Spatiotemporal Distribution of Asian Horseshoe Crab Eggs Are Highly Intermingled with Anthropogenic Structures in Northern Beibu Gulf,China

Identification,protection and restoration of spawning habitats are vital for protecting the depleted species.Asian horseshoe crabs are ecologically important macroinvertebrates in coastal and estuarine ecosystems.However,their spawning habitat studies were limited to several reports in tropical regions,possibly due to the lack of modified survey methods,particularly in habitats with a lower density of spawning adults,and/or intermingled with anthropogenic structures.In this study,the year-round egg distribution and spawning habitat baselines of Carcinoscorpius rotundicauda and Tachypleus tridentatus were determined in the northern Beibu Gulf,China.Our findings demonstrated that the peak spawning occurred in June–July and ceased in November–January when the average water temperature dropped below 20℃.Egg aggregations were found<10 cm beneath the sediment surface with regular tidal inundation,regardless of seasonal changes,in the vicinity of natural and artificial structures with elevated,mildly sloping substratum within the high tide zones.The nests were characterized by medium-sized sediment grains(0.5–0.9 mm),high temperatures(31–34℃),low water contents(0.8%–0.9%),and total organic carbon contents(0.5%–0.7%),which might maximize the hatching success.The identified nesting beaches were close to nursery habitats for juveniles,and tidal creeks were present as the possible corridor connecting these two important habitats through the dominant mangrove forests.The findings provide valuable insights in the scope of spawning behavior and nest-site selection of Asian horseshoe crabs under a mixture of natural and artificial structures,which could benefit future management efforts for the exploited spawning populations.

Influence of Tidal Cycles on Embryonic Rotation,Hatching and Emergence of Mangrove Horseshoe Crab,Carcinoscorpius rotundicauda

Horseshoe crabs are iconic and ecologically significant macroinvertebrates in coastal environments.The processes and mechanisms of larval hatching in Asian horseshoe crabs that occurs beneath the sand are largely unknown.The spawning and developmental ecology of Tachypleus tridentatus and T.gigas are assumed to be similar to their Atlantic counterpart Limulus polyphemus.However,Carcinoscorpius rotundicauda has been cited as an exception owing to their frequent sightings in muddy mangrove areas even during low tides.To reveal the larval hatching mechanisms,in this study,we examined varying hatching responses of C.rotundicauda embryos within the sediment to the environmental conditions under continuous tidal cycles.During the eight-week experiment,the count of hatched larvae ranged 4%–30%per week,while the cumulative emergence rate from the sediment was 0–47%.Embryos were observed to have the highest active rotation activity in the first two weeks after incubation.The inundation of tidal water significantly enhanced the occurrence of hatching,in which hydration,osmotic shock and possibly agitation had triggered or facilitated the eclosion.The larvae were found to remain in the sediment for approximately 2–6 weeks before emergence.In general,C.rotundicauda was found to share a similar hatching mechanism with L.polyphemus.Our findings provide insight into the developmental ecology of Asian horseshoe crabs exposed to varying tidal conditions,and are helpful to the management and protection of their spawning habitats.

High Microplastic Contamination in Juvenile Tri-Spine Horseshoe Crabs:A Baseline Study of Nursery Habitats in Northern Beibu Gulf,China

Microplastics(MPs)are ubiquitous in marine sedimentary environments.Their occurrence in horseshoe crabs and interactions with the sedimentary environment has not been determined.In this study,MPs,mostly microfibers,were found in all extracted gastrointestinal tract(GIT)samples of the juvenile Tachypleus tridentatus from the northern Beibu Gulf,China.The MP concentrations(4–53 items ind-1)were higher than those in most marine benthic invertebrates(<15 items ind-1)reported in Chinese waters,despite their generally low level in habitat sediments(9–1818 items kg-1).The correlation between the juveniles and sediments was not evident,but the abundance in juvenile horseshoe crab GITs decreased with ages.The findings were relevant to the life-history characteristics of the species,typically with limited dispersal capability and their spending nine years or longer time living in mangrove wetlands during low tides,with apparent ontogenetic changes in their diets.These baseline data enable a better understanding of MP availability in benthic macroinvertebrates,and the ecological risks present in the ecosystems.

红树植物白骨壤叶片凋落物浸提液对桐花树胚轴萌发与幼苗生长的化感作用

白骨壤与桐花树是我国南方潮间带红树林中2种常见的本土优势种红树植物。本研究以桐花树为实验材料,研究不同浓度白骨壤叶片凋落物浸提液(0、2、10、20和50 g·L^(-1))处理对其胚轴萌发与幼苗生长的化感作用。结果表明:随着白骨壤叶片凋落物浸提液处理浓度的升高,桐花树幼苗的细根数、营养根数与茎长逐渐降低,且桐花树胚轴的生根和发芽时间会逐渐变长;桐花树胚轴发芽率、幼苗叶干重与根冠比在2 g·L^(-1)浸提液处理时显著增加,而在10、20和50 g·L^(-1)浸提液处理下表现为下降的趋势。50 g·L^(-1)浓度的浸提液处理明显降低桐花树胚轴的生根率、幼苗的细根长、营养根长、叶片数、根干重与茎干重,而2、10和20 g·L^(-1)浓度的浸提液处理对桐花树上述生理指标的抑制作用均不明显。与胚轴萌发相比,桐花树幼苗生长对白骨壤叶片凋落物浸提液的化感响应更为敏感。