Simple mechanisms organise orientation of escape swimming in embryos and hatchling tadpoles of Xenopus laevis

MYH13, a superfast myosin expressed in extraocular, laryngeal and syringeal muscles

MYH13 is a unique type of sarcomeric myosin heavy chain (MYH) first detected in mammalian extraocular (EO) muscles and later also in vocal muscles, including laryngeal muscles of some mammals and syringeal muscles of songbirds. All these muscles are specialized in generating very fast contractions while producing relatively low force, a design appropriate for muscles acting against a much lower load than most skeletal muscles inserting into the skeleton. The definition of the physiological properties of muscle fibres containing MYH13 has been complicated by the mixed fibre type composition of EO muscles and the coexistence of different MYH types within the same fibre. A major advance in this area came from studies on isolated recombinant myosin motors and the demonstration that the affinity of actin-bound human MYH13 for ADP is much weaker than those of fast-type MYH1 (type 2X) and MYH2 (type 2A). This property is consistent with a very fast detachment of myosin from actin, a major determinant of shortening velocity. The MYH13 gene arose early during vertebrate evolution but was characterized only in mammals and birds and appears to have been lost in some teleost fish. The MYH13 gene is located at the 3' end of the mammalian fast/developmental gene cluster and in a similar position to the orthologous cluster in syntenic regions of the songbird genome. MYH13 gene regulation is controlled by a super-enhancer in the mammalian locus and deletion of the neighbouring fast MYH1 and MYH4 genes leads to abnormal MYH13 expression in mouse leg muscles.

Photobehaviours guided by simple photoreceptor systems

Light provides a widely abundant energy source and valuable sensory cue in nature. Most animals exposed to light have photoreceptor cells and in addition to eyes, there are many extraocular strategies for light sensing. Here, we review how these simpler forms of detecting light can mediate rapid behavioural responses in animals. Examples of these behaviours include photophobic (light avoidance) or scotophobic (shadow) responses, photokinesis, phototaxis and wavelength discrimination. We review the cells and response mechanisms in these forms of elementary light detection, focusing on aquatic invertebrates with some protist and terrestrial examples to illustrate the general principles. Light cues can be used very efficiently by these simple photosensitive systems to effectively guide animal behaviours without investment in complex and energetically expensive visual structures.

An early midbrain sensorimotor pathway is involved in the timely initiation and direction of swimming in the hatchling Xenopus laevis tadpole

Vertebrate locomotion is heavily dependent on descending control originating in the midbrain and subsequently influencing central pattern generators in the spinal cord. However, the midbrain neuronal circuitry and its connections with other brainstem and spinal motor circuits has not been fully elucidated. Vertebrates with very simple nervous system, like the hatchling Xenopus laevis tadpole, have been instrumental in unravelling fundamental principles of locomotion and its suspraspinal control. Here, we use behavioral and electrophysiological approaches in combination with lesions of the midbrain to investigate its contribution to the initiation and control of the tadpole swimming in response to trunk skin stimulation. None of the midbrain lesions studied here blocked the tadpole's sustained swim behavior following trunk skin stimulation. However, we identified that distinct midbrain lesions led to significant changes in the latency and trajectory of swimming. These changes could partly be explained by the increase in synchronous muscle contractions on the opposite sides of the tadpole's body and permanent deflection of the tail from its normal position, respectively. We conclude that the tadpole's embryonic trunk skin sensorimotor pathway involves the midbrain, which harbors essential neuronal circuitry to significantly contribute to the appropriate, timely and coordinated selection and execution of locomotion, imperative to the animal's survival.

