Can bryozoans scream, as the name suggests in this photo taken with a scanning electron microscope? Well… no: these sea creatures are absolutely silent!
Bryozoans are small, millimeter-sized animals that live in colonies that are on average the size of a fist. These colonies are made up of hundreds of individuals who live in mineral lodges formed from calcium carbonate (CaCO.)3) each individual in the colony builds its own lodge, a kind of box with openings, one of which is the main one that allows the animal to filter and feed on seawater. On this image, we see the tip of a colony’s skeleton, where we can distinguish a dozen of these “lodges”; Those at the top are still in production. Its astonishing size is the result of the progressive development of the colony.
So bryozoans are capable of this feat called “biomineralization”: rock formation from elements dissolved in water (here carbon and calcium). They are thus constituents of carbonate reefs: although they are a minority in tropical rocks, they may represent up to 80% by weight of carbonates present in some temperate sedimentary rocks.
Bryozoans are sometimes called low-profile organisms (i.e. not very visible in reefs) but high impact in providing micro-habitat and fortifying reefs. They are reefs that are birch to forests: a species that settles among the first. Among bryozoans, this is due, among other things, to their ability to settle on many surfaces, even soft surfaces.
Carbonate reefs, of which bryozoans are important components, play a major role in the carbon cycle. For this reason, the protection of these reefs has been identified as an important issue in the latest IPCC report. In fact, many events threaten the inhabitants of coastal reefs, known as corals, but also others, such as bryozoans.
Bryozoans in the face of climate change
Among these dangerous events is global warming, which causes coral bleaching and can be dangerous for some species of bryozoan. To this added CO . there is an increase in the amount of2 Which dissolves in sea water and causes acidification. However, calcium carbonate (the mineral that forms bryozoan colonies) is more difficult to form in these more acidic conditions: the colonies become less dense and therefore potentially more fragile. Added to this is the pollution of water and the mechanical destruction caused by human activities on rocks…
But the bryozoans have survived various crises even before 485 million years. For this they have advantages, including their ability to tolerate a wide range of temperatures, especially the lowest, and thus their ability to inhabit a wide variety of environments, from abyss to coasts. Could bryozoans thus tolerate the amplitude of climate change better than corals, or could they change their distribution more quickly? Or will they be very sensitive to acidification, and some species, like reef canaries, will emerge first?
These questions remain unanswered. To answer this, a better understanding of bryozoans is fundamental. That’s why we collected this colony of bryozoans on our shores, in Roskoff, where water temperature and acidity are continuously measured. Thus informed, for example, we can estimate the variation in their future distribution … and determine whether bryozoans are right to cry!
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