Testis structure and symplastic spermatid formation during spermatogenesis of pipefishes

The pipefishes Syngnathus abaster and S. acus have paired testes of atypical organization. Each testis is a hollow tube consisting of a single germinal compartment of the tubular type. During the reproductive period, the germinal epithelium consists of small spermatocysts containing spermatogonia or primary spermatocytes. Cysts of older germ cells, such as secondary spermatocytes and spermatids were never observed. Developing symplastic spermatids were found in the lumen of the tubule together with mature sperm and large droplet-containing cells. Most of the spermatids were giant cells with four nuclei at the same developmental stage. Symplastic spermatids, which presumably form by nuclear division not followed by cytokinesis, are a stage of spermatogenesis in pipefishes.