Indiscriminate Dietary Compositions of Two Asian Horseshoe Crabs,Tachypleus tridentatus and Carcinoscorpius rotundicauda:Evidence from Hemolymph Stable Isotopes

Knowledge of dietary compositions is essential to the recovery of the dwindling populations of horseshoe crabs in China.The feeding habits of Tachypleus tridentatus and Carcinoscorpis rotundicauda of Pearl Bay in southwestern China were studied by stable isotope analysis.Hemolymph samples of the two species were obtained from two age groups living in different habitats(inner,central,and outer Bay).In addition,their potential food source samples were collected in May 2019.Results showed that the mean values ofδ^(13)C andδ^(15)N in C.rotundicauda individuals ranged from−19.01‰to−16.47‰and from 10.49‰to 13.5‰,respectively,and those of T.tridentatus ranged from−19.12‰to−14.96‰and from 8.78‰to 13.48‰,respectively.These values indicated that the horseshoe crabs have a wide variety of food sources and therefore are highly omnivorous.No remarkably correlations were found betweenδ^(13)C andδ^(15)N values and individual widths in the two species.Thirteen potential food sources were selected,in which Sipunculus nudus,Batillaria zonalisi,and Ulva lactuca contributed largely to the dietary compositions of the two species of horseshoe crabs.No significant correlation was found between the feeding habits and habitat geochemical characteristics of the horseshoe crabs.All these results possibly indicate the high diversity of their food sources and the indiscriminate dietary compositions of the two Asian horseshoe crabs.

Transcriptome Analysis of the Digestive Tract of Tachypleus tridentatus and Carcinoscorpius rotundicauda

Transcriptome assemblies for the stomach,midgut,and fecal mucosa of Tachypleus tridentatus and Carcinoscorpius rotundicauda are reported for the first time in this study.Genome assembly resulted in 373069 unigenes with an N50 of 1314,while transcriptome assembly resulted in 736378 unigenes and an N50 of 2121.The annotated unigenes showed the highest similarity to fishes,amphibians,and reptiles,with most unigenes closely related to metabolism,translation,biogenesis,signal transduction,energy production,immune response,and secretion.Unigenes(4768)were mapped to KEGG pathways,and the most enriched pathways were involved in translation,environmental information signal processing,metabolism,endocrine system,immune system,nervous system,and varying metabolism.Totally 65889,2001,and 7162 differentially expressed genes(DEGs)were identified in the stomach,midgut,and fecal mucosa between the two horseshoe crab species,respectively.Compared with C.rotundicauda,99.95%of the DEGs in the T.tridentatus stomach were down-regulated,while 99.97%of the DEGs of T.tridentatus fecal mucosa and 55.42%of the DEGs of the T.tridentatus midgut were up-regulated.Most midgut DEGs were involved in hydrolase activity,protein metabolism,and cell cytoplasm,while most stomach and fecal mucosa DEGs were involved in catalytic activity and primary metabolic processes.Most stomach DEGs were assigned to the cellular component in cellular macromolecular complexes,while most fecal mucosa DEGs were assigned to the cellular component in cell and cell parts.These results will benefit the investigation of the molecular mechanisms of the digestive tract related to the feeding habits and environmental traces of horseshoe crabs.

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.

Asian Horseshoe Crab Bycatch in Intertidal Zones of the Northern Beibu Gulf:Suggestions for Conservation Management

Bycatch poses a severe challenge to global fishery management.Although studies have focused on marine megafauna bycatch,research on relevant issues regarding invertebrates is limited,particularly for the threatened horseshoe crabs.In this study,the distribution of fishing gears and the bycatch intensity of Asian horseshoe crabs in the intertidal zones of the northern Beibu Gulf that harbors the most abundant juvenile Asian horseshoe crabs populations in China were evaluated.Seven intertidal nursery habitats for Tachypleus tridentatus and Carcinoscorpius rotundicauda were surveyed from May to July in 2021.A transect that crossed the tidal creeks and tidal flats between the high tide embankment/vegetation and low tide line of a given habitat was surveyed during the ebb tides.The type,number,and GPS of fishing gears were recorded when sighted,and the number and prosomal width of each horseshoe crab species were measured.Bycatch intensities differed among habitats,ranging from 0.3 to 18.4 and 1.2 to 22.7 individuals per kilometer of transect for T.tridentatus and C.rotundicauda,respectively.Among the three identified fishing gear types,ground cages and stick net sets caused a stronger bycatch pressure on these two species.Most T.tridentatus bycatch and C.rotundicauda bycatch were large individuals in late juvenile and adult stages.Therefore,the ground cages and stick net sets along/near the tidal creeks,mangrove fringe,and low tide line should be regulated and removed to ensure the functionality of the intertidal zone as the spawning corridor and nursery habitat of Asian horseshoe crabs.