Multiscale architecture and composition of two South Australian Bryozoa: Celleporaria cristata and Densipora corrugata

Deep beneath the surface of the ocean, in the cool, temperate waters off the coast of Australia, tiny colonial animals known as bryozoans build intricate stone homes. These creatures, often called "moss animals," are not single individuals but vast communities of microscopic clones working together. For centuries, scientists have known that these colonies are made of calcium carbonate, the same mineral found in limestone and seashells. However, the story of how these animals construct their skeletons, and what secrets those skeletons might hold about the environment they live in, has remained largely hidden. The challenge lies in the scale: the structures are so small that they require powerful microscopes to see, and the chemical signals they contain are so faint that they are easily missed by standard tools. Understanding these tiny builders is crucial because they are major contributors to the world's carbonate deposits, and their skeletons could potentially serve as historical records of ocean conditions, provided we can learn to read the chemical language written within their stone walls.

A team of researchers set out to decode this language by studying two specific species of bryozoans found in South Australian seagrass meadows. Despite belonging to different evolutionary families that split apart hundreds of millions of years ago, these two species, Celleporaria cristata and Densipora corrugata, have evolved to look almost identical from the outside. Both form spindle-shaped colonies that grow along the stems of seagrass, featuring a series of prominent ridges running across their surfaces. The scientists wanted to know if this striking similarity in shape meant they built their homes in the same way, or if their ancient differences still dictated the microscopic details of their construction. To find out, they used a suite of advanced imaging techniques to examine the colonies from the size of the whole animal down to the individual grains of mineral that make up their walls.

The investigation began by looking at the overall shape and internal layout of the colonies. Using high-resolution 3D scanning, the researchers confirmed that while the external shapes were nearly twins, the internal rooms where the individual animals lived were fundamentally different. In one species, the living chambers were box-like, while in the other, they were long tubes. This difference reflected their distinct evolutionary histories. Yet, when the team zoomed in further to look at the walls themselves, they found a surprising amount of common ground. Both species built their walls using a mix of tiny, flat mineral plates and small, grainy clusters of calcium carbonate. At the very smallest level, both were constructed from nanogranules, tiny rounded particles ranging from 65 to 250 nanometers in size. This suggests that despite their different evolutionary paths, both animals use the same basic building blocks to assemble their stone homes, likely forming them by sticking these tiny particles together.

The researchers then turned their attention to the chemical composition of these stone walls, specifically looking for magnesium, an element that often gets trapped in calcium carbonate as it forms. They found that both species incorporated magnesium into their skeletons, but they did so in very different patterns. In the species with the tube-shaped chambers, the magnesium was concentrated in distinct bands that lined up perfectly with the external ridges on the colony's surface. This pattern led the scientists to a compelling hypothesis: these ridges might be annual growth marks, similar to the rings of a tree. Because magnesium levels in marine animals often rise during warmer summer months, the high-magnesium ridges could represent periods of rapid summer growth. If this is true, the colonies could be several years old, with the number of ridges indicating their age. In contrast, the other species showed magnesium scattered in irregular bands throughout its walls, with no clear connection to the external shape, making it impossible to use the same method to determine its age.

Beyond the minerals, the team also investigated the organic materials trapped within the stone. All living things that build hard shells mix in some organic molecules, like proteins and sugars, to guide the formation of the mineral. The researchers discovered that the two species used very different organic recipes. The species with the box-shaped chambers had walls rich in protein, which appeared to be part of a protective outer skin, or cuticle, that gets incorporated into the wall as the animal grows. The other species, which lacks this outer skin, had walls filled with carbohydrate-rich organic layers sitting between the mineral grains. This difference highlights how two animals that look the same on the outside can have completely different internal biological strategies for building their skeletons.

The study concludes that while these two bryozoans have converged on a similar shape to survive in the same environment, their internal construction remains a testament to their unique evolutionary past. The findings offer a new way to look at these tiny colonies, suggesting that the species with the magnesium-rich ridges could serve as a natural recorder of past ocean temperatures. By counting the ridges and analyzing their chemistry, scientists might be able to reconstruct the history of summer warming in these seagrass ecosystems. This work does not just describe how these animals build; it opens a door to using their stone homes as tools to monitor how our oceans are changing, turning a microscopic colony into a window on the climate of the past.