Hormonal control of sleep-appetitive behaviour and diurnal activity rhythms in the cleaner wrasse Labroides dimidiatus (Labridae, Teleostei)

The interplay between sleep and ecophysiology, behaviour and responses to environmental change in fish

Evidence of behavioural sleep has been observed in every animal species studied to date, but current knowledge of the behaviour, neurophysiology and ecophysiology associated with sleep is concentrated on mammals and birds. Fish are a hugely diverse group that can offer novel insights into a variety of sleep-related behaviours across environments, but the ecophysiological relevance of sleep in fish has been largely overlooked. Here, we systematically reviewed the literature to assess the current breadth of knowledge on fish sleep, and surveyed the diverse physiological effects and behaviours associated with sleep. We also discuss possible ways in which unstudied external factors may alter sleep behaviours. For example, predation risk may alter sleep patterns, as has been shown in mammalian, avian and reptilian species. Other environmental factors - such as water temperature and oxygen availability - have the potential to alter sleep patterns in fish differently than for terrestrial endotherms. Understanding the ecological influences on sleep in fish is vital, as sleep deprivation has the potential to affect waking behaviour and fitness owing to cognitive and physiological impairments, possibly affecting ecological phenomena and sensitivity to environmental stressors in ways that have not been considered.

Cleaner personality and client identity have joint consequences on cleaning interaction dynamics

Mutualistic interactions involve 2 species beneficially cooperating, but it is not clear how these interactions are maintained. In many mutualisms, one species interacts with multiple species, and since partners differ in terms of the commodities they trade, partner identity will directly influence the decisions and behaviors of interacting individuals. Here, we investigated the consequences of within and between-species diversity on a model cleaner-client interaction in a natural environment, by quantifying the behavior of both partners. We found that the predominant Caribbean cleaner fish, the sharknose goby (Elacatinus evelynae), shows personality variation as we documented repeatable individual differences in activity, boldness, and exploratory behaviors. Personality variation was associated with cleaner-client interactions: cleaner boldness and activity were significantly related to posing by clients and cleaning, respectively. Cleaner personality variation was also associated with the functional identity (sociality, mobility, body size, and trophic level) of clients posing and being cleaned. We thus demonstrate that partner identity can have consequences on mutualistic outcomes which will contribute to the context-dependency and highly heterogeneous patterns we observe at a population level. We also suggest that within- and between-species differences have consequences on partner choice, a feature that has been previously thought to be absent from these cleaner-client interactions.

Arginine vasotocin regulation of interspecific cooperative behaviour in a cleaner fish

In an interspecific cooperative context, individuals must be prepared to tolerate close interactive proximity to other species but also need to be able to respond to relevant social stimuli in the most appropriate manner. The neuropeptides vasopressin and oxytocin and their non-mammalian homologues have been implicated in the evolution of sociality and in the regulation of social behaviour across vertebrates. However, little is known about the underlying physiological mechanisms of interspecific cooperative interactions. In interspecific cleaning mutualisms, interactions functionally resemble most intraspecific social interactions. Here we provide the first empirical evidence that arginine vasotocin (AVT), a non-mammalian homologue of arginine vasopressin (AVP), plays a critical role as moderator of interspecific behaviour in the best studied and ubiquitous marine cleaning mutualism involving the Indo-Pacific bluestreak cleaner wrasse Labroides dimidiatus. Exogenous administration of AVT caused a substantial decrease of most interspecific cleaning activities, without similarly affecting the expression of conspecific directed behaviour, which suggests a differential effect of AVT on cleaning behaviour and not a general effect on social behaviour. Furthermore, the AVP-V1a receptor antagonist (manning compound) induced a higher likelihood for cleaners to engage in cleaning interactions and also to increase their levels of dishonesty towards clients. The present findings extend the knowledge of neuropeptide effects on social interactions beyond the study of their influence on conspecific social behaviour. Our evidence demonstrates that AVT pathways might play a pivotal role in the regulation of interspecific cooperative behaviour and conspecific social behaviour among stabilized pairs of cleaner fish. Moreover, our results suggest that the role of AVT as a neurochemical regulator of social behaviour may have been co-opted in the evolution of cooperative behaviour in an interspecific context, a hypothesis that is amenable to further testing on the potential direct central mechanism involved.

