By: RICHARD HARKER
The November 1997 issue of Aquarium Frontiers Online included an article entitled, “What are Natural Reef Salinities and Temperatures…Really…and Does It Matter?†(Shimek 1997). In the article, the author argued that hobbyists maintain reef tanks at artificially low temperatures and, “forcing animals to ’live on the edge’ of physiological disaster is doubtless the cause of many unnecessary deaths.†While the logic of the article appeared sound, the author’s conclusions are demonstrably false, and can be refuted by examination of his own references.
Were this solely an academic issue with no consequences to hobbyists, the article would not warrant a second look. However, hobbyists reading the article might conclude that maintaining a reef tank at the commonly recommended temperature of 74 to 77 degrees Fahrenheit (23 to 25 degrees Celsius) will have dire consequences to the tank’s inhabitants. Quite the contrary, following the recommendation of the author to raise tank temperatures to those found on a tropical reef could have serious negative consequences. Hobbyists need to fully understand the relationship between temperature and the health of hermatypic (reef building) corals before they follow the author’s advice.
The thesis of the author can be simply stated: Because coral diversity is greatest in the Malay Peninsula-Indonesia-New Guinea area, corals should be maintained in the conditions found in this area. While it is true that coral diversity is greatest near the equator, the reasons are far from clear. Veron (1995) lists 34 hypotheses that attempt to explain why the center of diversity lies where it does. Stehli and Wells (1971) proposed that coral species spread by adapting to less favorable conditions in marginal regions. This is essentially the author’s “adaptive radiation†argument. Veron writes of this hypothesis, “This classical view cannot now be supported: distributions of species display no evidence of geographic displacement and very little evidence of amphitropical distributions†(page 51). If there is any argument for raising reef tank temperatures, it should not be based on the theory of adaptive radiation.
The author argues that because the majority of imported corals come from reefs near the equator, we should maintain them in similar equatorial conditions. For most physical and chemical parameters, this is simply not practical. Corals on a typical equatorial reef receive three to 10 times the light found over a typical hobbyist’s reef (personal observation). Water movement in a typical reef tank is a fraction of the water movement encountered by a coral on even a back-reef, let alone a fore-reef (Harker 1998). Nutrient levels in a hobby tank are a magnitude or two greater than those found on a typical reef (Crossland 1983). Clearly, in most regards, the conditions found in our tanks do not bear any resemblance to the conditions found on the typical ntural reef. If, as the author argues, inhabitants in our tanks are forced to live on the edge of physiological disaster, it is not clear which compromised parameter is at work.
The author makes much of the fact that water temperatures near the equator average around 84 degrees Fahrenheit and that “the commonly advised mini-reef temperatures of 74 to 77 degrees are stressing most of the animals unnecessarily and, in some cases, severely.†Stress is never defined and “severely†is never explained.
The author’s long list of references in the same paragraph paints a somewhat different picture of the situation. Koehn (1989) defines stress as, “any environmental change that acts to reduce the fitness of an organism.†The references make it clear that based on that definition, corals maintained at the temperatures normally encountered in a hobbyist’s reef tank are not stressed because the temperatures do not reduce the fitness of corals.
Clausen and Roth (1975) studied the calcification rates of Pocillopora damicornis. They found that calcification rates peaked at 27 to 30 degrees Celsius (around 80 to 86 degrees Fahrenheit), but that calcification took place at temperatures as low as 17 degrees (63 degrees Fahrenheit). Coles and Jokiel (1977) looked at the relationship between temperature and coral metabolism and found that the two were positively correlated. In other words, as temperature declined, metabolism declined. However, they found that even at 18 degrees Celsius (64 degrees Fahrenheit), photosynthesis met the metabolic needs of the Pocillopora, Montipora, Porites, and Fungia studied. None of the studies cited by the author suggests that corals maintained at 74 to 77 degrees Fahrenheit are stressed. Quite the contrary, the studies portray corals living at these temperatures as normal, healthy corals.
