A window into the ghost shark's life: eye lenses reveal a biphasic trophic strategy driven by neonatal generalism

Holocephalans, or ghost sharks, are ecologically significant but poorly understood cartilaginous fishes facing intense yet poorly quantified pressure from fisheries. Effective management is hindered by a critical knowledge gap: the continuous, lifetime trophic development of individuals remains uncharacterized. While conventional methods provide dietary snapshots, an individual's complete life-history strategy has remained elusive. Here we tested the hypothesis that the plownose chimaera ( Callorhinchus callorynchus ) undergoes predictable ontogenetic shifts in its isotopic trophic niche and habitat use. We present the first continuous reconstruction of a holocephalan's life history by analyzing the isotopic archive preserved within concentric layers of the eye lens. Our analysis reveals a complex, biphasic trophic strategy. Counterintuitively, neonates exhibit an exceptionally broad, generalist isotopic niche, a likely strategy to minimize competition with adults during a vulnerable stage. This niche contracts sharply after the first year, marking an abrupt transition to a more specialized foraging strategy. Subsequently, individuals undergo a gradual, lifelong increase in trophic position and a systematic shift toward more pelagic habitats, culminating in a decoupled niche expansion where adults broaden their foraging habitat without altering their core trophic level. These findings deliver a quantitative framework for the ecosystem-based management of this and other data-poor species. By identifying distinct nursery habitats and clarifying the shifting ecological role of the adult population, our work provides the detailed biological information necessary to advance conservation strategies.