Body irregularly massive to digitate (up to 20–30 cm in diameter and height). Colour bright yellow in vivo and dramatically changing in a few minutes after collection or preservation (both alcohol and formalin, but also in dry conditions) into a very dark violet or most frequently pure black. Evident oscules on the top of sponge body or digitations. Sponge body surfaces seasonally covered by thin outgrowths (asexual propagules) up to 5 cm in length and 1 cm in diameter; outgrowths are lost by the mother-sponge as propagules at the end of summer. Consistency firm and fleshy. Surface smooth to slightly conulose, showing a fine (but evident) superficial fibrous network. Skeleton fragile, with fibres of a single dimensional class (80–150 µm) arranged in a regular three-dimensional scaffold. Fibre structure laminar with a large axial core (30–70 µm) inconspicuous in dry condition.