A search of Biological Abstracts found 34 articles on temperature and coral stress. Only one study addressed stress induced by below normal temperatures. Studying corals in the Florida Keys, Roberts et. al. wrote, “sustained temperatures below 16 degrees are detrimental to most reef building corals.†They noted that at 12.9 degrees Celsius (55 degrees Fahrenheit), corals died. They did not believe corals at 16 degrees Celsius were stressed.
The other 33 articles focus on stress induced by higher than normal temperatures. This is because corals are much more sensitive to above normal temperatures than below normal temperatures. Elevated water temperatures are known to induce bleaching, the loss of symbiotic zooxanthellae (Brown 1997) and oxidative stress (Lesser 1996).
The author wrote that “(at these low temperatures) coral growth is effectively minimal and will be hard to detect…After they use the last of their stored reserves, they will die slowly.†In spite of the plethora of citations offered as proof, none supports this claim.
The Houck et al. (1977) reference cited by the author measured linear growth rates of Montipora, Porites and Pocillopora over a temperature range of 21 to 28 degrees Celsius. Growth rates were lowest at 21 degrees (70 degrees Fahrenheit) for Pocillopora and Montipora, but the decline in growth from higher temperatures was less than 30 percent (for example, a decline from 7 to 5 millimeter extension in 14 weeks for Montipora). A growth rate of nearly 20 millimeters per year for Montipora growing at 70 degrees Fahrenheit can hardly be called “minimal.†There is no evidence that at these temperatures any of the corals died, slowly or otherwise.
In a study of Acropora pulchra, Yap and Gomez (1984) found that temperature and growth rates were positively correlated, but that the difference was less than 3 centimeters per year (cm/yr). Corals growing at cooler temperatures (79 degrees Fahrenheit) grew at a rate of 13.1 cm/yr rather than the 15.8 cm/yr at warmer temperatures. A growth rate of nearly a half-foot per year is clearly contrary to the picture the author paints of a slow death.
The author wrote “It (a coral) is metabolizing very slowly, and in effect, is almost in a state of suspended animation.†One measure of the physiological condition of a coral is whether it is capable of spawning. Babcock et al. (1994) wrote of mass spawning on the reefs surrounding the Houtman Abrolhos Islands of Western Australia. At a latitude of 28 degrees South, these reefs lie at the most southern limit of coral reefs. Water temperatures on the reefs range from 21.5 to 24.1 degree Celsius. Even at these temperatures, the authors found that 94 of 107 scleractinians on the reef spawned. Given the metabolic cost of spawning, no coral “in a state of suspended animation†could spawn, yet this takes place yearly in 70- to 75-degree Fahrenheit water.
At 24 degrees North, the reefs off Ryukyu Islands represent some of the most Northern reefs in the world. Typical water temperatures range from 20 to 30 degrees Celsius. Yet, even at this extreme latitude, the reefs are dominated by tabular and staghorn Acropora (Matsuda 1989). Contrary to the article’s claims, corals are calcifying, reproducing, and thriving in environments where temperatures are close to hobbyist tank temperatures.
Studying reefs off the Ryukyu Islands, Veron and Minchin (1992) concluded, “the minimum sea surface temperature for coral reef development is 18 degrees C (64 degrees F.)…Approximately half of all species tolerate temperatures 4 degrees below the 18 degree minimum (57 degrees F.).â€
The evidence is clear. Corals around the world grow in a wide range of conditions. While coral reefs can be found in waters much warmer than a typical reef tank, healthy corals can also be found in waters much cooler than a typical reef tank.
Negative consequences of higher temperatures
As shown above, the author’s contention that corals suffer at lower temperatures is clearly not true. While corals grow more slowly at lower temperatures, there are no other negative consequences. One might therefore conclude that higher temperatures in a captive system would be preferred over lower temperatures, if for no other reason than that corals will grow faster. The issue is more complex than that.