Fish sleeping under sandy bottom: interplay of melatonin and clock genes

Wrasse species exhibit a definite daily rhythm in locomotor activity and bury themselves in the sand at the bottom of the ocean at night. It remains unclear how their behavior in locomotor activity is endogenously regulated. The aim of the present study was to clarify the involvement of melatonin and clock genes (Per1, Per2, Bmal1, and Cry1) in daily and circadian rhythms of the threespot wrasse, Halichoeres trimaculatus, which is a common species in coral reefs. Daily and circadian rhythms in locomotor activity were monitored under conditions of light-dark cycle (LD=12:12), constant light (LL), and darkness (DD). Daily rhythms in locomotor activity were observed under LD and persisted under LL and DD. Melatonin from a cultured pineal gland showed daily variations with an increase during the nighttime and a decrease during daytime, which persisted under DD. Melatonin treatment induced decreases in locomotor activity and respiratory rate, suggesting that melatonin has a sleep-inducing effect. Per1 and Per2 mRNA abundance in the brain under LD showed daily rhythms with an increase around lights on. Robust oscillation of Per1 and Per2 mRNA expression persisted under DD and LL, respectively. Expression of Bmal1 and Cry1 mRNA also showed daily and circadian patterns. These results suggest that clock genes are related to circadian rhythms in locomotor activity and that melatonin plays a role in inducing a sleep-like state after fish bury themselves in the sand. We conclude that the sleep-wake rhythm of the wrasse is regulated by a coordination of melatonin and clock genes.

Dopaminergic activity in the brain of a tropical wrasse in response to changes in light and hydrostatic pressure

Many tropical wrasses show a daily pattern of spawning with gamete release typically near daytime high tide. The environmental cues the fish obtains from day-night and tidal cycles to ensure spawning synchrony and how those cues are transduced, however, are not fully understood. To gain insight into these issues, the involvement of monoamines in mediating endogenous day-night and tidal rhythms in the threespot wrasse, Halichoeres trimaculatus, were examined. Levels of dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC, a metabolite of DA), serotonin (5-HT), and 5-hydroxyindoleacetic acid (5-HIAA, a metabolite of 5-HT) in the brain of the fish were measured with high-performance liquid chromatography and electrochemical detection. DOPAC and the metabolic rate of DA activity (DOPAC/DA) were found to increase during the day and decrease during the night for fish held under a natural photoperiod. Fish acclimated to a 12:12 light-dark cycle and to constant dark conditions exhibited similar changes, whereas fish acclimated to constant light conditions exhibited little or no change. Intraperitoneal injection of melatonin resulted in a significant reduction in DOPAC/DA. Furthermore, DOPAC/DA was significantly lower in fish held at 3m compared to 0m depth, suggesting that hydrostatic pressure influences DA metabolic rate. These results indicate that light and hydrostatic pressure control dopaminergic turnover in the brain of threespot wrasse. Day-night and tidal changes in these two factors therefore may be the main environmental cues the fish uses to synchronize its spawning activity.

