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The hexactinellid sponge Asconema setubalense Kent, 1870is a large bathyal species of the North Atlantic Ocean with a funnel-like body and very large osculum. Populations of A. setubalense have a three-dimensional structure and increase the complexity and biodiversity in their habitat, and are therefore considered to be a habitat-forming species. Nevertheless, at present the information on the extension, biomass, density, population structure, and ecology of this species is scarce, and it could be susceptible to the longline fishing practices that take place in the Canary Islands. The main objectives of this study are to define a functional and accurate methodology to measure specimens of A. setubalense by comparing differences in users, techniques, and morphometric measurements; to describe the height-size relationship; to present the population size structure of the species, and to establish a relationship between the size of A. setubalense and the environmental variables that can be found at the "Banco de La Concepción" seamount (BC). The obtained results suggest that surface area is the most reliable measurement to define the size of this species, whilst also showing a clear correlation with the height of the species. The selected methodology has made it possible to measure the surface area of 1035 specimens and thus obtain the size structure of the population of A. setubalense in BC. The GAM model that was used to analyse the relationship between the size of A. setubalense and the geomorphologic variables of BC, shows areas where there is a high probability of finding large specimens of the species. The results of this study greatly enhance the knowledge of this species and its habitat, and should be considered in future conservation directives, or in the development of indicators to show the good environmental state of habitats. Additionally, the study improves analysis methodology that, with the appropriate morphometric measurements, can favour the development of future studies of this species, and indeed others with a similar morphology or growth pattern. • Asconema setubalense is one of the main habitat-forming species in the BC area. • Zenithal images are appropriate to measure size of some benthic species. • Area is the most reliable measurement to define the size of this sponge. • Area parameter presents a clear correlation with the height of the species. • The size structure of the species shows a young population and GAM model provides areas distribution size of the sponge. [ABSTRACT FROM AUTHOR]