Photobehaviour of Hydra (Cnidaria, Hydrozoa) and correlated mechanisms: a case of extraocular photosensitivity

Phototaxis is a satiety-dependent behavioral sequence in Hydra vulgaris

Understanding how internal states such as satiety are connected to animal behavior is a fundamental question in neuroscience. Hydra vulgaris, a freshwater cnidarian with only 12 neuronal cell types, serves as a tractable model system for studying state-dependent behaviors. We found that starved hydras consistently move towards light, while fed hydras do not. By modeling this behavior as a set of three sequences of head orientation, jump distance and jump rate, we demonstrate that the satiety state only affects the rate of the animal jumping to a new position, while the orientation and jump distance are unaffected. These findings yield insights into how internal states in a simple organism, Hydra, affect specific elements of a behavior, and offer general principles for studying the relationship between state-dependent behaviors and their underlying molecular mechanisms.

CLOCK evolved in cnidaria to synchronize internal rhythms with diel environmental cues

The circadian clock enables anticipation of the day/night cycle in animals ranging from cnidarians to mammals. Circadian rhythms are generated through a transcription-translation feedback loop (TTFL or pacemaker) with CLOCK as a conserved positive factor in animals. However, CLOCK's functional evolutionary origin and mechanism of action in basal animals are unknown. In the cnidarian Nematostella vectensis, pacemaker gene transcript levels, including NvClk (the Clock ortholog), appear arrhythmic under constant darkness, questioning the role of NvCLK. Utilizing CRISPR/Cas9, we generated a NvClk allele mutant (NvClkΔ), revealing circadian behavior loss under constant dark (DD) or light (LL), while maintaining a 24 hr rhythm under light-dark condition (LD). Transcriptomics analysis revealed distinct rhythmic genes in wild-type (WT) polypsunder LD compared to DD conditions. In LD, NvClkΔ/Δ polyps exhibited comparable numbers of rhythmic genes, but were reduced in DD. Furthermore, under LD, the NvClkΔ/Δ polyps showed alterations in temporal pacemaker gene expression, impacting their potential interactions. Additionally, differential expression of non-rhythmic genes associated with cell division and neuronal differentiation was observed. These findings revealed that a light-responsive pathway can partially compensate for circadian clock disruption, and that the Clock gene has evolved in cnidarians to synchronize rhythmic physiology and behavior with the diel rhythm of the earth's biosphere.

Diurnal and circadian regulation of opsin-like transcripts in the eyeless cnidarian Hydra

Opsins play a key role in the ability to sense light both in image-forming vision and in non-visual photoreception (NVP). These modalities, in most animal phyla, share the photoreceptor protein: an opsin-based protein binding a light-sensitive chromophore by a lysine (Lys) residue. So far, visual and non-visual opsins have been discovered throughout the Metazoa phyla, including the photoresponsive Hydra, an eyeless cnidarian considered the evolutionary sister species to bilaterians. To verify whether light influences and modulates opsin gene expression in Hydra, we utilized four expression sequence tags, similar to two classic opsins (SW rhodopsin and SW blue-sensitive opsin) and two non-visual opsins (melanopsin and peropsin), in investigating the expression patterns during both diurnal and circadian time, by means of a quantitative RT-PCR. The expression levels of all four genes fluctuated along the light hours of diurnal cycle with respect to the darkness one and, in constant dark condition of the circadian cycle, they increased. The monophasic behavior in the L12:D12 cycle turned into a triphasic expression profile during the continuous darkness condition. Consequently, while the diurnal opsin-like expression revealed a close dependence on light hours, the highest transcript levels were found in darkness, leading us to novel hypothesis that in Hydra, an "internal" biological rhythm autonomously supplies the opsins expression during the circadian time. In conclusion, in Hydra, both diurnal and circadian rhythms apparently regulate the expression of the so-called visual and non-visual opsins, as already demonstrated in higher invertebrate and vertebrate species. Our data confirm that Hydra is a suitable model for studying ancestral precursor of both visual and NVP, providing useful hints on the evolution of visual and photosensory systems.