Development of subdomains in the medial pallium of Xenopus laevis and Trachemys scripta: Insights into the anamniote-amniote transition

In all vertebrates, the most dorsal region of the telencephalon gives rise to the pallium, which in turn, is formed by at least four evolutionarily conserved histogenetic domains. Particularly in mammals, the medial pallium generates the hippocampal formation. Although this region is structurally different among amniotes, its functions, attributed to spatial memory and social behavior, as well as the specification of the histogenetic domain, appears to be conserved. Thus, the aim of the present study was to analyze this region by comparative analysis of the expression patterns of conserved markers in two vertebrate models: one anamniote, the amphibian Xenopus laevis; and the other amniote, the turtle Trachemys scripta elegans, during development and in adulthood. Our results show that, the histogenetic specification of both models is comparable, despite significant cytoarchitectonic differences, in particular the layered cortical arrangement present in the turtle, not found in anurans. Two subdivisions were observed in the medial pallium of these species: a Prox1 + and another Er81/Lmo4 +, comparable to the dentate gyrus and the mammalian cornu ammonis region, respectively. The expression pattern of additional markers supports this subdivision, which together with its functional involvement in spatial memory tasks, provides evidence supporting the existence of a basic program in the specification and functionality of the medial pallium at the base of tetrapods. These results further suggest that the anatomical differences found in different vertebrates may be due to divergences and adaptations during evolution.

Identification and functional evaluation of GRIA1 missense and truncation variants in individuals with ID: An emerging neurodevelopmental syndrome

GRIA1 encodes the GluA1 subunit of α-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptors, which are ligand-gated ion channels that act as excitatory receptors for the neurotransmitter L-glutamate (Glu). AMPA receptors (AMPARs) are homo- or heteromeric protein complexes with four subunits, each encoded by different genes, GRIA1 to GRIA4. Although GluA1-containing AMPARs have a crucial role in brain function, the human phenotype associated with deleterious GRIA1 sequence variants has not been established. Subjects with de novo missense and nonsense GRIA1 variants were identified through international collaboration. Detailed phenotypic and genetic assessments of the subjects were carried out and the pathogenicity of the variants was evaluated in vitro to characterize changes in AMPAR function and expression. In addition, two Xenopus gria1 CRISPR-Cas9 F₀ models were established to characterize the in vivo consequences. Seven unrelated individuals with rare GRIA1 variants were identified. One individual carried a homozygous nonsense variant (p.Arg377Ter), and six had heterozygous missense variations (p.Arg345Gln, p.Ala636Thr, p.Ile627Thr, and p.Gly745Asp), of which the p.Ala636Thr variant was recurrent in three individuals. The cohort revealed subjects to have a recurrent neurodevelopmental disorder mostly affecting cognition and speech. Functional evaluation of major GluA1-containing AMPAR subtypes carrying the GRIA1 variant mutations showed that three of the four missense variants profoundly perturb receptor function. The homozygous stop-gain variant completely destroys the expression of GluA1-containing AMPARs. The Xenopus gria1 models show transient motor deficits, an intermittent seizure phenotype, and a significant impairment to working memory in mutants. These data support a developmental disorder caused by both heterozygous and homozygous variants in GRIA1 affecting AMPAR function.

From decision to action: Detailed modelling of frog tadpoles reveals neuronal mechanisms of decision-making and reproduces unpredictable swimming movements in response to sensory signals

How does the brain process sensory stimuli, and decide whether to initiate locomotor behaviour? To investigate this question we develop two whole body computer models of a tadpole. The "Central Nervous System" (CNS) model uses evidence from whole-cell recording to define 2300 neurons in 12 classes to study how sensory signals from the skin initiate and stop swimming. In response to skin stimulation, it generates realistic sensory pathway spiking and shows how hindbrain sensory memory populations on each side can compete to initiate reticulospinal neuron firing and start swimming. The 3-D "Virtual Tadpole" (VT) biomechanical model with realistic muscle innervation, body flexion, body-water interaction, and movement is then used to evaluate if motor nerve outputs from the CNS model can produce swimming-like movements in a volume of "water". We find that the whole tadpole VT model generates reliable and realistic swimming. Combining these two models opens new perspectives for experiments.

