In situ observation of sponge trails suggests common sponge locomotion in the deep central Arctic

Located 10° further north than any sponge ground yet reported, the population, primarily comprising Geodia parva, G. hentscheli and Stelletta rhaphidiophora individuals, covers the great majority of the Langseth Ridge from ∼1,000 m upward, to a shallowest peak of 580 m depth, at very high abundance1. Sizes of individual sponges varied, with a median diameter of 17 cm (± 0.10 cm SE, n = 10,839, and IQR 11–25 cm); the maximum diameter observed was 110 cm and the smallest were just over 1 cm. This size range equates to a median wet weight of 1.4 kg (± 0.12 kg SE, n = 10,839; IQR 0.4–5 kg; Figure S1). Sponges are opportunistic feeders; although they efficiently retain particulates by filter feeding, dissolved organic matter in seawater often represents the main food source2. The low productivity and nutrient fluxes that characterize this region — due to high latitude, permanent sea ice cover, and distance from land — raise the question of how this abundant biomass is maintained. We analyzed the many trails of interwoven spicules across the seamount peaks. Trails were observed to be several centimeters in height and up to meters in length, often connected directly to living sponges. These trails were particularly visible in areas of low sponge density, that is, where trails crossed bedrock in areas sparsely covered by sediments or organic debris (Figure 1A–E and Video S1). Spicule trails in areas of denser sponge aggregation exhibited a higher vertical thickness than observed elsewhere (Figures 1F and S1A, and Video S1). Our observations also suggest that the spicule trails correlated with the distribution of smaller juvenile sponges (Figure S1A).