Decoupling behavioral and transcriptional responses to color in an eyeless cnidarian

Background

Animals have specific molecular, physiological, and behavioral responses to light that are influenced by wavelength and intensity. Predictable environmental changes – predominantly solar and lunar cycles – drive endogenous daily oscillations by setting internal pacemakers, otherwise known as the circadian clock. Cnidarians have been a focal group to discern the evolution of light responsiveness due to their phylogenetic position as a sister phylum to bilaterians and broad range of light-responsive behaviors and physiology. Marine species that occupy a range of depths will experience different ranges of wavelengths and light intensities, which may result in variable phenotypic responses. Here, we utilize the eyeless sea anemone Nematostella vectensis, an estuarine anemone that typically resides in shallow water habitats, to compare behavioral and molecular responses when exposed to different light conditions.

Results

Quantitative measures of locomotion clearly showed that this species responds to light in the blue and green spectral range with a circadian activity profile, in contrast to a circatidal activity profile in the red spectral range and in constant darkness. Differences in average day/night locomotion was significant in each condition, with overall peak activity during the dark period. Comparative analyses of 96 transcriptomes from individuals sampled every 4 h in each lighting treatment revealed complex differences in gene expression between colors, including in many of the genes likely involved in the cnidarian circadian clock. Transcriptional profiling showed the majority of genes are differentially expressed when comparing mid-day with mid-night, and mostly in red light. Gene expression profiles were largely unique in each color, although animals in blue and green were overall more similar to each other than to red light.

Conclusions

Together, these analyses support the hypothesis that cnidarians are sensitive to red light, and this perception results in a rich transcriptional and divergent behavioral response. Future work determining the specific molecular mechanisms driving the circadian and potential circatidal rhythms measured here would be impactful to connect gene expression variation with behavioral variation in this eyeless species.

Hydra vulgaris exhibits day-night variation in behavior and gene expression levels

BACKGROUND

Day-night behavioral variation is observed in most organisms, and is generally controlled by circadian clocks and/or synchronization to environmental cues. Hydra species, which are freshwater cnidarians, are thought to lack the core clock genes that form transcription-translation feedback loops in clock systems. In this study, we examined whether hydras exhibit diel rhythms in terms of behavior and gene expression levels without typical clock genes.

RESULTS

We found that the total behavior of hydras was elevated during the day and decreased at night under a 12-h light-dark cycle. Polyp contraction frequency, one component of behavior, exhibited a clear diel rhythm. However, neither total behavior nor polyp contraction frequency showed rhythmic changes under constant light and constant dark conditions. To identify the genes underlying diel behavior, we performed genome-wide transcriptome analysis of hydras under light-dark cycles. Using three different analytic algorithms, we found that 380 genes showed robust diel oscillations in expression. Some of these genes shared common features with diel cycle genes of other cnidarian species with endogenous clock systems.

CONCLUSION

Hydras show diel behavioral rhythms under light-dark cycles despite the absence of canonical core clock genes. Given the functions of the genes showing diel oscillations in hydras and the similarities of those genes with the diel cycle genes of other cnidarian species with circadian clocks, it is possible that diel cycle genes play an important role across cnidarian species regardless of the presence or absence of core clock genes under light-dark cycles.

The planarian TRPA1 homolog mediates extraocular behavioral responses to near-ultraviolet light

Although light is most commonly thought of as a visual cue, many animals possess mechanisms to detect light outside of the eye for various functions, including predator avoidance, circadian rhythms, phototaxis and migration. Here we confirm that planarians (like Caenorhabditis elegans, leeches and Drosophila larvae) are capable of detecting and responding to light using extraocular photoreception. We found that, when either eyeless or decapitated worms were exposed to near-ultraviolet (near-UV) light, intense wild-type photophobic behaviors were still observed. Our data also revealed that behavioral responses to green wavelengths were mediated by ocular mechanisms, whereas near-UV responses were driven by extraocular mechanisms. As part of a candidate screen to uncover the genetic basis of extraocular photoreception in the planarian species Schmidtea mediterranea, we identified a potential role for a homolog of the transient receptor potential channel A1 (TRPA1) in mediating behavioral responses to extraocular light cues. RNA interference (RNAi) to Smed-TrpA resulted in worms that lacked extraocular photophobic responses to near-UV light, a mechanism previously only identified in Drosophila These data show that the planarian TRPA1 homolog is required for planarian extraocular-light avoidance and may represent a potential ancestral function of this gene. TRPA1 is an evolutionarily conserved detector of temperature and chemical irritants, including reactive oxygen species that are byproducts of UV-light exposure. Our results suggest that planarians possess extraocular photoreception and display an unconventional TRPA1-mediated photophobic response to near-UV light.

