the individual plate morphology widely varies between species. These benthic fish are found in a range of habitats with a variety of predators and locomotor challenges (Kolmann et al., 2020; Kruppert et al., 2020). It is known that the more time fish species spend on the bottom the more likely they are to be scaleless, but this is the complete opposite of the intense full-body spiny armor of Agonids (Lemopoulos and Montoya-Burgos, 2021).

Having a whole bunch of pointy bony scales over the fish’s body affects the way that the fluid around interacts with the fish’s surface. A key factor that changes is the boundary layer around the fish, which is the layer of slow-moving fluid close to the surface of the fish caused by friction created in the layer of fluid in contact with the body. The fluid and flow starts as laminar – organized in nice sheets – and then at some point after coming in contact with the body, it transitions to turbulent due to this friction (Figure. 3). The turbulence is fluid going in many directions and can cause the boundary layer to separate from the body, creating a larger wake behind the fish and increasing its drag (lower part of Figure 3). Using Particle Image Velocimetry (PIV), I found that the larger spines on poacher armor decreases boundary layer separation and drag while providing protection, covering the vulnerable gaps between plates (upper part of Figure 3) (Vandenberg et al., 2024).

Like poachers, engineers add rugosity (roughness) to surfaces to reduce drag, e.g. on boat hulls, plane fuselages and even on swimsuits (Choi et al., 1989). However, poacher armor is not simplified like the traditional riblets used in engineering for drag reduction, and has a much more complex relationship with flow; the morphology of their armor varies from bumpy and holey, flat plates to grandiose projecting spines to intricate bumpy knobs, creating completely differently fluid interactions. In fact, the larger protruding spines with higher rugosities help to transition the flow from laminar to turbulent, increasing the boundary layer. However, we found that the spines also counteract the drag effects by creating vortices that pull the boundary layer closer, helping to avoid separation (Figure 4).

My work aims to understand poacher armor from all angles, exploring the form to understand the function. However, it has been the access to tools and wealth of knowledge at FHL that has allowed me to ask these questions and integrate different areas of biology from morphology to hydrodynamics to biomechanics into my research. There are few places that have access to such equipment and specimens, and especially where students are allowed to use state-of-the-art tools; these factors contribute to making FHL so special, and that is why I love being here.

Thank you to the Karel F. Liem Imaging Center at FHL for free use of the CT scanner and SEM. I am also grateful for the funding I have received from the Stephen and Ruth Wainwright Fellowship, the Frederic H. and Kirstin C. Nichols Endowed Graduate Fellowship and the Friday Harbor Laboratories Research Endowment Fellowship that have made it possible for me to spend as much time at the Labs as possible.

References:

Choi K.S., Gadd G.E., Pearcey H.H., Savill A. M., and S. Svensson.  1989.  Tests of drag-reducing polymer coated on a riblet surface.  Applied Scientific Research: 46, 209–216.

Fletcher T., Altringha J., Peakal J., Wignal P. and R. Dorrell.  2014.  Hydrodynamics of fossil fishes.  Proc. R. Soc. B.: 281, 20140703.

Kolmann M.A., Peixoto T., Pfeiffenberger J.A., Summers A.P. and C.M. Donatelli.  2020.  Swimming and defense: competing needs across ontogeny in armored fishes (Agonidae).  J. R. Soc. Interface.: 17, 20200301.

Kruppert S., Chu F., Stewart M.C., Schmitz L. and A.P. Summers.  2020.  Ontogeny and potential function of poacher armor (Actinopterygii: Agonidae).  Journal of Morphology: 281, 1018–1028.

Lees J., Märss T., Wilson M.V.H, Saat T. and H. Špilev.  2012.  The sculpture and morphology of postcranial dermal armor plates and associated bones in gasterosteiforms and syngnathiforms inhabiting Estonian coastal waters.  Acta Zoologica: 93, 422–435.

Lemopoulos A. and J.I. Montoya-Burgos.  2021.  From scales to armor: Scale losses and trunk bony plate gains in ray-finned fishes.  Evolution Letters: 5, 240–250.

Porter M.M., Novitskaya E., Castro-Ceseña A.B., Meyers M.A. and J. McKittrick.  2013.  Highly deformable bones: Unusual deformation mechanisms of seahorse armor.  Acta Biomaterialia: 9, 6763–6770.

Vandenberg M.L., Cohen K.E., Rubin R.D., Goldbogen J.A., Summers A.P., Paig‐Tran E.W.M. and S.R. Kahane‐Rapport.  2023.  Formation of a fringe: A look inside baleen morphology using a multimodal visual approach.  Journal of Morphology: 284, e21574.

Vandenberg M.L., Hawkins O.H., Chier E., Kahane-Rapport S.R., Summers A.P. and C.M. Donatelli.  2024.  How rugose can you go? Spiny Agonidae armour decreases boundary layer separation.  Biological Journal of the Linnean Society: 143, blae075.

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