Musings

A common statement seen in print and heard at marine aquarium conferences is something like, "Corals and other reef organisms are delicate and are consequently hard to keep." On th surface, this seems to be a reasonable statement. After all, reef aquarists invest a lot of money and time to provide what they consider appropriate conditions for these "delicate" animals. Corals, in particular, and coral reef animals, in general, do seem to be difficult for some hobbyists to care for. Of course, the difficulty in caring for them is compounded by the fact that many hobbyists seem to have their own preconceived ideas of what the conditions are on coral reefs and these preconceptions seem to intersect only rarely with reality.

Some of these preconceptions result from misinformation that has made its way into the popular aquarium literature. As in other aspects of human endeavor, in the reef aquarium hobby, bad ideas seem to have an allure that transcends reason. Possibly the worst of these ideas are the myths regarding temperature and salinity. Never mind that these basic physical conditions of coral reefs have been well-known since Charles Darwin's pioneering publication describing the method of coral reef formation (Darwin, 1842). Reef aquarists might find it interesting that Darwin published his first major work essentially as a geologist. In that book he correctly hypothesized the mode of coral reef formation, although the collection of the final data substantiating his theory had to wait until after the preparations for the thermonuclear tests on Pacific coral atolls. By the way, should an aquarist want a copy of this book for his or her reef library, facsimile editions are available at many booksellers. On the other hand, if the urge is for an original, a first edition copy inscribed in Darwin's hand, one recently became available at auction. It could be had provided, of course, the collector could meet the estimated starting bid of £20,000.

The longevity of this knowledge about reefs notwithstanding, many sources still recommend keeping coral reef aquarium temperatures in the mid-seventy degree Fahrenheit range. This is a range that most reef animals would seldom, if ever, see in their natural world. While some higher latitude reefs are found in areas where these temperatures occur, and though upwellings can cool reef waters periodically, the number of coral reef species that can thrive at such temperatures is relatively small, and in just about every group that could be examined, the number of species that are found in such regions is very low and limited (see, for example, the data on corals in Veron, 1986, 2000). What should be obvious to one and all seems to escape a lot of people; that is that animals in captivity do best under the same natural environmental conditions where their wild populations are doing the best (Clausen and Roth, 1975; Coles and Jokiel, 1977, 1978; Highsmith, 1979a, b; Highsmith, et al., 1983; Prosser and Heath, 1991; Sorokin, 1991; Carricat-Ganivet, 2004). Research on over 1000 reefs (summarized by Kleypas, et al., 1999) has confirmed that the average reef temperature is 81.7°F, and that is the temperature at which the average coral will do best. Keeping animals adapted for this average temperature at lower temperatures results in a drop in metabolic rate of about five percent for each degree Fahrenheit below the optimum temperature (Prosser and Heath, 1991). In other words, animals whose optimum is about 82°F are "running" at only about 75% "efficiency" at 77°F (Prosser and Heath, 1991). Such a reduction in metabolism reduces all physiological functions such as immunity, growth, and repair of injury. On the other hand, corals and coral reef animals sometimes exist very near the top of their thermal tolerance range as well. They really push life to the limits, so that temperature increases as minor as a few degrees above the normal natural temperatures for their habitats can cause problems. For animals from cold water reefs, this upper lethal limit may be as low as 86°F, while for most reef animals it is in the range of about 90°F.

Figure 1. Corals, such as this temperate cup coral, Balanophyllia elegans, are simple animals having only a few types of tissues and no organs. They lack the ability to adjust to swings in their physical environmental conditions, and must be kept near their optimal conditions for long-term survival. For this particular coral, that means a temperature around 50°F.

