Gutsy moss.

"There are more things in heaven and earth, Horatio, Than are dreamed of in your philosophy." (Hamlet)

Or, as R.L. Stevenson puts it, more simply;

"The world is so full of a number of things ..."

Forget heaven and earth, forget the world; there are more things in my little corner of it than I could have ever dreamed. The bryozoans I found on seaweed, for example.

Bugula pacifica (I think) and unidentified bryozoan.

Bryozoans. The "moss animals". The Latin name is derived from the Greek word βρύον (bruon), "tree-moss", from βρύω, "to be full to bursting, to teem" and from the Greek word ζῶον (zoon), "animal; a live thing". In other words, an animal that looks like moss. (The plants, Bryophytes). Which it does, sometimes. And often it doesn't.

The basics, first.

The moss animals are minute animals, each less than a millimetre long, living in tiny "boxes", usually calcified like barnacle shells, but sometimes leathery. Most of them live in colonies composed of many individuals; Wikipedia mentions millions in one colony. I am tempted to call them cities.

Each species arranges the colony in its own individual way. Some are like crusts, barely a scum on the rocks, kelp, or other animals. Others build mounds or coral shapes, still others attach themselves to each other in branch or leaf formations. These last are the "mossy" ones.

They are all aquatic, mostly marine dwellers.

I looked at my sampler under my hand microscope. Of course, all mine are dead; there is no sign of the animals inside the cases. But I got an idea of the shapes and arrangements. Apparent tubes, little barrels, balloons, and prickly pear cactus shapes, all made of white chalk or even glass. Breathtakingly beautiful; oh, for a microscope with a camera attached! (One day.)

Vocabulary: The body of the animal is called a zooid. (From zoon, life.) Its box or case, often called a "house", goes by the name of zooecium (zooecia, plural). Sessile: attached, not moving. Most bryozoans are sessile.

Anatomy: The bryozoan is basically a digestive tube in a box. At the open end, the lophophore, tentacles wave particles of food towards the mouth. On the same end, just beside the mouth, the anus discharges undigested remnants. Besides that, there is a muscle to pull the mouth closed (some have a lid, or operculum), and the reproductive organs (male and female; the animal is hermaphroditic).

This diagram, from Living Invertebrates, by Pearse and Buchsbaum, was helpful.

All that is background. Here's the weird stuff:

1. A zooid lives for a few days or weeks, then basically digests its innards, leaving a "brown body" inside the wall. Then the wall gives rise to a brand-new gut, etc. (That would be a handy trait for some of us whose "cast-iron stomachs" have rusted!) The brown bodies either stay inside the body, or are defecated through the anus, depending on the species.
   
   
2. A colony somehow keeps track and staggers this process so that the zooids with functioning guts can feed the rebirthing ones.

   Within a colony the individual zooids are not completely isolated. Each zooid is connected to its nearest neighbours by a strand of protoplasm. This enables nutrients to be transferred from one individual to another. Earthlife.net.

3. Each zooid has a nervous system composed of one bi-lobed ganglion (look on the diagram, just at the base of the tentacles) and a few nerves, connected to the internal organs and muscles, the body wall, and the tentacles. In many species, the ganglions also connect to a nerve network common to the whole colony.

   "Disturbance of the lophophore can result in rapid retraction of the lophophore by most or all of the zooids in the colony." Living Invertebrates.

   A hive mind, where we least expected it! Hail our moss animal overlords!
   

4. "The retractor muscles ..." (That long, straight line from the base of the tentacles to the bottom of the box) "... are among the most rapidly contracting muscles known, shortening more than 20 times their length per second, nearly twice as fast as the fastest vertebrate muscle."

5. Not all zooids of the same species look alike. They may take on different shapes and functions for the aid of the colony. In other words, the colony functions, in some way, as a single animal.
   
   
   • The basic zooid is the box with tentacles, mouth and anus. It becomes part of the digestive system of the colony; it passes nutrients to the other forms.
     
     
   • Some zooids become avicularia; they look like a bird head, with a snapping beak. (Look at the diagram again; there's one attached to the side of the large zooid.) They either bite invaders, or sometimes pin them down until they die and disintegrate. I found a fuzzy movie segment that shows their snapping action, here.
     
     
   • Some are vibricula, just a bunch of tentacles that sweeps the colony.
     
     
   • In some colonies, a few of the zooids at the base become little more than empty cases, serving as supports.
     
     
   • And when the zooids reproduce sexually, (They do this and also bud asexually. Can't be limited to one lifestyle.) some become gonozooids or ovicells, brood-chambers. I think those glassy "balloons" I saw would be these.
     
     
   • One more; zooids at the edge of some colonies sometimes produce spines. In some species they

     "form only on zooids ... after the prescence of nudibranch" (sea slug) "predators is detected ..."

   How they manage all that with one ganglion and a gut, I can't imagine.
   
   
6. Bryozoans are sessile. Except for those that aren't.

   "... but in some species, particularly those which live in freshwater, the whole colony is able to walk, or glide to a new locality. Species of Cristatella can move up 10 cm (4 inches) per day. However the fastest species is Selenaria maculata which can move up to 1 metre per hour." Earthlife
   

7. With (almost) anything you say about Bryozoans, you have to add, "except for these cases ..." No one rule holds for them all.

A few handy links for more info:

• Wikipedia, of course.
• The Earthlife Web
• A photo of Bugula pacifica, which looks like the one I found. From Marine Life on Floating Docks.
• A photo showing the branching glassy zooids of Bugula neritina.
• Marine Life of the Pacific Northwest has many good photos, too.
• And, of course, I always check Kozloff, Seashore Life ... first.
• * Update: Christopher Taylor, at Catalogue of Organisms, tells of a walking bryozoan colony.
  

.