Among the filter-feeding organisms and important bioeroders, sponges are one of the most scarcely known. Maybe it is because they are immobile, unattractive, they seem like squishy rocks and that is it. Apart from some species that are inhabited by microbes, such as the cyanobacteria, sponges are some really strange coloured individuals. As if it is not enough, they are also poorly appreciated in terms of food source. In fact, they are deterrent due to the production of specific chemical compounds (secondary metabolites) which work as a defense mechanism. To have a better idea, there are 71 species of Caribbean sponges that produce a deterrent against the reef fish Thalassoma bifasciatum.

Not very nice these sponges, are they? They sit still, are ugly, push away whatever and whoever comes around and, to top it off, they also stink. So what is the deal with them anyway?

Being sessile, sponges have to find the right substrate and bore into it for a greater attachment and stability. On a calcareous substrate, they do it both via mechanical and chemical ways by developing cytoplasmic extensions (cells move their cytoplasm towards the extremities, elongating themselves) which protrude into it, cutting out small particles called “chips.” The cell extensions probably release an acidic substance which dissolves the substrate, enabling a more efficient penetration. To maximise the efficiency, these sponges incorporate the calcareous chips.

Their body is made out of a cortical region, the ectosome, and an internal region, the choanosome. Focusing on the external region, flattened cells called exo-pinacocytes surround dense bundles of collagen fibrils. Chondrosia reniformis, a demosponge, incorporates exogenous material (grains and spicules) into the ectosome to strengthen its skeleton. This species is an exception among the demosponges because it does not produce opaline spicules for a stronger body.

Instead, it takes up the substrate chips and through the upper ectosome selectively chooses the minerals essential for its growth. The most abundant calcareous material gets eliminated, whereas the siliceous particles get engulfed. On the contrary, the lower ectosome, settled on the calcareous substrate, partially incorporates the predominant calcareous fragments. This has been proven to show the sponges body polarity. In other words, the upper part of the body engulfs quartz and silicates, while the lower part collects calcareous particles so that the body can successfully anchor itself to the substrate. Curiously enough, this particular body polarization could be explained by the expression of a silicate-dependent gene for collagen.

So hypothetically, Chondrosia reniformis is able to metabolise quartz and silicates, such as diatoms’ frustules (outer silicious involucres), and produce collagen. Under controlled conditions in sponge aquaculture, we could be able to produce collagen for pharmaceutical purposes. Sure one of the almost immediate questions is the cost of silicates and cultivation, although before that it is better to begin with why does Chondrosia reniformis use quartz and why, in particular, is the gene for collagen dependent on the silicate concentration?