Similar observations can be made about the salinity of the medium in which these creatures are kept. Diverse coral reefs are found in areas where the water has the salinity of non-diluted oceanic water (Sverdrup, et al, 1942; Veron, 1986; 2000; Longhurst, 1997). The effect of lowered salinity on both specific animals and ecological assemblages has been known for a long time (for some examples, see: Gosner, 1971; Zajac and Whitlach 1982a, b; Whitlatch and Zajac, 1985; Kato, 1987; Hoegh-Guldberg and Smith. 1989; Bulger, et al., 1990; Kozloff, 1990; Moberg, et al., 1997; Ferrier-Pagés, et al., 1999; Sakami, 2000; Alutoin, et al., 2001; Ruppert et al., 2003). In fact, the deleterious effects of lowered salinity are so well known that fluctuations in natural environmental conditions can be tracked by the reduction in growth and well-being of corals as recorded in their skeletal deposition (Hailey, et al., 1994; Guzman, H. M. and A. W. Tudhope, 1998). In spite of the preponderance of data showing the harmful effects of lowered salinity, some authors, and many manufacturers of artificial salt water mixes, still persist in stating in their instructions to mix artificial sea water to abnormally low levels; levels that will result in high stress for many coral reef animals (see, for example: Kirschner, 1991; but other examples in the same text, Prosser, 1991, are also informative).

The average salinity of more-or-less normal coral reefs ranges from 34 ppt to 38 ppt, with lower extremes being areas near large river mouths and the higher extremes being areas such as the Red Sea where extreme evaporation and subtidal hydrothermal activity tend to elevate the value (Kleypas et al., 1999). Incidentally, since about 1980, oceanographers have "officially" defined the salinity of seawater as a dimensionless unit, referred to as S, PS, PSU or PSS for various combinations of the words "Practical Salinity Units" or "Practical Salinity Scale." Salinity is now defined as the ratio of the seawater's conductivity to that of a specified potassium chloride solution, rather than as an actual measure of how much salt it contains. Most biologists and many biological oceanographers, however, have, in large part, ignored this change in nomenclature. Thus, the recent marine biological and oceanographic literature may express salinity either as S, PS, PSS; or, by traditionalists, as ppt (Pilson, 1998). For aquarists who depend on hydrometers for measuring their aquarium water's salinity, adequate salinity for their organisms is represented by specific gravity values in the range of 1.025 to 1.026. In spite of this, some manufacturers still recommend mixing artificial sea water to a specific gravity of about 1.022. Such a value results in salinities between 28 and 30 ppt, well outside the range for long-term survival of most coral reef organisms.

Coral reef animals are, indeed, delicate and hard to keep alive. Anybody can prove this by getting some of these animals and keeping them at temperatures that are abnormally low, abnormally high, or under salinity conditions that stress the animals to their maximum limits. Under such conditions, is it any surprise that the animals die when the slightest other factor goes wrong? It should not be, and neither should the observation that many of these animals also will simply just die from prolonged exposure to those conditions.

Survivors

Humans can live in all sorts of environments. We generally do this by altering our environment, by using clothing or buildings to alter our immediate microhabitats to contain the temperature range that our ancestors required during their evolution. Additionally, we are mammals, and mammals often can keep their metabolism constant over relatively wide ranges of external temperature variation. Most animals can't do this, of course, and humans often tend to regard these "limited" animals, such as corals, as "delicate." On the other hand, if corals and other coral reef animals are kept under conditions that approximate their natural and normal environment, not only are these animals not delicate, they are among the most resilient and hardy of all living things.

Delicate animals, indeed! These are animals whose life spans are so great that if they were handled competently, they could, in many cases, outlive their keepers. Relatively few animals can withstand the pounding of waves from hurricanes, but these are conditions periodically seen in virtually all reef habitats. And most of their animals survive them; indeed many acroporid corals may require such conditions for successful reproduction (Lafferty et al, 2004). Additionally, the shallowest of coral reefs are subjected to extremes of both infrared and ultraviolet radiation. Even with all of these risks to contend with, living corals that are absolutely ancient, over 3,000 years old, have been commonly found. Some other coral reef animals, such as sponges and many echinoderms, also know no defined life span. Unlike the corals, they can't be easily aged, but there is no a priori reason that they should not get as old, or older, than the corals. Sea urchins can exceed 200 years of age (Ebert and Southen, 2003), and sea stars probably get every bit as old. Many of the worms, such as the palola worms that reproduce by budding off swarmers, but whose body stays hidden in the rocks, should be able to live decades, at least, and perhaps longer. In temperate areas, many snails can live well over 50 years, and some clams have been aged at close to 200 years old. There is no reason to suggest that coral reef organisms shouldn't be as long lived as these.

