The evolution of feeding motor patterns in vertebrates

Prey capture kinematics of horned frogs (Anura: Ceratophryidae)

Horned frogs, members of the Ceratophryidae family, encompass a group of anurans varying in size and behavior, yet unified by morphological and behavioral traits enabling them to adopt a megalophagous diet (i.e., large prey feeding). Although the group has been the focus of numerous studies, our understanding of its feeding behavior remains limited. In this study, we characterize the feeding mechanism in five species representing the three extant genera of ceratophryid anurans, both in terrestrial and aquatic environments. We also explore the ability of Chacophrys pierottii to adjust feeding behavior based on prey type. Our findings show that all species are capable of wide mouth opening, displaying an asymmetric feeding cycle. While tongue usage is the primary method for capturing prey on land, ceratophryids may use their forelimbs to manipulate prey into their mouths, exhibiting different behavioral patterns. C. pierottii shows modulation of its feeding kinematics and is also capable of some modulation of its feeding in response to prey properties.

Studies of the Behavioral Sequences: The Neuroethological Morphology Concept Crossing Ethology and Functional Morphology

Simple Summary

Behavioral sequences analysis is a relevant method for quantifying the behavioral repertoire of animals to respond to the classical Tinbergen’s four questions. Research in ethology and functional morphology intercepts at the level of analysis of behaviors through the recording and interpretation of data from of movement sequence studies with various types of imaging and sensor systems. We propose the concept of Neuroethological morphology to build a holistic framework for understanding animal behavior. This concept integrates ethology (including behavioral ecology and neuroethology) with functional morphology (including biomechanics and physics) to provide a heuristic approach in behavioral biology.

Abstract

Postures and movements have been one of the major modes of human expression for understanding and depicting organisms in their environment. In ethology, behavioral sequence analysis is a relevant method to describe animal behavior and to answer Tinbergen’s four questions testing the causes of development, mechanism, adaptation, and evolution of behaviors. In functional morphology (and in biomechanics), the analysis of behavioral sequences establishes the motor pattern and opens the discussion on the links between “form” and “function”. We propose here the concept of neuroethological morphology in order to build a holistic framework for understanding animal behavior. This concept integrates ethology with functional morphology, and physics. Over the past hundred years, parallel developments in both disciplines have been rooted in the study of the sequential organization of animal behavior. This concept allows for testing genetic, epigenetic, and evo-devo predictions of phenotypic traits between structures, performances, behavior, and fitness in response to environmental constraints. Based on a review of the literature, we illustrate this concept with two behavioral cases: (i) capture behavior in squamates, and (ii) the ritualistic throat display in lizards.

Chewing or not? Intraoral food processing in a salamandrid newt

Food processing refers to any form of food mechanical breakdown prior to swallowing. Variations of this behaviour are found within all major gnathostome groups. Chewing is by far the most commonly used intraoral processing mechanism and involves rhythmic mandibular jaw and hyobranchial (tongue) movements. Chewing occurs in chondrichthyans (sharks and rays), actinopterygians (ray-finned fishes), dipnoi (lungfishes) as well as amniotes and involves similarities in the patterns of muscle activity and movement of the feeding apparatus. It has been suggested that amniote chewing, which involves the interaction of movements of the mandibular jaw and the muscular tongue, has evolved as part of the tetrapod land invasion. However, little is known about food processing mechanisms in lissamphibians, which might have retained many ancestral tetrapod features. Here, we identify a processing mechanism in the salamandrid newt, Triturus carnifex, which after prey capture displays cyclic head bobbing in concert with rhythmic jaw and tongue movements. We use high-speed fluoroscopy, anatomical reconstructions and analyses of stomach content to show that newts, although not using their mandibular jaws, deploy a derived processing mechanism where prey items are rasped rhythmically against the dentition on the mouth-roof, driven by a loop-motion of the tongue. We then compare patterns and coordination of jaw and tongue movements across gnathostomes to conclude that food processing in this newt species shares traits with processing mechanisms in fish as well as amniotes. This discovery casts salamanders as promising models for reconstructing the evolution of intraoral processing mechanisms at the fish-tetrapod split.

