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.

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.

Improving the aquaculture production efficiency by appropriate diets is an essential approach to meeting the increasing market demand for sea cucumbers. The feces of sea urchins, which contains various enzymes and microorganisms, is a potentially cost-effective food for sea cucumbers. To assess the usability of the fecal diet, a five-week laboratory simulation is conducted to investigate behaviors, digestion ability, growth and resistance ability of the sea cucumber Apostichopus japonicus fed with fecal diet at water temperatures of 15°C and 5°C. In the present study, A. japonicus fed with fecal diet shows an obvious preference to fecal diet rather than prepared feed at water temperatures of both 15°C and 5°C, which suggests that the feces is an applicable diet for A. japonicus. Furthermore, small A. japonicus fed with feces (group F) shows significant advantages in intestinal community richness, community diversity and intestine protease activity to A. japonicus fed with prepared feed (group S) at 15°C. These results indicate that the fecal diet provides benefits to digestion ability of small A. japonicus at 15°C. Weight gaining rate is significantly higher in the A. japonicus fed with feces than that in A. japonicus that were fed with feed or not fed with food (group C), which suggests that the direct improvement of the production efficiency at 15°C. The advantages in intestinal bacteria, protease activity, and growth are consistently found in group F compared with group S at 5°C. In addition, the composition of intestinal bacteria indicates that sea cucumbers may inherit the intestinal bacteria of sea urchins through fecal consumption. This suggests that the fecal diet enhances the digestion ability and enzyme activity at low water temperature and thus improves the growth of sea cucumbers. Furthermore, sea cucumbers fed with sea urchin feces have the highest survival rate among the three groups in exposure to an acute salinity decrease at both 5 and 15°C, indicating a better resistance to low salinity. This provides a new insight into the geographical expansion to low-salinity areas in sea cucumber aquaculture. In conclusion, the present study suggests that sea urchin feces have a great potential for the application in improving the production efficiency of sea cucumber aquaculture.

The energetic link in the benthic community is based on physiological characteristics of the low food absorption efficiency of sea urchins. Low food absorption efficiency of sea urchins is correlated with the activity of digestive enzymes and the duration of food in their gut. Thus, the digestive enzymes activities (pepsin and amylase enzyme activities) and gut emptying are important indicators in assessing nutrient digestion and absorption in sea urchins. In the present study, the relationship between these indicators and molecules related to digestive physiology were quantified in sea urchins. We found (1) an inter-regulatory relationship existed between Transient receptor potential cation channel, subfamily A, member 1 (TRPA1), and serotonin (5-hydroxytryptamine; 5-HT) in the gut of Strongylocentrotus intermedius; (2) digestive enzyme activities were negatively correlated with the TRPA1 and concentration of 5-HT in the gut of S. intermedius; (3) gut emptying rate was positively correlated with TRPA1 and concentration of 5-HT in the gut of S. intermedius. The present study revealed that the digestion and absorption of food are correlated with the TRPA1 and 5-HT in the gut of S. intermedius, which provides valuable information about the digestive physiology of sea urchins. This novel finding is relevant to understanding the low food digestibility of sea urchins. It also provides valuable information to the digestive physiology of sea urchins, which are key to maintaining the stability of food webs in the marine ecosystem.

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.

The Integrated Multi-Trophic Aquaculture (IMTA) of sea cucumbers and sea urchins has high commercial values. The consumption of sea urchin feces by sea cucumbers is important but largely unknown. The present study aims to evaluate and subsequently improve the fecal consumption in sea cucumbers. Sea cucumbers ingested the feces of sea urchins at both 15 degrees C and 5 degrees C, suggesting that the feces of sea urchins has potential to be an appropriate diet for the aquaculture of A. japonicus. Because feces has a complex process after the egestion, the age and temperature significantly affected the fecal consumption by sea cucumbers. The optimal ageing time after egestion was 0-48 h at 15 degrees C, 0-72 h at 5 degrees C, respectively. Thus, we recommend aqua-farmers feed A. japonicus with feces at the optimal ageing hours and remove the feces in time. Further, light intensity significantly affected the fecal consumption by A. japonicus. Sea cucumbers had significantly more fecal consumption in low light intensity than that in high light intensity at both 5 and 15 degrees C. Further, sea cucumbers fed significantly more in low light intensity with shelters than those in low light intensity without shelter, suggesting that shelter provides extra benefits for the fecal consumption by sea cucumbers besides the low light intensity. In addition, significant feeding benefits were found in both low light intensity and shelter at low temperatures. This may solve the problem that sea cucumbers cease feeding at 5 degrees C in winter. Organizing IMTA in sheltered environment and low light intensity can improve fecal consumption at both 15 degrees C and 5 degrees C. The present study provides valuable information in evaluating and improving consumption of sea urchin feces by A. japonicus in the IMTA of sea cucumbers and sea urchins.

