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These sponges form a yellow or orange cushion, up to 13 cm in diameter. The cushion is covered by tapered, sometimes semi translucent papillae. Sometimes a few papillae grow significantly bigger than the others.


The sponges are usually observed on rocky substrate, deeper than 15 meters.


These sponges are frequently reported from the British Isles and Scandinavia.


There seems to be some confusion regarding the classification of the most common Polymastia species in the North-East Atlantic. Two species, P. boletiformis and P. mammillaris, are mentioned among my sources. According to [1] and [2], the two are distinguished by the shape and degree of transparency of the papillae. P. boletiformis is claimed to have steeply tapered, opaque papillae.  In contrast, the papillae of P. mammillaris are gradually tapered and often semi translucent. In the pictures below, both forms are presented. Sponges of Britain & Ireland mentions only one species, P. boletiformis. The pictures presented there, seem very similar to the pictures of P. mammillaris in [1]. To add further to the confusion, the Marine Species Identification Portal, claims that P. mammillaris is a third species and that the name is often misused to identify P. penicillus. P. robusta is a synonym for P. boletiformis.

A bit confused? I suspect many biologists are too.