Urechis unicinctus has ventral and caudal chaetae.To explore the structure and composition of the chaetae of U.unicinctus,the internal and external morphology of the chaetae were observed via scanning electron microscopy.The main components were analyzed via Fourier transform infrared spectroscopy and X-ray diffraction.Based on existing transcriptome data,the gene types and potential pathways related to their metabolism were ana-lyzed.The relationship between the structural characteristics of chaetae and age was preliminarily analyzed.The results showed that U.unicinctus usually had 2 ventral chaetae and 8-13 caudal chaetae.The ventral chaetae were slender,and their curvature was large;conversely,the caudal chaetae were thick and short,and the curvature was gentle.With increasing age,the length and diameter of the chaetae increased.In addition,the older the age,the more obvious the nodular features on the surface of chaetae.The chaetae were composed of chitin microfibers ap-proximately 1 μm in diameter,which were longitudinally arranged in an inclined layered structure.The average width of each layer was(172±27)μm.Component analysis showed that the chaetae were mainly composed ofα-chitin and contained protein components.Through bioinformatics analysis,enzymes related to chitin metabolism were found in U.unicinctus,and the potential metabolic pathways were speculated.The results of this study provide a theoretical basis for determining the age and growth status of U.unicinctus.
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