底播增殖是一种提高中间球海胆(Strongylocentrotus intermedius)产量的有效方法.底播增殖受到多种内在和外在因素的影响,其中温度发挥着十分重要的作用,因此确定中间球海胆适宜增殖温度范围至关重要.本实验将大(23.29±0.27)mm、小(18.78±0.19)mm两种规格的中间球海胆放置在3个温度环境下(10℃、15℃、20℃)养殖6周,观察并测定其生存、生长和行为.实验结果表明,3个温度条件均不会造成海胆死亡.20℃组的两种规格海胆摄食量显著高于15℃和10℃组(P＜0.01).10℃、15℃组小规格海胆的觅食时间显著大于20℃组(P＜0.01),觅食距离显著小于20℃组(P＜0.05).在趋光实验中,20℃组小规格海胆的运动时间显著大于15℃组(P＜0.01);运动路程显著小于15℃、10℃组(P＜0.05).在避光实验中,两规格海胆20℃组的运动时间均显著大于15℃、10℃组(P＜0.05),运动路程均显著小于15℃、10℃组(P＜0.01);15℃、10℃组两种规格海胆口器咬合频率显著大于20℃组(P＜0.01).15℃、10℃组两种规格海胆的壳径、壳高、体重、性腺重、肠重、口器长、口器重、壳重、壳压力均大于20℃组;15℃、10℃组两种规格海胆的口器长/壳径、壳高/壳径均大于20℃组.综上所述,在10℃和15℃的水温下,中间球海胆均表现出显著更好的行为和生长,10～15℃是中间球海胆幼胆底播增殖的适宜增殖温度范围,在该环境下放流中间球海胆幼胆预期能够提升底播增殖生产效率.

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.

Mass mortality of commercially cultured marine species in summer is the most serious problem for the longline culture. Reducing crowding stress induced by high stocking density on the basis of high biomass is proposed to be an important approach to solving this issue. A laboratory experiment with multi-layer culture (18 sea urchins total, 6 urchins in each layer) was conducted to investigate gonad yield, gonadal development, thermal tolerance (CTmax) and disease transmission for the juvenile sea urchins Strongylocentrotus intermedius for -7 weeks at elevated temperatures. The present study found that precocious puberty occurred both in those sea urchins cultured in multi-layer and those not (control group), indicating that gonadal development is accelerated by crowding stress (even in a low density of 6 sea urchins/1834.56 cm3). The CTmax showed a significantly negative correlation with gonadal development, suggesting precocious puberty probably is responsible for the decrease of resistance ability. We further assessed whether segregation in multi-layer culture improves these important traits of sea urchins. This approach significantly reduced gonadal development, improved CTmax and lower mortality and morbidity after disease challenge assays in sea urchins than those not separated in multi-layer culture, which efficiently reduces the gametogenesis, improves resistance ability and decreases disease transmission. The present study establishes a new approach to improving the survival of cultured organisms in the longline culture at high temperatures.

It is important to study the fitness of marine invertebrates in exposure to high water temperature. We studied whether the long-term high temperatures work on the fitness-related traits (righting behavior, covering behavior, foraging behavior, Aristotle's lantern reflex, body size) of S. intermedius whose parents (males and females) were exposed to ambient or high temperatures (~3 °C higher than the ambient) for a long period of time. The present study found that test diameter, wet body weight and test weight of offspring were not significantly different between temperature treatments, indicating that the parental sea urchins in exposure to high temperatures develop no carryover effects on the body size of the offspring sea urchins. We found no significant difference in foraging behavior, Aristotle's lantern reflex, lantern length and lantern weight of sea urchins after their parents had experienced long-term high temperatures. In addition, no significant change was found in the righting and covering behaviors of sea urchins whose parents were at long-term high temperatures. These results indicate that no significant lasting effects exhibited in the fitness-related behaviors and tissue size after their parents were exposed to high temperatures for a long time. The crushing force of test and test thickness showed no significant difference in the offspring of S. intermedius, no matter whether their parents were exposed to long-term high temperatures or not. The current results enrich our understanding that the parental sea urchin experiencing long-term high temperatures probably develop no carryover effects on the test of their offspring. We found that sea urchins whose parents were exposed to long-term elevated temperatures showed a significantly higher lantern length/test diameter and a significantly lower test height/test diameter in offspring sea urchins due to the thermal experience of their parents, showing the plasticity of lantern and test of offspring sea urchins in response to the thermal experience of their parents. Together with our previous investigation, the present study indicates that small sea urchins are less susceptible to the carryover effects of high temperatures in comparison with the developmental stages of embryos and larvae.