Schooling in Xenopus laevis Tadpoles as a Way to Assess Their Neural Development

Escape behaviors, orienting reflexes, and social behaviors in Xenopus laevis tadpoles have been well-documented in the literature. Schooling behavior experiments allow for the observation of tadpole social interactions and in the past have been used to characterize behavioral deficits in models of neurodevelopmental disorders. Unlike other species of frogs, Xenopus tadpoles show polarized schooling. Not only do tadpoles aggregate, they also swim in the same direction. Quantifying both aggregation and relative swim angle can give us an important measure of social behavior and sensory integration. Past iterations of these experiments have required the continued presence of an experimenter throughout the duration of each trial and relied on expensive software for subsequent data analysis. The instrument configuration and analysis protocol outlined here provide an automated method to assess schooling by delivering a series of timed vibratory stimuli to a group of tadpoles to induce swimming behavior and then controlling a camera to document their positions via still images. Both stimulus delivery and image acquisition are automated using the Python programming language. Analysis is done using ImageJ and custom Python scripts, which are provided in this protocol. The specific equipment configuration and scripts shown here provide one solution, but other equipment and custom scripts can be substituted.

Reconciling the functions of even-skipped interneurons during crawling, swimming, and walking

In all bilaterally symmetric animals, movements across the body are coordinated by interneurons that traverse the midline. Recent work is beginning to tease apart the functional complexity of interneurons labeled by the homeodomain transcription factor even-skipped, which provide a phylogenetically-conserved source of commissural excitation during locomotion in both vertebrates and invertebrates. Here we review recent studies of the roles of even-skipped neurons during locomotion in flies (EL neurons), fishes, frogs, and mice (V0v neurons). Comparisons across species reveal commonalities, which include the functional organization of even-skipped circuits based on birth order, the link between increased muscular complexity and even-skipped neuron diversity, and the hierarchical organization of even-skipped circuits based on their control of escape versus exploratory movements. We discuss how stronger links between different species enable testable predictions to further the discovery of principles of locomotor network organization.

Xenopus embryos to study fetal alcohol syndrome, a model for environmental teratogenesis

Vertebrate model systems are central to characterize the outcomes of ethanol exposure and the etiology of fetal alcohol spectrum disorder (FASD), taking advantage of their genetic and morphological closeness and similarity to humans. We discuss the contribution of amphibian embryos to FASD research, focusing on Xenopus embryos. The Xenopus experimental system is characterized by external development and accessibility throughout embryogenesis, large clutch sizes, gene and protein activity manipulation, transgenesis and genome editing, convenient chemical treatment, explants and conjugates, and many other experimental approaches. Taking advantage of these methods, many insights regarding FASD have been obtained. These studies characterized the malformations induced by ethanol including quantitative analysis of craniofacial malformations, induction of fetal growth restriction, delay in gut maturation, and defects in the differentiation of the neural crest. Mechanistic, biochemical, and molecular studies in Xenopus embryos identified early gastrula as the high alcohol sensitivity window, targeting the embryonic organizer and inducing a delay in gastrulation movements. Frog embryos have also served to demonstrate the involvement of reduced retinoic acid production and an increase in reactive oxygen species in FASD. Amphibian embryos have helped pave the way for our mechanistic, molecular, and biochemical understanding of the etiology and pathophysiology of FASD.

A method for detailed movement pattern analysis of tadpole startle response

Prolonged space flight, specifically microgravity, presents a problem for space exploration. Animal models with altered connections of the vestibular ear, and thus altered gravity sensation, would allow the examination of the effects of microgravity and how various countermeasures can establish normal function. We describe an experimental apparatus to monitor the effects of ear manipulations to generate asymmetric gravity input on the tadpole escape response. To perform the movement pattern analysis, an imaging apparatus was developed that uses a high-speed camera to obtain time-resolved, high-resolution images of tadpole movements. Movements were recorded in a temperature-controlled test chamber following mechanical stimulation with a solenoid actuator, to elicit a C-start response. Temperature within the test cell was controlled with a recirculating water bath. Xenopus laevis embryos were obtained using a standard fertilization technique. Tadpole response to a controlled perturbation was recorded in unprecedented detail and the approach was validated by describing the distinct differences in response between normal and one-eared tadpoles. The experimental apparatus and methods form an important element of a rigorous investigation into the response of the tadpole vestibular system to mechanical and biochemical manipulations, and can ultimately contribute to improved understanding of the effects of altered gravity perception on humans.

