The occurrence and feeding of a critically endangered shark species, Oxynotus centrina in the Sea of Marmara

Abstract

The angular roughshark, Oxynotus centrina (Linnaeus, 1758), is a rarely encountered deep-sea demersal shark species. It is listed as “Critically Endangered” in the IUCN’s regional assessment for the Mediterranean. Like other sharks and rays, the angular roughshark is subject to bycatch in demersal trawl and longline fisheries. This study contributes new knowledge on: i) Its occurrence, size, and sex information, and ii) its feeding ecology in the Sea of Marmara. Data were collected on total length (TL), total weight (TW), and sex from unpublished research survey reports performed intermittently from 1994 to 2020. According to the data, O. centrina was mainly recorded from the muddy sand bottom type in the southwest region of the Marmara Sea, which has higher biodiversity of macrobenthic species than other regions. Different methodological approaches were conducted to understand its feeding, such as DNA metabarcoding and isotope analysis to identify its prey spectra and assign a trophic level. Sampled specimens were full of liquid in their stomachs without any discernible visually identifiable prey items. The metabarcoding analyses were also unsuccessful in identifying any prey items. Trophic position calculation based on nitrogen isotope in muscles highlighted that O. centrina has the highest trophic position compared to the other sharks and rays in the Sea of Marmara. Although it was found mainly feeding on polychaetes and sipunculids species from previous studies, our δ15N values made us cautious about the possibility of this species feeding on low trophic level benthic invertebrates. Since we could not observe the presence of shark vitellus in the stomachs and could not succeed in DNA identification for prey, our study highlights high δ15N values similar to top predators for O. centrina. We conclude that this species strategically feeds mainly on shark eggs to fulfill its nutritional requirements with minimum energy expenditure in line with its slow-moving behavior and mouth morphology.