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The parasitic dinoflagellate Amyloodinium ocellatum (AO) is known to cause lethal outbreaks at aquaculture facilities, resulting in nearly 100% mortality events in just a few days (Byadgi et al., 2021;Nozzi et al., 2016). Consequently, in recent years, AO has become a major concern in marine fish farming (Francis-Floyd & Floyd, 2011). AO can infect many types of fishes, such as the gilthead bream (Sparus aurata Linnaeus) and seabass (Dicentrarchus labrax) (Alvarezpellitero, 2008;Paperna, 2010). Mature trophonts detach from the host and form motile dinospores. Briefly, reproduction occurs via binary fission, and actively swimming dinospores were released to infect new fish hosts (Diansheng et al., 2015). Considering the impact of parasites on aquaculture, AO is considered to be the most pathogenic protozoan parasite, affecting the culture of marine and brackish water fish (Woo, 2006).

The silver pomfret (Pampus argenteus) is a widely distributed and economically important marine fish in the Indo‐Pacific. In this study, we acquired the second generation of wild P. argenteus by artificial breeding and further studied the testicular development and ultrastructure of spermatogenesis. The results of gonadosomatic index (GSI) showed the spawning period of this marine fish was from April to June. Besides, through morphological analysis, we found that P. argenteus had an anastomosing tubular testis surrounded by a layer of tunica albuginea, in which spermatogenesis occurred in cysts where the synchronous germ cells were completely surrounded by the cytoplasmic projection of Sertoli cells. Meanwhile, based on submicroscopic characteristics, the germ cells are classified into nine different types. During the ontogenesis of testis, both the early stage of spermatogenesis and sperm were observed in P. argenteus. At sperm maturation stage, different types of spermatozoa and activation of sperms occurred non‐synchronously in the tubules. Cytoplasmic bridges also were observed among synchronous germ cells within the cysts, suggesting an interrelated and differentiated relationship among these germ cells.