Speed dependency in α-motoneuron activity and locomotor modules in human locomotion: indirect evidence for phylogenetically conserved spinal circuits

Coordinated locomotor muscle activity is generated by the spinal central pattern generators (CPGs). Vertebrate studies have demonstrated the following two characteristics of the speed control mechanisms of the spinal CPGs: (i) rostral segment activation is indispensable for achieving high-speed locomotion; and (ii) specific combinations between spinal interneuronal modules and motoneuron (MN) pools are sequentially activated with increasing speed. Here, to investigate whether similar control mechanisms exist in humans, we examined spinal neural activity during varied-speed locomotion by mapping the distribution of MN activity in the spinal cord and extracting locomotor modules, which generate basic MN activation patterns. The MN activation patterns and the locomotor modules were analysed from multi-muscle electromyographic recordings. The reconstructed MN activity patterns were divided into the following three patterns depending on the speed of locomotion: slow walking, fast walking and running. During these three activation patterns, the proportion of the activity in rostral segments to that in caudal segments increased as locomotion speed increased. Additionally, the different MN activation patterns were generated by distinct combinations of locomotor modules. These results are consistent with the speed control mechanisms observed in vertebrates, suggesting phylogenetically conserved spinal mechanisms of neural control of locomotion.

A method for discrimination of noise and EMG signal regions recorded during rhythmic behaviors

Analyses of muscular activity during rhythmic behaviors provide critical data for biomechanical studies. Electrical potentials measured from muscles using electromyography (EMG) require discrimination of noise regions as the first step in analysis. An experienced analyst can accurately identify the onset and offset of EMG but this process takes hours to analyze a short (10-15s) record of rhythmic EMG bursts. Existing computational techniques reduce this time but have limitations. These include a universal threshold for delimiting noise regions (i.e., a single signal value for identifying the EMG signal onset and offset), pre-processing using wide time intervals that dampen sensitivity for EMG signal characteristics, poor performance when a low frequency component (e.g., DC offset) is present, and high computational complexity leading to lack of time efficiency. We present a new statistical method and MATLAB script (EMG-Extractor) that includes an adaptive algorithm to discriminate noise regions from EMG that avoids these limitations and allows for multi-channel datasets to be processed. We evaluate the EMG-Extractor with EMG data on mammalian jaw-adductor muscles during mastication, a rhythmic behavior typified by low amplitude onsets/offsets and complex signal pattern. The EMG-Extractor consistently and accurately distinguishes noise from EMG in a manner similar to that of an experienced analyst. It outputs the raw EMG signal region in a form ready for further analysis.

Comparisons of chewing rhythm, craniomandibular morphology, body mass and height between mothers and their biological daughters

OBJECTIVE

To study and compare the relationships between mean chewing cycle duration, selected cephalometric variables representing mandibular length, face height, etc., measured in women and in their teenage or young-adult biological daughters.

DESIGN

Daughters were recruited from local high schools and the University of Michigan School of Dentistry. Selection criteria included healthy females with full dentition, 1st molar occlusion, no active orthodontics, no medical conditions nor medication use that could interfere with normal masticatory motor function. Mothers had to be biologically related to their daughters. All data were obtained in the School of Dentistry. Measurements obtained from lateral cephalograms included: two "jaw length" measures, condylion-gnathion and gonion-gnathion, and four measures of facial profile including lower anterior face height, and angles sella-nasion-A point (SNA), sella-nasion-B point (SNB) and A point-nasion-B point (ANB). Mean cycle duration was calculated from 60 continuous chewing cycles, where a cycle was defined as the time between two successive maximum jaw openings in the vertical dimension. Other variables included subject height and weight. Linear and logistic regression analyses were used to evaluate the mother-daughter relationships and to study the relationships between cephalometric variables and chewing cycle duration.

RESULTS

Height, weight, Co-Gn and Go-Gn were significantly correlated between mother-daughter pairs; however, mean cycle duration was not (r(2)=0.015). Mean cycle duration was positively correlated with ANB and height in mothers, but negatively correlated with Co-Gn in daughters.

CONCLUSIONS

Chewing rate is not correlated between mothers and daughters in humans.

