Unravelling the trophic ecology of the ocean sunfish Mola mola in the western Mediterranean Sea through integrated approaches

Abstract

Understanding the trophic ecology of marine megafauna is essential for elucidating their ecological roles and supporting effective conservation strategies. The ocean sunfish ( Mola mola ) is the second-largest bony fish worldwide and has a broad distribution, including the Mediterranean Sea. Despite its size and widespread occurrence, many aspects of its biology remain poorly understood. Traditionally regarded as an obligate feeder on gelatinous zooplankton, recent evidence instead points to a more generalist feeding strategy. Here, we applied a multi-method approach integrating stomach content analysis, DNA metabarcoding, and stable isotope analysis to investigate the feeding ecology of ocean sunfish in the western Mediterranean Sea. Our results revealed that, indeed, ocean sunfish mainly consume gelatinous zooplankton, particularly siphonophores such as Chelophyes appendiculata . However, substantial arthropod, mollusc and chordate contributions were also detected by molecular or observational methods, suggesting a more generalist diet. Stable isotope analysis showed high δ 15 N values compared to other sympatric marine predators reported in the literature, while δ 13 C values showed that ocean sunfish feed extensively in oceanic/pelagic environments. Our results suggest that ocean sunfish in the western Mediterranean Sea occupy an intermediate trophic position but are isotopically segregated from other sympatric marine predators, reflecting a distinct trophic niche. Our integrated approach shows that ocean sunfish play an important role as intermediate predators, facilitating energy transfer from low pelagic trophic levels to higher-level predators, and potentially contributing to the regulation of gelatinous plankton populations.

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