Unique aspects of reproductive energetics and endocrinology among Chondrichthyes

Chondrichthyan fishes (sharks, rays, skates, and chimaeras) boast an array of at least ten reproductive strategies ranging from oviparity (egg-laying) to viviparity (live-bearing). Although reproduction in this taxon has been a topic of interest since the early 1900s, ongoing research continues to uncover nuances in the way these fishes reproduce. Indeed, over time, the focus on classifying reproductive strategies has shifted toward an interest in female maternal energetic strategies rather than discrete categories. This article presents an updated array of reproductive strategies, focusing on a spectrum of maternal energetic inputs. It underscores that the classification of reproductive strategies is becoming increasingly complex as more species are studied. This article also highlights gaps in chondrichthyan reproductive endocrinology, noting that most basic research has been limited to three sex steroid hormones and a few model species. Further work is needed to deepen the understanding of reproductive endocrine mechanisms, which can then be applied to reproduction in wild populations for improved management and conservation. Finally, the article outlines new avenues to address the long-standing gaps in the reproductive literature of chondrichthyan fishes.