Transcriptome analysis of phototransduction-related genes in tentacles of the sea cucumber Apostichopus japonicus

Sea cucumber physiological response to abiotic stress: Emergent contaminants and climate change

The ocean is facing a multitude of abiotic stresses due to factors such as climate change and pollution. Understanding how organisms in the ocean respond to these global changes is vital to better predicting consequences. Sea cucumbers are popular echinoderms with multiple ecological, nutritional, and pharmaceutical benefits. Here, we reviewed the effects of environmental change on an ecologically important echinoderm of the ocean, aiming to understand their response better, which could facilitate healthy culture programs under environmental changes and draw attention to knowledge gaps. After screening articles from the databases, 142 studies were included on the influence of emergent contaminants and climate variation on the early developmental stages and adults of sea cucumbers. We outlined the potential mechanism underlying the physiological response of sea cucumbers to emerging contaminants and climate change. It can be concluded that the physiological response of sea cucumbers to emergent contaminants differs from their response to climate change. Sea cucumbers could accumulate pollutants in their organs but are aestivated when exposed to extreme climate change. Research showed that the physiological response of sea cucumbers to pollutants indicates that these pollutants impair critical physiological processes, particularly during the more susceptible early phases of development compared to adults, and the accumulation of these pollutants in adults is often observed. For climate change, sea cucumbers showed gradual adaptation to the slight variation. However, sea cucumbers undergo aestivation under extreme conditions. Based on this review, critical suggestions for future research are presented, and we call for more efforts focusing on the co-occurrence of different stressors to extend the knowledge regarding the effects of environmental changes on these economically and ecologically important species.

Sea cucumbers: an emerging system in evo-devo

A challenge for evolutionary developmental (evo-devo) biology is to expand the breadth of research organisms used to investigate how animal diversity has evolved through changes in embryonic development. New experimental systems should couple a relevant phylogenetic position with available molecular tools and genomic resources. As a phylum of the sister group to chordates, echinoderms extensively contributed to our knowledge of embryonic patterning, organ development and cell-type evolution. Echinoderms display a variety of larval forms with diverse shapes, making them a suitable group to compare the evolution of embryonic developmental strategies. However, because of the laboratory accessibility and the already available techniques, most studies focus on sea urchins and sea stars mainly. As a comparative approach, the field would benefit from including information on other members of this group, like the sea cucumbers (holothuroids), for which little is known on the molecular basis of their development. Here, we review the spawning and culture methods, the available morphological and molecular information, and the current state of genomic and transcriptomic resources on sea cucumbers. With the goal of making this system accessible to the broader community, we discuss how sea cucumber embryos and larvae can be a powerful system to address the open questions in evo-devo, including understanding the origins of bilaterian structures.

Circadian rhythm regulation in the sea cucumber Apostichopus japonicus: Insights into clock gene expression, photoperiod susceptibility, and neurohormone signaling

Sea cucumber Apostichopus japonicus displays the typical circadian rhythms. This present study investigated the molecular regulation of clock genes, as well as monoamines and melatonin, in multiple tissues of A. japonicus, responding to the photoperiod. In order to determine their pivotal role in circadian rhythms, the crucial clock genes, namely AjClock, AjArnt1, AjCry1, and AjTimeless, were identified and a comprehensive analysis of their expressions across various tissues in adult A. japonicus was conducted, revealing the potential existence of central and peripheral oscillators. Results demonstrated that the tissues of polian vesicle and nerve ring exhibited significant clock gene expression associated with the orchestration of circadian regulation, and that environmental light fluctuations exerted influence on the expression of these clock genes. However, a number of genes, such as AjArnt1 and AjCry1, maintained their circadian rhythmicity even under continuous light conditions. Moreover, we further investigated the circadian patterns of melatonin (MT), serotonin (5-HT), and dopamine (DA) secretion in A. japonicus, data that underscored the tissue-specific regulatory differences and the inherent adaptability to dynamic light environments. Collectively, these findings will provide the molecular mechanisms controlling the circadian rhythm in echinoderms and the candidate tissues playing the role of central oscillators in sea cucumbers.

Swing Regulates Movement Direction in the Sea Cucumber Apostichopus japonicus in the Presence of Food Cue: New Insights into Movement Patterns

Regulating movement direction is essential in the locomotion of animals. Sea cucumbers, as eyeless animals, do not have eyes for the perception of the surrounding environment and food. They have a unique way of swinging their bodies when a food cue is detected, although they lack an important perceptual tool. The present study investigated the movement patterns of the sea cucumber Apostichopus japonicus in the absence of a food cue (experiment 1) and in the presence of a food cue (experiment 2). In experiment 1, we found that the movement of sea cucumbers was close to a linear motion (motion linearity 0.91 ± 0.01). In experiment 2, sea cucumbers most frequently adjusted the movement direction when being exposed to food (84 times/216 min), indicating that sea cucumbers adjusted the direction of movement in the swing state but not the motion state. In experiment 2, we found significantly lower time in the immobility state in the sea cucumbers in the presence of food cues compared to that of those without being exposed to food cues, and the frequency of the motion state in response to food cues was 1.6 times than that of those without food cue. This suggests that food cues cause the change in motion state in sea cucumbers. Swing frequency was 1.7 times higher in sea cucumbers exposed to food cues than that of those not exposed to food cues. Further, sea cucumbers in the presence of food showed significantly better performances in swing angle and swing velocity compared to those not exposed to food cues. This suggests that food cue significantly affects the swing state of sea cucumbers. Notably, the present study described the movement patterns of sea cucumbers when they detected food cues, and other factors (such as the detection of predators) need to be further studied. The present study provides new insights into the regulation of movement direction in eyeless organisms.