Knowledge of broodstock manipulation, gametogenesis, artificial fertilization and larval nutrition is a prerequisite to reach a large‐scale production of the polychaete Hediste diversicolor . In this study, the characteristics of oogenesis and spermatogenesis and embryonic and larval development were observed. Moreover, the effect of temperature on embryonic development and effect of diets and sediment on earlier juvenile development were tested. Mature oocytes were characterized by a spherical shape with a diameter of about 200 μm. They showed radial symmetry with a large centric nucleus surrounded by four distinctive layers. Embryo cleavage started between 4 and 10 hr after fertilization, and developed to the swimming trochophore stage after 5–6 days after fertilization at 11°C. The rate of embryonic development increased with a temperature increase from 6.1 to 21.2°C. The trochophore larvae increased in length from day 9 and commenced differentiation into 3‐setiger stage larvae at day 12. The length growth of larvae until 7‐setiger stage was 0.43 mm/day when fed with fish feed, while those fed shellfish diet and smolt sludge both grew around 0.21–0.23 mm/day. The results suggest that fish feed is a superior diet compared to shellfish diets and smolt sludge for the early larval stage of H. diversicolor .

Three different experiments were conducted to examine the oocyte development of the polychaete Hediste diversicolor in response to changing photoperiod and temperature at three different periods of oogenesis. In experiment I, worms collected during summer were reared under constant or decreasing photoperiod and temperatures to test the combined effects of the summer–autumn photoperiod and temperature transition on oogenesis. The result showed females collected during summer showed the highest oocyte growth when exposed to constant temperatures combined with decreasing photoperiod and decreasing temperature combined with constant photoperiod. In experiment II, worms collected in late autumn were under mimicked or shortened seasonal changes in photoperiod to evaluate the effect of accelerated change on oocyte growth. The result showed worms had 1.5 times faster oocyte growth rates when exposed to accelerated rate of change in photoperiod (2.5 times faster). In experiment III, worms collected in spring were exposed to different temperature regimes to examine the effect of raising temperature on the synchronization of oocyte growth and maturation. The results showed worms collected in spring showed increased temperatures will increase oocyte maturation synchronicity. It was concluded temperature and photoperiod transition can increase the oocyte development.

Polychaetes are a promising intertidal resources due to their widespread distribution, considerable biomass, and high nutrition value. The nutritional profiles of two novel tropical polychaetes species (Marphysa sp. and Halla sp.) were determined. Additionally, comparing nutritional levels of polychaetes with other species and requirements of main aquatic animals, this study aimed to explore their potential as aquaculture feed ingredients. The results revealed that both polychaetes species are nutrient‐rich with significant interspecific variation. Marphysa sp. showed higher protein, total essential amino acid (EAA) ratio (17.32 ± 0.46 mg/g), lipid, eicosapentaenoic acid (EPA; 10.99%), α‐Linolenic acid (ALA; 1.25%), ash and energy content compared to Halla sp. (p < 0.05). Conversely, Halla sp. had a significantly higher docosahexaenoic acid (DHA; 5.68%) index compared to Marphysa sp. (DHA; 2.57%). Meanwhile, the levels of macroelements (K, Ca, Na) and trace elements (Mg, Mn, Cu) also exhibited differences in the two species. Compared with other polychaetes and aquatic feed, both Marphysa sp. and Halla sp. are promising high‐quality feed ingredients, as they contain high levels of minerals, essential amino acids, and beneficial lipid profiles. This study will support the identification of alternative substitutes to meet the growing demand for aquafeed ingredients.

Siphonosoma australe, Phascolosoma arcuatum, and Sipunculus nudus are three important sipunculan species in tropical intertidal zones. In this study, the particle size, organic matter content, and bacterial community composition in the gut contents of three different sipunculans and their surrounding sediments were analyzed. The grain size fractions of sipunculans’ guts were significantly different from those of their surrounding sediments; particle size fractions < 500 μm were favored by the sipunculans. As for the total organic matter (TOM), higher contents of organic matter were observed in the guts than in the surrounding sediments in all three sipunculan species. The bacterial community composition of all the samples was investigated by 16S rRNA gene sequencing, in which a total of 8974 OTUs were obtained from 24 samples based on a 97% threshold. The predominant phylum identified from the gut contents of three sipunculans was Planctomycetota, while the predominant phylum in their surrounding sediments was Proteobacteria. At the genus level, the most abundant genus was Sulfurovum (average 4.36%) in the surrounding sediments, while the most abundant genus was Gplla (average 12.76%) in the gut contents. The UPGMA tree showed that the samples from the guts of three different sipunculans and their surrounding sediments were clustered separately into two groups, which showed that these three sipunculans had a different bacterial community composition with their surrounding sediments. The grain size and total organic matter (TOM) had the greatest impacts on the bacterial community composition at both the phylum and genus levels. In conclusion, the differences in particle size fractions, organic matter content, and bacterial community composition between the gut contents and surrounding sediments in these three sipunculan species might be caused by their selective ingestion.