Characterization of deep-sea sponge ground (Asconema setubalense) using structure from motion methodology.

The hexactinellid sponge Asconema setubalense Kent, 1870 is a deep-sea species characterized by its expansive cup-shaped morphology, which contributes significantly to the three-dimensional complexity of the marine ecosystems. This sponge forms grounds that offer protection to pelagic organisms and juvenile stages. Despite its ecological relevance, there is a lack of information on the ecology, behavior and population structure of this species. The research aimed to conduct a multitemporal analysis of this sponge in the Aviles Canyon System over a ten-year period, focusing on spatial distribution and abundance. Changes in fishing pressure were estimated using the presence of fishing gears as a proxy. Additionally, the study sought to provide a detailed morphometric description through 3D photogrammetric reconstructions based on the latest data. The multi-temporal analysis revealed a subtle increase in sponge density, particularly at depths ranging from 320 to 390 m, exhibiting an irregular spatial distribution in 2022, with maximal values of 0.08 individuals/m². Despite a small decrease on the loss of fishing gears in the overlapping area between samplings in 2012 and 2022, there was not a clear indication of a decline in fishing pressure over the years. Notably, more sightings of fishing gears were found in transect IC222TV_16 (0.07 gears/m²) than in IC222_TV02 (0.04 gears/m²) in 2022, suggesting potential spatial preferences for fishing activities. Visual analysis of temporal populations revealed an increase of 7% in specimen perturbations over ten years, with the population in IC222_TV16 being healthier (9% of the individuals presenting severe deformations) than in IC222_TV02 (40%). High-density fishing gear locations coincided with areas inhabited by sponges displaying the highest perturbation levels in both transects. Morphometric analysis using data from 2022 indicated a prevalence of individuals with heights concentrated between 0.18 and 0.38 m, osculum and flounce diameters reaching 0.4 and 0.56 m and osculum and flounce surfaces of 0.02–0.04 and 0.06–0.19 m² respectively. Over 80% of measured specimens exhibited a high degree of asymmetry. Strong correlations were observed between heights and osculum and flounce surfaces, but external factors may be included for explaining wall deformities. These findings contribute valuable insights into the characterization of A. setubalense, serving as a foundation for future research in the area. Moreover, this work highlights the promising potential of photogrammetry as an efficient tool for monitoring of vulnerable marine ecosystems (VME) and marine protected areas.