AHABS... Issue Nine
The Rise and Fall Of Coral Reefs: The Evolution Of, And Impending Extinction Of, Coral Reefs In The Late 21 st Century. What Can Hobbyists Do?
Ronald L. Shimek, Ph. D.
Reefs Are Very Ancient
Reefs, defined as geological structures that are ridges, hills or rocks extending up from an ocean bottom into the overlying water mass, are quite ancient. Even if the condition is added that such structures must be biogenic in origin, in other words, the reefs must have been made by living things, it is still possible to find evidence in the fossil record of such structures dating back to around a billion years ago. Those most ancient of reefs did not look much like today's coral reefs. These reefs, structures called stromatolites, were much smaller than modern barrier, atoll, or fringing coral reefs, and were more akin to patch reefs. The largest were a few hundred meters long and only a few meters in height, but they WERE biogenic “reefs.”
Although many very ancient fossils are recognizable as the remains of life, those fossils are frequently difficult to interpret; are they plant, animal, or something in-between. Those ancient remains are often so different in structure from anything alive today that it is really impossible to tell what they were and how they lived. This lack of understanding is particularly evident in the fossils found in rocks formed prior to the so-called “Cambrian Explosion,” that wonderful blossoming of shelled fossils that began about 530 million years ago. Fortunately, these first reef forming organisms provide one of the few easily recognizable fossils from this long-ago world. There is a very simple reason for this; obvious stromatolites are being made today at various places around the world. Ockham's Razor , the principle that the simplest and most straight-forward answer is often the best answer, leads us to conclude that effectively identical structures are created by very similar organisms, and in a similar manner; hence, the ancient stromatolites are similar to modern stromatolites. If so, it is possible to deduce what these first reefs were. Interestingly enough, similar structures also may be found in some reef aquaria!!!
These first reefs had a very simple structure (Wood, 1999). The formative organisms were photosynthetic prokaryotes – or in the language of reef aquarists – cyanobacteria. It is impossible to deduce the color of the ancient ones, but the modern stromatolite forming organisms are typically green, dark blue, black or a combination of those colors. A stromatolite forms when a bacterial mat forms. These mats form identically to the layers of red or pink...
Ronald L. Shimek, Ph. D.
Some Other (Paleozoic) Reef Forming Organisms
In broadly keeping with this month's theme of reef evolution and extinction though time, this article will briefly discuss a fascinating group of organisms, the Bryozoa, that occasionally make it into hobbyist tanks. Unfortunately, bryozoans seem to be effectively impossible for hobbyists to keep, even though they are frequently imported on live rock. Bryozoans are colonial animals that look superficially like corals, but they have a soft-part morphology that is significantly more complicated than that of corals; in some regards they are reminiscent of feather duster worms. In others… well, they are reminiscent of bryozoans. While they have a relatively close relationship to the brachiopods, a group of clam-like animals, and the phoronids, an odd and interesting group of odd tubeworms, they are - as are all discrete life forms - unique to themselves. While many of them form colonies that are simply crusts, some of them also form colonies of an exceptionally delicate beauty that would be welcome in any marine reef aquarium.
Bryozoans are often overlooked by casual observers, such as beachcombers or amateur naturalists, and probably because of this, I don't know of any species that has a useful common name. The group is known taxonomically, both as the Phylum Bryozoa or, as the Phylum Ectoprocta. Both of these terms refer exactly to the same group, and neither name is probably “better” than the other. I prefer to use Bryozoa when referring to the group, but that is simply a matter of personal choice to avoid the term Ectoprocta, which is similar to the name of yet another phylum called the Entoprocta. I have found that while I am teaching, if I use the name Ectoprocta, then invariably I will misspeak or sometimes the students get confused. For me, it has turned out to be best, simply, to stick with Bryozoa as the term for these animals.
The Bryozoa is not one of the larger animal groups; there are only an estimated 4,000 extant species, compared to some 45,000 species for chordates (fish, birds, mammals) or 150,000 for mollusks (clams, snails, squids). On the other hand, it is a much more species-rich group than such phyla as the Phoronids, which have less than 25 worldwide species. Because many of the bryozoans have a hard colonial exoskeleton, they have a good fossil record, with over 15,000 fossil species having been described. Interestingly, they are the last phylum to appear in the fossil record. And before we get excited about their modernity, let me state that that appearance is still close enough to being half a billion years old as to make the difference between those dates a trivial concern. Additionally, although they only have a moderate diversity, they are quite abundant in...
Ronald L. Shimek
Lophing Around …
Some of the oddest types of animals found on today's coral reefs belong to what are called, by biologists, the “Lophophorate” phyla. There are several discrete and different types of animals found in these groups, but they all share a specialized feeding and respiratory structure called a lophophore which is made of a circular, or horseshoe-shaped, band of thin tentacles. Most taxonomic authorities consider that there are three to five discrete groups of animals that possess such a structure, and because that structure is rather “peculiar” and conservative, that is to say, not found in other animals, these groups are considered to be relatively closely related. Elsewhere in this issue I have described one of the lophophorate groups, the Bryozoa, and in this article will explore some of the properties and intricacies of another one, the Brachiopoda .
In its most basic form as seen in the bryozoans, the lophophore is a simple band of ciliated tentacles arising from a ridge which itself is slightly elevated from the animal's surface. This ridge is centered on the mouth resulting in the tentacles being found in a symmetrical pattern either on either side of the mouth or surrounding it. In most of the bryozoans (discussed elsewhere in this issue of AHABS) those tentacles are arranged in a simple ring, resulting in the animal, or zooid, having a superficial appearance that is quite similar to that of a hydroid. In the brachiopods, however, the lophophore is of fundamentally different construction. In the brachiopods, it is shaped like a “U” or “horse-shoe,” and then it is suspended above and away from the animal's body. Then each end of the “U” is coiled in a helix. As you might guess, this is not a simple structure to describe.