Study On Gut Morphological Structure And Food Sources Of Juvenile Sea Cucumber Stichopus Monotuberculatus

Stichopus monotuberculatus belongs to Holothuroidea of Echinodermata,which is one of the economic sea cucumber species inhabiting the tropical coral reefs sea area.The large-scale breeding of S.monotuberculatus has been broken through,and some bottom enhancement experiments of S.monotuberculatus have been carried around Wuzhizhou Island sea area,Sanya.However,the lack of food sources and behavioral ecology research limit the feed research and the construction of suitable culture environment of S.monotuberculatus.In this paper,the morphological characteristics of the feeding and digestive organs of S.monotuberculatus were studied,and the procaryotic and eukaryotic food sources based on high-throughput sequencing were also researched.The main contents are as following:1.Morphology and histology of feeding and digestive organs in juvenile sea cucumber S.monotuberculatusIn this study,the morphological characteristics of the tentacles and the tissue structure of different parts of the intestine in juvenile S.monotuberculatus were examined.The results showed that the mouth is surrounded by 20 branched equal length tentacles,and each tentacle was composed of a stalk and some peltate tentacles with five or six ramified processes.The main branches keep forming secondary and tertiary branches,and on the ventral side of the branches,there are many mastoids,and 2-3 mastoids(69.99±20.97 μm)are mostly gathered together to form mastoid clusters(104.99±24.15 μm)which is suitable for the uptake of fine sediment particles.Histological analysis showed that the thickness of mucosa in foregut(127.42±102.09 μm)and midgut(114.10±35.47 μm)are significantly higher than that in hindgut(18.18±8.38 μm)(P<0.05).The thickness of submucosa in hindgut were significantly higher than that in foregut and midgut(P<0.05).There is no significant difference in the muscular and serosal layer thickness among the three sections of the intestine.The villi length and density of the foregut and midgut were significantly higher than that of the hindgut,and the crypt depth of the hindgut was significantly lower than that of the foregut and midgut(P<0.05).The results suggested that the digestion and absorption in S.monotuberculatus mainly occurred in foregut and midgut,and the secretion function of the hindgut is comparatively high.2.Eukaryotic food sources of juvenile sea cucumber(S.monotuberculatus)in Wuzhizhou Island sea areaIn this study,the eukaryotic composition of gut contents and sediments in the surrounding environment of juvenile S.monotuberculatus were analyzed based on 18SrRNA gene V4 loci sequences and high-throughput sequencing.Gut group sequences were assigned to 20±2 phyla,and those in sediments group were assigned to 19±4 phyla.Diatomea(41.61±11.10%),Arthropoda(24.27±12.17%),Mollusca(7.89±11.71%),Apicomplexa(3.71±2.32%),and Chlorophyta(2.17±0.28%)are the dominant phyla in gut of S.monotuberculatus.A total of 274 genera were obtained from all the samples,of which Psammodictyon,Arcocellulus,Amphidinium and Protaspis have the relatively high contents in gut.The clustering tree UPGMA and heatmap figure showed that the gut and sediment samples have significant differences in eukaryotic food sources(P<0.05),and this result suggest that S.monotuberculatus could feed selectively.The results could provide references for feed research and the construction of the suitable breeding environment of S.monotuberculatus.3.Procaryotic food sources of juvenile sea cucumber(S.monotuberculatus)in Wuzhizhou Island sea areaIn this study,the 16SrRNA gene sequences and high-throughput sequencing were used to study the prokaryotic food sources of juvenile sea cucumber S.monotuberculatus.Gut group sequences were assigned to 26±3 phyla,and those in sediments group were assigned to 37±1 phyla.Proteobacteria(63.51±7.12%),Actinobacteria(11.24±3.58%),Bacteroidetes(8.23±4.89%),Cyanobacteria(3.65±2.23%)and Verrucomicrobia(2.58±2.16%)are the dominant phyla in gut samples of S.monotuberculatus.A total of 264 genera were obtained from all the samples,of which Ruegeria,Ilumatobacter,Actibacterium,Woeseia,Halioglobus and Dinoroseobacter have the high contents in gut samples.The clustering tree UPGMA and heatmap figure showed that the gut and sediment samples have significant differences in bacterial community composition(P<0.05),which indicate selective feeding of S.monotuberculatus from the perspective of bacterial community composition.