Semiconducting polymers are light nanotransducers in eyeless animals

Current implant technology uses electrical signals at the electrode-neural interface. This rather invasive approach presents important issues in terms of performance, tolerability, and overall safety of the implants. Inducing light sensitivity in living organisms is an alternative method that provides groundbreaking opportunities in neuroscience. Optogenetics is a spectacular demonstration of this, yet is limited by the viral transfection of exogenous genetic material. We propose a nongenetic approach toward light control of biological functions in living animals. We show that nanoparticles based on poly(3-hexylthiophene) can be internalized in eyeless freshwater polyps and are fully biocompatible. Under light, the nanoparticles modify the light response of the animals, at two different levels: (i) they enhance the contraction events of the animal body, and (ii) they change the transcriptional activation of the opsin3-like gene. This suggests the establishment of a seamless and biomimetic interface between the living organism and the polymer nanoparticles that behave as light nanotransducers, coping with or amplifying the function of primitive photoreceptors.

The two nerve rings of the hypostomal nervous system of Hydra vulgaris-an immunohistochemical analysis

In Hydra vulgaris, physiological and pharmacological evidence exists for a hypostomal circumferential neuro-effector pathway that initiates ectodermal pacemaker activity at tentacular-hypostomal loci coordinating body and tentacle contractions. Here, we describe an ectodermal nerve ring that runs below and between the tentacles, and an anti-GABA_B receptor antibody-labeled ring coincident with it. The location of this ring is consistent with the physiology of the hypostomal pacemaker systems of hydra. We also describe a distally located, ectodermal ring of nerve fibers that is not associated with anti-GABA_B receptor antibody labeling. The neurites and cell bodies of sensory cells contribute to both rings. The location of the distal ring and its sensory cell neurites suggests an involvement in the behavior of the mouth. Between the two rings is a network of anastomosing sensory and ganglion cell bodies and their neurites. Phase contrast, darkfield, and antibody-labeled images reveal that the mouth of hydra comprises five or six epithelial folds whose endoderm extensively labels with anti-GABA_B receptor antibody, suggesting that endodermal metabotrobic GABA receptors are also involved in regulating mouth behavior.

Cubozoan genome illuminates functional diversification of opsins and photoreceptor evolution

Animals sense light primarily by an opsin-based photopigment present in a photoreceptor cell. Cnidaria are arguably the most basal phylum containing a well-developed visual system. The evolutionary history of opsins in the animal kingdom has not yet been resolved. Here, we study the evolution of animal opsins by genome-wide analysis of the cubozoan jellyfish Tripedalia cystophora, a cnidarian possessing complex lens-containing eyes and minor photoreceptors. A large number of opsin genes with distinct tissue- and stage-specific expression were identified. Our phylogenetic analysis unequivocally classifies cubozoan opsins as a sister group to c-opsins and documents lineage-specific expansion of the opsin gene repertoire in the cubozoan genome. Functional analyses provided evidence for the use of the Gs-cAMP signaling pathway in a small set of cubozoan opsins, indicating the possibility that the majority of other cubozoan opsins signal via distinct pathways. Additionally, these tests uncovered subtle differences among individual opsins, suggesting possible fine-tuning for specific photoreceptor tasks. Based on phylogenetic, expression and biochemical analysis we propose that rapid lineage- and species-specific duplications of the intron-less opsin genes and their subsequent functional diversification promoted evolution of a large repertoire of both visual and extraocular photoreceptors in cubozoans.