Surviving the Worst of Times

There is another, altogether more profound, reason to regard all these animals as survivors, though. All animals have an evolutionary history that dates back to a common ancestor. Changes in the genetic materials found in all animals suggest that life that was recognizably "animal" probably appeared on Earth around 1,000 million years ago. Definitely small, and probably wormy, such animals may have looked similar to either acoel worms or the planula larvae found as early life stages in many cnidarians such as corals.

The fossil record of animals unmistakably recognizable as Cnidarians first appeared in the early Cambrian period about 500 million years ago. Although they didn't immediately proliferate, reefs of one sort or another were present by the middle of the Paleozoic Era. These reefs were formed by corals, but they were corals that may have been decidedly unlike modern corals in structure. Two different types of stony corals developed in the Paleozoic, the rugose and tabulate corals. As in modern scleractinian corals, the coral polyp sat in a skeletal cup, or corallite. In the modern or scleractinian corals, the corallites were subdivided internally by septal ridges, radially formed calcareous plates. Similar patterns are seen in the fossils derived from these ancient animals. However, the patterns of ridges seen in all of these types of corals are very different from each other.

Fossils attributable to tabulate corals were first found in rocks of the Ordovician age, but in some regards they are difficult to characterize fully. Moore, et al. (1952) state, "The tabulates are a group of unknown origin or exact zoological relationships, which may well include ancestors zoantherian and alcyonarian coelenterates." In other words, this group may be the group that gave rise to both stony and soft corals. Their skeletons were typically tubular and had table-like internal platforms or "floors" called tabulae. There were a lot of variations on these themes, and this group proliferated in the middle of the Paleozoic era, from the Lower Ordovician age. In a lot of regards, many tabulate coral skeletons (= fossils) look like the skeleton of the pipe organ coral, Tubipora musica.

Tabulate corals built reefs throughout the Ordovician, and are found in what now covers a geographical region from Alaska, to Baffin Island, and Texas. These particular types of corals declined after the Ordovician and while locally present, never seemed particularly dominant after that (Tasch, 1975). Even if they were reduced in dominance, however, they were a major reef-building component on reefs that were dominated by rugose corals as well as some massive sponges called stromatoporids.

Rugose corals looked a bit more like modern corals. The skeletons of some looked rather like a cow's horn with septa coming from the side. Presumably a more-or-less "normal" looking coral animal lived in the cavity. The rugose corals were similar to the tabulate corals, in showing an amazing amount of derivation and evolution of forms. In some cases, the forms have been tracked through several thousand feet of rocks deposited by sediments. Carruthers (1910) followed the evolution of one genus, Zaphrentites (1) through 4000 feet of Scottish rocks showing the change from the oldest species, Zaphrentites delanouei, through several intermediates to the youngest representative in those rocks, Zaphrentites disjuncta.

By the middle to late Paleozoic era, huge reefs were being formed. These were dominated by rugose and tabulate corals, and by thousands of species of associated animals. The scale of these reefs can be visualized by considering that the fossil remnants of such reefs constitute whole mountain ranges in Texas and elsewhere in the southwestern United States.

Then it All Changed

Over a very short geological time frame, possibly as short as a single really bad afternoon, but more likely over a period of about 10,000 to 30,000 years (Bowring, et al., 2001), animal life on Earth almost ceased. It has been estimated that up to 95% of all species in the seas went extinct. Life on land was hit similarly hard. Whole rich lineages became extinct or were severely reduced in numbers, including for example, ancestral mammals. As a typical example, all the sea urchins alive today are believed to be descendent from one genus, Miocidaris, which survived the extinction event. Similar reductions occurred in almost every group that survived the event (Benton, 2003).

