We examined the growth response of a brackish snail (Hydrobiidae) against multiple temperature treatments in a mesocosm located beside the Alfred Wegener Institute Wadden Sea Station on Sylt (55°01′19.2″N, 8°26′17.7″E). Bulk sediments were collected south of Pellworm (54° 31' 55.83"N, 8° 42' 40.36"E) at low tide on March 22, 2022, transferred to mesh-lined crates and introduced to mesocosm tanks (170 cm × 85 cm × 1800 L). Experimental warming treatments were conducted using three heaters per tank (Titanium heater 500 W, Aqua Medic, Bissendorf, Germany). The full specifications for the mesocosms are already published (Pansch et al., 2016). Throughout the experimental warming period, four sampling events (March 30, April 25, May 24, June 20) were conducted to core sediments. Sediment cores were washed and sieved (1mm mesh size) to disaggregate infauna. Individuals were separated for the common hydrobiid mudsnail, which were subsequently imaged in groups on a typical petri plate under stereomicroscopy. A semi-automatic object segmentation and size measurement approach was developed to rapidly differentiate and measure individuals from images. Segmentation was highly accurate and precise against manual length measurements (end-to-end; mm) collected in ImageJ for 4595 snails. Scaling the segmentation method across the full dataset estimated >40,000 snails and presented a complex species-specific response to warming. The enclosed dataset represents all raw, processed, and segmented images (n= 3201) produced by this study.

3201 .tif files (2880 x 2048 px, RGB, 96 DPI). Equal number of raw, processed, and segmented files. The naming convention follows: Species_Sample_Treatment_Event_Tank_ImageNumber_Type.tif