Look, if you absolutely must know about the tiny colonial creatures cluttering up the planet's waterways, fine. Don't say I didn't warn you. It's not exactly a thrilling saga of triumph and betrayal. It's mostly just... filtering.
Bryozoa
Bryozoans, which some insist on calling Polyzoa, Ectoprocta, or the offensively twee "moss animals," are a phylum of aquatic invertebrate animals. Because the universe has a cruel sense of humor, these creatures are typically minuscule, measuring about 0.5 millimeters (or 0.020 inches, if you prefer archaic units) in length. They've compensated for their individual insignificance by becoming obsessive colonialists. They form colonies, known as zooids, which are not so much a cooperative society as they are a collection of clones tethered together in a shared existential crisis. These colonies can be charmingly described as "mossy" growths, but they also form sheet-like or branching structures that can, on occasion, be mistaken for coral. Just don't tell the coral. It would be insulted.
These colonies can range from a few millimeters to, in some overachieving cases, over a meter across. Most are found in marine environments, clinging to rocks, shells, kelp, or basically any surface that isn't moving fast enough to escape them. A few, however, have braved the questionable waters of freshwater. To date, humanity, in its infinite quest for things to categorize, has identified over 4,000 living species. One genus, Monobryozoon, apparently missed the memo on colonial living and exists as solitary individuals, which is frankly the only sensible choice.
The defining feature of a bryozoan, if you can call it that, is the lophophore. This is a crown of tentacles—ciliated, naturally—that they use to filter food particles out of the water. It’s their equivalent of a fine dining experience, which for them involves passively waiting for microscopic debris to drift into their mouths. Each individual animal, or zooid, lives inside a tiny box or tube-shaped compartment made of calcium carbonate or an organic polymer like chitin. This exoskeleton is called a zoecium. In any given colony, most zooids are autozooids, the ones tasked with the tedious jobs of feeding and excretion. Some colonies, however, exhibit a disturbing level of specialization, with different types of zooids—heterozooids—taking on specific roles. These can include kenozooids, which are basically living structural supports, and avicularia, which are unsettling pincer-like zooids that do nothing but snap at intruders. Imagine having a cousin whose entire existence is being a doorstop. That’s a kenozooid.
Bryozoans reproduce sexually, producing larvae that then go on to found new colonies through asexual budding. They also have a rather dramatic survival mechanism: when conditions become unbearable—which, let's face it, is often—entire colonies can degenerate into a dormant state called a brown body, waiting for the environment to become slightly less awful before regenerating. Freshwater species have an even more theatrical trick, producing statoblasts, which are hardy masses of cells that can survive freezing and drying. It’s a lot of effort for an organism that spends its life glued to a rock.
Their impact on the world is, like their existence, subtle and vaguely annoying. Some species are pests on ships and piers, a phenomenon known as biofouling. Others have proven useful to humanity, providing a source for unique chemical compounds used in medical research. Their most significant contribution, however, is their extensive fossil record. They first appeared in the Lower Ordovician period and proceeded to build vast underwater landscapes of calcareous skeletons, becoming major builders of limestone beds. Geologists find this fascinating. The rest of us can feign interest.
Anatomy and Physiology
Colonies and Zooids
The foundational principle of bryozoan life is that misery loves company. A colony consists of a series of individual animals called zooids, which are genetically identical clones because creativity is clearly not their strong suit. These zooids are all interconnected, sharing nutrients through a system of pores in their body walls. This arrangement, while efficient, sounds like a nightmare of forced socializing.
The basic zooid, the autozooid, is responsible for the thankless tasks of feeding and getting rid of waste. Its body, the "polypide," lives inside a mineral or organic shell called the "zoecium." The entire unit—the living parts and the shell—is what we call a zooid. The body wall of the zoecium, known as the "cystid," secretes the exoskeleton. When a zooid decides it's had enough, it can retract its lophophore and tentacles back into the zoecium, usually by contracting a set of retractor muscles. This is probably the most relatable thing about them.
Some colonies get ambitious and develop specialized zooids, or heterozooids. The avicularia are perhaps the most bizarre: modified zooids that have been reduced to little more than a muscular beak on a stalk. They can’t feed. Their only purpose is to snap at things, presumably out of sheer frustration. Then there are the vibracula, which are even more pathetic—long, bristle-like structures that sweep debris off the colony. And of course, the aforementioned kenozooids, which are just empty structural filler. It’s a society built on a caste system of drudgery and specialized aggression.
Lophophore and Feeding
The main event for a bryozoan is the lophophore, a ring of hollow tentacles used to trap food. The tentacles are lined with cilia, tiny hairs that beat in a coordinated fashion to create a water current. This current flows down into the center of the lophophore and then outwards between the tentacles. Phytoplankton and other microscopic morsels are trapped by the tentacles and funneled toward the mouth, which is located, conveniently, at the center of the lophophore.
This feeding method is a classic example of suspension feeding, a strategy employed by countless other aquatic organisms who couldn't be bothered to hunt. The gut is U-shaped, with the mouth at the base of the lophophore and the anus located outside of it—hence the older name "Ectoprocta," meaning "outside anus." This is considered a major evolutionary step up from creatures like cnidarians, who use the same opening for both business and pleasure. A small victory, I suppose. They lack a traditional circulatory system, instead using a fluid-filled body cavity, the coelom, to move nutrients around. Because why invest in complex plumbing when you can just slosh things about?
Respiration and Excretion
Given their diminutive size and the fact that they're basically hollow, bryozoans have no need for specialized respiratory or excretory organs. Gas exchange occurs directly across the entire surface of the body, particularly the tentacles of the lophophore. It’s the biological equivalent of leaving a window open. Metabolic waste products, like ammonia, also diffuse out into the surrounding water. Larger waste particles are simply bundled up by phagocytic cells and left to accumulate into a "brown body"—a dense, pigmented mass of cellular refuse. When a zooid degenerates, this brown body is either left behind in the zoecium or expelled when a new zooid regenerates in its place. It’s like they just shove all their garbage into a corner until someone else deals with it.
Evolutionary History
Fossil Record
For creatures so unassuming, bryozoans have left an astonishingly detailed diary of their existence in the form of fossils. They burst onto the scene in the Early Ordovician, about 480 million years ago, and quickly became one of the most abundant and diverse groups of fossil-forming organisms. They were major contributors to the formation of carbonate hardgrounds and buildups, particularly during the Paleozoic Era. Their fossil record is so robust that it’s used extensively in biostratigraphy for dating sedimentary rocks.
The two major groups of bryozoans, the Stenolaemata and Gymnolaemata, have starkly different histories. The Stenolaemates dominated during the Paleozoic, creating intricate, branching colonies that formed reef-like structures. They were hit hard by the Permian–Triassic extinction event, that great cosmic reset button that wiped out most life on Earth. While some Stenolaemates survived and persist today (mostly in the order Cyclostomatida), they never regained their former glory.
The Gymnolaemata, meanwhile, were minor players in the Paleozoic but seized their opportunity after the extinction. They diversified dramatically during the late Jurassic and Cretaceous periods, giving rise to the Cheilostomata, which is now the most dominant and diverse group of bryozoans. Their success is attributed to innovations like the operculum (a little trapdoor for their orifice) and the evolution of those bizarre avicularia. Apparently, having tiny defensive beaks was a game-changer. Who knew?