Research

IMAGING BIOLOGICAL SURFACES

Surfaces provide many important functions when organisms interact with their surroundings, and I am interested in how evolution and function have shaped the diversity of organismal surfaces, and how we can accurately image biological surfaces to better understand their diversity. A major need in studying biological surfaces is finding a method that can deliver three-dimensional topographic information for surfaces that may be soft, wet, shiny, or transparent. To that end, I have pioneered the use of gel-based stereo profilometry to rapidly image biological surfaces in situ and in vivo. This method works on many biological surfaces but my research focuses on aquatic animals, especially fishes. At left are some images of scales from Osmerus mordax. See blog post on the 3D surface topography method I use here.

EVOLUTIONARY PATTERNS OF FISH SURFACES

Biologists have long wondered why fish surfaces are so diverse. Fish surfaces come in a large variety of forms and although we think surfaces serve some particular functions (e.g. protection from predators, parasites, and disease), we do not understand how changes in surface form connect with changes in function and performance. I am working to explore this topic by using comparative studies of scale and surface morphology - combining modern phylogenetic comparative methods with 3D surface topography. So far, I've studied damselfish surfaces (a species-rich group of reef fishes) and shown that they respond to changes in ecology and body shape in ways that suggest ecological connections to scale morphology. Damselfishes also show differences in surface form when a species' flow ecology is different - suggesting a hydrodynamic role for scales. These results hold promise for continuing this line of research in more fish groups, and for generating new hypotheses for biomechanical testing. Understanding the generation and maintenance of diversity is a major question in evolutionary biology, and fish surfaces are an interesting system for exploring this topic.

This image shows a phylogeny of different species used in this study on damselfish surface and scale morphology. The phylogeny has 59 tips and each is colored according to the flow habitat that the species is often found to feed in. There are three categories - an open-water category, a structure or reef-associated category, and an intermediate

BIOMECHANICS & HYDRODYNAMICS OF AQUATIC SURFACES

Fishes and other aquatic animals are constantly interacting with the viscous water around them that provides buoyancy and allows them to generate lift and drag. Learning about how aquatic animals move efficiently through water is a focus of both marine biology, comparative biomechanics, and bio-inspired engineering, and I have studied this topic throughout my work on biological surfaces in groups such as bony fishes, sharks, and cetaceans (whales and dolphins). In some cases I use topographic information to calculate if surfaces are likely to influence boundary-layer hydrodynamics, whereas in other cases I take a direct experimental approach to test how mucus causes drag reduction in fish skin. This area of research has implications for understanding form-function relationships for biological surfaces, but also for discovering new bio-inspired mechanisms for drag reduction that could be interest to engineering research and technology.

Boundary layer flow over fish scales.

This image shows three species of fishes - the yellow tang, niger triggerfish, and brook trout. For each species, two images are shown - one image of the species' scaled surface without mucus present and another image with mucus present. The yellow tang's surface looks very similar with mucus present - the scale are very small and have many small spines pointed posteriorly. The triggerfish has large scales with many small bumps and one large central bump. With mucus present the triggerfish's surface still shows bumps although the barrier between scales are less distinct. The trout shows the largest differences with mucus present. Small round scales are visible when mucus isn't present, but with mucus and epidermis present the scales are no longer visible and the surface looks flat and featureless.

Surface of fishes with and without mucus; cool colors are low heights. Mucus has different effects, but often obscures the microstructure on fish scales.

This figure shows the surface texture of two species of toothed whales - a bottlenose dolphin and a killer whale. Both species have small parallel ridges on their surface although the killer whale has much larger ridges - both bigger in width but also in height. Graphs of height profiles are show for transects that are perpendicular to the ridges. Killer whale ridges have an apparent height of about 60-70 microns and there are around three ridges every 5 mm according to the graph. Bottlenose dolphin ridges are shorter than 10 microns and there are around ten in 5mm.

Ridges on cetacean skin are too small on most species to effect the boundary layer.

