By Chris Jury

Introduction

I began writing this series near the end of last winter, which was a doozy if you happened to find yourself in Eastern North America or Northern Europe. It was bloody cold and snowy too, breaking records in many places. At the same time that cold and snow records were being broken in these places, record hot and snowless conditions could be found from Western North America to Australia, South-East Asia, and beyond. These strange weather patterns were the result of a combination of factors, including an El Nino in the Pacific (which leads to cool, wet conditions in the Eastern U.S.), and a very negative Arctic Oscillation, (which poured cold polar air over the U.S. and Europe, and brought warmer mid-latitude air up over the Arctic). Lucky, lucky, similar conditions have developed this winter over North America and Europe, but for somewhat different reasons. We’ve had a La Nina in the Pacific since last summer, but globally we had very warm conditions during most of 2010 (in fact, 2010 is tied for the warmest year on the instrumental record), and especially in the Arctic. Sea ice in the Arctic grows and shrinks every year, in keeping with the seasons, but has been steadily shrinking to smaller and smaller sizes during the summer melt season over the last few decades. Last summer Hudson Bay largely melted, and was still substantially ice-free into November. Suffice to say, this is very unusual, and instead of a normal polar vortex and normal Jet Stream (which keeps frigid Arctic air in the Arctic, and warmer mid-latitude air in the mid-latitudes), the Jet Stream has dipped very far south this winter, letting that frigid Artic Air dump into North America and Europe, and allowing warmer mid-latitude air into Northern Canada. As New York, Boston, Dallas, and London have been frozen and buried in snow, the barely inhabited stretches of Northern Canada and the Arctic have been comparatively balmy. Oy what a winter!

Sandwiched between these two winters was, for most places, a brutally hot summer. Once again, the Eastern U.S. set lots of temperature records, but this time due to the record heat! Last year we saw large-scale coral bleaching in the Indian Ocean, throughout South-East Asia, and the Western Pacific. Prolonged high seawater temperatures lead to coral bleaching, and seawater temperatures were quite high in these places (often 2-5 F above normal summer maximums). As I discussed last time, local weather conditions can work to either enhance or reduce the risk of bleaching. In some places, such as the Northern Caribbean, local weather patterns cooled conditions down a bit, and little bleaching was reported. In the Eastern Caribbean, conditions ended up hotter, and there was significant bleaching there. While I’ve discussed it a bit, let’s look a bit more closely at bleaching, and what it means for corals.

Coral-algal symbiosis

Reef-building, tropical corals are a complex association of an animal (coral), a plant (dinoflagellates of the genus Symbiodinium referred to as zooxanthellae), and a variety of other microrganisms (including perhaps thousands of species of bacteria, cyanobacteria, archaea, and viruses). In healthy associations, these associations allow the coral animals to rapidly build calcium carbonate skeletons. In the classical classification scheme of animal, vegetable, or mineral, corals get a “yes, yes, and yes!”

Not all corals host symbiotic algae – only about half of described species do – but all of the species that build tropical coral reefs have algal symbionts. A small subset of coral species are facultatively zooxanthellate: some individuals of the species host zooxanthellae, while other individuals do not, depending on the conditions in which they grow. For example, several such species, (including those of the genera Oculina and Astrangia) grow along the East Coast of the U.S., from Florida to Massachusets. Individuals that grow in areas with enough light host zooxanthellae whereas those that grow in dark areas (e.g., deep water, or under overhangs) don’t.

Not long after zooxanthellae were discovered in corals, some thought that the corals were diseased, infected by the algae. As a hold-over of this historic view, when newly settled baby corals take up their first zooxanthellae from the environment we say that they become “infected” with zooxanthellae. Over time we came to realize that the zooxanthellae are not harmful infections for the corals and in fact benefit them. For most tropical coral species the zooxanthellae are absolutely essential for their survival. Corals kept in dark conditions for months would lose most of their zooxanthellae and many species would ultimately die, even when fed zooplankton. Zooxanthellate corals kept in the light grew (= calcified) faster than those in the dark. Indeed, when experiments were performed with some of the few facultatively zooxanthellate species it was found that individuals that had zooxanthellae and were given light grew faster than those that either did not have zooxanthellae, or those that had zooxanthellae, but were kept in the dark. Somehow, it was clear, zooxanthellae benefit the corals that host them, and allow those corals to grow much faster than they otherwise could, if given sufficient light. In fact, most tropical corals didn’t seem to be able to survive without their symbiotic zooxanthellae, but at first it wasn’t clear why.