Spinal control of motor outputs by intrinsic and externally induced electric field potentials

Despite numerous studies on spinal neuronal systems, several issues regarding their role in motor behavior remain unresolved. One of these issues is how electric fields associated with the activity of spinal neurons influence the operation of spinal neuronal networks and how effects of these field potentials are combined with other means of modulating neuronal activity. Another closely related issue is how external electric field potentials affect spinal neurons and how they can be used for therapeutic purposes such as pain relief or recovery of motor functions by transspinal direct current stimulation. Nevertheless, progress in our understanding of the spinal effects of electric fields and their mechanisms has been made over the last years, and the aim of the present review is to summarize the recent findings in this field.

Intraspinal Sensory Neurons Provide Powerful Inhibition to Motor Circuits Ensuring Postural Control during Locomotion

In the vertebrate spinal cord, cerebrospinal fluid-contacting neurons (CSF-cNs) are GABAergic neurons whose functions are only beginning to unfold. Recent evidence indicates that CSF-cNs detect local spinal bending and relay this mechanosensory feedback information to motor circuits, yet many CSF-cN targets remain unknown. Using optogenetics, patterned illumination, and in vivo electrophysiology, we show here that CSF-cNs provide somatic inhibition to fast motor neurons and excitatory sensory interneurons involved in the escape circuit. Ventral CSF-cNs respond to longitudinal spinal contractions and induce large inhibitory postsynaptic currents (IPSCs) sufficient to silence spiking of their targets. Upon repetitive stimulation, these IPSCs promptly depress, enabling the mechanosensory response to the first bend to be the most effective. When CSF-cNs are silenced, postural control is compromised, resulting in rollovers during escapes. Altogether, our data demonstrate how GABAergic sensory neurons provide powerful inhibitory feedback to the escape circuit to maintain balance during active locomotion.

Huntingtin is required for ciliogenesis and neurogenesis during early Xenopus development

Huntington's Disease (HD) is a neurodegenerative disorder that results from the abnormal expansion of poly-glutamine (polyQ) repeats in the Huntingtin (HTT) gene. Although HTT has been linked to a variety of cellular events, it is still not clear what the physiological functions of the protein are. Because of its critical role during mouse embryonic mouse development, we investigated the functions of Htt during early Xenopus embryogenesis. We find that reduction of Htt levels affects cilia polarity and function and causes whole body paralysis. Moreover, Htt loss of function leads to abnormal development of trigeminal and motor neurons. Interestingly, these phenotypes are partially rescued by either wild-type or expanded HTT. These results show that the Htt activity is required for normal embryonic development, and highlight the usefulness of the Xenopus system for investigating proteins involved in human diseases.

Excitation and inhibition in recurrent networks mediate collision avoidance in Xenopus tadpoles

Information processing in the vertebrate brain is thought to be mediated through distributed neural networks, but it is still unclear how sensory stimuli are encoded and detected by these networks, and what role synaptic inhibition plays in this process. Here we used a collision avoidance behavior in Xenopus tadpoles as a model for stimulus discrimination and recognition. We showed that the visual system of the tadpole is selective for behaviorally relevant looming stimuli, and that the detection of these stimuli first occurs in the optic tectum. By comparing visually guided behavior, optic nerve recordings, excitatory and inhibitory synaptic currents, and the spike output of tectal neurons, we showed that collision detection in the tadpole relies on the emergent properties of distributed recurrent networks within the tectum. We found that synaptic inhibition was temporally correlated with excitation, and did not actively sculpt stimulus selectivity, but rather it regulated the amount of integration between direct inputs from the retina and recurrent inputs from the tectum. Both pharmacological suppression and enhancement of synaptic inhibition disrupted emergent selectivity for looming stimuli. Taken together these findings suggested that, by regulating the amount of network activity, inhibition plays a critical role in maintaining selective sensitivity to behaviorally-relevant visual stimuli.

