Sponges are not what you’d normally think of as “exciting” animals. They are simple creatures that spend the entirety of their adult lives sitting around, patiently sifting immense amounts of water for microscopic food. The closest most of them get to “doing” anything is popping out a few babies every now and then. (Exception: deadly shrimp-killin’ predators :o) However, these (mostly) placid filter feeders have a lot to offer once we move past the usual coolness filters that make our inner ten-year-old a Velociraptor fan*.
I’ve been getting quite fond of sponges recently. It’s mostly a byproduct of the reading I do for my work, which partly concerns the mineralised hard parts of animals. All sponges have skeletons, and the majority of them make hard(ish) skeletons from one of two minerals: either amorphous silica (think glass) or calcium carbonate (think chalk, limestone, clam shell, etc.) (The rest, including bath sponges, use proteins.) Siliceous sponges in the class Hexactinellida (= glass sponges proper) can have beautiful, intricate skeletons like this one from a Venus’s flower basket (Euplectella sp. by NEON ja, Wikimedia Commons):
They are not only gorgeous, but, at least in some cases, also insanely strong and bendy – nature’s fibreglass fishing rods, if you like. See this photo from sponge guru Werner Müller’s group for a demonstration. That glass rod is the skeleton of Monorhaphis chuni, a deep-sea glass sponge that anchors itself with the largest known single structure made of silica in the living world. This “giant spicule” can be up to 3 m long, and flexible enough to bend around in a circle (Levi et al., 1989).
Some sponges have both glassy and calcareous (or “chalky”, if you like) skeletons. And such sponges are giving me all kinds of squee moments lately. Something I’ve only learned recently is that sponges often live in close association with a variety of bacteria. Now it turns out that these symbiotic bacteria contribute to their skeleton-building abilities!
Last year, Dan Jackson and his team published evidence that a sponge species stole a gene it uses to make its calcareous skeleton from a bacterium (Jackson et al., 2011). The gene in question occurs only in bacteria – and sponges. While the sponge species used in the study does harbour bacteria in the cells that produce the calcareous portion of its skeleton, multiple lines of evidence indicate that the gene in question sits in its own genome, and has done so for a long time. It is only active in the skeleton-forming cells, and its protein product is present in bits of skeleton isolated from the animal, suggesting that it does in fact function in building the skeleton. (As of that study, its exact role is still unknown.)
(Above: Astrosclera willeyana, coralline sponge and convicted gene thief. The living animal forms a crust over an ever-growing bulk of dead skeleton. From Jackson et al. )
Most recently, another “spongy” research team found that members of a different sponge lineage have the actual bacteria in their cells make their skeletons for them. Uriz et al. (2012) examined three species of crater sponges, belonging to the “siliceous” sponge genus Hemimycale. In certain cells of the animals, they saw tiny round objects that molecular genetic tests revealed to be bacteria. The bacterial cells were surrounded by a coat of varying thickness that, when the researchers probed its elemental composition using X-rays, proved to be made of calcium carbonate. According to their observations, the bacteria live and divide inside membrane-enclosed vacuoles. They accumulate calcareous material as they mature, and finally the host cell spits them out to form a mineral crust around the animal. (Below: colonies of Hemimycale columella, one of the three species used in the study, from the Encyclopedia of Marine Life of Britain and Ireland via Encyclopedia of Life)
The bacteria look like they’ve had a long-standing partnership with their host sponges. They were abundant in all examined individuals of all three species. Unlike free-living bacteria, they appear to lack cell walls. They are also inherited by baby sponges. Mother Hemimycale sponges nurture their embryos in their bodies (apparently this is common among sponges). Sponges provide their embryos with so-called nurse cells, which, in the case of these species, contain some mineral-making bacteria. The young sponge eventually eats the nurse cells, thereby acquiring the bacteria. By the time it becomes independent and settles on a comfortable rock, its body is littered with tiny mineral spheres made by its inherited symbionts.
On closer examination, it seems that Hemimycale is far from the only sponge genus to harbour similar hired skeleton-builders. Uriz and colleagues tell us that they have found previously overlooked evidence of such “calcibacteria” in several other sponges – one of which is only distantly related to Hemimycale. Could calcibacteria be ancient partners of these animals, inherited by many different sponges from a distant common ancestor? Could bacteria even hold the key to the origin of calcareous animal skeletons?
(FWIW, I don’t really buy the second idea. As far as I know, all non-sponge animals that have been investigated make their skeletons with their own genes – nothing suspiciously bacterial-like the way Jackson et al.‘s spherulin is. [Caveat: there remain plenty of groups that haven’t been investigated in sufficient molecular detail.] However, the idea that sponges as a whole may have acquired their calcareous skeletons this way is fascinating. Incidentally, though the ID isn’t 100% certain yet, the calcibacteria may belong to the same bacterial class as mitochondria and these insidious bastards. Do alpha-proteobacteria have a special knack for endosymbiosis?)
*Not to say Velociraptor isn’t cool, but being a vicious toothed, raptor-clawed killer bird is, well, not the only road to coolness 😛
ETA: 42nd post, yay! (Also yay: random Hitchhiker’s Guide reference in completely unrelated post :D)
Jackson DJ et al. (2011) A horizontal gene transfer supported the evolution of an early metazoan biomineralization strategy. BMC Evolutionary Biology 11:238
Levi C et al. (1989) A remarkably strong natural glassy rod: the anchoring spicule of theMonorhaphis sponge. Journal of Materials Science Letters 8:337-339
Uriz MJ et al. (2012) Endosymbiotic calcifying bacteria: a new cue to the origin of calcification in Metazoa? Evolution early online view, doi: 10.1111/j.1558-5646.2012.01676.x