Introduction
The japanese ocean cucumber Apostichopus japonicus ( Selenka, 1867 ), an Echinoderm of the class Holothuroidea, is wide distributed along the slide of southern far-east Russia, South Korea, northern China, Japan, and United States ( Choo, 2008 ). A. japonicus is one of the common temperate sea cucumber species that has been harvested, traded, and traditionally consumed as seafood and a tonic in southeast Asia for centuries ( Akamine, 2015 ). Japan has exported ocean cucumbers, including A. japonicus, to China as a major commodity at least for 350 years ( Akamine, 2004 ). In particular, A. japonicus harvested from Hokkaido, the northern island of Japan, is regarded as the highest quality due to characteristics of a greater total, large lined-up warts, and chummy flesh ( Brown and Eddy, 2015 ). The declining wild population of ocean cucumber by overfishing, habitat loss, and human-induced pollution have been described by Purcell et aluminum. ( 2013 ), and A. japonicus was besides added to the ICUN Red List of endanger species as “ Endangered ” in the same year ( Mercier and Hamel, 2013 ). Although Japan has taken the inaugural in conservation measures such as regulating the full annual catch, harvest size and adopting a shut season to promote reproduction, the wild stocks of A. japonicus have declined at least 30 % in the past 30 years ( Choo, 2008 ). The release of hatchery-produced juveniles is promoted to enhance the crazy population of A. japonicus ( Battaglene, 1999 ; Purcell, 2004 ). To rebuild the wild stocks around the coast of Hokkaido, the vigorous publish of juveniles has been carried out ( hokkaido Government, 2020 ) .
predation is one of the determining factors that influence the benthic invertebrate community structure in aquatic ecosystems ( Bell et al., 2005 ). numerous reports have documented that early juveniles of invertebrates such as bivalves, decapod crustacean crustaceans, and echinoderms, and so forth, are vulnerable to predation, and a meaning number of juveniles are assumed to be killed by predators ( Gosselin and Qian, 1997 ). aquatic animals, including marine mammals, ocean birds, pisces, asteroids, crustaceans, and gastropods, have besides been documented to prey on holothurians ( Francour, 1997 ). however, few studies have documented the significance of predators on A. japonicus ( Yu et al., 2015 ). The kelp cancer Pugettia ferox, recently reclassified as a new species, is a much larger cancer species than once assigned Pugettia quadridens, which distributes more north ( Ohtsuchi and Kawamura, 2019 ). P. quadridens is known as a ill-famed marauding crab negatively affecting fishery resources such as juveniles of Ezo abalone and sea urchins in Japan ( Shibui, 1971 ; Kawai and Agatsuma, 1996 ; Hoshikawa, 2003 ). On our report locate, a remarkable addition of the P. ferox population within artificial intercede sea cucumber reef for releasing hatchery-produced adolescent A. japonicus has been observed ( Figures 1A–C ), and the ossicles of A. japonicus were detected endlessly from the abdomen contents of P. ferox ( Figure 1D ) deoxyadenosine monophosphate well as active predation ( Figure 1E and Supplementary Movie 1 ). To our best cognition, there is no literature on quantitative depredation on A. japonicus by P. ferox. This report provides novel findings of predation on A. japonicus juveniles by P. ferox through microscopic observations of abdomen contents in the natural population and depredation experiments .
Reading: Predation of Juvenile Japanese Sea Cucumber Apostichopus japonicus by Kelp Crab Pugettia ferox
FIGURE 1
Figure 1. (A) An artificial intercede ocean cucumber reef for releasing hatchery-produced adolescent Apostichopus japonicus. (B) Attached juvenile A. japonicus ( 
) on scallop shells as a substrate within the reef. (C) Observed Pugettia ferox ( ) within the same reef. (D) Ossicles of A. japonicus detected from the stomach contents of P. ferox. (E) active predation of adolescent A. japonicus by P. ferox. (F) Results of the lab predation experiments. An aquarium with no P. ferox was used as a operate ( C ). (G) painting of A. japonicus killed by a female P. ferox within the first 2 heat content. (H) Decoration of P. ferox using pieces of chopped sea cucumber .
Materials and Methods
Field Sampling and Observation of Stomach Content
A entire of 68 P. ferox were collected in December 2018 from artificial intercede sea cucumber reef, deployed at a astuteness of 4–5 m within a fish port ( 41.2744N, 140.1452E ) located in southwest Hokkaido, Japan. Samples were preserved in 70 % ethyl alcohol immediately after sampling in the field for late measurements and stomach subject observations. utmost carapace width ( CW ) and post-pseudorostral carapace length ( PCL ), and wet weight were measured as body size indexes ( Figure 2 ). After the measurement, the digest of each P. ferox was dissected using a scalpel. The stomach contents were placed into a microtube and dissolved in a sodium hypochlorite solution ( NaClO, 5 % ). then, microscopic observation using Axiovert 135 ( Zeiss, Germany ) was performed to check whether the ossicles of A. japonicus exist in the samples .