Extraocular spectral photosensitivity in the tentacles of Hydra vulgaris

Previous electrophysiological studies on the cnidarian Hydra vulgaris have shown that hydra have a highly developed and specific photoresponse despite their lack of any structure recognizable as a traditional photoreceptor. In an effort to identify the site of hydra's photoreceptors, we recorded extracellularly from single excised tentacles and from ablated hypostomes lacking tentacles in absolute darkness and during exposure to light of various wavelengths. During recording, after an initial period of absolute darkness, tentacles or hypostomes were exposed to light from 450nm to 600nm, red, and white light. Exposure to light caused a change in the pattern and frequency of impulses in the tentacles that varied with color. The number of large tentacle pulses (TPs) increased at 550 and 600nm relative to darkness, whereas the number of small tentacle pulses (STPs) tended to decrease in 500nm light. Impulse frequency was significantly different among the different wavelengths. In addition to bursts of tentacle contraction pulses, long trains of pulses were observed. A change in lighting caused a switch from bursting to trains or vice versa. In contrast to excised tentacles, no change in electrical activity was seen in ablated hypostomes at any of the wavelengths relative to each other or relative to darkness. These results indicate that isolated tentacles can distinguish among and respond to various colors across the visible spectrum and suggest that electromagnetic information is transmitted from the tentacles to the hypostome where it may be integrated by the hypostomal nervous system, ultimately contributing to hydra's photoreceptive behavior.

Circadian clocks in the cnidaria: environmental entrainment, molecular regulation, and organismal outputs

The circadian clock is a molecular network that translates predictable environmental signals, such as light levels, into organismal responses, including behavior and physiology. Regular oscillations of the molecular components of the clock enable individuals to anticipate regularly fluctuating environmental conditions. Cnidarians play important roles in benthic and pelagic marine environments and also occupy a key evolutionary position as the likely sister group to the bilaterians. Together, these attributes make members of this phylum attractive as models for testing hypotheses on roles for circadian clocks in regulating behavior, physiology, and reproduction as well as those regarding the deep evolutionary conservation of circadian regulatory pathways in animal evolution. Here, we review and synthesize the field of cnidarian circadian biology by discussing the diverse effects of daily light cycles on cnidarians, summarizing the molecular evidence for the conservation of a bilaterian-like circadian clock in anthozoan cnidarians, and presenting new empirical data supporting the presence of a conserved feed-forward loop in the starlet sea anemone, Nematostella vectensis. Furthermore, we discuss critical gaps in our current knowledge about the cnidarian clock, including the functions directly regulated by the clock and the precise molecular interactions that drive the oscillating gene-expression patterns. We conclude that the field of cnidarian circadian biology is moving rapidly toward linking molecular mechanisms with physiology and behavior.

Evidence for multiple photosystems in jellyfish

Cnidarians are often used as model animals in studies of eye and photopigment evolution. Most cnidarians display photosensitivity at some point in their lifecycle ranging from extraocular photoreception to image formation in camera-type eyes. The available information strongly suggests that some cnidarians even possess multiple photosystems. The evidence is strongest within Cubomedusae where all known species posses 24 eyes of four morphological types. Physiological experiments show that each cubomedusan eye type likely constitutes a separate photosystem controlling separate visually guided behaviors. Further, the visual system of cubomedusae also includes extraocular photoreception. The evidence is supported by immunocytochemical and molecular data indicating multiple photopigments in cubomedusae as well as in other cnidarians. Taken together, available data suggest that multiple photosystems had evolved already in early eumetazoans and that their original level of organization was discrete sets of special-purpose eyes and/or photosensory cells.

Nitric oxide signaling pathways at neural level in invertebrates: functional implications in cnidarians

Nitric oxide (NO) is a small molecule with unconventional properties. It is found in organisms throughout the phylogenetic scale, from fungi to mammals, in which it acts as an intercellular messenger of main physiological events, or even as an intracellular messenger in invertebrates. In both vertebrates and invertebrates, NO is involved in many processes, regulated in part by cyclic guanosine monophosphate (cGMP), and reacts with different oxygen molecular species. The presence of NO in the early-diverging metazoan phylum of Cnidaria, of which Hydra represents the first known species having a nervous system, supports a role of this molecule as an ancestral neural messenger with physiological roles that remain to be largely elucidated. Therefore, our novel findings on the presence of NO in Hydra are here integrated in such a comparative frame.