Modulation in the feeding prey capture of the ant-lion, Myrmeleon crudelis

Ant-lions are pit-building larvae (Neuroptera: Myrmeleontidae), which possess relatively large mandibles used for catching and consuming prey. Few studies involving terrestrial arthropod larva have investigated prey capture behavior and kinematics and no study has shown modulation of strike kinematics. We examined feeding kinematics of the ant-lion, Myrmeleon crudelis, using high-speed video to investigate whether larvae modulate strike behavior based on prey location relative to the mandible. Based on seven capture events from five M. crudelis, the strike took 17.60 ± 2.92 msec and was characterized by near-simultaneous contact of both mandibles with the prey. Modulation of the angular velocity of the mandibles based on prey location was clearly demonstrated. M. crudelis larvae attempted to simultaneously contact prey with both mandibles by increasing mean angular velocity of the far mandible (65 ± 21 rad sec(-1) ) compared with the near mandible (35 ± 14 rad sec(-1) ). Furthermore, kinematic results showed a significant difference for mean angular velocity between the two mandibles (P<0.005). Given the lengthy strike duration compared with other fast-striking arthropods, these data suggest that there is a tradeoff between the ability to modulate strike behavior for accurate simultaneous mandible contact and the overall velocity of the strike. The ability to modulate prey capture behavior may increase dietary breadth and capture success rate in these predatory larvae by allowing responsive adjustment to small-scale variations in prey size, presentation, and escape response.

Neural mechanisms underlying the evolvability of behaviour

The complexity of nervous systems alters the evolvability of behaviour. Complex nervous systems are phylogenetically constrained; nevertheless particular species-specific behaviours have repeatedly evolved, suggesting a predisposition towards those behaviours. Independently evolved behaviours in animals that share a common neural architecture are generally produced by homologous neural structures, homologous neural pathways and even in the case of some invertebrates, homologous identified neurons. Such parallel evolution has been documented in the chromatic sensitivity of visual systems, motor behaviours and complex social behaviours such as pair-bonding. The appearance of homoplasious behaviours produced by homologous neural substrates suggests that there might be features of these nervous systems that favoured the repeated evolution of particular behaviours. Neuromodulation may be one such feature because it allows anatomically defined neural circuitry to be re-purposed. The developmental, genetic and physiological mechanisms that contribute to nervous system complexity may also bias the evolution of behaviour, thereby affecting the evolvability of species-specific behaviour.

A preliminary analysis of correlations between chewing motor patterns and mandibular morphology across mammals

The establishment of a publicly-accessible repository of physiological data on feeding in mammals, the Feeding Experiments End-user Database (FEED), along with improvements in reconstruction of mammalian phylogeny, significantly improves our ability to address long-standing questions about the evolution of mammalian feeding. In this study, we use comparative phylogenetic methods to examine correlations between jaw robusticity and both the relative recruitment and the relative time of peak activity for the superficial masseter, deep masseter, and temporalis muscles across 19 mammalian species from six orders. We find little evidence for a relationship between jaw robusticity and electromyographic (EMG) activity for either the superficial masseter or temporalis muscles across mammals. We hypothesize that future analyses may identify significant associations between these physiological and morphological variables within subgroups of mammals that share similar diets, feeding behaviors, and/or phylogenetic histories. Alternatively, the relative peak recruitment and timing of the balancing-side (i.e., non-chewing-side) deep masseter muscle (BDM) is significantly negatively correlated with the relative area of the mandibular symphysis across our mammalian sample. This relationship exists despite BDM activity being associated with different loading regimes in the symphyses of primates compared to ungulates, suggesting a basic association between magnitude of symphyseal loads and symphyseal area among these mammals. Because our sample primarily represents mammals that use significant transverse movements during chewing, future research should address whether the correlations between BDM activity and symphyseal morphology characterize all mammals or should be restricted to this "transverse chewing" group. Finally, the significant correlations observed in this study suggest that physiological parameters are an integrated and evolving component of feeding across mammals.

