Constant light disrupts the circadian but not the circatidal rhythm in mangrove crickets

Rhythms and Clocks in Marine Organisms

The regular movements of waves and tides are obvious representations of the oceans' rhythmicity. But the rhythms of marine life span across ecological niches and timescales, including short (in the range of hours) and long (in the range of days and months) periods. These rhythms regulate the physiology and behavior of individuals, as well as their interactions with each other and with the environment. This review highlights examples of rhythmicity in marine animals and algae that represent important groups of marine life across different habitats. The examples cover ecologically highly relevant species and a growing number of laboratory model systems that are used to disentangle key mechanistic principles. The review introduces fundamental concepts of chronobiology, such as the distinction between rhythmic and endogenous oscillator-driven processes. It also addresses the relevance of studying diverse rhythms and oscillators, as well as their interconnection, for making better predictions of how species will respond to environmental perturbations, including climate change. As the review aims to address scientists from the diverse fields of marine biology, ecology, and molecular chronobiology, all of which have their own scientific terms, we provide definitions of key terms throughout the article.

Role of Bmal1 in mediating the cholinergic system to regulate the behavioral rhythm of nocturnal marine molluscs

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The circadian differential expression of AchE was identified using TMT quantitative proteomics;

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It was found that the Ach concentration and the expression levels of AchE and Bmal1 exhibit circadian cosine rhythm;

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The full-length sequences of AchE and nAchR were obtained by cloning technique and made available for phylogenetic analysis;

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The movement distance and duration of abalone increased after the injection of neostigmine methylsulfate as the AchE inhibitor;

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Bmal1 as the core circadian clock gene was proven to bind to AchE and nAchR, thereby regulating the movement behavior of abalone.

The circadian rhythm is one of the most general and important rhythms in biological organisms. In this study, continuous 24-h video recordings showed that the cumulative movement distance and duration of the abalone, Haliotis discus hannai, reached their maximum values between 20:00–00:00, but both were significantly lower between 08:00–12:00 than at any other time of day or night (P < 0.05). To investigate the causes of these diel differences in abalone movement behavior, their cerebral ganglia were harvested at 00:00 (group D) and 12:00 (group L) to screen for differentially expressed proteins using tandem mass tagging (TMT) quantitative proteomics. Seventy-five significantly different proteins were identified in group D vs. group L. The differences in acetylcholinesterase (AchE) expression levels between day- and nighttime and the key role in the cholinergic nervous system received particular attention during the investigation. A cosine rhythm analysis found that the concentration of acetylcholine (Ach) and the expression levels of AchE tended to be low during the day and high at night, and high during the day and low at night, respectively. However, the rhythmicity of the diel expression levels of acetylcholine receptor (nAchR) appeared to be insignificant (P > 0.05). Following the injection of three different concentrations of neostigmine methylsulfate, as an AchE inhibitor, the concentration of Ach in the hemolymph, and the expression levels of nAchR in the cerebral ganglia increased significantly (P < 0.05). Four hours after drug injection, the cumulative movement distance and duration of abalones were significantly higher than those in the uninjected control group, and the group injected with saline (P < 0.05). The expression levels of the core diurnal clock Bmal1 over a 24-h period also tended to be high during the day and low at night. First, a co-immunoprecipitation assay demonstrated the binding between Bmal1 and AchE or nAchR. A dual-luciferase gene test and electrophoretic mobility shift assay showed that Bmal1 bound to the promoter regions of AchE and nAchR. Twenty-four hours after silencing the Bmal1 gene, the expression levels of AchE and nAchR decreased significantly compared to those of the dsEGFP and PBS control groups, further showing that Bmal1 mediates the cholinergic system to regulate the behavioral rhythm of abalone. These findings shed light on the endocrine mechanism regulating the rhythmic behavior of abalone, and provide a reference for understanding the life history adaptation strategies of nocturnal organisms and the proliferation and protection of bottom dwelling economically important organisms.

Towards an Understanding of Circatidal Clocks

Circadian clocks are an intrinsic element of life that orchestrate appropriately timed daily physiological and behavioural rhythms entrained to the solar cycle, thereby conferring increased fitness. However, it is thought that the first archaic ‘proto-clocks’ evolved in ancient cyanobacteria in a marine environment, where the dominant time cues (zeitgebers) probably would have been lunar-driven and included tidal cycles. To date, non-circadian ‘marine clocks’ have been described with circatidal (~12.4 h), circasemilunar (~14.8 days), and circalunar (~29.5 days) periodicity, mostly studied in accessible but temporally complex intertidal habitats. In contrast to the well-described circadian clock, their molecular machinery is poorly understood, and fundamental mechanisms remain unclear. We propose that a multi-species approach is the most apposite strategy to resolve the divergence that arose from non-circadian clockwork forged in an evolutionary environment with multiple zeitgebers. We review circatidal clock models with a focus on intertidal organisms, for which robust behavioural, physiological, or genetic underpinnings have been explicated, and discuss their relative experimental merits. Developing a comprehensive mechanistic understanding of circatidal clocks should be a priority because it will ultimately contribute to a more holistic understanding of the origins and evolution of chronobiology itself.

De novo assembly and annotation of the mangrove cricket genome

Objectives

The mangrove cricket, Apteronemobius asahinai, shows endogenous activity rhythms that synchronize with the tidal cycle (i.e., a free-running rhythm with a period of ~ 12.4 h [the circatidal rhythm]). Little is known about the molecular mechanisms underlying the circatidal rhythm. We present the draft genome of the mangrove cricket to facilitate future molecular studies of the molecular mechanisms behind this rhythm.

Data description

The draft genome contains 151,060 scaffolds with a total length of 1.68 Gb (N50: 27 kb) and 92% BUSCO completeness. We obtained 28,831 predicted genes, of which 19,896 (69%) were successfully annotated using at least one of two databases (UniProtKB/SwissProt database and Pfam database).

Circatidal gene expression in the mangrove cricket Apteronemobius asahinai

The mangrove cricket Apteronemobius asahinai is endemic to mangrove forest floors. It shows circatidal rhythmicity, with a 12.6-h period of locomotor activity under constant conditions. Its free-running activity also has a circadian component; i.e. it is more active during the subjective night than during the day. In this study, we investigated rhythmic gene expression under constant darkness by RNA sequencing to identify genes controlled by the biological clock. Samples collected every 3 h for 48 h were analysed (one cricket per time-point). We identified 284 significant circatidal cycling transcripts (period length 12–15 h). Almost half of them were annotated with known genes in the NCBI nr database, including enzymes related to metabolic processes and molecular chaperones. There were less transcripts with circadian rhythmicity than with circatidal rhythmicity, and the expression of core circadian clock genes did not show significant rhythmicity. This may reflect the nature of the mangrove cricket or may be due to the paucity of the sampling repeats: only two periods for circadian cycle with no replications. We evaluated for the first time the rhythmic transcriptome of an insect that shows circatidal rhythmic activity; our findings will contribute to future studies of circatidal clock genes.