The cause or causes of this extinction event, the worst in our planet's history, are unknown. The two leading contenders are an impact event similar to the one that eliminated the Dinosaurs 160,000,000 years later, and massive volcanic eruptions in an area of present day Siberia which could have liberated huge amounts of toxic gases into the atmosphere; still other causes also are being considered. Or, frankly, it could have been a combination of several different types of these catastrophic events occurring in close proximity, as there is some fairly good evidence for each. Whatever caused this extinction event had a profound effect on the environment; these effects were profound enough that they are still evident after a quarter of a billion years. It appears that there was a period of global warming, possibly caused by the massive eruptions producing the flood of basalt deposits called the Siberian traps. These eruptions were the largest volcanic eruptions on land in the last 600 million years, producing about 2,000,000 km³ (480,000 mi³) of basalt (Benton, 2003); this is roughly an area 2,000 miles long, by 1,000 miles wide covered half a mile deep in lava! Such eruptions, and there have been several much smaller ones within human history, produce significant amounts of what we can call "greenhouse" gases. The greenhouse effect of these eruptions was coupled with a period of reduced oxygen in the atmosphere (Grice, et al., 2005; Ward, et al., 2005). The atmospheric oxygen level was estimated to have dropped to about 16% and that oceans became anoxic below about 400 feet. Even in shallow waters, the oxygen tension of the water was likely quite low. These are decidedly NOT good conditions for life and whole groups of organisms perished. Among them were the rugose corals, and the tabulate corals were severely affected as well. Coral reefs disappeared. For over 10 million years after this extinction event, at the end of the Permian era, few coral fossils are found and no reefs existed.

Corals recognizable as modern scleractinian corals began to appear in the middle Triassic period. They proliferated widely and rapidly in the Mesozoic. At the time of the dinosaurs' extinction, the corals were about as diverse as they are today. Several groups went extinct with the dinosaurs, but others soon replaced them. However, the real survivors of this story are their ancestors; those hardy animals that survived the crash in life at the Permian period. During and for several score million years after this extinction event, the world's ocean was not the relatively benign place we see today. It was a harsh, demanding environment, and these hardy animals made it through.

Conclusion:

Marine animals that survived that horrible period of extinctions gave rise to all the animals we see in the seas today. The ruggedness of those ancestral animals lives on in modern reef animals, the corals and all others. Give them half a chance in an aquarium, and they will do well. All they need is a warm, nicely illuminated little sea to call their own, with some food to eat, and they will be hardier than most aquarists can imagine.

We are now likely in the midst of another great extinction event. It began on land with the extinction of most of the large mammals between 10,000 and 100,000 years ago (Beck, 1996; Ward, 1997), and has continued in the seas. Recently, it has been demonstrated that literally thousands of species have been eliminated from the North Atlantic in the last few thousand years by human overfishing (Jackson, 2001). Additionally, we are well into the process of raising the world's temperature to record high levels, and to suspect that this will lead to a global disaster such as occurred at the end of the Paleozoic is reasonable. Rational estimates are that over 100 species a day are now going extinct (Benton, 2003), so the planet's biota at the end of this century will be very different from when it began on 1 January, 2001. These changes will particularly impact coral reefs. Although many corals will likely persist (Hughes, et al., 2003), it is highly unlikely that anything familiar to one of us as a coral reef will be alive at the end of this century (Pandolfi, et al., 2003). It will be interesting to see how long the hardy survivors of the coral reef can withstand this new and present peril.

Humanity is, of course, the cause of the present extinction event. This makes it all the more important that we treat our aquarium animals with the care and concern that they need. By keeping our animals in their optimal physical conditions we can at least ameliorate a small portion of the human degradation of the reef environment.

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