External and internal controls of lunar-related reproductive rhythms in fishes

Reproductive activities of many fish species are, to some extent, entrained to cues from the moon. During the spawning season, synchronous spawning is repeated at intervals of c. 1 month (lunar spawning cycle) and 2 weeks (semi-lunar spawning cycle) or daily according to tidal changes (tidal spawning cycle). In species showing lunar-related spawning cycles, oocytes in the ovary develop towards and mature around a specific moon phase for lunar spawners, around spring tides for semi-lunar spawners and at daytime high tides for tidal spawners. The production of sex steroid hormones also changes in accordance with synchronous oocyte development. Since the production of the steroid hormones with lunar-related reproductive periodicity is regulated by gonadotropins, it is considered that the higher parts of the hypothalamus-pituitary-gonad axis play important roles in the perception and regulation of lunar-related periodicity. It is likely that fishes perceive cues from the moon by sensory organs; however, it is still unknown how lunar cues are transduced as an endogenous rhythm exerting lunar-related spawning rhythmicity. Recent research has revealed that melatonin fluctuated according to the brightness at night, magnetic fields and the tidal cycle. In addition, cyclic changes in hydrostatic pressure had an effect on monoamine contents in the brain. These factors may be indirectly related to the exertion of lunar-related periodicity. Molecular approaches have revealed that mRNA expressions of light-sensitive clock genes change with moonlight, suggesting that brightness at night plays a role in phase-shifting or resetting of biological clocks. Some species may have evolved biological clocks in relation to lunar cycles, although it is still not known how lunar periodicities are endogenously regulated in fishes. This review demonstrates that lunar-related periodicity is utilized and incorporated by ecological and physiological mechanisms governing the reproductive success of fishes.

Effects of photic environment on ocular melatonin contents in a labrid teleost, the wrasse Halichoeres tenuispinnis

The wrasse Halichoeres tenuispinnis is a labrid teleost that exhibits robust circadian rhythms in locomotor activity under constant light (LL). This fish buries itself in the bottom sand during the subjective-night, thereby suggesting that behaviorally it adjusts its circadian clock to avoid photoreception. In this study, we determined ocular melatonin contents of the wrasse under various photic environments and used ocular melatonin to indicate photoreception. Under light-dark (LD) cycles, ocular melatonin contents of the wrasse exhibited a daily rhythm, with higher levels during the dark phase than those during the light phase. The duration of nocturnal melatonin elevation was longer under LD 9:15 than under LD 15:9. Acute exposure to 2-h light during the dark phase resulted in a significant decrease in ocular melatonin at mid-dark in an intensity-dependent manner. However, acute exposure to different intensities of light for 2h during the light phase had only a small effect on ocular melatonin contents at mid-day. Under LL, ocular melatonin contents in the wrasse reared with bottom sand present exhibited circadian rhythms and were significantly higher than those with transluscent acryl pellets on the bottom. These results indicate that the ocular melatonin rhythm in the wrasse is driven both by the photic environment and by a circadian clock, and that the wrasse that buries itself in the bottom sand can perceive low intensity of light.

Social behavior functions and related anatomical characteristics of vasotocin/vasopressin systems in vertebrates

The neuropeptide arginine vasotocin (AVT; non-mammals) and its mammalian homologue, arginine vasopressin (AVP) influence a variety of sex-typical and species-specific behaviors, and provide an integrational neural substrate for the dynamic modulation of those behaviors by endocrine and sensory stimuli. Although AVT/AVP behavioral functions and related anatomical features are increasingly well-known for individual species, ubiquitous species-specificity presents ever increasing challenges for identifying consistent structure-function patterns that are broadly meaningful. Towards this end, we provide a comprehensive review of the available literature on social behavior functions of AVT/AVP and related anatomical characteristics, inclusive of seasonal plasticity, sexual dimorphism, and steroid sensitivity. Based on this foundation, we then advance three major questions which are fundamental to a broad conceptualization of AVT/AVP social behavior functions: (1) Are there sufficient data to suggest that certain peptide functions or anatomical characteristics (neuron, fiber, and receptor distributions) are conserved across the vertebrate classes? (2) Are independently-evolved but similar behavior patterns (e.g. similar social structures) supported by convergent modifications of neuropeptide mechanisms, and if so, what mechanisms? (3) How does AVT/AVP influence behavior - by modulation of sensorimotor processes, motivational processes, or both? Hypotheses based upon these questions, rather than those based on individual organisms, should generate comparative data that will foster cross-class comparisons which are at present underrepresented in the available literature.

