Bryozoa

Image: National Museums Northern Ireland and its licensors

Bryozoa, sometimes called Ectoprocts or Moss Animals, is an entire phylum of some 5,000 species that are probably life's only chance against the all-conquering weirdness of Cnidaria.

The thing about these "Moss Animals" is that they turn themselves into underwater cities in a manner similar to coral. A single Bryozoan is a tiny thing just 0.5 mm (0.020 in) long, but with their identical twin brethren and sistren they make up colonies that can sometimes be over a metre (3.3 feet) across!

Most of these cities are encrusting, meaning they spread out across rocks and other surfaces. A bit like London, but they probably do it with more thought and care.

Image: National Museums Northern Ireland and its licensors

Some create impressive structures that look like strange plants attached to the ground by a stem.

Image: Wikimedia
Christatella sp. a mobile Bryozoan.
Or a hairy slug.

There's one genus where they look like a hairy slug slowly creeping along in freshwater bodies. Another species is ball shaped and floats in the sea. Then there's a whole genus of renegades and hermits who go it alone as tiny, solitary Bryozoans.

Image: National Museums Northern Ireland and its licensors
A Bryozoan encrusting.
It's a bit like moss, only it's an animal.

Most Bryozoans make their home in the warmth of shallow, tropical oceans. Adventurous types have made their way as far as the Antarctic and the deep sea, with others in brackish and fresh water. 10 cm (4 in) is a normal size for Bryozoans, but as I say, some of them are ten times bigger.

Colonies are hard or soft to the touch because each individual Bryozoan secretes an exoskeleton for itself. In some, it's gelatinous, in others it's chitin like an insect. Some use minerals to produce a hard, heavy lime exoskeleton. It's all about protection.

Great things are afoot when the word "zooid" comes into play!

Image: Bioimages
Row upon row of tiny feeding zooids.

A zooid is the name for an individual in this sort of colonial animal. It's the zooids that make the exoskeleton for the whole city, each acting like a single brick in the city wall. But this is hungry work. When's dinner?

The zooids that collect food are known as autozooids, and many colonies are made up entirely of these feeding zooids.

They have a lophophore, which is a crown of tentacles that pokes up out of the exoskeleton. If predators come along the whole lophophore can be pulled back inside for protection and some Bryozoans even have a lid called a operculum.

mikrographica

When the coast is clear they will feed on tiny plankton. The tentacles have cilia that wave like tiny hairs and mucus for drifting food to stick onto. Then it's passed down to a mouth at the centre of the lophophore. From there, a simple U-shaped gut expels waste through an anus to the side of the tentacles.

Image: Natural History Museum, London
Lophophores

Some waste products are too complicated for such a simple digestive system and they accumulate in the gut. Eventually, there's simply too much of the stuff. The solution is drastic. Half of the autozooid just dies! The gut, the tentacles and various nerves and muscles associated with the lophophore die and start to shrivel up.

Here's a thing:

Coral, jellyfish and other Cnidaria are made up of 2 layers of cells with non-living gelatinous stuff in between. Bryozoans are more like other animals in that they have outer cells (body wall), a space (coelom), more cells (gut) and then the space where the food goes. So Cnidaria are like a bag, whereas most animals are like a tube with another tube inside. The Bryozoa is allowing the inner tube to die away, leaving only the outer one.

It seems like a bit much! Killing off your guts because of a bit of tummy trouble. Laxatives? Anyway, the body wall of the autozooid simply regrows all its organs and the old ones are passed out along with all that difficult waste. Talk about puking your guts up!

The other reason they can do without guts for a while is that all the zooids are connected by pores or gaps in the body wall, so they're all sharing food among themselves. This means that some Bryozoans have zooids that don't catch their own food at all.

Image: WoRMS for SMEBD
Beware the bite!

Many of these don't have the lophophore or tentacles. Some of them have a sort of bird's beak at the end of a flexible stalk. This thing snaps at tiny predators like shrimp and amphipods, maiming or even killing them. Other zooids have just a vibrating bristle for clearing silt and sand from the feeding zooids. It'd be terrible to valiantly defeat enemies at the city wall only to slowly starve to death for want of some simple housework!

GirlScientist
Bryozoan larvae under a microscope. They'll never move this much again!

Pretty much all the things that make Bryozoa so weird are involved in reproduction. For some, each zooid is a hermaphrodite with both male and female organs. For others, zooids start off male and become female later. Either way, the colony as a whole can produce both eggs and sperm.

Image: National Museums Northern Ireland and its licensors

Sperm is released through pores in the lophophore. Most Bryozoans don't release their eggs, instead they capture sperm the same way they get their food. The eggs then develop and again, autozooids can sacrifice their own innards, this time to provide nourishment for developing larvae.

Other Bryozoans have zooids whose sole purpose is to serve as brooding chambers for their young.

Eventually, the larvae are released and they soon settle on the ground somewhere. They have no mouth, so they can't feed and will immediately metamorphose into a founding zooid.

Image: Ken-ichi via Flickr
Lacy Bryozoa

Other Bryozoans produce much smaller eggs and a lot more of them. These are released into the sea along with the sperm. They will become larvae that have mouths and can eat as they float around as plankton for a while. Eventually they'll sink to the floor and turn into the founding zooid.

Either way, the first zooid plants a flag and begins cloning and budding new zooids to form a whole new colony. So easy! They don't have to call a massive field of snow "Greenland" to try and attract new colonists.

There's one other thing involving freshwater Bryozoans. These ones have a really cool way of surviving the vagaries of lakes and rivers. Interestingly, it's almost exactly the same as the methods of freshwater Sponges, indicating the difficulties these unmoving marine creatures face in conquering waters beyond the vastness of the sea.

Image: Wikimedia
A statoblast

They clone themselves and create thousands of survival pod called statoblasts. These contain lots of food for the larval Bryozoan inside, and are surrounded in a bivalve shell made of chitin. It's like a tiny oyster or mussel.

When conditions go bad, these things can survive freezing temperatures or drying out. Some fall to the floor, some float away to pastures new. while others remain attached to their parents.

Should the city of origin fall, her ironclad daughters will survive the worst of the elements. With the return of clemency, new cities will arise from the ruins of the old, while others expand the empire into unknown waters.

Image: klmontgomery via Flickr
A freshwater Bryozoan.
Some day, our cities will look just like this.

Sometimes you got to be weird to survive. And when the going gets tough, you got to get even weirder.