FIGURE 2
Figure 2. Maximum carapace width ( CW ), post-pseudorostral carapace length ( PCL ), and cheliped propodus duration ( CPL ) of P. ferox measured as consistency size indexes .
Laboratory Predation Experiment
Pugettia ferox ( n = 9, four Males, five Females ) collected in the same reef mentioned above at the end of June 2019 were brought back to the testing ground animated. P. ferox were kept individually in clear aquarium ( 31 curium distance × 18 cm width ) filled to a depth of 5 centimeter of seawater at 13 ± 1°C which is the local water temperature of releasing A. japonicus juveniles, under a 12 henry light/12 heat content dark cycle ( 12:12 LD ) without feeding for a workweek before the predation experiments. An aquarium with no P. ferox was besides prepare as operate ( C ). CW, PCL, cheliped propodus length ( CPL ), and wet weight of P. ferox were measured as indexes of body size ( Figure 2 and Table 1 ). Body length, width, and moisture weight of A. japonicus juveniles used as prey were besides measured in advance. All A. japonicus were immersed in L-menthol for 20–30 min ( Hatanaka and Tanimura, 1994 ), and the body length and width were measured, then standard consistency length ( SBL ), which is a widely used method for accurate measurement of sea cucumbers, was estimated ( Yamana et al., 2011 ). A digital vernier caliper was used for all the measurements to the nearest 0.1 millimeter. A. japonicus juveniles used in this cogitation were purchased from Hokkaido Aquaculture Promotion Corporation. Ten A. japonicus individuals were randomly selected and placed into each aquarium for depredation experiments. The average SBL and wet weight of A. japonicus used were 15.35 ± 2.47 millimeter ( Mean ± SD ) varied from 10.82 to 19.99 millimeter, and 0.1 ± 0.04 g varied from 0.03 to 0.23 thousand, respectively. A. japonicus were counted after 12 henry ( the end of a light motorbike ) and 24 h ( the end of a dark cycle ). All the P. ferox decorated with a nibble of A. japonicus were observed continuously for another week in the lapp aquarium after removing the remaining A. japonicus and organic residuals. The densities of A. japonicus juveniles per unit area adopted in the experiments were based on the densities observed in the field. The depth of seawater in the aquarium applied was to eliminate the inaccessibility due to vertical attachment by A. japonicus during the experiments. The experiments mentioned above were repeated three times and then preserved in 70 % ethyl alcohol to examine the ontogenetic stages ( Ohtsuchi and Kawamura, 2019 ) .
table 1
Table 1. Measured carapace width at the widest part ( CW ), post-pseudorostral carapace length ( PCL ), an average of the correct and left cheliped propodus distance ( CPL ), and besotted weight of Pugettia ferox ( n = 9, four Males, five Females ) used in the lab depredation experiments .
Statistical Analysis
The differences in body size indexes of CW, PCL, and wet weight between the ossicle-detected and not-detected groups from the field sampling and mortality rate between day and night, male and female, and ontogenetic stages ( adolescent and adult ) in the depredation experiments were assessed by using a scholar ’ mho t -test after confirming the population of two quiz groups are normally distributed with peer variation. The occurrence pace of ossicle-detected and not-detected was compared between male and female using a chi-square test. Relationships between the mortality pace and all four body size indexes ( CW, PCL, CPL, and wet weight ) were examined using correlation analyses ( Pearson ’ s correlation coefficient ) to determine if two numeral variables are importantly linearly related. The statistical analyses were performed using StatView statistical software ( translation 5.0 ) and R version 3.6.2 .
