The anthropogenic construction activities on the coasts, such as pile-driving, generate vibrations that propagate through the substrate. Such substrate-borne vibrations could potentially affect marine organisms inhabiting the benthic environments. However, there is a lack of documented studies on the effects of vibrations on benthic animals. To investigate whether anthropogenic substrate-borne vibrations such as pile-driving operation influence the fiddler crab, Austruca lactea, we measured their locomotion response under vibrations of 35, 120, 250, 500, and 750 Hz generated by a vibrator. We compared the locomotion of crabs between control and vibrationtreatment groups using videography. The duration of movements was significantly lower under 120 Hz vibrations compared to the control. Moreover, crab velocity was significantly higher under vibrations of 120 Hz and 250 Hz compared to the control group. Our result suggests that A. lactea can detect low-frequency substrateborne vibrations and experience stress, leading to increased energy consumption.

Coastal ecosystems, including tidal flats, are exposed to recreational activity-driven trampling. The white-clawed fiddler crab, Austruca lactea, is endangered in South Korea majorly due to reclamation but recreational trampling also emerged as a potential threat. To assess short-term behavioral responses of individual A. lactea to a single trampling and the long-term population changes under cumulative trampling, we conducted two field experiments for three semilunar tidal cycles. First, we compared the behaviors of two crab individuals, one of whose burrow openings had undergone a single trampling event. Results revealed that trampling delayed the initiation of surface activity, reduced travel distance, and altered the behaviors of the crabs from courting or feeding to vigilance and burrow repair. Second, we constantly exerted trampling on subsets of the crab population for two trampling periods (four and two weeks) and monitored the restoration of their collective behaviors for an additional two weeks. Daily trampling decreased the number of crabs active on the surface and the degree of male courtship indicators, and the rhythms collapsed. After trampling cessation, only courtship indicators recovered after longer trampling treatment. These results imply that continuous coastal visitation without any regulation might aggravate A. lactea population decline in heavily used habitats. Because A. lactea is an indicator species for coastal ecosystem health, we suggest conservation of the habitat area of these crabs based on their behavioral changes caused by human trampling. Our study underscores the importance of assessing the behavioral ecology of benthic animals for tidal flat ecosystem monitoring and conservation areas establishment.

Continuous emissions of anthropogenic CO2 are changing the atmospheric and oceanic environment. Although some species may have compensatory mechanisms to acclimatize or adapt to the changing environment, most marine organisms are negatively influenced by climate change. In this study, we aimed to understand the compensatory mechanisms of the Antarctic clam, Laternula elliptica, to climate-related stressors by using archived shells from 1995 to 2018. Principal component analysis revealed that seawater pCO2 and salinity in the Antarctic Ocean, which have increased since the 2000's, are the most influential factors on the characteristics of the shell. The periostracum thickness ratio and nitrogen on the outermost surface have increased, and the dissolution area (%) has decreased. Furthermore, the calcium content and mechanical properties of the shells have not changed. The results suggest that L. elliptica retains the mechanism of protecting the shell from high pCO2 by thickening the periostracum as a phenotype plasticity.

Coastal ecosystems, including tidal flats, are exposed to recreational activity-driven trampling. To assess the effect of trampling on the tidal flat ecosystem, it is pertinent to investigate changes in the activity of its dominant inhabitant. The fiddler crab, Austruca lactea, is abundant in some coastal areas, but is currently endangered owing to exposure to anthropogenic activities, including trampling. To assess the short-term behavioral responses of individual fiddler crabs to a single trampling and the long-term changes in the population under cumulative trampling, we conducted two types of field experiments for three semilunar tidal cycles during their reproductive period. First, we compared the behaviors of a pair of crabs, one of whose burrow openings had undergone a single trampling event. Results revealed that trampling delayed the initiation of surface activity, reduced travel distance, and altered the behaviors of the crabs from courting or feeding to vigilance and burrow repair. Second, we constantly exerted trampling on subsets of the crab population for two trampling periods (four and two weeks) and monitored the restoration of their collective behaviors for an additional two weeks. Daily trampling events decreased the degree of total surface activities and male courtship indicators, and the rhythms collapsed. After the cessation of trampling, only the courtship indicators recovered after longer trampling treatment. These results imply that continuous coastal visitation without any regulation might aggravate the decrease in the population of A. lactea and affect the tidal flat ecosystem function. Thus, we suggest the management of the habitat area of these crabs based on their behavioral changes caused by human trampling. Our study verified the substantial role of the behavioral ecology of benthic animals in monitoring the tidal flat ecosystem function and installing conservation area.

