Shark eggs contribute to the trophic ecology of a cold-seep chemosynthetic ecosystem

Cold seeps host oasis-type ecosystems sustained by microorganisms such as chemosymbiotic bacteria, fueled by reduced gasses like hydrogen sulfide and methane. These habitats are characterized by a wealth of carbon and nutrient sources, substantial microbial turnover of key nutrients, and unknown metabolic interactions between symbionts and their hosts. Thus, the trophic ecology of cold seeps is not fully understood. Recent discoveries of massive shark nurseries and extensive chemotones in the southeastern Mediterranean Sea (SEMS) hint at a previously unknown complexity of food webs in this habitat. To provide insights into the trophic ecology of SEMS seeps, we collected symbiont-bearing (Lamellibrachia anaximandri tubeworms, Idas modiolaeformis mussels, and Lucinoma kazani clams) and other fauna, such as eggs of Galeus melastomus sharks, Gracilechinus elegans echinoids, Clelandella myriamae gastropods, and Calliax lobata ghost shrimps, from the Palmahim Disturbance seeps (~1000-1150 m water depth, Levantine basin in the SEMS). We obtained bulk and compound-specific values (δ¹³C and δ¹⁵N), using isotope ratio mass spectrometry and compound-specific isotopic analysis of amino acids. Glutamic-acid-phenylalanine trophic position (TP_Glu-Phe) and the contribution of reworked organic matter (microbial resynthesis index, ΣV) were estimated for individual specimens. Our findings indicate a wealth of nutrition strategies and trophic interactions, as chemosynthetic productivity and external sources sustain these communities. Collagen-rich eggs of G. melastomus appear to sustain the opportunistic detritivores/carnivores such as G. elegans (maximum TP_Glu/Phe = 4.7; higher than that of G. melastomus, maximum TP_Glu/Phe = 3.8), but also supplement the chemosynthetic nutrition of Idas mussels, likely through heterotrophic symbionts.