Field collection. Adults of Pleuromamma xiphias were collected on the Atlantic Meridional Transect Cruise 24 (AMT24), between 4 Oct and 28 Oct 2014 (Fig. 1, Table S1). Animals from 20 stations along the cruise transect between 31°N and 43°S were included in this study. Oblique bongo tows (200 mm nets) were conducted between on average 310 m and the sea surface. Upon recovery at the surface, plankton from one net was diluted into chilled seawater, and sorted immediately. At some stations, plankton was split quantitatively in a Folsom plankton splitter prior to sorting. Adult females of Pleuromamma xiphias were sorted live under the microscope, flash frozen and stored in liquid nitrogen at sea, followed by transfer to -80°C in the laboratory. Only adult females were included in this study, in order to ensure comparable measurements across the oceanographic transect. Adult males have larger body size than females (Bradford-Grieve 1999), and are expected to have higher carbon and nitrogen content as well as dry weight relative to females.

Cryopreserved Pleuromamma xiphias females were stored on ice during all initial handling steps, with 5-10 animals processed at a time. Prosome length (mm) of each animal was measured in right lateral view. Individual females were then blotted dry and transferred to pre-weighed tin capsules for wet weight and dry weight measurements. Dry weight: copepods were individually weighed using a Mettler Toledo UMX2 ultramicrobalance, dried at 60°C for 24-72 hours, and weighed again.

Carbon and Nitrogen content: One or two individual copepods were included in elemental and isotope analyses, depending on dry weight. The C content, N content and bulk isotope composition of P. xiphias was determined using a Costech elemental combustion system (Model 4010) coupled to a ThermoFinnigan Delta Plus XP Isotope Ratio Mass Spectrometer (IRMS) through a Conflo IV interface. Isotope values are reported in standard δ-notation relative to the atmospheric N2 standard (AIR) as δ15N (‰) = [(15N:14Nsample / 15N:14NAIR) - 1] x 1000 or relative to Vienna Pee Dee Belemnite (VPDB) as δ13C (‰) = [(13C:12Csample / 13C:12CVPDB) - 1] x 1000. To ensure accuracy, glycine and ground tuna reference samples with well-characterized δ15N values were analyzed every 10 samples. The standard deviation of samples analyzed in duplicate was ≤ 0.2‰. Body size and weight measurements were made on a total of 237 animals, with 106 carbon and nitrogen content and isotopic measurements made across the transect.

Condition index (CFI) defined as CFI = 1000 x DW/(PL)3 (Ikeda et al. 2006). Stable isotope values measured on single individuals or in pools of N=2 (as noted in the SIA identifier column).

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