Microplastics (MPs) are found in every ocean and are frequently ingested by marine animals. This study analyzed MPs in the stomachs and intestines of 12 large marine animals comprising one fin whale (Balaenoptera physalus), seven finless porpoises (Neophocaena asiaeorientalis), two loggerhead turtles (Caretta caretta), one Indo-Pacific bottlenose dolphin (Tursiops aduncus), and one common dolphin (Delphinus delphis) that were stranded off the Republic of Korea between 2019 and 2021. MPs were detected with a mean abundance of 3.42 ± 3.2 items/g and were predominantly of transparent-white, fragment-shaped polypropylene smaller than 200 μm. The abundance of MPs found did not correlate with the biological information (maturity, body length) of the finless porpoises and there were no significant differences in the abundance of MPs between the stomachs and intestines. These results cannot accurately assess the impact of MPs on large marine animals, so further studies are necessary to understand how MPs can potentially affect them.

Ghost fishing via a derelict fishing gear (DFG) is a critical threat to marine organisms. To explore the effect of DFG on sea turtle strandings, the DFG distribution was compared at two sites on Jeju Island (South Korea) with a contrasting number of strandings. Coastal areas in northern Jeju Island were surveyed during dives with scuba equipment, and the DFG from two sites, Gwideok-ri and Sinchang-ri was collected and compared in terms of quantity and size of the items. Fishing line was more common, longer, and thicker in Gwideok-ri than in Sinchang-ri, while other types of DFG did not differ between the two sites. In addition, necropsies on two loggerhead sea turtles discovered on Jeju Island found fishing lines with fishing hooks in the oral cavity of both carcasses. This suggests that derelict recreational fishing lines may pose a significant threat to sea turtles in coastal areas.

We investigated the response of the Manila clam Venerupis philippinarum to possible temperature and salinity changes in a holding facility. First, clams were exposed to four temperatures for 15 days. Valve closure and survival of clams exposed to seawater at 18 °C were higher than that of those exposed to seawater at 24 °C. Second, clams were exposed to six salinities for 15 days. Survival of clams exposed to two salinity fluctuation conditions (24–30 and 27–24 psu) was lower than that of clams exposed to constant 30 psu conditions. Valve closures of clams exposed to constant low salinity conditions (24 psu) and two salinity fluctuation conditions (24–30 and 27–24 psu) were higher than those exposed to constant 30 psu conditions. Lastly, clams were exposed to two different temperatures and three different salinity conditions for 8 days. Valve closure and survival decreased significantly under the combination of 24 °C and 18 psu. These results suggest that an increase in temperature or a wider range of salinity fluctuations are detrimental to the survival of the Manila clam. The synergistic effect of temperature and salinity stressors may decrease the survival period of clams compared to the effect of a single stressor.

The characteristics of the sedimentary environment are considered significant for increasing the aquaculture production of Manila clams, Venerupis philippinarum. Among these, the grain size and sorting of the sediment, which determine the amount of pore-water-dissolved oxygen and organic matter content, can affect the growth and survival of clams. To determine the optimal grain size and sediment sorting in relation to the conditions of Manila clams (e.g., survival, growth, burrowing behavior, and condition index), we conducted three experiments. First, juvenile clams were incubated in four different grain sizes (coarse sand, 0.335-710 mm; medium sand, 0.180-0.335 mm; fine sand, 0.090-0.180 mm; and very fine sediment, < 0.09 mm). Survival was significantly higher in medium sand than in very fine sediment. The condition index of the Manila clams was higher in the medium sand than in the coarse and very fine sediments. However, the sediment grain size did not influence the shell growth or clam emergence behavior. Second, juvenile clams were incubated in three different treatments of sediment sorting (well-sorted sediment: 0.58 mm; moderately-sorted sediment: 0.46 mm; and poorlysorted sediment: 0.37 mm). The shell growth rate was significantly higher in the poorly-sorted sediment than in any other treatment, while sediment sorting did not influence the survivorship, condition index, or emergence behavior. Third, juvenile Manila clams were incubated for 30 days in the sediment taken from two locations on the clam culturing mudflat, where a structure was installed to facilitate the settling of clam larvae (which was more poorly-sorted) or without a structure (the control). Although the survival and emergence behavior were not significantly different between the two treatments, the shell growth rate in the sediment under the structure was higher than that in the sediment with no structure. Moreover, the condition index of the Manila clams in the sediment under the structure was significantly higher than that under no structure. The results suggest that poorly-sorted sediment with an average grain size of medium sand is optimal for the survival and growth of Manila clams.

Marine pollution caused by plastic litter can threaten the survival and health of marine organisms. In 2019, a juvenile fin whale (Balaenoptera physalus, length: 13.02 m, weight 12,000 kg) was found dead floating on the sea near Jeju Island, Republic of Korea. During the dissection, 45 plastic particles were found in the body of the whale, including fishing lines, plastic filaments, pieces of fishing nets, and Styrofoam particles. The largest item found was a piece of fishing line (1180 mm in length and 1.15 mm in thickness). Filaments, both bundled and separated, were more frequent. Some of the filaments found were entangled with the baleen plate bristles. These observations suggest that plastic pollution is a potential risk for baleen whale species. This is the first record of plastic ingestion by a vulnerable baleen whale species in the sea off East Asia.