December 1, 2010 | 4
In the world of biology, there is plant, there is animal, and there is plant-animal. Specifically, moss–animals, the bryozoans.
[Public Domain Haeckel illustration—click to see large]
I mention this because someone in Virginia recently had a run-in with these creatures that was startling enough to result in a press release. And when a bryozoan generates a press-release, that is in itself kind of news-worthy.
It all started when someone in Newport News, VA, was out walking their boss’s dog in October and discovered something floating in a man-made lake. Something squishy. Something blobby. Something alive. Though this isn’t the same specimen, here is a youtube video that shows what you might have seen had you been walking the dog that day.
After much scratching of heads and exchanging of emails by scientists at the Virginia Marine Institute at the College of William and Mary, the blob was deemed a giant freshwater bryozoan—Pectinatella magnifica, "the magnificent bryozoan". There are two interesting things about this: 1. It’s a freshwater bryozoan—they almost all grow in saltwater. And 2. it’s a bryozoan! Bryozoans are like the whales of the coral world—they are not coral, but have evolved into the same filter-feeding polyp-like niche. They’re really old—like 500,000,000 years old. And some bizarre details of their biology have helped them elude the best efforts of biologists to try and pin them down in the Earth family tree.
Bryozoans evolved in the Ordovician, the geological period that followed the Cambrian Explosion about 500 million years ago, about the same time as the corals they superficially resemble. Bryozoa are colonial or collective organisms not unlike a colony of bees, Volvox, or marine salps. Each individual colony member is a little animal that sometimes secretes a calcium carbonate shell that fuses with that of those all around it. Others, like our magnificent bryozoan, are mostly jelly and water (the latin name Pectinatella is a good clue to this).
The little animal – adorably called a zooid – is very simple: it has a U-shaped gut and a crown of ciliated tentacles called the lophophore that it can evert to feed. The cilia on the tentacles beat to draw in food, an effect you can see vividly at about 1:35 in this stirringly-(for bryozoa)scored video, after lophophore eversion at about :42. Here’s a different look. The mouth is inside the ring or horseshoe-shaped crown. A simple body cavity, a secreted protective coating, a few nerve and muscle cells, and perhaps an ovary and/or testis is about everything else there is. To breathe, the zooid exchanges gas through the large surface area created by its ciliated tentacles. To excrete waste (the equivalent of urine, which in vertebrates is collected by the intricate structure of the kidneys), it does the same.
[Photo of lophophore from wikimedia commons—Creative Commons license—click to see large]
As mentioned, when the little animal gets hungry, it sticks its lophophore into the water and beats the cilia on its tentacles. But there’s more – the whole zooid can also waggle back and forth and twist its lophophore around to increase its chances of catching food. That’s right: they do a little feeding dance (Youtube video of dancing zooid set to “Stayin’ Alive” or “Hungry Like the Wolf” appearing in 4, 3, 2 …). Since many bryozoa today are crust-like mats, when they stick out their lophophores, they can look like moss. Hence, moss–animals. No chlorophyll required.
In some bryozoans, individual animals specialize their function for feeding, defense (some bryozoans can even “walk” along on their little defensive spines), or reproduction. In others, everyone performs the same tasks as everyone else. They can settle on virtually any immobile surface and several mobile: rocks, kelp blades, wandering hermit crab shells, etc.
[fossil bryozoan—click to see large. Photo by Park Service, licensed under creative commons]
Not all are crusty mats; their forms can be fantastic—look once more at the print by 19-th century naturalist illustrator Ernst Haeckel of living byrozoa at the beginning of this post. 500 million years is a lot of time to work with, and judging by the fossils we have, bryozoa may have even been more spectacularly diverse and weird in the deep past. Bryozoans were shaped anywhere from tiny needle-like zooid high-rises, to lace-like perforated encrusting sheets, to the fantastic Archimedes, which, true to its name, looks like an Archimedes’ screw with fenestrate (window-like) fans housing zooids likely perched atop.
[Drawing of coral polyp—public domain]
Superficially, a coral polyp, or individual coral animal, is a very similar thing: a little animal that secretes a hard shell and pokes a crown of filter-feeding tentacles into and out of its stony home. But for all their similarities with the colonial filter-feeding corals, which are relatives of jellyfish and sea anemones in the phylum Cnidaria, they are not.
They are in their own little group (look for, and click on, Bryozoa here), an orphan taxon (biological classification unit based on evolutionary relatedness) that is so mystifying both morphologically and genetically that scientists can’t even seem to make up their mind whether the group belongs in the fundamental animal group the protostomes (the insects, crustaceans, arthropods, water bears, and nematodes) or the deuterostomes (vertebrates, echinoderms, and tunicates).
Why? Comparison of the genes of these organisms with those of the other fundamental animal groups, or phyla, have not clearly identified how they fit in. And the physical characteristics of development and anatomy we normally rely on to classify animals are often ambiguous at best in bryozoans. It’s as if the taxon is giving taxonomists the proverbial taxonomic finger. For instance, scientists often use the tissue types animals use to make their internal organs to classify them, figuring this property is highly conserved. Bryozoans inconsiderately destroy all their larval internal organs, and fabricate entirely new ones as adults. So much for that.
In most animals, a small dent forms in the hollow ball of cells formed after a fertilized egg begins dividing. In protostomes, the first little dent becomes the organism’s mouth. In deuterostomes, the first dent becomes the anus and the second the mouth (proto = first, deutero= second, stoma=mouth) In bryozoans, the first little dent in the hollow ball of cells disappears. They form their mouth from a different dent that forms later nearby. The list goes on, but you get the idea.
In any case, our particular P. magnfica — a firm, fuzzy sheet of zooids surrounding a big gelatinous ball – did not seem particularly bothered by its in-the-closet, phylum-bending status. It was massive for one of its kind — they typically grow one to two feet across, and the press-release bryozoan was around four. To see a specimen up close in all its gelatinous glory, see here (and above, with the permission of the author) for some gorgeous up-close pictures of the lophophores of Pectinatella and a rather-disturbing view of the colony when placed on land (yes, P. magnifica, land makes your colony look fat) and here for some wonderful drawings of the anatomy of this species.
Finally, it’s worth noting this thing was found in a man-made lake. According to the scientists, its presence is a good sign: one of high water quality. As bryozoans eat algae, it probably means that the often-pesky microorganisms are under control. But how did such a weird, huge thing get there? Look at the structure in the upper left of the photograph at the beginning of this post. That’s a resting structure produced by some bryozoans called a statoblast. You can see more up close here and here. Note the hooks. Think about how they might interact with dog fur or duck feathers. After that, just add water.
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Gould, J., and Keeton, W. Biological Science. 6th ed. New York: W.W. Norton & Company, 1996.
About The Author: Jennifer Frazer is a biodiversity blogger and AAAS Science Journalism Award-winning science writer who has written for the Wyoming Tribune-Eagle, The Boston Globe, The (Louisville) Courier-Journal, High Country News, and Fungi Magazine. She holds two biology degrees from Cornell University and a Master’s Degree in science writing from MIT. She writes about the spectrum of life on Earth at her blog, The Artful Amoeba, and tweets (occasionally) at @JenniferFrazer. Like David St. Hubbins, she is concerned about the potential world domination plans of slime molds.
The views expressed are those of the author and are not necessarily those of Scientific American.