Body and Tail Coordination in the Bluespot Salamander (\u3cem\u3eAmbystoma laterale\u3c/em\u3e) During Limb Regeneration

Animals are incredibly good at adapting to changes in their environment, a trait envied by most roboticists. Many animals use different gaits to seamlessly transition between land and water and move through non-uniform terrains. In addition to adjusting to changes in their environment, animals can adjust their locomotion to deal with missing or regenerating limbs. Salamanders are an amphibious group of animals that can regenerate limbs, tails, and even parts of the spinal cord in some species. After the loss of a limb, the salamander successfully adjusts to constantly changing morphology as it regenerates the missing part. This quality is of particular interest to roboticists looking to design devices that can adapt to missing or malfunctioning components. While walking, an intact salamander uses its limbs, body, and tail to propel itself along the ground. Its body and tail are coordinated in a distinctive wave-like pattern. Understanding how their bending kinematics change as they regrow lost limbs would provide important information to roboticists designing amphibious machines meant to navigate through unpredictable and diverse terrain. We amputated both hindlimbs of blue-spotted salamanders (Ambystoma laterale) and measured their body and tail kinematics as the limbs regenerated. We quantified the change in the body wave over time and compared them to an amphibious fish species, Polypterus senegalus. We found that salamanders in the early stages of regeneration shift their kinematics, mostly around their pectoral girdle, where there is a local increase in undulation frequency. Amputated salamanders also show a reduced range of preferred walking speeds and an increase in the number of bending waves along the body. This work could assist roboticists working on terrestrial locomotion and water to land transitions

Internal Vertebral Morphology of Bony Fishes Matches the Mechanical Demands of Different Environments

Fishes have repeatedly evolved characteristic body shapes depending on how close they live to the substrate. Pelagic fishes live in open water and typically have narrow, streamlined body shapes; benthic and demersal fishes live close to the substrate; and demersal fishes often have deeper bodies. These shape differences are often associated with behavioral differences: pelagic fishes swim nearly constantly, demersal fishes tend to maneuver near the substrate, and benthic fishes often lie in wait on the substrate. We hypothesized that these morphological and behavioral differences would be reflected in the mechanical properties of the body, and specifically in vertebral column stiffness, because it is an attachment point for the locomotor musculature and a central axis for body bending. The vertebrae of bony fishes are composed of two cones connected by a foramen, which is filled by the notochord. Since the notochord is more flexible than bony vertebral centra, we predicted that pelagic fishes would have narrower foramina or shallower cones, leading to less notochordal material and a stiffer vertebral column which might support continuous swimming. In contrast, we predicted that benthic and demersal fishes would have more notochordal material, making the vertebral column more flexible for diverse behaviors in these species. We therefore examined vertebral morphology in 79 species using micro-computed tomography scans. Six vertebral features were measured including notochordal foramen diameter, centrum body length, and the cone angles and diameters for the anterior and posterior vertebral cones, along with body fineness. Using phylogenetic generalized least squares analyses, we found that benthic and pelagic species differed significantly, with larger foramina, shorter centra, and larger cones in benthic species. Thus, morphological differences in the internal shape of the vertebrae of fishes are consistent with a stiffer vertebral column in pelagic fishes and with a more flexible vertebral column in benthic species

It Pays to Be Bumpy: Drag Reducing Armor in The Pacific Spiny Lumpsucker, \u3cem\u3eEumicrotremus Orbis\u3c/em\u3e

Armor is a multipurpose set of structures that has evolved independently at least 30 times in fishes. In addition to providing protection, armor can manipulate flow, increase camouflage, and be sexually dimorphic. There are potential tradeoffs in armor function: increased impact resistance may come at the cost of maneuvering ability; and ornate armor may offer visual or protective advantages, but could incur excess drag. Pacific spiny lumpsuckers (Eumicrotremus orbis) are covered in rows of odontic, cone-shaped armor whorls, protecting the fish from wave driven impacts and the threat of predation. We are interested in measuring the effects of lumpsucker armor on the hydrodynamic forces on the fish. Bigger lumpsuckers have larger and more complex armor which may incur a greater hydrodynamic cost. In addition to their protective armor, lumpsuckers have evolved a ventral adhesive disc, allowing them to remain stationary in their environment. We hypothesize a tradeoff between the armor and adhesion: little fish prioritize suction while big fish prioritize protection. Using micro-CT we compared armor volume to disc area over lumpsucker development and built 3D models to measure changes in drag over ontogeny. We found that drag and drag coefficients decrease with greater armor coverage and vary consistently with orientation. Adhesive disc area is isometric but safety factor increases with size, allowing larger fish to remain attached in higher flows than smaller fish

