To look at a rock sponge, which usually has all the visual appeal of a potato, you would never guess that inside lies the Notre Dame of animal skeletons. But so it is. Here are a few:

The rock sponges (named for their notable lack of squish) build their skeletons out of tiny bits of silica called spicules. The special silica spicules of rock sponges are called desmas ("articulating choanosomal megascleres of various geometry and usually complex morphology, often secondarily modified and very irregular"). Rock sponges are found in tropical and temperate water from reefs to the deep sea, where they're often found hanging out with their good buddies and cousins the glass sponges, who build their skeletons out of silica spicules of a different sort.

They grow their spicules inside cells called sclerocytes, which form them by first building a scaffolding of organic molecules and then depositing silicon onto their surface. The shape of the spicule varies greatly by species, and the same sponge may grow several types. Here are some of the individual spicules of glass sponges, from a recent paper exploring rock sponge evolution. Some look like hairpins, some look like nails, some look like mutant jacks. All are interesting, and some look downright industrial.

Large sponge spicules visible to the naked eye are called megascleres, tiny ones visible only with a microscope are called microscleres. As mentioned above, a single sponge can make many kinds and sizes of spicules. The rock sponge Neoschrammeniella norfolki makes up to six different types of spicules including both megascleres and microscleres.

The vocabulary for these structures is almost as jaw dropping as the structures themselves. There is a veritable alphabet soup of rock sponge spicule names: anatriaenes, rhabds, oxeas, amphiasters, spirasters, microxeas, raphides, and sigmaspires among them. If you expand the scope to include all demosponges, you can add to that strongyles, tornotes, tylotes, anisochelas, acanthostyles, euasters, forceps, isochelas, oxyasters, sigmas, and sphaerasters.

And here they are.

Spicules NOT to scale and range between .01 and 1 mm. "Demospongiae spicule diversity" by Rob W. M. Van Soest, Nicole Boury-Esnault, Jean Vacelet, Martin Dohrmann, Dirk Erpenbeck, Nicole J. De Voogd, Nadiezhda Santodomingo, Bart Vanhoorne, Michelle Kelly, John N. A. Hooper - Van Soest RWM, Boury-Esnault N, Vacelet J, Dohrmann M, Erpenbeck D, et al. (2012) Global Diversity of Sponges (Porifera). PLoS ONE 7(4): e35105. doi:10.1371/journal.pone.0035105. Licensed under CC BY 2.5 via Wikimedia Commons.

I think I can guess which ones are the forceps and which are the sigmas.

Ultimately, the spicules are fused together by the sponge into a scaffolding of intricate beauty. Here are scanning electron micrographs of some more few rock sponge skeletons:

Today there are 13 families, 41 genera (the classification above species), and more than 300 species of rock sponges, but in former times the number was larger. The fossil record shows that there were at least 34 families with over 300 genera. We know this because unlike other sponges whose skeletons are made of biodegradable protein called spongin(the source of the squishy bath sponges), the hard silicious skeletons of rock sponges fossilize well, as their name implies. It was the ability to examine their fossil record and compare it to the results of genetic studies of modern species that made the group a tempting target for the scientists interested in sponge evolution who took these photographs.

The results of their study were not surprising. Although sponges were traditionally classified based on their physical appearance and spicule medley, that was because that was all scientists had to go on. As has been found for many other groups of organisms, a comparison of two sponge genes showed that the physical characteristics of these sponges are not a reliable guide to their actual evolutionary relationships. The same spicule forms seem to have evolved multiple times, and in other cases, the ability to make certain spicules has been lost repeatedly.

The result makes mincemeat of the existing taxonomy and effectively blows up the now-archaic rock sponge group "Lithistida", but that is of small consequence to them, or to our appreciation of their way of life and beauty. They will go on making magical scaffoldings on which to hang their communes of semi-independent filter-feeding cells, completely unperturbed by our frustrations in attempting to trace their family tree.

Reference

Schuster A., Andrzej Pisera, John Hooper, Monika Bryce, Jane Fromont & Gert Wörheide (2015). Deceptive Desmas: Molecular Phylogenetics Suggests a New Classification and Uncovers Convergent Evolution of Lithistid Demosponges, PLoS ONE, 10 (1) e116038. DOI: http://dx.doi.org/10.1371/journal.pone.0116038