Faster growth on an open reef has only positive consequences. Competition for space is fierce, and a coral that can grow faster than other sessile organisms will out-compete organisms that grow more slowly (Rinkevich et al. 1985). In a closed system, however, there are negative as well as positive consequences.
Metabolic waste products are produced as by-products of the process of photosynthesis and respiration. On the open reef, metabolic waste products are quickly consumed by other organisms or carried off by the currents. In a closed system, they will build up unless they are removed in some fashion. Denitrification by live rock and sand, foam fractionization and activated carbon each can aid in the removal or breakdown of particulate and dissolved organics. Higher temperatures increase metabolism, which in turn increases metabolic waste products. The increased metabolic waste products put an additional load on the system and increase the need for mechanisms capable of removing the increased waste.
The increased biological load resulting from faster growth is similar to the increased load associated with adding too many corals to a tank. As the tank load increases, it becomes increasingly difficult to control the buildup of organic compounds. Overstocking a tank and increasing the water temperature at the same time can place such a high load on a reef tank that the system cannot remove the waste products as quickly as they build up. A hobbyist who is guilty of packing a tank full of corals would be wise to maintain lower tank temperatures so that the mechanisms in place to remove organics from the water will not be overloaded.
Periodic algae outbreaks is a sign of excessive organics in the system and evidence of an unstable tank. Raising the tank temperature before stabilizing the tank could make the battle with algae more difficult. If a hobbyist is suffering loses of inhabitants due to unclear reasons, raising the tank temperature could increase the stress of the remaining inhabitants and thus losses.
Maintaining higher tank temperatures also increases the possibility of coral bleaching. Mid-80s temperatures are close to the lethal limits of corals (Coles and Jokiel 1976). It has also been shown that corals can bleach even below the temperatures normally associated with bleaching if the temperature changes rapidly (Jokiel and Coles 1990). Higher tank temperatures require closer monitoring and greater control to see that the tank temperature does not go higher than intended.
A hobbyist who decides to increase the temperature of his or her tank needs to make sure that it is a stable tank with healthy corals, no sign of algae and has equipment necessary to efficiently remove the increased waste products. Under these conditions, it would be safe to increase the tank’s temperature.
Increasing the tank temperature will increase calcium needs of the tank. The hobbyist who wants to maximize growth by raising the tank temperature will need to increase calcium supplementation and more carefully monitor calcium and alkalinity levels.
There is one final concern that a hobbyist should be aware of before raising the temperature of his or her tank. A pathogen of undetermined origin has been attacking corals in numerous reef tanks. Dubbed RTN (rapid tissue necrosis) by the hobby, the pathogen has been blamed for causing the death of many small-polyped stony corals — in some cases entire tanks of coral. While the root cause remains uncertain, corals have responded to the use of antibiotics. This implicates a bacteria, and some tissue tests have found the bacterium Vibrio on dead coral tissue.
If Vibrio or some other bacteria is ultimately linked to RTN, higher tank temperatures could worsen RTN outbreaks by increasing the rate at which the bacteria reproduce. In experiments infecting Oculina corals with Vibrio, at 26 degrees Celsius, 90 percent of corals eventually became infected, while none did at 16 degrees (Kushmaro et al. 1996). Consequently, hobbyists who are fighting an outbreak of RTN would be wise to maintain a lower tank temperature.
Conclusion
The commonly recommended reef tank temperature range of 74 to 77 degrees Fahrenheit (23 to 25 degrees Celsius) is well within the temperature range in which healthy reproducing corals thrive. There is no evidence in the scientific literature that supports the belief that corals living at these temperatures are stressed, or that their health is in any way compromised. In contrast, the scientific literature makes clear that excessively high temperatures can be detrimental to corals.
Rather than maintain a captive reef at the high end of the temperature range in which coral reefs are found, hobbyists would be wise to maintain their reef tanks at the mid-point temperature of coral reefs, approximately 79 degrees Fahrenheit (26 degrees Celsius). This temperature provides the largest margin of safety for the hobbyist, as corals have been shown to thrive in water several degrees on either side of this temperature.
