Dietary tryptophan decreases the impacts of seawater temperature changes on behavior and gut health of the sea cucumber Apostichopus japonicus.

Seawater temperature change is an important concern for seed production and pond culture of sea cucumbers. The present study found that tentacle activity frequency was significantly lower in sea cucumbers exposed to continuous and rapid temperature increases than that of those at ambient temperature. Feeding behavior directly determines food intake, and further affects physiology and growth efficiency of sea cucumbers. This means that the decline in feeding caused by continuous and rapid temperature increases needs to be addressed in sea cucumber aquaculture. However, a sudden temperature change of 5 °C had no significant effect on behaviors of sea cucumbers. This indicates that continuous temperature increases, rather than a sudden increase, result in behavioral impacts on sea cucumbers. Therefore, we recommend aqua-farmers reduce the feeding amount for sea cucumbers during continuous and rapid temperature increases. In the present study, feeding behavior was significantly higher in sea cucumbers fed with 3% dietary tryptophan than that of those fed with 0% and 5% dietary tryptophan. This indicates that 3% dietary tryptophan increases the food intake of sea cucumbers, and mitigates the feeding decline caused by continuous and rapid temperature increase. This indicates that tryptophan has the potential to promote the feeding of sea cucumbers in seed production and pond culture. Adhesion capacity of sea cucumbers fed with 5% dietary tryptophan was significantly higher than that of individuals fed with 0% and 3% dietary tryptophan. This suggests that dietary tryptophan increases the feeding of sea cucumbers exposed to continuous and rapid temperature increases in pond culture and seed production. In addition, this study found that sea cucumbers fed with 3% dietary tryptophan had higher intestinal colony richness under the continuously rapid temperature change. The present study provides an effective method to improve adhesion behavior and to alleviate the impacts on feeding behavior for seed production and pond culture of sea cucumbers exposed to continuous and rapid temperature increases.

Behavior, intestinal health, and growth of small sea cucumbers Apostichopus japonicus in different color morphs.

Selecting high-quality seeds with long-term advantages in behavior, intestinal health, and growth are the key to improve production efficiency of sea cucumber aquaculture. It is proposed to distinguish the seed quality of sea cucumbers by color morphs. In the present study, we carried out a 6-week experiment to investigate behavior, intestinal health, and growth of small sea cucumbers Apostichopus japonicus in different color morphs. We found that dark-colored seeds of sea cucumber were significantly more adhesive than those with light-colored seeds. This indicates that the dark-colored seeds of A. japonicus are more adaptive in complex environments in stock enhancement. Food consumption and defecation outputs of dark-colored seeds were significantly higher than those of light-colored seeds. In addition, the feces of dark-colored seeds of sea cucumber had significantly lower crude protein content and better intestinal morphology, but there was no advantage in digestive enzyme activities. This suggests that there are potential digestive benefits in dark-colored seeds. Further, dark-colored seeds of A. japonicus showed significantly better intestinal microbiota composition and faster growth rate than that of light-colored seeds. In conclusion, the present results prove that dark-colored seeds of sea cucumber have long-term advantages in behavior, intestinal health and growth. Overall, this study provides important information for the early selection of seeds and the consequent production efficiency in sea cucumber aquaculture.

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.

Bacillus licheniformis reduces mortality, improves symptoms, and controls residual pathogenicity in juvenile sea cucumbers Apostichopus japonicus infected with skin ulceration syndrome.

Mass mortality caused by skin ulceration syndrome (SUS) is the bottle-neck for the sustainable aquaculture of the sea cucumber Apostichopus japonicus. In the present study, probiotic Bacillus licheniformis (0.25 × 109 CFU/g) was used as the treatment for A. japonicus infected with the SUS that caused by Vibrio harveyi. We found that B. licheniformis significantly reduced the number of infected sea cucumbers 5 days and 7 days after the treatment (group B), compared to those without B. licheniformis treatment (group C) (P < 0.001; P < 0.001). Further, the sea cucumbers fed B. licheniformis had significantly lower mortality at the end of the experiment (<10%) than that of those without the B. licheniformis treatment (>60%) (P < 0.001). These results suggest that the treatment of B. licheniformis is an effective method to reduce the mass mortality resulted from SUS in sea cucumber aquaculture. Further, 3-5 days of treatment significantly improved the adverse symptoms of SUS on the physiology and behavior of sea cucumbers, including the righting behavior, adhesion behavior, food consumption, fecal output and mobility. This indicates B. licheniformis treatment has the advantage in the recovery of sea cucumbers after SUS. Moreover, there was no significant difference observed in the physiology and behavior of sea cucumbers between the SUS infected sea cucumbers after the 7-day treatment of B. licheniformis and the healthy individuals. SUS infected sea cucumbers effectively returned to a stage of normalcy. Further, we found a significantly lower infected rate in sea cucumbers exposed to the culture water of group B (∼5%) than that of those in exposure to the culture water of group C (∼60%). This indicates that the treatment of B. licheniformis efficiently controls the residual pathogenicity of SUS in culture water. The present study demonstrated the effectiveness of B. licheniformis treatment as an environmentally friendly approach to reducing mortality, improving symptoms, and controlling residual pathogenicity in sea cucumber aquaculture.

