Description

 Hemimycale columella is a sessile sponge that forms a thick encrusting layer of at least 1 cm, and reaches widths of up to 30 cm. It has a fairly soft texture, a skeleton of siliceous spicules (spines) and spongin fibre. The surface of Hemimycale columella has a honeycomb appearance created by numerous cavities, the rims of which are supported by the spicules. Within each of the cavities is at least one small, inhalant pore (oscula) of up to 0.1 cm in diameter. In general the oscula are infrequent. Hemimycale columella is distinguished from similar species by its large megascleres (spicules) that are club-shaped at the ends, and by the absence of smaller spicules.

In its encrusting form Hemimycale columella grows outwards from the edge of the colony. However, this species also exhibits an irregular massive form in which growth takes place on the surface. When disturbed, the oscules contract within approximately 15 seconds - (Forester, 1955). Both reproduction and growth are dependent on the time of season. Hemimycale columella is a filter-feeder, using internal choanocytes to circulate water through its pores.

The taxonomy of Hemimycale columella is unclear with some authorities placing it in the Haliclonidae family, others in the Biemnidae with Biemna variantia. This species can be mistaken for Phorbas fictitus which lacks a light-coloured circle around the depression craters.