SCALE DIVERSITY IN FISHES

Scales are bony plates that cover the bodies of most fishes and many years of studies have shown us that scales show a myriad of forms in both different species and locations on the body. I have worked to modernize our understanding of scale diversity by using methods like micro computed tomography (µCT) scanning and gel-based profilometry to reveal undiscovered 3D diversity in fish scales. It is crucial to understand 3D structure of scales to allow us to better study the functional morphology of fish scales. My studies in this area have demonstrated that scales are diverse across different body regions according to a number of different morphological measurements. I also discovered that tunas show extreme diversity of scale shapes and sizes across their bodies, and interestingly, some tuna scales are thickened and filled with fat (shown in video at right) - possibly to act as surface insulation for the raised core temperatures of endothermic tunas.

SHARK SKIN DIVERSITY AND HYDRODYNAMICS

Sharks have small complex scales called denticles that cover their bodies. Denticles have been of great interest to biologists and engineers alike in part because they are known to decrease drag and increase swimming performance under certain hydrodynamic conditions. My work in this area has mostly been with undergraduate collaborators and I have worked with them to study patterns of 3D structure across sharks bodies. We have discovered that shark denticles occur in many forms across the body but that they show repeated features from leading-edge to trailing-edge surfaces with respect to flow. These morphological patterns might indicate hydrodynamic tuning to local flows at these locations. I am also working to better connect denticle morphology and hydrodynamic function by directly measuring boundary layer flows (just a few millimeters from the skin) in live sharks.

This figure depicts the surface texture and denticle (or scale) morphology across different regions of the thresher shark tail. We can see repeated patterns where regions on the leading edge of the tail have larger and more blocky denticles that also lack prominent surface ridges. In contrast, denticles on the lateral side of the tail and the trailing edge of the tail are closely packed and shaped like squares that are turned so that their diagonal runs in the anterior to posterior direction on the tail. These denticles also have proiminent ridges that run in the streamwise direction from head to tail. This pattern in denticle form holds true across and down the length of the tail.

FUNCTIONAL MORPHOLOGY OF TUNAS

This image shows a 3D reconstruction of the bones in the last few vertebrae of a tuna. These verebrae have enlarged lateral processes that form plate-like projections laterally and are called a bony keel. Tunas also have a fleshy keel made of soft tissue, and an image of the fleshy keel is overlayed with the image of the bones, showing that the widest part of the bony keel is more anterior than the widest part of the fleshy keel.

Tuna lateral peduncle keels from µCT data.

Tunas are one of the most economically-important groups of fishes and their biology as large, high-performance, open-ocean, warm-bodied predators makes them biologically interesting and important as well. Tunas are thought of as highly specialized for swimming fast and they have a number of novel morphological features. I used µCT scanning to study some of these features and then created physical models of these features to test their hydrodynamic function using fish-like locomotion in a flow tunnel. Generally I found that tuna features like lateral peduncle keels and finlets decrease lateral forces and torques, which could make tuna swimming more efficient.

BIO-INSPIRED ADHESION

There are a handful of fish clades that have independently evolved the ability to adhere to underwater surfaces - sometimes to hitch a ride on another organism, or sometimes to simply stay-put in high-flow habitats. I have worked on a number of these systems to explore their adhesive performance and relevant morphology, and I have also collaborated with experts in bio-inspired robotics to both emulate and understand adhesive mechanisms and behavior. This work helps to build transferrable knowledge with potential uses in engineered technology. (Video below shows a volume rendering of part of the head and adhesive disc of a remora; from µCT data with soft-tissue contrast enhancement).

This photograph shows a northern clingfish attached to a rock and out of water. The rock has a rough surface with encrusting organisms on it, such as algae. The clingfish is presented in mostly a lateral view - it is a dark mottled color and appear wet and scale-less. It has a tapered shape with a larger anterior body and a narrow tail region.

Northern clingfishes can adhere to rough and slippery surfaces using an adhesive disc on their belly. [Photo courtesy of Dr. Thomas Kleinteich]