Sponges are outliers in biology’s big bang, the Cambrian explosion. Their embryos appear in Precambrian strata, leading some to consider them primitive. That’s an illusion. New studies of how they construct their skeletons with silica “spicules” have revealed design principles remarkable enough to inspire biomimicry.
The punch line first — here’s how a news item from Current Biology concludes:
“This work not only sheds new light on skeleton formation of animals, but also might inspire interdisciplinary studies in fields such as theoretical biology, bioengineering, robotics, and architectural engineering, utilizing mechanisms of self-constructing architectures that self-adjust to their environments, including remote environments such as the deep sea or space,” the researchers write. [Emphasis added.]
Goodness! What are these simple animals doing to arouse such commotion? Just watch the video clip in the article of sponge cells at work. Then, look at the Graphical Abstract in the source paper and see the steps diagrammed in well-organized stages: (1) spicules are manufactured in specialized cells, then transported to the construction site; (2) the silica spicules pierce the epithelial tissue; (3) they are then raised up into position; (4) the bases are cemented by collagen provided by basal epithelial cells.
This simple animal knows, in short, how to build a house with pole-and-beam architecture in a way that self-adjusts to its environment.
That’s Pretty Impressive
Sponge skeletons, with their unique spicules, have been studied for a long time, but the manner of construction has been a mystery till now. What’s new, according to the Japanese researchers, is the identification of specialized “transport cells” that carry and finally push the spicules through the epithelia, and cementer cells that fasten them in place like poles. The process reveals division of labor and an overall plan.
Here we report a newly discovered mode of skeleton formation: assembly of sponges’ mineralized skeletal elements (spicules) in locations distant from where they were produced. Although it was known that internal skeletons of sponges consist of spicules assembled into large pole-and-beam structures with a variety of morphologies, the spicule assembly process (i.e., how spicules become held up and connected basically in staggered tandem) and what types of cells act in this process remained unexplored. Here we found that mature spicules are dynamically transported from where they were produced and then pierce through outer epithelia, and their basal ends become fixed to substrate or connected with such fixed spicules. Newly discovered “transport cells” mediate spicule movement and the “pierce” step, and collagen-secreting basal-epithelial cells fix spicules to the substratum, suggesting that the processes of spiculous skeleton construction are mediated separately by specialized cells. Division of labor by manufacturer, transporter, and cementer cells, and iteration of the sequential mechanical reactions of “transport,” “pierce,” “raise up,” and “cementation,” allows construction of the spiculous skeleton spicule by spicule as a self-organized biological structure, with the great plasticity in size and shape required for indeterminate growth, and generating the great morphological diversity of individual sponges.
This method of skeleton construction differs greatly from arthropods and vertebrates. It doesn’t appear to follow a set of rules or a preordained pattern, but it is very effective for sponges, “whose growth is plastic (i.e. largely depends on their microenvironment) and indeterminate, with great morphological variations among individuals.” Nevertheless, design and coordination is evident in the division of labor, the specialization of cells, and the end result that is good enough to inspire architects. If it were so simple, the authors would not have left many questions unanswered:
Many precise cellular and molecular mechanisms still remain to be elucidated, such as how transport cells can carry spicules, or how one end of pierced spicules is raised up. Additionally, one of the further questions that need to be answered is how sponges fine-tune their skeleton construction according to conditions of their microenvironment, such as water flow or stiffness of the substratum, since it is reported that the growth form of marine sponges changes according to the water movement of their environment.
Design is also evident in the self-organizational principles encoded in sponge DNA that make these results successful. Human intelligent designers would like to benefit from this knowledge. The authors conclude, repeating the “punch line”:
Intriguingly, our study revealed that the spiculous skeleton of sponges is a self-organized biological structure constructed by collective behaviors of individual cells. A chain of simple and mechanical reactions, “transport-pierce (by transport cells)-raise up (by yet unknown cells and/or mechanisms)-cementation (using collagenous matrix secreted by basopinacocytes and possibly by spicule-coating cells),” adds a spicule to the skeleton, and as a result of the iteration of these sequential behaviors of cells, the spiculous skeleton expands. As far as we know, this is the first report of collective behaviors of individual cells building a self-organized biological structure using non-cellular materials, like the collective behaviors of individual termites building mounds. Thus, our work not only sheds new light on skeleton formation in animals but also might inspire interdisciplinary studies in fields such as theoretical biology, bioengineering, robotics, and architectural engineering, utilizing mechanisms of self-constructing architectures that self-adjust to their environments, including remote environments such as the deep sea or space.
The reference to termite mounds is apt. The journal Science has described how these mounds, built by hundreds of individual termites, are able to “breathe” like an “external lung”:
Here’s how it works: Inside the hill is a large central chimney connected to a system of conduits located in the mound’s thin, flutelike buttresses. During the day, the air in the thin buttresses warms more quickly than the air in the insulated chimney. As a result, the warm air rises, whereas the cooler, chimney air sinks — creating a closed convection cell that drives circulation, not external pressure from wind as had been hypothesized. At night, however, the ventilation system reverses, as the air in the buttresses cools quickly, falling to a temperature below that of the central chimney. The reversal in air flow, in turn, expels the carbon dioxide-rich air — a result of the termites’ metabolism — that builds up in the subterranean nest over the course of the day, the researchers report online this week in the Proceedings of the National Academy of Sciences.
We know that some caves “breathe” as the temperature changes, but this is different. Termites construct their mounds for a purpose: to control the temperature and remove carbon dioxide for their health. It’s a bit like active transport in cells that draws in what the cell needs and removes what it doesn’t need, using machines that work against natural concentration gradients.
We all know that some beautiful things can self-organize without programming (snowflakes are a prime example). What we see here, though, are systems working from genetic programs for a purpose. In the case of sponges, its specialized cells cooperate in a plan to build a skeleton that adapts to the environment. In the case of termites, each individual insect’s genetic program makes it behave in a cooperative enterprise to build an air-conditioned mound. Such things do not arise by unguided natural forces.
If functional self-organization were simple, why did five European countries take years “working to design the European Union’s first autonomously deployed space and terrestrial habitat”? The effort, called the “Self-deployable Habitat for Extreme Environments” (SHEE) project, has a goal of programming elements for “autonomous construction” of housing for astronauts on Mars or other hostile locales. It took years of work in design, prototyping, construction, and optimization to get these buildings to “self-deploy” with no humans in the loop.
So when a sponge can do it, we should see intelligent design behind the scenes — not the sponge’s intelligence, which admittedly is minuscule, but intelligence as a cause for the genetic information that allows the sponge to run a program that leads to a functional result.
Those of us who appreciate the spectacular genetic programs that built the Cambrian animals should take note of the level of complex specified information in the lowly sponge. We can also notice that the sponge’s mode of construction bears no evolutionary ancestral relationship with the diverse, complex body plans that exploded into existence in the Cambrian strata. Sponges did well. They’re still with us.
This article was originally published in 2015.