The need for a simple animal model to understand sleep

Simple animal models have allowed biologists to apply the tools of modern molecular genetics to such complex behaviors as circadian rhythms and long-term memory consolidation. The mechanisms and molecules discovered in these simple animals are evolutionarily conserved in other species, including mammals. Sleep research lacks a simple animal model because criteria based on the electroencephalogram have been met only in birds and mammals. We argue that straightforward behavioral criteria could allow the identification of a sleep-like rest state that might be useful for molecular investigations to understand the regulation and function of sleep. Candidate model systems are discussed, leading to the conclusion that several species have complementary strengths. Specifically, techniques developed for larval zebrafish can be used to visualize neural firing patterns in the living animal, and the fruit fly Drosophila melanogaster has been used successfully for molecular and genetic dissection of complex behaviors. We conclude with a hypothesis that one putative function of sleep, the optimization of neural plasticity, would also have adaptive value in simple organisms and might therefore be evolutionarily conserved.

Electrolocation-communication discharges of the fish Gymnotus carapo L. (Gymnotidae: Gymnotiformes) during behavioral sleep

Technical problems have hampered the study of sleep in teleosts. The electrical discharges of Gymnotus carapo L. (Gymnotidae: Gymnotiformes) were monitored to evaluate their ease and reliability as parameters to study sleep. The discharges were detected by electrodes immersed in a glass aquarium and were recorded on a conventional polygraph. G. carapo showed conspicuous signs of behavioral sleep. During these periods, opercular beat rates were counted, electric discharges recorded, and the "sharp discharge increase" (SDI) of the orienting reflex was investigated. All 20 animals monitored maintained electrical discharges during behavioral sleep. The discharge frequencies during sleep (50.3 +/- 10.4 Hz) were not significantly different from those observed when the fish was awake and inactive (57.2 +/- 12.1 Hz) (Wilcoxon matched-pairs signed-ranks test, P>0.05). However, the SDI, which was prevalent in the awake fish, was not observed during periods of behavioral sleep. Additional observations showed that the species had cannibalistic habits. When presented with electrical discharges from a conspecific, the sleeping fish showed an initial decrease or pause in discharge frequency, while the awake fish did not have this response. We conclude that the electrical discharges of G. carapo were not conspicuous indicators of behavioral sleep. Discharges may have been maintained during sleep for sensory purposes, i.e., conspecific detection and avoidance of cannibalistic attacks.

Effect of vasotocin on locomotor activity in bullfrogs varies with developmental stage and sex

Although neurohypophysial peptides are present in many regions of the developing and adult bullfrog (Rana catesbeiana) brain, the function of these peptides remains unclear. To investigate possible behavioral actions, we examined locomotor activity following peptide injection in bullfrogs at various developmental stages. An intraperitoneal (ip) injection of arginine vasotocin (AVT) in tadpoles (stages V, X, or XVII) produced an immediate and dose-dependent inhibition of locomotor activity. On the other hand, AVT stimulated activity when administered ip to juvenile or adult female bullfrogs, but did not influence activity in juvenile or adult males. The minimum effective dose of AVT, when injected directly into the brain of tadpoles, was 100-fold less than that observed when injected ip, suggesting a central nervous system site of action for this peptide. A vasopressin receptor antagonist (d(CH2)5[Tyr(Me)2]AVP administered ip or icv) significantly increased locomotor activity in tadpoles, compared to controls. Oxytocin, vasopressin, and AVP4-9 inhibited activity in tadpoles while mesotocin, des Gly(NH2)AVP, and pressinoic acid had no significant effect. Injection of PGF2 alpha also significantly decreased activity levels in tadpoles. However, pretreatment of tadpoles with indomethacin, a prostaglandin synthesis inhibitor, did not prevent the behavioral effects of AVT, suggesting that prostaglandin synthesis is not required for this response. In summary, AVT influenced locomotor activity in bullfrog tadpoles and female frogs. This effect shifted during development from an inhibitory action in tadpoles to a stimulatory effect in metamorphosed female frogs. The effect of AVT on juvenile and adult frog locomotion was sexually dimorphic, as this peptide altered female behavior but not male behavior.