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Abstract: Hymeniacidon sinapium (De Laubenfels, 1930) (Demospongiae, Halichondrida, Halichondriidae) collected from China''s Yellow Sea has been a model sponge species for extensive studies of in vitro cultures of sponge cells and explants in our group since 2002. To better understand its reproductive cycle, the aim of this study is to elucidate the temporal changes in microbial community, the spiculogenesis activity (in terms of silicatein gene expression) and the stress response (in terms of hsp70 gene expression) during its annual life cycle. Histological studies by light microscopy revealed that H. sinapium is a characteristic seasonal species with four distinctive developmental stages of resuscitation, bloom, decline and dormancy during its annual life cycle. The sexual reproductive processes were observed from August to October (summer), with all the reproductive elements (oocytes, sperm cysts, embryos and larvae) documented. There is no direct correlation between seawater temperature and gametogenesis. The gene expression of silicatein and hsp70 was investigated by quantitative real-time RT-PCR during its annual life cycle. Silicatein expression correlated with the histogenesis and reached the highest value in late July (bloom stage). While hsp70 expression correlated with the seawater temperature and also reached the highest value at the same time. We hypothesize that after reaching the maximum biomass growth, sponge should adjust energy distribution to meet the need of gametogenesis and embryogenesis in the reproductive season. Bacterial community variability in a life cycle ranged from 18% to 98% (dice similarity of each two samples) using DGGE, and formed four distinctive clusters consistent with seasons. After sequencing, twelve typical bands from DGGE were compared to GenBank and basically clustered into five phyla: Actinobacteria, α-Proteobacteria, γ-Proteobacteria, Bacteroidetes and Cyanobacteria. This study reported, for the first time, high temporal variability in silicatein, hsp70 and bacterial community during the annual life cycle of H. sinapium. This knowledge is significant for the development of sponge explants culture under controlled laboratory conditions and large-scale sponge seed-rearing technology. [Copyright &y& Elsevier]