Evolution of muscle activity patterns driving motions of the jaw and hyoid during chewing in Gnathostomes

Although chewing has been suggested to be a basal gnathostome trait retained in most major vertebrate lineages, it has not been studied broadly and comparatively across vertebrates. To redress this imbalance, we recorded EMG from muscles powering anteroposterior movement of the hyoid, and dorsoventral movement of the mandibular jaw during chewing. We compared muscle activity patterns (MAP) during chewing in jawed vertebrate taxa belonging to unrelated groups of basal bony fishes and artiodactyl mammals. Our aim was to outline the evolution of coordination in MAP. Comparisons of activity in muscles of the jaw and hyoid that power chewing in closely related artiodactyls using cross-correlation analyses identified reorganizations of jaw and hyoid MAP between herbivores and omnivores. EMG data from basal bony fishes revealed a tighter coordination of jaw and hyoid MAP during chewing than seen in artiodactyls. Across this broad phylogenetic range, there have been major structural reorganizations, including a reduction of the bony hyoid suspension, which is robust in fishes, to the acquisition in a mammalian ancestor of a muscle sling suspending the hyoid. These changes appear to be reflected in a shift in chewing MAP that occurred in an unidentified anamniote stem-lineage. This shift matches observations that, when compared with fishes, the pattern of hyoid motion in tetrapods is reversed and also time-shifted relative to the pattern of jaw movement.

Overview of FEED, the feeding experiments end-user database

The Feeding Experiments End-user Database (FEED) is a research tool developed by the Mammalian Feeding Working Group at the National Evolutionary Synthesis Center that permits synthetic, evolutionary analyses of the physiology of mammalian feeding. The tasks of the Working Group are to compile physiologic data sets into a uniform digital format stored at a central source, develop a standardized terminology for describing and organizing the data, and carry out a set of novel analyses using FEED. FEED contains raw physiologic data linked to extensive metadata. It serves as an archive for a large number of existing data sets and a repository for future data sets. The metadata are stored as text and images that describe experimental protocols, research subjects, and anatomical information. The metadata incorporate controlled vocabularies to allow consistent use of the terms used to describe and organize the physiologic data. The planned analyses address long-standing questions concerning the phylogenetic distribution of phenotypes involving muscle anatomy and feeding physiology among mammals, the presence and nature of motor pattern conservation in the mammalian feeding muscles, and the extent to which suckling constrains the evolution of feeding behavior in adult mammals. We expect FEED to be a growing digital archive that will facilitate new research into understanding the evolution of feeding anatomy.

Chewing rates among domestic dog breeds

The mammalian masticatory rhythm is produced by a brainstem timing network. The rhythm is relatively fixed within individual animals but scales allometrically with body mass (M(b)) across species. It has been hypothesized that sensory feedback and feed-forward adjust the rhythm to match the jaw's natural resonance frequency, with allometric scaling being an observable consequence. However, studies performed with adult animals show that the rhythm is not affected by jaw mass manipulations, indicating that either developmental or evolutionary mechanisms are required for allometry to become manifest. The present study was performed to tease out the relative effects of development versus natural selection on chewing rate allometry. Thirty-one dog breeds and 31 mass-matched non-domestic mammalian species with a range in M(b) from approximately 2 kg to 50 kg were studied. Results demonstrated that the chewing rhythm did not scale with M(b) among dog breeds (R=0.299, P>0.10) or with jaw length (L(j)) (R=0.328, P>0.05). However, there was a significant relationship between the chewing rhythm and M(b) among the non-domestic mammals (R=0.634, P<0.001). These results indicate that scaling is not necessary in the adult animal. We conclude that the central timing network and related sensorimotor systems may be necessary for rhythm generation but they do not explain the 1/3rd to 1/4th allometric scaling observed among adult mammals. The rhythm of the timing network is either adjusted to the physical parameters of the jaw system during early development only, is genetically determined independently of the jaw system or is uniquely hard-wired among dogs and laboratory rodents.

Different functions for homologous serotonergic interneurons and serotonin in species-specific rhythmic behaviours