Cost-effective reefs improve the yield of sea cucumber Apostichopus japonicas cultured in ponds. The reaction to reef models was studied in laboratory experiments regarding selectivity and fitness-related behaviors (crawling, feeding and defecation) in different roughness (rough and smooth), angles (30°, 60° and 90°) and heights (3.5, 7, 10.5 and 14 cm). The present study found that significantly more sea cucumbers selected the rough model, with significantly better crawling frequency, tentacles activity frequency and defecation behavior compared with those on the smooth model. This indicates that rough materials in artificial reefs are appropriate for the fitness-related behaviors of sea cucumbers. Regarding the angles, the 30° model attracted significantly more sea cucumbers than those of 60° and 90°. Among the three angles, sea cucumbers in the 30° model showed significantly better defecation behavior, while showed higher frequencies of crawling and tentacles activity in comparison with the 90° model. In other words, the models of 30° and 60° serve as the ''toilet'' and the ''kitchen'' of sea cucumbers better in defecation and food intake. No significant difference was found in the number of sea cucumbers selecting any model of three heights. The 7-cm model sees significantly higher frequencies of crawling and tentacles activity than the ones of 3.5 and 14 cm. These results indicate that the higher reef may not necessarily be valuable in the pond culture and a shorter one may help boost the feeding efficiency of sea cucumbers.

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 m 2 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/m 2 ). 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/m 2 and 17% in the density of 30 ind/m 2 . This interaction resulted in significantly higher randomness in the movement direction and lower movement linearity in 15 and 30 ind/m 2 than in 1 ind/m 2 . 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/m 2 than those in 15 ind/m 2 . 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.

A cost-effective approach to controlling foraging and feeding behaviors of sea urchins is essential for the management of kelp beds. Laboratory experiments were designed to investigate whether alarm cues from crushed conspecific urchins can effectively prevent the foraging and feeding behaviors of the sea urchin Mesocentrotus nudus under the static seawater condition. The present study found that the number of M. nudus that foraged successfully was significantly lower when alarm cues were placed between the kelp and the sea urchins. This result indicates that alarm cues could play an important role in critical kelp-bed areas. It probably prevents sea urchins from foraging by acting as a barrier. Further, we found that alarm cues around the kelp significantly affected foraging behavior of M. nudus, indicating that the alarm cues around the kelp are a potential effective way to prevent sea urchins from foraging for the kelp. In addition, the number of sea urchins that stopped feeding was significantly higher in the group in the presence of alarm cues than that in the control group. This indicates that alarm cues may have an application in stopping sea urchins from feeding. However, there was no significant difference of Aristotle's lantern reflex between the groups with and without alarm cues. These results indicate that alarm cues greatly affect foraging behavior, but not Aristotle's lantern reflex of M. nudus. All together, the present study suggests that alarm cues have an application potential in the management of the kelp beds as green engineering. Future studies are essential to further investigate the chemical basis of the alarm cues of sea urchins for the application in large-scale.

The detection of light intensity and subsequent phototaxis are essential for the fitness of sea urchins in intertidal and shallow subtidal waters, where light intensity varies in accordance with the depth and other hydrographic conditions. The molecular basis of photoresponse, however, remains largely unknown. We compared the expression of SiOpsin4, SiPax6, and SiOpsin5 among tube feet, coelomocytes, gonads, and gut of the sea urchin Strongylocentrotus intermedius. All three genes showed the significantly highest expression in tube feet. To test whether r-opsin genes (for example, Opsin4) and Pax6 in tube feet, rather than chaopsin genes (for example, Opsin5), are involved in the molecular responses to light intensity, we investigated the expression of Opsin4, Pax6, and Opsin5 among four light intensities. Both Opsin4 and Pax6 expressions were significantly higher in the sea urchins at 1,500 lx than at 100, 300, and 700 lx, indicating a co-upregulation of Opsin4 and Pax6 expressions in sea urchins exposed to the high light intensity (for example, 1,500 lx). However, there was no significant difference of SiOpsin5 expression among the four light intensity groups. Consistent with our hypothesis, there was no significant difference in either Opsin4 or Pax6 expression between the two light intensity environments (low light intensity environment: 50-100 lx; high light intensity environment: 330-1400 lx). This indicates that positive phototaxis and negative phototaxis probably regulate the expression of Opsin4 and Pax6 to a similar level when sea urchins are exposed to the high and low light intensities, respectively. The present study provides new insights into the molecular basis of light intensity detection and subsequent phototaxis of eyeless species.