In due time it was discovered not only that zooxanthellae live in coral tissues, but that they actually feed their coral hosts! Zooxanthellae are fairly typical dinoflagellates, roughly similar to those that occasionally make a nuisance of themselves in reef tanks. Given light, a source of carbon dioxide, water, and nutrients they produce simple carbohydrates via photosynthesis. Under ordinary circumstances, a fraction of this energy-rich food (effectively sugar-water) leaks out of the zooxanthellae into the coral tissues, and is used by the corals as food. In fact, the first estimates of this process suggested that up to 95% of the food produced by the zooxanthellae is given to the corals; that’s pretty stiff rent! Later estimates have suggested that that percentage translocated to the corals is usually less than that (typically 50-75%, but in some circumstances can approach zero).

Healthy zooxanthellae generally feed their coral hosts, and that seems to be part of the reason zooxanthellate corals grow so quickly, but not all of the reason. Even heavily fed corals without zooxanthellae do not grow as fast as those that have zooxanthellae, suggesting that there is some additional benefit(s) to corals that have zooxanthellae beyond receiving food. Recent work points to the oxygen that zooxanthellae produce as very important for supporting high rates of coral calcification (in addition to food), but they may provide some additional benefits as well. These are problems we’ve been studying for more than 50 years. Progress has been slow largely because these problems are, logistically, very difficult to figure out, though there has been renewed interest in recent years.

Coral bleaching

Certain kinds of stress, especially high temperature and high light stress, damage or kill the zooxanthellae living in coral tissues. Zooxanthellae range from brown to golden brown, and corals that are predominately some shade of brown get the majority of their coloration from their zooxanthellae. Corals that are other colors (e.g., red, blue, green, purple, etc.) take their pretty colors from pigment proteins made by the corals themselves in combination with the brownish shades of the zooxanthellae. When corals experience stress that causes serious damage to the zooxanthellae, many of the zooxanthellae are expelled by the corals (never 100% though even severely bleached corals still have at least 10% of their normal density of zooxanthellae). In the case of brown corals, the loss of most of the algae or algal pigments leaves the coral tissues fairly transparent and the white skeleton beneath becomes clearly visible, giving the corals an overall white appearance. Colorful corals usually become pastel shades of their original colors, owing to the pigment proteins made by the corals without much contribution of the dark zooxanthellae. If the stress is severe, many colorful corals ultimately lose or metabolize their colorful pigment proteins, giving them the same overall whitish appearance. This severe lightening or whitening of the coral’s appearance is what we call coral bleaching.

An important point to understand is that coral “bleaching” is a general description of a symptom, and that several types of stress and physiological processes can lead to a bleached appearance. Bleaching is a question of degree, and not of kind. That is, corals are not either bleached or non-bleached/bleaching is a sliding scale, with a range of severity. We now understand a great deal about what happens to corals and zooxanthellae when exposed to stressors that cause bleaching, especially high temperature and high light stress, though there is a great deal more left to learn. Photosynthesis normally produces lots of molecular oxygen (O2), but also produces small amounts of what are called reactive oxygen species (ROS). These include chemical species such as oxygen radicals or peroxides and cause oxidative stress, cellular damage, and in large amounts can be quite deadly. All organisms have physiological safeguards to protect against normal amounts of ROS production. For instance, when we eat foods that are high in antioxidants (like blueberries) they give us the building blocks to protect our bodies from oxidative stress. When corals undergo high temperature or high light stress the photosynthetic processes in the zooxanthellae begin to run amok, producing far more ROS than normal, and overwhelming the protective systems in place. This stress harms corals and their zooxanthellae in the same way that drinking chlorox bleach would harm us. Low temperatures can also lead to coral bleaching, though prolonged low temperatures are as likely to kill the corals directly as cause bleaching.

Bleaching is a doubly whammy for most corals. Firstly, without the food they would normally get via photosynthesis, bleached corals can very quickly begin to starve. Second, bleaching (especially due to high temperature or high light) causes serious oxidative damage to corals. Bleaching is often deadly, though many corals can recover if the stress is removed (e.g., temperature is lowered) and they are kept in good conditions. However, even under the best scenario bleaching reduces coral growth rates substantially (even as low as zero) and it can take months or even years for them to fully recover. Suffice to say we want to avoid bleaching our corals like the proverbial plague. To do so we need to know the temperatures that will lead to coral bleaching. What are the upper and lower temperature limits for corals? Ah, there lies the rub.