Effects of circadian rhythm on behavior and physiology of the sea cucumber Apostichopus japonicus.

The present study investigated the behavioral and physiological responses to the circadian rhythm in the sea cucumber Apostichopus japonicus. We found that righting behavior of sea cucumbers was significantly faster at night than that in daytime (P < 0.001). We thus suggest aqua-farmers carry out seeding at night in stock enhancement. The number of tentacle swings was significantly higher at night than that in daytime (P = 0.005). Thus, we suggest aqua-farmers provide diets before the peak of sea cucumber feeding at night. There was no significant difference in foraging behavior and defecation behavior during the day and at night. This indicates that not all behaviors have different characteristics in circadian rhythm. In addition, we found that cortisol concentration was significantly higher at night than that in daytime (P = 0.021). This suggests that sea cucumbers are probably more prone to be stressed at night. However, there was no significant difference in 5-HT and melatonin during the day and at night, suggesting that 5-HT and melatonin are probably not affected by circadian rhythm. The present study clarifies the behavioral and physiological responses to circadian rhythm and provides valuable information into sea cucumber aquaculture.

The feces of sea urchins as food improves survival, growth, and resistance of small sea cucumbers Apostichopus japonicus in summer.

Mass mortality and low growth highly decrease the production efficiency and sustainable aquaculture development of the sea cucumber Apostichopus japonicus in summer. Sea urchin feces was proposed to address the summer problems. A laboratory study was conducted for ~ 5 weeks to investigate survival, food consumption, growth and resistance ability of A. japonicus cultured with the feces of sea urchins fed kelp (KF feces, group KF), the feces of sea urchins fed prepared feed (FF feces, group FF), and the prepared sea cucumber feed (group S) at high temperature (25 °C). The sea cucumbers of group KF had better survival (100%) than those of the group FF (~ 84%), higher CTmax (35.9 °C) than those of the group S (34.5 °C), and the lowest skin ulceration proportion (0%) when  they were exposed to an infectious solution among the three groups. These results suggest that the feces of sea urchins fed kelp is a promising diet for improving the survival and enhancing the resistance in A. japonicus aquaculture in summer. Sea cucumbers fed significantly less FF feces after 24 h of ageing than the fresh FF feces, suggesting this kind of feces became unsuitable for A. japonicus in a short time (within 48 h). However, the 24 h of ageing at 25 °C for the high fiber feces of sea urchins fed kelp had no significant effects on the fecal consumption of sea cucumbers. In the present study, both fecal diets provide better individual growth to sea cucumbers than the prepared feed. Yet, the feces of sea urchins fed kelp provided the highest weight gain rate (WGR) to sea cucumbers. Therefore, the feces of sea urchins fed kelp is a promising food to reduce the mortality, to address the problems of summer, and to achieve higher efficiency in A. japonicus aquaculture in summer.

Ecological niche models for the assessment of site suitability of sea cucumbers and sea urchins in China.

In the present study, the maximum entropy model (MaxEnt) based on the data of sea surface temperature (SST) and published information was used to assess the site suitability for the aquaculture expansion of the sea cucumber Apostichopus japonicus and the sea urchin Strongylocentrotus intermedius in China. According to the current assessment, the coastal areas of Hebei province and Tianjin have great prospects for A. japonicus aquaculture, while is currently being underutilized. In the south, more than 94% of the coastal areas in Zhejiang, Fujian, and Guangdong provinces are suitable for the growth of A. japonicus for six months, especially the coastal areas of Lianjiang, Changle, Fuqing and Putian in Fujian province. The water temperatures in more than 94% of China's coastal areas are higher than 25 °C in July and August, which probably results in the mortality of S. intermedius in aquaculture. This clearly indicates that high water temperature is the bottleneck of S. intermedius aquaculture and well explains the limited expansion of this commercially important exotic species since the introduction in 1989. We suggest a new aquaculture model of S. intermedius that extends the seed production to November to avoid the mass mortality in summer. In the south, 64% of coastal areas in Zhejiang and Fujian provinces are suitable for the transplantation of S. intermedius to the south. The present study suggests the ecological niche model MaxEnt based on the data of SST and published information as a new tool for the assessment of the site suitability of sea cucumbers and sea urchins in China. This provides new insights into the aquaculture expansion of native and exotic species.

