Thursday, 19 April 2012

Mysterious Bryozoa

Some pond animals might not look attractive at first glance. The majority of people who encounter a bryozoa colony most likely wouldn't recognize what it is. A hairy warm? A weird hydrozoa? After all, even the mention of words "bryozoa"and "moss animals" alone provokes only one question"what the heck is that?" The truth is, molecular taxonomists have more or less the same question when approaching this group.

Bryozoa are among the least studied invertebrates. According to phylogeny analysis they are not relatives of cnidaria or, in fact, any known group of invertebrates. Some evidence suggests that they might be related to even less studied group of organismsEntoprocta. But leaving phylogenetic and taxonomic speculations aside, bryozoa are colonial filter-feeding animals that are really abundant in freshwater and marine habitats. Their polyps are tiny and almost invisible to the naked eyes. Yet their colonies can reach up to several meters is size, although such giants are rare.

The most interesting fact about the skein of tentacles you see on the photos is the bryozoan's peculiar body organization. Cristatella mucedo forms mobile colonies. Pretty rare thing in the animal kingdom. The vast majority of colonial organisms are sessile. Even the diverse brozoa that form many types of colonies on the sea floor are almost always attached to the substrate. But Cristatella can move like a snail.

While most of images on this page are made through the microscope, the one below is a macro shot in which you can make out the general organization of this organism.

  But its uniqueness doesn't stop here. This species can make anyone think about what is a unit in an organism. We have believed that all life forms are made of cells for almost two centuries after Schwann and Schleiden postulated their theory based on observations made by multiple researchers. We now know that this rule can be broken as cells can form a syncytium (multinucleate cell). Some organisms went as far as not forming cellular structures at all, for example, even more weird form of life called "Xenophyophore." Spoiler: they belong to Foraminifera.

  The cellular theory becomes even more obscure when we think of symbioses. After all, some symbiotic bacteria can live inside eukaryotic cells, which is how we presumably got mitochondria. Aside from that, most multicellular organisms seem to form symbioses with prokaryotes, which poses a touch, mostly philosophical question of whether or not one organism is formed by cells of different species that cannot exist without each other.

  Perhaps that's why Richard Dawkins proposed that that genes, not cells, are units of life. Biologists, in general, treat this point of view differently. Some radically disagree because genes cannot exist, or at least reproduce, independently from the cell. But those speculations are not the main point here. In the previous examples living organisms consisted of cells, mostly differentiated ones and while they form very close relationships with cells of different life forms, the whole system, which we could call a holobiont, is still composed of cells, even when if sometimes these cells possess multiple nuclei.

   Cristatella, though, is one step ahead.

  This freshwater species of bryozoa consists of cells too, at least at first glance. What makes it stand out is the fact that individual zooids in the colony are differentiated the same way as cells are differentiated in a multicellular organism. These differentiated zooids play different roles and participate in feeding, reproduction, or colony motion. The feeding zooids, which are the most apparent on the pictures, have fancy tentacles. Each tentacle has cilia that create water motion directing food into the zooid's mouth. A microscope reveals a lot of peculiar details in their complex body and, not surprisingly, I spent a lot of time just looking at this magnificent colony.

  They might be an overlooked group of animals, yet they stand one level higher in organization than us, "simple" multicellular beings. The colonies are, essentially, super-organisms. I wonder why such body organization hasn't appeared more often in evolution and if it has any future.

 Because very few people study them and express any interest, you won't find a lot of information, easy ID guides, or detailed photos. That is why not only the ocean conceals a lot of mysteries, but the nearest pond too. We are selective on species that we care about or study, leaving such fascinating life forms next to us without attention.

No comments: