The data set contains compound-specific carbon isotope measurements of the lipid biomarkers sitosterol and cholesterol in the gutless annelids Olavius algarvensis and Olavius ilvae, which were collected from the sediment by scuba diving in the Bay of Sant' Andrea on the Island of Elba in Italy. Further, compound-specific carbon isotopes of sitosterol was measured for the seagrass plants Posidonia oceanica, which were collected at the seagrass meadows from the same site where the gutless annelids were collected. For Posidonia oceanica, leaves and rhizomes were measured separately for their compound-specific δ13C values . Finally, porewater samples were collected near the seagrass meadows and filtered porewater samples were measured for the compound-specific δ13C values of cholesterol and sitosterol as well. Generally, sitosterol is known as lipid biomarker of plants, whereas cholesterol is produced by animals. In the studied gutless annelids, however, sitosterol is the most abundant sterol, accompanied by cholesterol. The aim was to find out if the gutless annelids aquire their sterols from the environment by passive diffusion, or if they can produce sitosterol de novo. The δ13C values of sitosterol in the gutless annelids was ranging from -38 to -36‰, whereas sitosterol in the seagrass and porewater was between -30 to -29‰ and -19 to -15‰, respectively. The difference in the isotope signature of the sitosterol in both Olavius species and their environment (Seagrass and Porewater) excludes, that the gutless annelids aquired their sterols form the environment. Instead, the gutless worms get all their nutrition from chemosynthetic bacterial symbionts, which is reflected by the bulk carbon isotopic composition of the worms (-31‰). The δ13C values of the cholesterol and sitosterol varies by 1 to 10‰ to bulk biomass in the gutless annelids. It is hypothesized, that the worms synthesize their sterols de novo, using carbon sources from their chemosynthetic symbionts.

The data file contains δ13C values of sitosterol and cholesterol, which were measured in duplicates on the GC-IRMS. The resulting sterols were identified on a Thermo Electron Trace DSQ II coupled gas chromatograph mass spectrometer (GC-MS). The GC-MS was equipped with a 30 m HP-5 MS UI fused silica capillary column (0.25 mm diameter, 0.25 μm film thickness). The carrier gas was helium. The GC temperature program was as follows: 60°C (1 min), from 60°C to 150°C at 10°C min-1, from 150°C to 325°C at 4°C min-1, 25 min isothermal. Identification of compounds was based on retention times and published mass spectral data. Compound-specific carbon stable isotope compositions of sterols were measured on a gas chromatograph (Agilent 6890) coupled with a Thermo Finnigan Combustion III interface to a Finnigan Delta Plus XL isotope ratio mass spectrometer (GC-IRMS). The GC conditions were identical to those described above for GC-MS analyses. All sterols were corrected for the additional carbons introduced by derivatization with BSTFA. The standard deviation of the isotope measurements was < 0.8‰.