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Male members of the seaweed pipefish, Syngnathus schlegeli, incubate eggs in the brood pouch located on the tail. The eggs are directly spawned into the brood pouch by inverted copulation after intensive courtship, and the fertilization takes place in the brood pouch. During gestation, the brood pouch is filled with viscous fluid that seems to be of maternal origin, indicated by the absence of mucous secretion of the brood pouch epithelium. The spermatozoa are considered to swim in viscous ovarian fluid during fertilization. These findings indicate that the environment for fertilization is equivalent to that for internal fertilization. The pipefish spermatozoa had a bullet-shaped nucleus (3×0.6 μm), a spiral mitochondrion and an elongate flagellum (ca. 85 μm) with centrioles embedded in deep basal fossa. Based on the morphological features, the pipefish spermatozoon may be categorized in introsperm (internally fertilizing sperm). The spermatozoa swim straight by beating the entire length of the flagellum (84.1±43.8 μm/sec, ±SD, n = 3). The number of spermatozoa in the testis was extremely small (1–2 nuclei per whole transverse sectional area). The mode of fertilization is considered to enable the reduction of the spermatozoan density without deteriorating the success of fertilization. Apart from the typical spermatozoa, another type of spermatozoa with the head about 3 times as large as that of typical spermatozoa was observed. The atypical spermatozoa swim in circles (45 turn/min). Possible natures of the atypical spermatozoa are discussed.

Male Internal Fertilization and Introsperm-like Sperm of the Seaweed Pipefish (Syngnathus schlegeli)