Evolving visual pigments: hints from the opsin-based proteins in a phylogenetically old "eyeless" invertebrate

Visual pigments are photosensitive receptor proteins that trigger the transduction process producing the visual excitation once they have absorbed photons. In spite of the molecular and morpho-functional complexity that has characterized the development of animal eyes and eyeless photoreceptive systems, opsin-based protein family appears ubiquous along metazoan visual systems. Moreover, in addition to classic rhodopsin photoreceptors, all Metazoa have supplementary non-visual photosensitive structures, mainly located in the central nervous system, that sense light without forming an image and that rather regulate the organism's temporal physiology. The investigation of novel non-visual photopigments exerting extraretinal photoreception is a challenging field in vision research. Here we propose the cnidarian Hydra as a useful tool of investigation for molecular and functional differences between these pigment families. Hydra is the first metazoan owning a nervous system and it is an eyeless invertebrate showing only an extraocular photoreception, as it has no recognized visual or photosensitive structures. In this paper we provide an overview of the molecular and functional features of the opsin-based protein subfamilies and preliminary evidences in a phylogenetically old species of both image-forming and non-visual opsins. Then we give new insights on the molecular biology of Hydra photoreception and on the evolutionary pathways of visual pigments.

Peduncle of Hydra and the heart of higher organisms share a common ancestral origin

The heart is assumed to have evolved as the organ for pumping blood. Here we report a pumping phenomenon in Hydra, a member of the phylum Cnidaria. We find that the peduncle, lower quarter of the body column, stores most of the gastrovascular fluid when the animal is an elongate form. Upon contraction of the polyp, the peduncle contracts and transfers the fluid into the rest of the cavity. We also find that Hydra RFamide III, a homolog of cardioexcitatory RFamide neuropeptides in higher organisms, elevates this transfer activity. Further, CnNk-2, a homolog of a cardiomuscular tissue marker Nkx-2.5, is expressed in the endodermal tissue of the peduncle. These observations indicate that the transfer of fluid by the peduncle has a similar neurological and genetic basis to the pumping of blood by the heart, suggesting that the Hydra peduncle and the heart of higher organisms share a common ancestral origin.

Photoreceptors of cnidarians

Cnidarians are the most primitive present-day invertebrates to have multicellular light-detecting organs, called ocelli (eyes). These photodetectors include simple eyespots, pigment cups, complex pigment cups with lenses, and camera-type eyes with a cornea, lens, and retina. Ocelli are composed of sensory photoreceptor cells interspersed among nonsensory pigment cells. The photoreceptor cells are bipolar, the apical end forming a light-receptor process and the basal end forming an axon. These axons synapse with second-order neurons that may form ocular nerves. A cilium with a 9 + 2 arrangement of microtubules projects from the receptor-cell process. Depending on the species, the membrane covering the cilium shows several variations, including evaginating microvilli. In the cubomedusae stacks of membranes fill the apical regions of the photoreceptor cells. Pigment cells are rich in pigment granules, and in some animals the distal regions of these cells form tubular processes that project into the cavity of the ocellus. Microvilli may extend laterally from these tubular processes and interdigitate with the microvilli from the ciliary membranes of photoreceptor cells. Photoreceptor cells respond to changes in light intensity with graded potentials that are directly proportional to the range of the changes in light intensity. In the Hydrozoa these cells may be electrically coupled to each other through gap junctions. Light affects the behavioral activities of cnidarians, including diel vertical migration, responses to rapid changes in light intensity, and reproduction. Medusae with the most highly modified photoreceptors demonstrate the most complex photic behaviors. The sophisticated visual system of the cubomedusan jellyfish Carybdea marsupialis is described. Extraocular photosensitivity is widespread throughout the cnidarians, with neurons, epithelial cells, and muscle cells mediating light detection. Rhodopsin-like and opsin-like proteins are present in the photoreceptor cells of the complex eyes of some cubomedusae and in some neurons of hydras. Neurons expressing glutamate, serotonin, γ-aminobutyric acid, and RFamide (Arg-Phe-amide) are found in close proximity to the complex eyes of cubomedusae; these neurotransmitters may function in the photic system of the jellyfish. Pax genes are expressed in cnidarians; these genes may control many developmental pathways, including eye development. The photobiology of cnidarians is similar in many ways to that of higher multicellular animals.