Closely related species can exhibit different behaviours despite homologous neural substrates. The nudibranch molluscs Tritonia diomedea and Melibe leonina swim differently, yet their nervous systems contain homologous serotonergic neurons. In Tritonia, the dorsal swim interneurons (DSIs) are members of the swim central pattern generator (CPG) and their neurotransmitter serotonin is both necessary and sufficient to elicit a swim motor pattern. Here it is shown that the DSI homologues in Melibe, the cerebral serotonergic posterior-A neurons (CeSP-As), are extrinsic to the swim CPG, and that neither the CeSP-As nor their neurotransmitter serotonin is necessary for swim motor pattern initiation, which occurred when the CeSP-As were inactive. Furthermore, the serotonin antagonist methysergide blocked the effects of both the serotonin and CeSP-As but did not prevent the production of a swim motor pattern. However, the CeSP-As and serotonin could influence the Melibe swim circuit; depolarization of a cerebral serotonergic posterior-A was sufficient to initiate a swim motor pattern and hyperpolarization of a CeSP-A temporarily halted an ongoing swim motor pattern. Serotonin itself was sufficient to initiate a swim motor pattern or make an ongoing swim motor pattern more regular. Thus, evolution of species-specific behaviour involved alterations in the functions of identified homologous neurons and their neurotransmitter.

Stereotypy, flexibility and coordination: key concepts in behavioral functional morphology

Animal movement and its muscular control are central topics in functional morphology. As experimentalists we often manipulate stimuli in a controlled setting or compare species to observe the degree of variation in movement and motor control of particular behaviors. Understanding and communicating the biological significance of these sources of variability requires a universal terminology that is presently lacking in the functional morphology literature. We suggest that `stereotypy' be used to refer to the degree of variability observed in a behavior across trials under a given set of conditions. The ability of an organism to alter its behavior across experimental treatments is referred to as `flexibility'. We discuss how there has been a tendency to confound the phenomenon of a behavior exhibiting low variability, which we refer to as stereotyped, with inflexibility, or the inability to alter the behavior in response to a change in stimulus. The degree of stereotypy and flexibility in a behavior need not be correlated, nor need they have a common underlying basis. Coordination, a term used to describe the relationship between different body parts during movement, can be stereotyped and can show flexibility. Stereotypy of coordination can be assessed by the strength of correlations between movements of two body parts. The influence of coordination coherence on behavioral performance has rarely been considered,and could shed light on how taxa differ in their ability to perform behaviors. We suggest definitions of the terms stereotypy, flexibility and coordination,and provide examples of how and when these terms could be used when discussing behavioral changes in functional morphology.

Fluid dynamics of feeding behaviour in white-spotted bamboo sharks

Although the motor control of feeding is presumed to be generally conserved, some fishes are capable of modulating the feeding behaviour in response to prey type and or prey size. This led to the `feeding modulation hypothesis', which states that rapid suction strikes are pre-programmed stereotyped events that proceed to completion once initiated regardless of sensory input. If this hypothesis holds true, successful strikes should be indistinguishable from unsuccessful strikes owing to a lack of feedback control in specialized suction feeding fishes. The hydrodynamics of suction feeding in white-spotted bamboo sharks (Chiloscyllium plagiosum) was studied in three behaviours: successful strikes, intraoral transports of prey and unsuccessful strikes. The area of the fluid velocity region around the head of feeding sharks was quantified using time-resolved digital particle image velocimetry (DPIV). The maximal size of the fluid velocity region is 56%larger in successful strikes than unsuccessful strikes (10.79 cm2vs 6.90 cm2), but they do not differ in duration,indicating that strikes are modulated based on some aspect of the prey or simply as a result of decreased effort on the part of the predator. The hydrodynamic profiles of successful and unsuccessful strikes differ after 21 ms, a period probably too short to provide time to react through feedback control. The predator-to-prey distance is larger in missed strikes compared with successful strikes, indicating that insufficient suction is generated to compensate for the increased distance. An accuracy index distinguishes unsuccessful strikes (–0.26) from successful strikes (0.45 to 0.61). Successful strikes occur primarily between the horizontal axis of the mouth and the dorsal boundary of the ingested parcel of water, and missed prey are closer to the boundary or beyond. Suction transports are shorter in duration than suction strikes but have similar maximal fluid velocity areas to move the prey through the oropharyngeal cavity into the oesophagus (54 ms vs 67 ms).