Results
Measurement of P. ferox and A. japonicus Ossicle in Stomach Content
Averages of CW, PCL, and wet weight of the collected P. ferox were 13.40 ± 5.40 millimeter ( Mean ± SD ) varied from 4.08 to 25.26 mm, 18.45 ± 6.34 millimeter varied from 6.42 to 34.71 millimeter, and 1.73 ± 2.05 g varied from 0.03 to 9.42 thousand, respectively. The ossicles of A. japonicus were detected from the stomach contents at a rate of 32.4 %, and averages of CW, PCL, and wet weight of P. ferox containing ossicles were 12.53 ± 3.00 mm varied from 6.80 to 20.33 mm, 18.10 ± 3.17 millimeter varied from 11.53 to 24.92 millimeter, and 1.12 ± 0.85 g varied from 0.18 to 4.11 thousand, respectively. Although there were no statistically significant differences in body size indexes of CW, PCL in both male and female cancer, and wet weight in females between the ossicle-detected and not-detected groups ( p > 0.05 ), the average of besotted weight between the ossicle-detected and not-detected groups in male cancer ( p < 0.05 ) were significantly different ( Figure 3 ). furthermore, the ossicles were detected more frequently in females ( 1:1.44 ) despite the sex ratio of male and female P. ferox examined stomach contents was 1:0.94. No statistical significance was seen in the happening rate of ossicle-detected and not-detected groups between male and female crab ( p > 0.05 ) .
FIGURE 3
Figure 3. Box plot of wet weight between the ossicle-detected and not-detected groups in male and female P. ferox collected within artificial intercede sea cucumber reef for releasing hatchery-produced juveniles in December 2018. An open circle ( 
) represents the average wet weight. The peak and the bed channel of the corner show the first base and third quartiles, and the middle line is the median. The upper ( lower ) whisker displays the utmost ( minimal ) value. Asterisk ( ∗ ) indicates statistical meaning ( p < 0.05 ) .
Mortality Rate and Predatory Behavior
The high mortality rate [ 7.7 ± 2.4 individuals day–1 ( Mean ± SD ) ] of juvenile A. japonicus was confirmed despite the respective sizes and sexes of P. ferox beside no deathrate in control ( Figure 1F ). tied though there were cocksure relationships between the mortality rate and all four body size indexes ( CW, PCL, CPL, and moisture weight ) in the male cancer, no significant linear correlation coefficient was detected from either male or female crab ( p > 0.05 ). All crabs actively searched for food right after the start of experiments, and a maximum of five ocean cucumbers was confirmed to be killed by a female P. ferox and cut into little pieces within the inaugural 2 planck’s constant ( Figure 1G ). Although test P. ferox were confirmed to forage for food during the day, the number of A. japonicus killed during the nox was significantly higher ( p < 0.001 ). No statistical significance was confirmed in mortality pace between male and female and ontogenetic stages ( p > 0.05 ). During experiments, all of the smaller and younger individuals ( adolescent ) between CWs of 14.2–17.8 mm actively decorated themselves using chopped sea cucumber pieces during and/or after feeding ( Figure 1H ). however, attached pieces of sea cucumber were observed to be in full eaten within a week in all experiments .
Discussion
In general, holothurians, including A. japonicus, are known to have few predators ( Francour, 1997 ), owing to wide-ranging anti-predator behaviors, including separation, body shape change, shedding body parts, and toxicity, in change for their slow-moving behavioral characteristics ( Mosher, 1956 ; Bakus, 1968 ; Kropp, 1982 ; Bingham and Braithwaite, 1986 ; Morton, 1991 ; Aminin, 2016 ; Kamyab et al., 2020 ). indeed, upon attack by P. ferox, frequently note antipredator behaviors by A. japonicus were the contractions of their body and detachments from the attached surface in this study. The contraction of A. japonicus is caused by external stimuli using catch conjunction weave, which controls the body wall ’ s stiffness for resisting predators ( Motokawa, 1984 ). Some studies in Holothuria scabra and Stichopus horrens demonstrated successful escapes from predators by detachment ( Kropp, 1982 ; Morton, 1991 ). On the other hand, Dance et alabama. ( 2003 ) reported heavy predations by pisces after releasing juveniles of tropical ocean cucumber Holothuria scabra to the wild, indicating the importance of evaluating impact by predators. carnivorous fish, crab, gastropods, and sea stars are suggested to be candidates as predators for A. japonicus juveniles ( Francour, 1997 ; Yu et al., 2015 ), and sea ace Asterina pectinifera is considered as the principal marauder of A. japonicus adolescent, ingesting 1.8 individuals with an average distance of 15.9 mm on average per day in lab experiments ( Hatanaka et al., 1994 ). In the present learn, the calculate mortality rate of 7.7 ± 2.4 A. japonicus individuals ( 15.35 ± 2.47 millimeter ) per day by P. ferox was over four times higher than Asterina pectinifera, suggesting the importance of specifying likely predators arsenic well as evaluating their impact. Considering spatial distribution and habitat overlap of the two species ( Liu, 2015 ; Ohtsuchi and Kawamura, 2019 ), the presented phenomenon may be widespread in the lifelike environment. frankincense, the future probe of relatively obscure ecological data of P. ferox may be crucial for the areas putting efforts into recovering wild stocks of A. japonicus, and/or establishing efficient release methods of hatchery-produced A. japonicus juvenile .