Macroalgae and interspecific alarm cues regulate behavioral interactions between sea urchins and sea cucumbers.

Sea urchins and sea cucumbers are mutually beneficial organisms in kelp ecosystem. As herbivores, sea urchins process kelp through feeding and egestion, providing inaccessible food for benthic consumers such as sea cucumbers. Sea urchins in turn profit from the sediment cleaned by sea cucumbers. However, behavioral interactions between them remain poorly understood, which greatly hampers our understanding on the relationship between ecologically important benthic species in marine ecosystems and the regulating mechanism. The present study investigated behavioral interactions between sea urchins Strongylocentrotus intermedius and sea cucumbers Apostichopus japonicus in laboratory conditions. We revealed that the presence of sea urchins caused significant higher speed movement of A. japonicus. Interestingly, the negative effects of S. intermedius on A. japonicus were significantly reduced in the shared macroalgal area. For the first time, we found the interspecific responses to alarm cues between sea cucumbers and sea urchins. Conspecific responses were significantly larger than the interspecific responses in both sea urchins and sea cucumbers. This indicates that interspecific response to alarm cues is an efficient approach to anti-predation and coexistence in mutually beneficial organisms. The present study shed light on the interspecific relationships and coexistence between sea urchins and sea cucumbers in kelp ecosystem.

High fitness areas drive the aggregation of the sea urchin Mesocentrotus nudus.

Sea urchin aggregation is a common phenomenon in coastlines. However, it remains controversial whether sea urchins form resource aggregations or behavioral aggregations in a non-spawning season. To clarify, we studied the aggregative responses to food and predators in the sea urchin Mesocentrotus nudus when high fitness areas (HFAs) were scarce versus sufficient. By taking the occupied area of each sea urchin (test diameter + spines =  4.5 cm) as a square (4.5 cm × 4.5 cm), we set scarce HFAs for the sea urchins in Experiment 1 (the squares of HFAs: the area occupied by experimental sea urchins = 1:1) and sufficient HFAs for the sea urchins in Experiment 2 (the squares of HFAs: the area occupied by experimental sea urchins = 2:1). If M. nudus form resource aggregations, they would aggregate passively under the scarce HFAs conditions, but not in the sufficient HFAs conditions. Conversely, if M. nudus form behavioral aggregation, aggregation would occur in both scarce and sufficient HFAs. The present results showed that in the scarce HFAs, M. nudus in the food and predator groups were significantly closer to the food and further from predators, and had significantly more aggregated numbers in HFAs than those in the control group. Sea urchins did not aggregate in response to food or predators under the sufficient HFAs, although significantly more sea urchins of the experimental group was found in HFAs than that of the control group. Sea urchins (at least M. nudus) form resource aggregations that are driven by the scarce HFAs. This provides valuable information into the mechanisms of the aggregation of sea urchins.

Foraging behavior of the sea urchin Mesocentrotus nudus exposed to conspecific alarm cues in various conditions.

Conspecific alarm cues crushed from Mesocentrotus nudus prevent sea urchins from foraging the kelp, but do not repel them far away from the kelp. However, it remains largely unknown of whether this phenomenon was affected by conspecific alarm cues or by the attraction of the kelp. The present study found no significant difference in the duration in the danger area with or without the kelp around conspecific alarm cues. This suggests that the phenomenon is the strategy of sea urchins but not by the attraction of kelp. We found that conspecific alarm cues appearing between the kelp and sea urchins significantly affected foraging behavior of sea urchins fasted for 21 days. This indicates that conspecific alarm cues can effectively prevent fasted sea urchins from foraging the kelp. Further, there was no correlation between foraging velocity and the duration in the danger area. Pearson correlation analysis revealed no significant correlation between foraging velocity and the duration in the safety area close to different amounts of conspecific alarm cues, suggesting that conspecific alarm cues prevent sea urchins with strong foraging ability to forage. Collectively, the present results indicate that conspecific alarm cues as highly available biological barriers are cost-effective approaches to preventing overgrazing of sea urchins in the protection of kelp beds ecosystems. Notably, the present study is a short-term laboratory investigation that does not consider the complexity of natural conditions. Future studies are essential to test the present findings in the field.

Interaction among sea urchins in response to food cues.