Electromyography and the evolution of motor control: limitations and insights

Electromyography (EMG), or the study of muscle activation patterns, has long been used to infer central nervous system (CNS) control of the musculoskeletal system and the evolution of that control. As the activation of the muscles at the level of the periphery is a reflection of the interaction of descending influences and local reflex control, EMG is an important tool in integrated investigations of the evolution of coordination in complex, musculoskeletal systems. Yet, the use of EMG as a tool to understand the evolution of motor control has its limitations. We here review the potential limitations and opportunities of the use of EMG in studying the evolution of motor control in vertebrates and provide original previously unpublished data to illustrate this. The relative timing of activation of a set of muscles can be used to evaluate CNS coordination of the components in a musculoskeletal system. Studies of relative timing reveal task-dependent variability in the recruitment of different populations of muscle fibers (i.e., different fiber types) within a single muscle, and left-right asymmetries in activation that need to be taken into account in comparative studies. The magnitude of muscle recruitment is strongly influenced by the instantaneous demands imposed on the system, and is likely determined by local reflex-control systems. Consequently, using EMG to make meaningful inferences about evolutionary changes in musculoskeletal control requires comparisons across similar functional tasks. Moreover, our data show that inferences about the evolution of motor control are limited in their explanatory power without proper insights into the kinematics and dynamics of a system.

Is a convergently derived muscle-activity pattern driving novel raking behaviours in teleost fishes?

Behavioural differences across prey-capture and processing mechanisms may be governed by coupled or uncoupled feeding systems. Osteoglossomorph and salmonid fishes process prey in a convergently evolved tongue-bite apparatus(TBA), which is musculoskeletally coupled with the primary oral jaws. Altered muscle-activity patterns (MAPs) in these coupled jaw systems could be associated with the independent origin of a novel raking behaviour in these unrelated lineages. Substantial MAP changes in the evolution of novel behaviours have rarely been quantified so we examined MAP differences across strikes, chewing and rakes in a derived raking salmonid, the rainbow trout, Oncorhynchus mykiss. Electromyography, including activity onset timing, duration, mean amplitude and integrated area from five feeding muscles revealed significant differences between behaviour-specific MAPs. Specifically, early activity onset in the protractor hyoideus and adductor mandibularis muscles characterised raking, congruent with a recent biomechanical model of the component-mechanisms driving the raking preparatory and power-stroke phases. Oncorhynchus raking MAPs were then compared with a phylogenetically derived osteoglossomorph representative, the Australian arowana, Scleropages jardinii. In both taxa, early onset of protractor hyoideus and adductor mandibularis activity characterised the raking preparatory phase, indicating a convergently derived MAP, while more subtle inter-lineage divergence in raking MAPs resulted from onset-timing and duration differences in sternohyoideus and hypaxialis activity. Convergent TBA morphologies are thus powered by convergently derived MAPs, a phenomenon not previously demonstrated in feeding mechanisms. Between lineages, differences in TBA morphology and associated differences in the functional coupling of jaw systems appear to be important factors in shaping the diversification of raking behaviours.

Interspecific variation in sternohyoideus muscle morphology in clariid catfishes: Functional implications for suction feeding

Depression of the hyoid apparatus plays a crucial role in generating suction, especially in fishes with a dorso-ventrally flattened head shape. It is generally assumed that shortening of the sternohyoideus muscle, which connects the hyoid to the pectoral girdle, contributes to hyoid depression. However, a recent study on the clariid catfish Clarias gariepinus has shown that this muscle does not shorten but elongates during this phase through retraction of the pectoral girdle. Here, we test whether this pattern is general among clariid catfish, or if variation in the morphology of the sternohyoideus may result in a different sternohyoideus behavior during hyoid depression. First, sternohyoideus mass, effective cross-sectional area, fiber length and fiber diameter were measured and compared for four clariid species. Next, velocity and magnitude of hyoid depression during prey capture (from high-speed videos), as well as patterns of sternohyoideus strain were analyzed (from high-speed X-ray videos) in these species. While morphology and hyoid depression performance varied considerably among these species, only the species with the most massive sternohyoideus, Gymnallabes typus, showed shortening of the sternohyoideus muscle during the initial part of the expansive phase. The data for Channallabes apus demonstrate that increasing the magnitude of hyoid depression does not necessarily require a shortening of the m. sternohyoideus, as it shows elongation of this muscle during hyoid depression.

