Bryozoans: Moss Animals of the Tidepools

It’s that time of the year when brown turns to green. With a little rain, crannies of aged walls, the muddy veins of cobbled streets, the balcony of my brothers’ rooms, the insides of the well in our house — they all undergo a makeover and green starts sprouting up like seasonal graffiti. For me, the monsoon is incomplete without bending over and feeling these tufts of green with my finger or madly zooming in with my phone, prying on the lives of these microscopic gardens. For me, it’s moss time of year.

Thinking about mosses is always followed by the thought of another nondescript group of animals: Bryozoans, commonly known as moss animals.

Moss animals or bryozoans get their name because of the likeness they share with mosses or bryophytes; which is, in the way they also grow snugly on the ground, sharing the same interface between land and air. Like their photosynthesising counterparts, bryozoans grow as mats on rocks or form little mounds and are found at the base of everything else that grows and lives on them. But unlike mosses, bryozoans are very much a part of the animal kingdom and are a group of aquatic invertebrates that are largely marine. The easiest way to see them is to take a walk on a rocky shore. 

Bryozoans and I became acquainted on the rocky shores of Ratnagiri in Maharashtra. I admit it took us a while to hit it off. As somebody on a rocky shore for the first time, distractions were plenty. The first was just to get across the boulder field of Mandavi and avoid yet another fall. And so, when I started exploring this marine space, one scooch at a time, it was nothing like the sea I had imagined. It was not an expanse but a dense aggregation of life forms. Every rock I turned was wild and full of life. From the most outlandish sea slugs to crabs that looked like fine china; from saintly anemones to small hillsides of sponges. To have all this and then some on footlong rocks was bewildering. And so, in all the intertidal bustle, where did I find the bryozoans? Almost as if I had come of age as a tide-pooler, I started picking up on them once I had made my way through the more conspicuous organisms of the intertidal community. Because under all the dynamicity of the rocks, bryozoans grew as small, colourful mats, which were easy to miss if you did not know what you were looking for.

You can’t really tell much about a bryozoan colony without taking a close look at them. And I say colony because it’s not one, but a million tiny organisms called zooids, fused and living together as one unit. Only when I saw this under a microscope for the first time did they really have me.

The closer and longer I looked upon these colonies, the more about themselves they would give away. One of the first things was that the mats I saw were not the animal, but the calcareous skeleton inside which it lived, and the small speckles were tiny holes out of which their tentacled head (lophophore) would come out to filter feed in the moving waters.

Like bees in a hive, every zooid in a Bryozoan colony has a role to play. Some zooids are foragers, some with modified spikes are armed guards that protect the colony, others with microscopic bristles help keep it neat, and still others look at the expansion of the colony, filling in empty spaces and even creating boundary walls. All this is accomplished by animals that are smaller than a grain of sand. It is these levels of intricacy, at scales so minute, that astound me even today. And it’s probably this minute nature that has allowed these animals to retain their space in this tumultuous habitat for 500 million years. Almost like an ancient civilisation much ahead of its time.

And as it is with civilisations, they all start with a wanderer. Bryozoans, too, just did not come to be on a rock. In fact, the very first individual of the colony is a single free-swimming larva released from a parent colony. The nomad that it is, this larva journeys the surf until it comes ashore to a rock that feels just right. Once settled, it is from here that the first individual, aptly named ancestrula, buds off the rest of its family of millions! Only to live happily ever after.

Well, not really. Bryozoans are part of a habitat where there’s constant sharing and invading of space. Without the ability to move, they, like the rocks they grow on, end up becoming a substrate (base) for several species. If you observed these colonies long enough, you are most likely to come across a cowrie snail or the Okenia sea slug moving on a colony. Far from being mere passersby, they are on the colony for a very specific reason — feeding. Several molluscs and flatworms make a meal of bryozoans by sucking out the animals from the zooids. And while they are at it, they might even lay frilly eggs on the colonies. It may seem like bryozoans are taking it all lying down, but that’s hardly the case. As long as the outer edges of the colony remain intact, the zoans will continue to expand slowly but surely. One zooid at a time. They will take over and grow upon anything that comes in their way. From polychaete tubes to shells of crabs and gastropods to even algae. Yet another secret behind their evolutionary longevity.

Being as old as they are, bryozoans have not remained unchanged. So yes, the flat, encrusting ones are the most common, but there is more. Some, like Amathia or Crisia, are as pretty as their names and grow like little branches in a rock pool. Then there’s a species of Electra which seems to have taken an oath of loyalty because it will only grow on sargassum. Then, we have Alcyonidium, which looks like a glove balloon.

Bryozoans look like a lot of things on the shore, and they have fooled me into thinking they are hydroids, corals, algae, etc. So diverse are their forms that it’s as if they are vying for attention in their own small way.

And attention is what I gave. For three years, I pried on these small hypnotic lives. Sometimes, I just observed them being a part of the salty kingdom they lived in, and sometimes, I obsessed over them through a microscope, trying to know a little more. Eavesdropping, on the “dialect of the moss on stone” as the American botanist Robin Wall Kimmerer puts it. “An interface of immensity and minuteness, of past and present, softness and hardness, stillness and vibrancy, yin and yan.”