Modeling human neurodevelopmental disorders in the Xenopus tadpole: from mechanisms to therapeutic targets

The Xenopus tadpole model offers many advantages for studying the molecular, cellular and network mechanisms underlying neurodevelopmental disorders. Essentially every stage of normal neural circuit development, from axon outgrowth and guidance to activity-dependent homeostasis and refinement, has been studied in the frog tadpole, making it an ideal model to determine what happens when any of these stages are compromised. Recently, the tadpole model has been used to explore the mechanisms of epilepsy and autism, and there is mounting evidence to suggest that diseases of the nervous system involve deficits in the most fundamental aspects of nervous system function and development. In this Review, we provide an update on how tadpole models are being used to study three distinct types of neurodevelopmental disorders: diseases caused by exposure to environmental toxicants, epilepsy and seizure disorders, and autism.

The firing patterns of spinal neurons: in situ patch-clamp recordings reveal a key role for potassium currents

Neuron firing patterns underpin the detection and processing of stimuli, influence synaptic interactions, and contribute to the function of networks. To understand how intrinsic membrane properties determine firing patterns, we investigated the biophysical basis of single and repetitive firing in spinal neurons of hatchling Xenopus laevis tadpoles, a well-understood vertebrate model; experiments were conducted in situ. Primary sensory Rohon-Beard (RB) neurons fire singly in response to depolarising current, and dorsolateral (DL) interneurons fire repetitively. RB neurons exhibited a large tetrodotoxin-sensitive sodium current; in DL neurons, the sodium current density was significantly lower. High-voltage-activated calcium currents were similar in both neuron types. There was no evidence of persistent sodium currents, low-voltage-activated calcium currents, or hyperpolarisation-activated currents. In RB neurons, the potassium current was dominated by a tetraethylammonium-sensitive slow component (I(Ks) ); a fast component (I(Kf) ), sensitive to 4-aminopyridine, predominated in DL neurons. Sequential current-clamp and voltage-clamp recordings in individual neurons suggest that high densities of I(Ks) prevent repetitive firing; where I(Ks) is small, I(Kf) density determines the frequency of repetitive firing. Intermediate densities of I(Ks) and I(Kf) allow neurons to fire a few additional spikes on strong depolarisation; this property typifies a novel subset of RB neurons, and may activate escape responses. We discuss how this ensemble of currents and firing patterns underpins the operation of the Xenopus locomotor network, and suggest how simple mechanisms might underlie the similar firing patterns seen in the neurons of diverse species.

Pharmacology of currents underlying the different firing patterns of spinal sensory neurons and interneurons identified in vivo using multivariate analysis

The operation of neuronal networks depends on the firing patterns of the network's neurons. When sustained current is injected, some neurons in the central nervous system fire a single action potential and others fire repetitively. For example, in Xenopus laevis tadpoles, primary-sensory Rohon-Beard (RB) neurons fired a single action potential in response to 300-ms rheobase current injections, whereas dorsolateral (DL) interneurons fired repetitively at 10-20 Hz. To investigate the basis for these differences in vivo, we examined drug-induced changes in the firing patterns of Xenopus spinal neurons using whole cell current-clamp recordings. Neuron types were initially separated through cluster analysis, and we compared results produced using different clustering algorithms. We used these results to develop a predictive function to classify subsequently recorded neurons. The potassium channel blocker tetraethylammonium (TEA) converted single-firing RB neurons to low-frequency repetitive firing but reduced the firing frequency of repetitive-firing DL interneurons. Firing frequency in DL interneurons was also reduced by the potassium channel blockers 4-aminopyridine (4-AP), catechol, and margatoxin; 4-AP had the greatest effect. The calcium channel blockers amiloride and nimodipine had few effects on firing in either neuron type but reduced action potential duration in DL interneurons. Muscarine, which blocks M-currents, did not affect RB neurons but reduced firing frequency in DL interneurons. These results suggest that potassium currents may control neuron firing patterns: a TEA-sensitive current prevents repetitive firing in RB neurons, whereas a 4-AP-sensitive current underlies repetitive firing in DL interneurons. The cluster and discriminant analysis described could help to classify neurons in other systems.

