Reproductive Morphology of Oarfish (Regalecus russellii).

Reproduction is a critical aspect of understanding the biology of fishes. Relatively little is known about oarfish (Regalecus russellii) reproduction; however, strandings of dead animals have provided a rare opportunity to investigate the gonadal morphology of four fish: two females and two males. A female collected in June 2015 (4.32 m TL) had bifurcated ovaries 2.14 m in length and 2.14 kg. The gonadosomatic index (GSI) was 11.8% and the fish was determined to be spawning capable/spawning reproductive phase. A female that stranded in Sept. 2015 (5.20 m TL) had bifurcated ovaries 1.43 m in length and 1.28 kg with a GSI of 1.55%. The Sept. female was in a regressing phase of reproduction. A male collected in Aug. 2015 (4.30 m TL) had 64.7-cm-long testes that weighed 40.1 g. The GSI was 0.05% representing a regressing phase of reproduction. A male collected in Nov. 2015 (4.10 m TL) had testes 104.0 cm in length and 467.0 g with a GSI of 0.59%. The Nov. male was in a spawning/spawning capable phase of reproduction. We described ovarian follicles and sperm cells based on size classes and cytological characteristics. We concluded that oarfish are likely batch spawners that undergo periods of regression after a spawning event or season. While this study is not complete with respect to the annual reproductive cycle of oarfish, it markedly contributes to our overall understanding of this rare, mesopelagic fish. Anat Rec, 300:1695-1704, 2017.