Noninvasive and stress-free ultrasound scans were successfully conducted to determine the spawning cycle of an oviparous catshark. The ultrasound scans were conducted every day on the same catshark for approximately six months (Image 1). As well as the ultrasound scans, blood samples were taken every three days, and these showed periodic fluctuations in the levels of sex hormones that synced with the spawning cycle. Testosterone levels in the blood declined before fertilization and eggshell formation, while progesterone levels in the blood rose briefly right after (Image 2). Based on knowledge from previous daily monitoring of hormone levels in the blood, we know this elevation in testosterone occurs two days before eggshells are found in the oviduct. This suggests testosterone plays an important role in ovulation, fertilization, and eggshell formation. The results of the research help shed light on the reproductive mechanisms of oviparous and viviparous cartilaginous fish.

Journal

General and Comparative Endocrinology, 327: 114076.

Authors

Inoue, T., Shimoyama, K., Saito, M., Wong, M K-S., Ikeba K., Nozu. R., Matsumoto, R., Murakumo, K., Sato, K., Tokunaga, K., Kofuji, K., Takagi, W. and Hyodo, S. 2022.

Ultrasound conducted on the catshark, ultrasound scans.
(White arrows on the right image shows the eggs’ shells seen with ultrasound)

Fluctuation of levels of testosterone and progesterone in the blood during spawning cycle of a catshark. (The areas in color show the periods when egg shells were observed in the body, the red line shows levels of progesterone in the blood, the blue line shows levels of testosterone in the blood, and the horizontal axis is the number of days.)