A New Metric for Characterizing Swimming Kinematics in Elongate Fishes

Many species of elongate fishes use Anguilliform swimming to propel themselves through the water (Gillis 1996, Long 1998). A fish using this method passes a wave of motion from the head, through the body, to the tail causing thrust. This type of swimming is the only one in which the entire body is used as opposed to just the caudal end such as in Thunniform swimmers (Tytell 2010). When watching certain species of elongate fishes swim, an interesting rotation in the body can be observed. If the fish is being looked at dorsally as it swims, there is a clear view of the lateral side of the fish as the tail beats back and forth. This view changes as the fish passes the wave from its head to its tail. The current work will describe a new method for measuring this rotation, or wobble, in the fish as it is swimming

A New Metric for Characterizing Swimming Kinematics in Elongate Fishes

Many species of elongate fishes use Anguilliform swimming to propel themselves through the water (Gillis 1996, Long 1998). A fish using this method passes a wave of motion from the head, through the body, to the tail causing thrust. This type of swimming is the only one in which the entire body is used as opposed to just the caudal end such as in Thunniform swimmers (Tytell 2010). When watching certain species of elongate fishes swim, an interesting rotation in the body can be observed. If the fish is being looked at dorsally as it swims, there is a clear view of the lateral side of the fish as the tail beats back and forth. This view changes as the fish passes the wave from its head to its tail. The current work will describe a new method for measuring this rotation, or wobble, in the fish as it is swimming

Bent out of shape: Bioinspired vertebral column morphology and mechanics

The vertebral column plays an essential role in body stiffness and swimming. Greater body stiffness results in greater mechanical outputs from the body on the environment. Previous work in bioinspired systems has shown that changing the length of the intervertebral joint changes the mechanics of the whole column. Our goals were to test the effects of centrum morphology and intervertebral joint length on mechanical outputs of the vertebral column using bioinspired models. We expected that centrum angle, joint length, and bending angle would all be significant effects in our statistical models. We designed five 3-D models inspired by vertebral morphology of fishes, humans, and marine mammals, and models were printed on a 3-D printer. We constructed 12 motion segments (centrum – joint – centrum) of varying joint length for each centrum morphology. A moment arm was added to one end of the motion segment to insure pure bending and eliminate shear. We tested mechanical properties of motion segments on a materials testing system measuring force (N). From the force outputs we calculated moment (Nm), Work (J), and Bending Stiffness (Nm2). Increasing the bending angle during testing, increased the moment and work produced by the system, while it decreased the bending stiffness. By increasing joint length we measured decreases in each of those mechanical properties. The marine marine mammal models, regardless of joint length, always had the largest mechanical outputs while the fish model with an intermediate centrum angle had the lowest. These data suggest that convex and flat models are consistently stiffer than concave models. Our data show the relation between centrum shape, joint length, and their associated mechanical outputs is perhaps not linear. Building additional models will allow us to further explore morphospace and better understand the conserved centrum shapes we see in among vertebrate groups. This research was funded by NSF grant DBI 1262239

Long Axis Twisting During Locomotion of Elongate Fishes

Fish live in a complex world and must actively adapt their swimming behavior to a range of environments. Most studies of swimming kinematics focus on two-dimensional properties related to the bending wave that passes from head to tail. However, fish also twist their bodies three dimensionally around their longitudinal axis as the bending wave passes down the body. We measured and characterized this movement, which we call ‘wobble’, in six species of elongate fishes (Anoplarchus insignis, Xiphister mucosus, Lumpenus sagitta, Pholis laeta, Apodichthys flavidus and Ronquilus jordani) from three different habitats (intertidal, nearshore and subtidal) using custom video analysis software. Wobble and bending are synchronized, with a phase shift between the wobble wave and bending wave. We found that species from the same habitats swim in similar ways, even if they are more closely related to species from different habitats. In nearshore species, the tail wobbles the most but, in subtidal and intertidal species, the head wobbles more than or the same as the tail. We also wanted to understand the relationship between wobble and the passive mechanics of the fish bodies. Therefore, we measured torsional stiffness and modulus along the body and found that modulus increases from head to tail in all six species. As wobble does not correlate with the passive properties of the body, it may play a different role in swimming behavior of fishes from different habitats