Biomedical subjects

M A R Koehl

Publications and source records attributed to M A R Koehl.

6 recordsLinked to original sources

PubMed2005

The fluid mechanics of arthropod sniffing in turbulent odor plumes.

Many arthropods capture odorant molecules from the environment using antennae or antennules bearing arrays of chemosensory hairs. The penetration of odorant-carrying water or air into the spaces between these chemosensory hairs depends on the speed at which they are moved through the surrounding fluid. Therefore, antennule flicking by crustaceans and wing fanning by insects can have a profound impact on the odorant encounter rates of the chemosensory sensilla they bear; flicking and fanning are examples of sniffing. Odors are dispersed in the environment by turbulent wind or water currents. On the scale of an antenna or antennule, an odor plume is not a diffuse cloud but rather is a series of fine filaments of scent swirling in odor-free water. The spatiotemporal pattern of these filaments depends on distance from the odor source. The physical interaction of a hair-bearing arthropod antennule with the surrounding fluid affects the temporal patterns of odor concentration an animal intercepts when it sniffs in a turbulent odor plume.

Animals↗

PubMed2005

How to make a spiral bacterium.

The motility of some kinds of bacteria depends on their spiral form, as does the virulence of certain pathogenic species. We propose a novel mechanism for the development of spiral shape in bacteria and the supercoiling of chains ('filaments') of many cells. Recently discovered actin-like proteins lying just under the cell wall form fibers that play a role in maintaining cell shape. Some species have a single actin-like fiber helically wrapped around the cell, while others have two fibers wrapped in the same direction. Here, we show that if these fibers elongate more slowly than growth lengthens the cell, the cell both twists and bends, taking on a spiral shape. We tested this mechanism using a mathematical model of expanding fiber-wound structures and via experiments that measure the shape changes of elongating physical models. Comparison of the model with in vivo experiments on stationary phase Caulobacter crescentus filaments provide the first evidence that mechanical stretching of cytoskeletal fibers influences cell morphology. Any hydraulic cylinder can spiral by this mechanism if it is reinforced by stretch-resistant fibers wrapped helically in the same direction, or shortened by contractile elements. This might be useful in the design of man-made actuators.

Actins↗

PubMed2004

Biomechanics of microscopic appendages: functional shifts caused by changes in speed.

Many diverse animals use arrays of hair-like structures to perform important jobs such as feeding, gas exchange, smelling, and swimming. Since these functions involve hair interactions with the surrounding water or air, analysis of the fluid dynamics of diverse hair-bearing appendages reveals how the morphology of an array of hairs affects it performance. Mathematical and physical models of flow between cylinders have shown that arrays of large, rapidly moving cylinders are leaky sieves, whereas little fluid moves through a row of small, slow rods. The purpose of the present study was to test this prediction for realistic appendage morphologies and to elucidate whether the design of a hairy leg can affect the range of speeds in which this transition in function occurs. We studied flow through hairy food-capturing appendages (second maxillae) of calanoid copepods, abundant planktonic crustaceans whose feeding on unicellular algae forms an important link in many marine food webs. Using dynamically scaled physical models, we found that hairy appendages undergo a transition between paddle- and sieve-like function at a critical range of sizes and speeds. The coarser the mesh of hairs on second maxillae, the smaller the size and speed at which this functional shift occurs. Thus, a simple increase in size (ontogenetic or evolutionary) or speed can generate a novel function (a paddle can become a filter), but the morphology of a hairy appendage determines the size and speed range at which leakiness to fluid flow can be affected by behavior or growth.

Animals↗

PubMed2004

Rapid behavioral responses of an invertebrate larva to dissolved settlement cue.

Larvae of the nudibranch Phestilla sibogae were used to study whether a natural dissolved settlement cue (from their prey, Porites compressa, an abundant coral on Hawaiian reefs) induces behavioral responses that can affect larval transport to suitable settlement sites. As cue and larvae are mixed in the turbulent flow over a reef, cue is distributed in fine-scale filaments that the larva experiences as rapid (seconds) on/off encounters. To examine larval responses in this setting, individual larvae were tethered in a small flume with flow simulating water velocity relative to a freely swimming larva, and their responses to realistic temporal patterns of cue encounter were videotaped. Competent larvae quickly ceased swimming in cue filaments and resumed swimming after exiting filaments. The threshold cue concentration eliciting a response was 3%-17% of concentrations within heads of P. compressa in nature. When moving freely in filtered seawater, competent larvae swam along straight paths in all directions at approximately 0.2 cm s(-1), whereas in water conditioned by P. compressa, most ceased swimming and sank at approximately 0.1 cm s(-1). The ability of larvae to rapidly respond (by sinking) to brief encounters with dissolved settlement cues can enhance their rapid transport to the substratum, even in wave-driven turbulent flow.

Analysis of Variance↗

PubMed2003

Physical modelling in biomechanics.

Physical models, like mathematical models, are useful tools in biomechanical research. Physical models enable investigators to explore parameter space in a way that is not possible using a comparative approach with living organisms: parameters can be varied one at a time to measure the performance consequences of each, while values and combinations not found in nature can be tested. Experiments using physical models in the laboratory or field can circumvent problems posed by uncooperative or endangered organisms. Physical models also permit some aspects of the biomechanical performance of extinct organisms to be measured. Use of properly scaled physical models allows detailed physical measurements to be made for organisms that are too small or fast to be easily studied directly. The process of physical modelling and the advantages and limitations of this approach are illustrated using examples from our research on hydrodynamic forces on sessile organisms, mechanics of hydraulic skeletons, food capture by zooplankton and odour interception by olfactory antennules.

Animals↗

PubMed2003

Fine-scale patterns of odor encounter by the antennules of mantis shrimp tracking turbulent plumes in wave-affected and unidirectional flow.

Many marine animals track odor plumes to their source. Although studies of plume-tracking behavior have been performed in unidirectional flow, benthic animals such as crustaceans live in coastal habitats characterized by waves. We compared signal encounters by odor-plume-tracking stomatopods (mantis shrimp) in wave-affected and unidirectional flow in a flume. Stomatopods are small enough that we can study their natural behavior in a flume. They sample odors by flicking their antennules. A thin sheet of laser light illuminating an odor plume labeled with dye [planar laser induced fluorescence (PLIF) technique] permitted us to measure the instantaneous odor concentration encountered by the animal's chemosensory organs (antennules) while it tracked the plume. We simultaneously measured behavior and the high-resolution odor signal at the spatial and temporal scale of the animal. We found that the navigating animal encountered odor filaments more often in wave-affected flow than in unidirectional flow. Odor filaments along the animals' antennules were significantly wider and of higher concentration in waves than in unidirectional flow.

Animals↗