Interaction among sea urchins remains largely uninvestigated, although the aggregation of sea urchins is common. In the present study, 1, 15 and 30 sea urchins Strongylocentrotus intermedius (11.06 ± 0.99 mm in test diameter) were placed in a 1 m2 circular tank, respectively. Movement behaviors were recorded for 12 min to investigate potential interactions among sea urchins. After the 12-min control period, we added food cues into the tank and recorded the changes in sea urchins' behaviors. For the first time, we here quantified the interactions among sea urchins in laboratory and found that the interactions varied with food cues and with different densities. The sea urchins dispersed in random directions after being released. There was no significant difference in the movement speed and the displacement of sea urchins among the three density groups (1, 15 and 30 ind/m2). The interaction occurred when sea urchins randomly contacted with the conspecifics and slowed down the movement speed. The speed of sea urchins after physical contacts decreased by an average of 40% in the density of 15 ind/m2 and 17% in the density of 30 ind/m2. This interaction resulted in significantly higher randomness in the movement direction and lower movement linearity in 15 and 30 ind/m2 than in 1 ind/m2. After the introduction of food cues, the movement speed, displacement and dispersal distance of sea urchin groups decreased significantly in all the three densities. The dispersal distance and expansion speed of sea urchins were significantly lower in 30 ind/m2 than those in 15 ind/m2. The present study indicates that the interaction among sea urchins limits the movement of individual sea urchin and provides valuable information into how large groups of sea urchins are stable in places where food is plentiful.

Fitness benefits and costs of shelters to the sea urchin Glyptocidaris crenularis.

Understanding the ecological role of shelters is greatly hampered by the scarcity of long-term laboratory experiments on the trade-off between fitness benefits and costs. This lack probably leads to an underestimation of the negative and/or positive effects on behaviors and growth of marine invertebrates in benthic ecosystems. Although our previous study revealed a significant effect on fitness-related traits of Glyptocidaris crenularis after 31 months, the present study extended it and investigated fitness benefits and/or costs of long-term sheltering on sea urchins to over 7 years. The present long-term study suggests that the previously reported reduction in feeding rate probably resulted from a reduction in reflexive feeding motions (Aristotle's lantern reflex) rather than changes in foraging behavior. Actively seeking sheltering behavior was negatively impacted in individuals with continuous access to shelters. However, covering and righting behaviors did not differ in sheltered sea urchins, indicating that these behaviors are maintained to escape from adverse environments regardless of shelter. Body size of sea urchins in the group with shelters was significantly lower than those without shelters after 7 years. Weights of gonads and gut were not significantly different after 7 years despite previous observations of differences after ~2.5 years. The present study provides valuable information on the trade-off between fitness benefits and costs to sea urchins residing in shelters. However, the present study is only a laboratory investigation for one urchin species (G. crenularis) which does not consider the complexity of natural environments. Field studies should be carried out with G. crenularis and other sea urchin species, before a more universal conclusion can be drawn.

Light intensity regulates phototaxis, foraging and righting behaviors of the sea urchin Strongylocentrotus intermedius.

Small sea urchins Strongylocentrotus intermedius (1-2 cm of test diameter) are exposed to different environments of light intensities after being reseeded to the sea bottom. With little information available about the behavioral responses of S. intermedius to different light intensities in the environment, we carried out an investigation on how S. intermedius is affected by three light intensity environments in terms of phototaxis, foraging and righting behaviors. They were no light (zero lx), low light intensity (24-209 lx) and high light intensity (252-2,280 lx). Light intensity had obvious different effects on phototaxis. In low light intensity, sea urchins moved more and spent significantly more time at the higher intensity (69-209 lx) (P = 0.046). S. intermedius in high light intensity, in contrast, spent significantly more time at lower intensity (252-690 lx) (P = 0.005). Unexpectedly, no significant difference of movement (average velocity and total distance covered) was found among the three light intensities (P > 0.05). Foraging behavior of S. intermedius was significantly different among the light intensities. In the no light environment, only three of ten S. intermedius found food within 7 min. In low light intensity, nine of 10 sea urchins showed successful foraging behavior to the food placed at 209 lx, which was significantly higher than the ratio of the number (two of 10) when food was placed at 24 lx (P = 0.005). In the high light intensity, in contrast, significantly less sea urchins (three of 10) found food placed at the higher light intensity (2,280 lx) compared with the lower light intensity (252 lx) (10/10, P = 0.003). Furthermore, S. intermedius showed significantly longer righting response time in the high light intensity compared with both no light (P = 0.001) and low light intensity (P = 0.031). No significant difference was found in righting behavior between no light and low light intensity (P = 0.892). The present study indicates that light intensity significantly affects phototaxis, foraging and righting behaviors of S. intermedius and that ~200 lx might be the appropriate light intensity for reseeding small S. intermedius.