In general, body size, ontogenetic stages, and sex are essential determinants influencing the feed behaviors of brachyuran crab ( Kolts et al., 2013 ; Williner and Collins, 2013 ). For example, larger males could consume larger prey, but female and smaller male cancer may be more efficient feeders, depending on their treatment ability and lastingness ( Kolts et al., 2013 ; Tina et al., 2015 ). In the stage report, the ossicles were detected more frequently in females with a broader size roll, and no ossicle was found in male crab over 15 mm in CW ( ca. 1.56 guanine in wet weight unit ). together with the gamey mortality rate of female crab confirmed in lab cogitation, these results indicate female P. ferox pose a greater risk on adolescent A. japonicus than males. Regarding the size of sea cucumber, Purcell and Simutoga ( 2008 ) documented that size at dismissal significantly affected the survival of adolescent sandfish Holothuria scabra by predation. The depredation by ocean star topology Asterina pectinifera was well reduced when A. japonicus juveniles with an average length of 30.1 mm compared with an average duration of 15.9 millimeter ( Hatanaka et al., 1994 ). therefore, arouse and ontogeny-related feed behaviors and the predator-prey body size relationships may need to be cautiously investigated as a future undertaking for a more accurate evaluation of the affect on A. japonicus by P. ferox .
Pugettia ferox is referred to as decorator crab, using their specify seta to hook external materials from the environment for consumption as disguise ( Ohtsuchi and Kawamura, 2019 ). The functions of decoration camouflage are to reduce the probability of detection, recognition, and pulmonary tuberculosis by predators ( Hultgren and Stachowicz, 2011 ). P. ferox utilize versatile combinations of pieces of red and brown alga, branched colonies of bryozoans, or hydrozoans for their decoration materials ( Ohtsuchi and Kawamura, 2019 ). In the confront survey, all of the smaller and younger ( adolescent ) P. ferox actively used chopped ocean cucumber pieces to decorate themselves during and/or after feeding ( Figure 1H and Supplementary Movie 2 ). Our observation was reproducible with the decoration behavior of other brachyuran crabs that juvenile decorated more frequently than adult ( Hultgren and Stachowicz, 2009 ; De Carvalho et al., 2016 ), explained with their susceptibility to predation and habitat changes ( Todd et al., 2009 ). There are besides crab known to decorate with noxious organisms, such as alga, sponges, or other invertebrates, to deter predators ( Stachowicz and Hay, 1999 ). A. japonicus grow saponins, which act as a eating hindrance and/or warning consequence to electric potential predators ( Van Dyck et al., 2011 ). Assuming that P. ferox exhibit the lapp deck behavior using A. japonicus in nature, the risk of feeding pressure by predators may be importantly reduced. however, decorated pieces of ocean cucumbers were in full eaten within a week when no food was given thereafter. In this respect, our observation was preferably alike to the results presented by Woods and Mclay ( 1994 ) that the attach materials take a character as short-run food repositing and/or play multiple roles ( Hay, 1992 ). It is intriguing to deepen our cognition on these novel and alone ecological traits of decoration in reaction to the bearing or absence of predators and food resources in orderliness to understand adaptation and natural survival of P. ferox, deoxyadenosine monophosphate well as offering an important perspective on A. japonicus fishery .
Data Availability Statement
The original contributions presented in the study are included in the article/ Supplementary Material, far inquiries can be directed to the equate writer .
Author Contributions
NI and HK conceived and designed the research. NI, TM, and YA performed the fieldwork. NI and KM analyzed the data and wrote the manuscript with substantive contribution from all authors .
Conflict of Interest
YA was employed by the company Ocean Construction Co., Ltd .
The remaining authors declare that the inquiry was conducted in the absence of any commercial or fiscal relationships that could be construed as a potential conflict of pastime .
Acknowledgments
We express sincere gratitude to the members of Fisheries Engineering Research Team, Civil Engineering Research Institute for Cold Region ( CERI ), Japan, for administrative support, particularly Sawako Shirai, a technical adjunct. We would besides like to show our capital taste to Emeritus professor Ichiro Imai from Hokkaido University for critical advice and Naoya Ohtsuchi from International Coastal Research Center, University of Tokyo, for essential advice about P. ferox identification. Our sincere taste goes to Atsushi Yamaguchi from Hokkaido University for providing testing ground space during the investigation .
Supplementary Material
The Supplementary Material for this article can be found on-line at : hypertext transfer protocol : //heyreviewfood.com/articles/10.3389/fmars.2021.684989/full # supplementary-material
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