Taking advantage of the seasonally appropriate water temperature in southern China is an important approach to expanding Strongylocentrotus intermedius aquaculture for the increasing market demand. However, appropriate diets remain largely uninvestigated and seed supply is in shortage, leaving the most serious problems of S. intermedius aquaculture in southern China unaddressed. Here, a feeding experiment was conducted for ∼6 months (from November 2018 to May 2019) to compare the somatic and gonadal growth, as well as gonadal development of S. intermedius (∼30 mm of test diameter) fed either macroalgae Gracilaria lemaneiformis or Saccharina japonica at the temperatures simulating water those off southern China coast (Fuzhou, 119°50′ E, 26°24′ N). Test size, wet body weight, specific growth rate, lantern size, wet gut weight and gonad yield of S. intermedius fed S. japonica were all significantly larger than those of sea urchins fed G. lemaneiformis. The present results indicate that long-term feeding G. lemaneiformis is unacceptable for aquacultural S. intermedius in southern China, because of the inefficiency of supporting somatic and gonadal growth. Saccharina japonica may suffice as a suitable diet for the aquaculture of S. intermedius in southern China. Further, spent gonads were observed in S. intermedius fed G. lemaneiformis in May 2019, meanwhile all gonads of individuals fed S. japonica remained in the premature stage. This suggests that G. lemaneiformis displays a promising potential for accelerating gonadal development of S. intermedius and subsequently supports the seed production of sea urchins in spring in southern China. The present study provides valuable information into the aquaculture management of S. intermedius in southern China.

AbstractThe effects of flow velocity on the fitness-related behaviours of Mesocentrotus nudus remain largely unknown, greatly hampering the efficiency of stock enhancement. To explore the appropriate velocities for stock enhancement, we investigated dislodgement and immobilization velocities up to 90 cm s−1. The experimental results showed that M. nudus (test diameter of ~30 mm) were dislodged at 73.50 ± 7.7 cm s−1 and that M. nudus movement occurred only when the flow velocity was less than 33.40 ± 2.7 cm s−1. Three flow velocities less than 33.40 ± 2.7 cm s−1 (2, 10 and 20 cm s−1) were subsequently used to study the effects of flow velocities on covering behaviour and the righting response time of M. nudus. The downstream movement velocity of M. nudus was significantly larger than that upstream at 2 cm s−1 (P = 0.016) and 10 cm s−1 (P = 0.008), but not at 20 cm s−1 (P = 0.222). The righting response time of M. nudus was significantly longer at 20 cm s−1 than that at 2 cm s−1 (P = 0.015). The present study indicates that a flow velocity less than 20 cm s−1, preferably 2–10 cm s−1, is probably appropriate for the stock enhancement of M. nudus. Notably, the current study is a laboratory investigation without considering the hydrographic complexity in the field. Further studies should be carried out to investigate the long-term effects of water flow on feeding and growth of M. nudus both in the laboratory and the field.

Light is an important environmental factor for the fitness of small sea urchins. It is thus important to reveal the appropriate light environments for their stock enhancement. However, the effects of light spectra on fitness related behaviors of sea urchins remain mostly unknown. To reveal appropriate light spectra for stock enhancement of the sea urchin Strongylocentrotus intermedius, we investigated the effects of light spectra on the foraging behavior, phototaxis, Aristotle's lantern reflex of S. intermedius (test diameter: 10.35 +/- 0.05 mm). There were five light spectra, including red (630-670 nm), yellow (570-600 nm), green (510-550 nm), blue (440-490 nm) and white light (a mix of blue-based and a little red). We found that successful foraging proportion under blue light (65 %) was obviously higher than those under other light spectra, although no significant difference was detected (P > 0.05). The foraging time of S. intermedius was significantly higher under red light than under blue (P < 0.05) and white light (P < 0.01). Light spectra did not significantly affect the movement distance of S. intermedius (P> 0.05). Light spectra showed no significant effect on phototaxis of S. intermedius (P > 0.05). This indicates that the effect of light spectra on the foraging behavior is not due to phototaxis and movement distance in S. intermedius. Further, Aristotle's lantern reflex of S. intermedius under white light was significantly higher than that under red light (P < 0.01). In summary, S. intermedius showed significantly better foraging and feeding ability, when they were exposed to blue and white light. Since most of the spectrum is short-wavelength light (blue light) in the water below 10 m, we suggest that small S. intermedius are appropriate to be reseeded into clear water areas with the depths more than 10 m.

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.

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.

Transgenerational effects are important for phenotypic plasticity and adaptation of marine invertebrates in the changing ocean. Ultraviolet-B (UV-B) radiation is an increasing threat to marine invertebrates. For the first time, we reported positive and negative transgenerational effects of UV-B radiation on egg size, fertilization, hatchability and larval size of a marine invertebrate. Strongylocentrotus intermedius exposed to UV-B radiation showed positive transgenerational effects and adaptation on egg size, hatching rate and post-oral arm length of larvae. Negative transgenerational effects were found in body length, stomach length and stomach width of larvae whose parents were exposed to UV-B radiation. Sires probably play important roles in transgenerational effects of UV-B. The present study provides valuable information into transgenerational effects of UV-B radiation on fitness related traits of sea urchins (at least Strongylocentrotus intermedius ).