Coral temperature limits

Most tropical corals can tolerate exposure to temperatures as low as 70-72 F for several weeks, and to 65 F for several hours to a few days without bleaching or dying. However, most tropical species are killed when exposed to 65 F for more than a week or two. Some tropical to subtropical corals are able to tolerate somewhat lower temperatures, but these are rarely collected for reef tanks. I don’t know any aquarists that would intentionally let their tank drop to 65 F or below, nor would I recommend it. I would suggest that the lowest safe long-term temperature limit for our reef tanks is about 72 F.

Where do high temperature bleaching thresholds lie? As nice as it would be if I could just give a number, unfortunately this isn’t possible. As we’ve seen, coral reefs in different regions see somewhat different temperature regimes, and corals that grow in different regions have different temperature tolerances. A constant temperature of 85 °F for a month causes severe bleaching in most Hawaiian corals, whereas those growing in American Samoa can easily tolerate these conditions, and might not show serious bleaching until temperatures hit 88 F for a month.

Upper temperature limits of corals depend on many factors. Corals in different regions have adapted to distinct temperature regimes over evolutionary time. Even though the same species of coral might be found here in Hawaii (relatively cool) and in American Samoa (relatively warm) the individual corals that live in these two places have physiological differences that allow them to tolerate somewhat different temperature ranges. Those individuals that are able to tolerate higher temperatures have an advantage in American Samoa, and have proliferated there, whereas those with lower temperature tolerances have an advantage in Hawaii, and have proliferated here. Previous experience also affects coral temperature tolerances. For instance, corals that have experienced a little bit of high temperature stress are less likely to bleach later than corals that have not recently experienced higher temperatures. Lastly, the zooxanthellae that corals host vary in their own temperature tolerances. Some corals are able to host several different types of zooxanthellae, whereas other corals are less flexible. Those corals that are able to host types of zooxanthellae with higher temperature tolerances themselves able to tolerate higher temperatures as a result. Considering all these sources of variation, the bleaching thresholds for most tropical corals fall somewhere within the range of about 85-92 F (depending on the coral), sustained for a month.

You may have noticed that I keep qualifying these bleaching thresholds by saying “sustained for a month.” Bleaching thresholds in actuality depend on many interacting factors, not just temperature. For instance, many Hawaiian corals bleach when exposed to 85 F for a month. However, they tolerate exposure to 90 F for a couple hours without bleaching, whereas exposure to 9F for several days not only causes bleaching, it kills them dead. A little too hot for a long time and a lot too hot for a short time can be equally bad. Most corals are more than capable of tolerating brief (minutes to hours) exposure to even rather high temperatures though. Another major factor that plays into bleaching thresholds is light intensity. Corals exposed to high light intensity incur much more photodamage and have lower bleaching thresholds than those exposed to lower light intensity. For example, in the bleaching episode in Kaneohe Bay that I discussed last time, corals in turbid water close to streams did not bleach even though they were exposed to the same temperature stress for the same period of time as corals in clear water that did bleach.

Most hobbyists keep corals, reef fish, and other organisms that originated on different reefs in different regions in the same aquaria. For example, lots of corals come from the relatively hot Indo-Pacific, but lots of corals also come from somewhat cooler Fijian waters. Fijian corals generally have lower thermal limits than Indo-Pacific corals. Reef fish also come from diverse locations, including the hot Philippines, and the cooler Hawaiian archipelago. Even though Indonesian corals and Filipino fishes may very well have higher temperature tolerances than their Fijian and Hawaiian counterparts, if we approach those tolerances in captivity we’ll very likely be stressing and killing our animals from slightly cooler regions. For these reasons I would suggest that the highest safe long-term temperature limit in our aquaria is about 84 F. This is the highest temperature that I would feel fairly safe maintaining a reef tank at for more than a few weeks, since the tank is likely to contain corals and other animals that would suffer at higher temperatures.

Should we keep our reef tanks at 84 F (my recommended upper temperature limit) for long stretches of time? In my opinion, no, we should not because that is hotter than most corals and other reef animals “want” to be most of the time.