Probing spinal circuits controlling walking in mammals

Locomotion in mammals is a complex motor act that involves the activation of a large number of muscles in a well-coordinated pattern. Understanding the network organization of the intrinsic spinal networks that control the locomotion, the central pattern generators, has been a challenge to neuroscientists. However, experiments using the isolated rodent spinal cord and combining electrophysiology and molecular genetics to dissect the locomotor network have started to shed new light on the network structure. In the present review, we will discuss findings that have revealed the role of designated populations of neurons for the key network functions including coordinating muscle activity and generating rhythmic activity. These findings are summarized in proposed organizational principles for the mammalian segmental CPG.

Genetics moving to neuronal networks

Neuronal circuits are essential components of the nervous system and determine various body functions. To understand how neuronal circuits operate it is necessary to identify the participating neuronal subpopulations and to dissect the function of the neurons at the molecular level. The locomotor central pattern generator that coordinates body movements is well suited for elucidating the assembly and identity of the participating neurons. Remarkable advances in the field of genetics are making studies in neuroscience more efficient and precise so that now, using nematode worms, fruit flies, zebrafish and mice as model organisms, a genetic approach can be used to identify molecules and neurons crucial for locomotor network functionality.

The morphology, behavior, and biomechanics of swimming in ascidian larvae

Ascidian larvae use a simple set of locomotor behaviors during dispersal and settlement. The swimming performance and the ability of an ascidian larva to orient within its environment depend on the biomechanics of its rudimentary locomotor morphology. The undulatory motion of the tail generates fluid forces that propel the body with a rate and direction determined by body mass and its spatial distribution. Differences in morphology and behavior among species influence these dynamics and create differences in swimming performance. Furthermore, the ability of a larva to orient within its environment depends on its ability to coordinate the motion of its body with respect to perceived sensory cues. Research on the swimming of ascidian larvae demonstrates the biomechanical basis of tactic orientation and interspecific differences in performance in a large and diverse group of animals.

Mechanisms and significance of reduced activity and responsiveness in resting frog tadpoles

Hatchling Xenopus laevis tadpoles spend most of their time attached to objects or the water surface by mucus secreted by a gland on the head. While attached, swimming activity and responsiveness to swim-initiating stimuli are reduced over long periods of time. We have investigated the mechanisms and significance of this apparent long-term inhibition. In behavioural experiments we show, firstly, that innervation of the cement gland and GABA(A)-mediated inhibition are necessary for attachment to reduce responsiveness, and secondly, that denervation of the cement gland increases tadpole activity and increases their predation by damselfly nymphs (Zygoptera). To investigate the neuronal pathway from the cement gland to GABA(A) inhibition, we have devised an immobilized, inverted tadpole preparation where a weight attached to the mucus simulates the force as it hangs. Simulated attachment reduces responsiveness and spontaneous fictive swimming activity. We have recorded the activity and responses of trigeminal neurons innervating the cement gland. They are spontaneously active and simulating attachment results in a sustained increase in this activity. We propose that hanging from a mucus strand increases firing in cement gland afferents. This leads to tonic GABA inhibition that reduces tadpole activity and responses, and leads to fewer attacks by predators.

