Acarnus is a genus of sponges belonging to the family Acarnidae.

[2] Species are non-fistulose sponges with encrusting to massive growth forms.

[3] The choanosomal skeleton is isodictyal (triangular mesh in which each side is one spicule long), isotropic (no differentiation into primary or secondary tracts, lines or fibers), or anisotropic (primary and secondary tracts, lines or fibers).

In encrusting species, it may occasionally be plumose (skeletal structures radiate from primary fibres or spicule tracts) or hymedesmioid (monoaxon megascleres arranged singly with the heads fixed to a spongin base and the points directed outwards).

[3] The skeletal tracts are covered with smooth styles, which may be spined by cladotylotes.