Feeding with speed: prey capture evolution in cichilds

The diversity of both the locomotor and feeding systems in fish is extensive, although little is known about the integrated evolution of the two systems. Virtually, all fish swim to ingest prey and all open their buccal cavity during prey capture, but the relationship between these two ubiquitous components of fish feeding strikes is unknown. We predicted that there should be a positive correlation between ram speed (RS) and maximum gape (MG) because the accuracy of a predatory strike goes down with an increase in RS and fish with larger mouths eat larger, more evasive prey. For 18 species of neotropical cichlids, we used phylogenetic-independent contrasts to study the relationship between the predator closing speed (RS) and mouth size (MG) during prey capture. To provide a robust comparative framework, we augmented existing phylogenetic information available from the mitochondrial cytochrome b gene with sequences from the S7 nuclear ribosomal intron for these species. Then, we captured high-speed (500 images per second), lateral view feeding sequences of each species by using a digital video camera and measured both RS and MG. Uncorrected species values of MG and RS were positively and significantly correlated. When accounting for any of the set of phylogenetic relationships recovered, the independent contrasts of RS and MG remained significantly, and positively, correlated. This tight evolutionary coupling highlights what is likely a common relationship between locomotor behaviour and feeding kinematics in many organisms.

Homologues of serotonergic central pattern generator neurons in related nudibranch molluscs with divergent behaviors

Homologues of a neuron that contributes to a species-specific behavior were identified and characterized in species lacking that behavior. The nudibranch Tritonia diomedea swims by flexing its body dorsally and ventrally. The dorsal swim interneurons (DSIs) are components of the central pattern generator (CPG) underlying this rhythmic motor pattern and also activate crawling. Homologues of the DSIs were identified in six nudibranchs that do not exhibit dorsal-ventral swimming: Tochuina tetraquetra, Melibe leonina, Dendronotus iris, D. frondosus, Armina californica, and Triopha catalinae. Homology was based upon shared features that distinguish the DSIs from all other neurons: (1) serotonin immunoreactivity, (2) location in the Cerebral serotonergic posterior (CeSP) cluster, and (3) axon projection to the contralateral pedal ganglion. The DSI homologues, named CeSP-A neurons, share additional features with the DSIs: irregular basal firing, synchronous inputs, electrical coupling, and reciprocal inhibition. Unlike the DSIs, the CeSP-A neurons were not rhythmically active in response to nerve stimulation. The CeSP-A neurons in Tochuina and Triopha also excited homologues of the Tritonia Pd5 neuron, a crawling efferent. Thus, the CeSP-A neurons and the DSIs may be part of a conserved network related to crawling that may have been co-opted into a rhythmic swim CPG in Tritonia.

Anatomy and functional morphology of the feeding apparatus of the lesser electric ray,Narcine brasiliensis (Elasmobranchii: Batoidea)

Protrusion of the jaws during feeding is common in Batoidea (rays, skates, sawfishes, and guitarfishes), members of which possess a highly modified jaw suspension. The lesser electric ray, Narcine brasiliensis, preys primarily on polychaete annelids using a peculiar and highly derived mechanism for jaw protraction. The ray captures its prey by protruding its jaws beneath the substrate and generating subambient buccal pressure to suck worms into its mouth. Initiation of this protrusion is similar to that proposed for other batoids, in that the swing of the distal ends of the hyomandibulae is transmitted to Meckel's cartilage. A "scissor-jack" model of jaw protrusion is proposed for Narcine, in which the coupling of the upper and lower jaws, and extremely flexible symphyses, allow medial compression of the entire jaw complex. This results in a shortening of the distance between the right and left sides of the jaw arch and ventral extension of the jaws. Motion of the skeletal elements involved in this extreme jaw protrusion is convergent with that described for the wobbegong shark, Orectolobus maculatus. Narcine also exhibits asymmetrical protrusion of the jaws from the midline during processing, accomplished by unequal depression of the hyomandibulae. Lower jaw versatility is a functional motif in the batoid feeding mechanism. The pronounced jaw kinesis of N. brasiliensis is partly a function of common batoid characteristics: euhyostylic jaw suspension (decoupling the jaws from the hyoid arch) and complex and subdivided cranial musculature, affording fine motor control. However, this mechanism would not be possible without the loss of the basihyal in narcinid electric rays. The highly protrusible jaw of N. brasiliensis is a versatile and maneuverable feeding apparatus well-suited for the animal's benthic feeding lifestyle.