Physiological, anatomical and genetic identification of CPG neurons in the developing mammalian spinal cord

The basic motor patterns underlying rhythmic limb movements during locomotion are generated by neuronal networks located within the spinal cord. These networks are called Central Pattern Generators (CPGs). Isolated spinal cord preparations from newborn rats and mice have become increasingly important for understanding the organization of the CPG in the mammalian spinal cord. Early studies using these preparations have focused on the overall network structure and the localization of the CPG. In this review we concentrate on recent experiments aimed at identifying and characterizing CPG-interneurons in the rodent. These experiments include the organization and function of descending commissural interneurons (dCINs) in the hindlimb CPG of the neonatal rat, as well as the role of Ephrin receptor A4 (EphA4) and its Ephrin ligand B3 (EphrinB3), in the construction of the mammalian locomotor network. These latter experiments have defined EphA4 as a molecular marker for mammalian excitatory hindlimb CPG neurons. We also review genetic approaches that can be applied to the mouse spinal cord. These include methods for identifying sub-populations of neurons by genetically encoded reporters, techniques to trace network connectivity with cell-specific genetically encoded tracers, and ways to selectively ablate or eliminate neuron populations from the CPG. We propose that by applying a multidisciplinary approach it will be possible to understand the network structure of the mammalian locomotor CPG. Such an understanding will be instrumental in devising new therapeutic strategies for patients with spinal cord injury.

Mechanisms of helical swimming: asymmetries in the morphology, movement and mechanics of larvae of the ascidian Distaplia occidentalis

A great diversity of unicellular and invertebrate organisms swim along a helical path, but it is not well understood how asymmetries in the body shape or the movement of propulsive structures affect a swimmer’s ability to perform the body rotation necessary to move helically. The present study found no significant asymmetries in the body shape of ascidian larvae (Distaplia occidentalis) that could operate to rotate the body during swimming. By recording the three-dimensional movement of free-swimming larvae, it was found that the tail possessed two bends, each with constant curvature along their length. As these bends traveled posteriorly, the amplitude of curvature changes was significantly greater in the concave-left direction than in the concave-right direction. In addition to this asymmetry, the tail oscillated at an oblique angle to the midline of the trunk. These asymmetries generated a yawing moment that rotated the body in the counterclockwise direction from a dorsal view, according to calculations from hydrodynamic theory. The tails of resting larvae were bent in the concave-left direction with a curvature statistically indistinguishable from the median value for tail curvature during swimming. The flexural stiffness of the tails of larvae, measured in three-point bending, may be great enough to allow the resting curvature of the tail to have an effect on the symmetry of kinematics. This work suggests that asymmetrical tail motion is an important mechanism for generating a yawing moment during swimming in ascidian larvae and that these asymmetries may be caused by the tail’s bent shape. Since helical motion requires that moments also be generated in the pitching or rolling directions, other mechanisms are required to explain fully how ascidian larvae generate and control helical swimming.

Responses of young Xenopus laevis tadpoles to light dimming: possible roles for the pineal eye

When the light is dimmed, the pineal eye of hatchling Xenopus laevis tadpoles excites the central pattern generator for swimming, but the behavioural significance of pineal excitation is unclear. We show that tadpoles spend 99 % of their time hanging from the surface meniscus or solid objects using mucus secreted by a cement gland on the head. Attachment inhibits swimming, but unattached tadpoles swim spontaneously. Provided that their pineal eye is intact, they attach closer to the water surface in the dark than in the light and attach preferentially to the underside of floating objects that cast shadows. Dimming causes tadpoles swimming horizontally to turn upwards and is very effective in initiating upward swimming in unattached tadpoles. Similar pineal-dependent responses during swimming are present up to stage 44. Pinealectomy blocks responses to dimming at all stages. Recordings from immobilised tadpoles reveal that light dimming induces faster fictive swimming and that pineal activity is increased for up to 20 min during sustained light dimming. We suggest that the increase in pineal discharge during dimming increases the probability of upward swimming and, in this way, increases the probability of tadpoles attaching to objects higher in the water column that cast shadows.