Methodology:

Specimens of Astarte crenata and Ctenodiscus crispatus were collected in July 2017 (JR16006, RRS James Clark Ross; Hopkins 2018) using a 1.25 m Agassiz trawl (AGT) towed for 15 minutes at a ship speed of 1 knot, at three stations along the 30E meridian across the approximate location of the Polar Front (B13, 74° 49 N (South of the polar front); B14, 76° 50 N (approximate location of the polar front); B16, 80° 06 N (North of the polar front)). Four trawls were conducted at each station to ensure sufficient spatial replication. Ctenodiscus crispatus were found at all stations, whereas A. crenata were only found at stations B13 and B16. Fauna were sieved over a 1 cm mesh and retained and fixed in 10% phosphate buffered formalin (4% formaldehyde) prior to morphological and histological examination.

Morphology: Astarte crenata. To assess population dynamics, each individual (n = 159) was measured using a digital calliper (± 0.01mm), to record maximum shell length, height, and width. Soft tissue was removed from the shell with a scalpel and weighed (± 0.01 g). Observation of the dissected bivalves and a preliminary histological analysis revealed that this population of A. crenata do not have discrete gonads, but have germinal tissues infiltrating the visceral mass, particularly within the digestive diverticulum. This means that gonad index cannot be reliably calculated. Hence, to ensure reproductive maturity had been reached, only specimens >20 mm shell length were used for reproductive analysis. Whole animal histology was necessary, and it was not possible to calculate a gonad index or measure of energy storage in the digestive diverticulum.

Ctenodiscus crispatus. To assess population dynamics, reproductive, and digestive condition of each animal, we measured each specimen, and calculated gonad and pyloric caeca indices. We evacuated sediment within the body cavity through the mouth by applying pressure to the dorsal surface whilst rinsing with seawater. In total, 324 individuals were measured (± 0.01 mm) from the centre of the mouth to the tip of the longest arm, and from the centre of the mouth to the madreporite interradius, blotted dry and weighed (± 0.01 g). Dissection of the dorsal epithelium of 151 individual C. crispatus above 15 mm arm length revealed the pyloric caeca and gonads as discrete paired organs, which were subsequently removed from two interradial sections by dissection and used to determine the total gonad and pyloric caeca indices, i.e., the ratio of gonad or pyloric caeca mass to whole body wet weight, expressed as a percentage.

Reproductive traits: Sections of A. crenata demonstrated dense areas of gametogenesis. To ensure near maximum cross-sectional diameter was quantified, unique oocytes were measured only when a nucleus was visible. For, C. crispatus, as the nucleus remained visible across multiple 6 µm sections, oocytes were only measured when the nucleolus was visible. For comparison of oocyte sizes between each female and station (A. crenata, n = 24 (B13, n = 12; B16, n = 12); C. crispatus, n = 24 (B13, n = 8; B14, n = 8; B16, n = 8), we calculated the Equivalent Circular Diameter (ECD) by measuring the area of 100 oocytes of each female (i.e., 1200 and 800 oocytes per station). Full descriptions and additional information provided in Reed et al. 2021 Ecology and Evolution in main manuscript and supplementary information.

Data collection:

Instrumentation:

Digital vernier caliper

Balance (+/-0.000 g)

Rotary microtome

Nikon D5000 digital SLR camera

Olympus BHS (BH-2) stereomicroscope

Data quality:

Standard protocols were followed, and data entry double checked by an independent person.

Biometric measurements (i.e., arm length, interradial length, shell length, shell height and tumidity (width)) to two decimal places.

Wet weight to four decimal places.

Oocyte Equivalent Circular Diameter to two decimal places.