Hadromerid sponges do not have spongy protein fibers (spongin). They just have simple, pin-like or star-shaped skeletal parts (spicules) made of silica. Yet, their texture is quite hard. They get their firmness from proteins called fibrillar collagens (which also make up fibrous tissues such as tendons and ligaments of humans). While you can squish some sponge skeletons and they will spring back into shape, a hadromerid skeleton is more likely to break into pieces, especially a dead one that has dried out. A hadromerid would not make a good bath sponge.

Sponges host many organisms living on their surface or even inside them. They make great homes because they do not move much, may live a long time (some for hundreds of years), and have hollow bodies with lots of cavities and canals. A single sponge might provide a condominium for organisms as tiny as bacteria and as large as sea stars. The nature of the relationship between the host sponge and its occupants varies. A "parasitic" occupant, such as a mite that lays eggs inside a freshwater sponge, harms the sponge. In many cases, though, a sponge has commensal occupants, who benefit from living there but do not hurt the sponge. The sponge may even have a "mutualistic" relationship, such as with shrimp that keep it clean by living in its canals and eating debris or with bacteria that make toxic chemicals that repel sponge predators. In any case, a sponge is rarely alone.