Polymastia rubens is a species of demosponge

Demosponge

The Demospongiae are the largest class in the phylum Porifera. Their "skeletons" are made of spicules consisting of fibers of the protein spongin, the mineral silica, or both. Where spicules of silica are present, they have a different shape from those in the otherwise similar glass sponges...

 belonging to the family Polymastiidae

Polymastiidae

Polymastiidae is a family of demosponges found in oceans throughout the world. A useful diagnostic characteristic of members of this family is the presence of numerous surface papillae although this feature is shown by some other sponges.-References:*...

. It is only known from rocky subtidal habitats around Kawau Island

Kawau Island

Kawau Island is an island in the Hauraki Gulf, close to the north-eastern coast of the North Island of New Zealand. At its closest point it lies off the coast of the North Auckland Peninsula, just south of Tawharanui Peninsula, and about by sea journey from Sandspit Wharf, and shelters Kawau Bay...

 off North Island

North Island

The North Island is one of the two main islands of New Zealand, separated from the much less populous South Island by Cook Strait. The island is in area, making it the world's 14th-largest island...

, New Zealand

New Zealand

New Zealand is an island country in the south-western Pacific Ocean comprising two main landmasses and numerous smaller islands. The country is situated some east of Australia across the Tasman Sea, and roughly south of the Pacific island nations of New Caledonia, Fiji, and Tonga...

.

This is an encrusting sponge with a very firm texture, growing in patches up to 7 cm across. The outer layer is bright red with a brown interior. The surface is smooth apart from small wart-like papillae.