If corals could talk!

What would they say? First, they might tell us to quite hacking them apart and gluing them to rocks. What would they tell us about their preferred temperatures though? Unfortunately they can’t tell us directly what temperatures they want, but there are ways that we can make educated guesses about what they would tell us if they could.

Most reef aquarists want their corals and other animals not only to grow, but to grow quickly. Tissue and skeletal growth depends on a large number of interacting physiological processes. Maximizing growth rates therefore depends on maximizing the net effect of these interacting processes. Corals and all sorts of other organisms grow fastest at particular temperatures, and more slowly at temperatures above or below these thermal optima. If we were to plot their growth rates as a function of temperature, we would see a roughly bell-shaped relationship. Only a handful of studies have directly characterized these temperature-growth relationships for corals, but they are extremely useful in helping us decide where to maintain temperature in our aquaria.

Some of the earliest work examining the effects of temperature on coral physiology were performed by Paul Jokiel and Stephen Coles here in Hawaii. They worked with several species of coral including Pocillopora damicornis, Fungia scutaria, and Montipora capitata. These three species collected in Hawaiian waters all showed rather similar growth responses to temperature. They attained the highest growth (= calcification) rates at a temperature of about 78.5 F, with lower growth rates significantly above or below that optimum. As is always true in the real world, the actual data are a bit scattered around the mathematical fit we use to describe them. It therefore makes little sense to say that the thermal optimum is precisely 78.5 F. High growth rates of 80-100% of the maximum rate were attained within the range of 75-82 F, whereas low growth rates <25% of the maximum rate were obtained below about 72 °F or above 84 F. Very similar results were obtained by Marshall and Clode with the corals Galaxea fascicularis and Dendrophyllia sp. collected from the cool, southern part of the Great Barrier Reef. High growth rates of 80-100% of the maximum rate were obtained within the range of about 75.5-79.5 F with a thermal optimum at about 77 F. Low growth rates of <25% of the maximum rate were obtained below about 72.5F or above about 84 F. In contrast to these cooler reefs, Carricart-Ganivet has shown higher thermal optima for the coral Montastraea annularis on somewhat warmer reefs. Populations from the Gulf of Mexico have thermal optima in the neighborhood of 80-81 F and experience low growth rates below about 75 F. Caribbean populations have thermal optima closer to 82-83 F and experience low growth rates below about 77 °F. Similar results have been found for Porites spp. and other corals growing in other warm areas.

On the cool end of the spectrum we find corals that grow most rapidly at temperatures in the range of about 76-81 F and slowly at temperatures below 73 F or above 84 F. On the warm end we find corals that grow most rapidly in the range of about 79-84 F and slowly at temperatures below 75-77 F or above 86 F or so. Different corals have different thermal optima, and if we are keeping them in the same aquaria (which we all are) we cannot possibly achieve the thermal optima for every coral at the same time. However, there is overlap in the ranges where growth is high for corals from cool and warm reefs: about 77-82 F. Below 77 F temperature limits the growth rate of some corals adapted to warm conditions, whereas above 82 F temperature limits the growth rate of some corals adapted to cooler conditions. Within the range of 77-82 F, and especially near the mid-point of that range (79-80 F) corals from diverse regions are at or near their thermal growth optima, and should be able to attain high growth rates as a result. These temperatures are very typical for coral reefs worldwide and are clearly acceptable for tropical reef fish and invertebrates as well as corals.

My recommendation as the most appropriate temperature for reef tanks is therefore somewhere in the range of 77-82 F, with 72 F and 84 F as lower and upper limits, respectively. I base these recommendations on the data available about the effects of temperature on coral physiology, on real coral reef temperatures, and on experience. Suffice to say, these temperatures work. They have worked for years in my own reef tanks and in those of countless aquarists that I have communicated with. While my recommended target temperature is about 79-80 F (but anywhere in the range of 77-80 F as totally acceptable in my opinion), I leave it to you, the aquarist to determine if this recommendation makes sense for your own aquarium. I have seen reef tanks run long-term at temperatures of 74-76 F and at 82-84 F that were extremely successful, and amongst the most beautiful tanks I’ve ever seen. A temperature below 77 F or above 82 F does not necessarily imply failure or even undue problems. Instead, the temperature range I recommend is, in my mind, the best compromise available to keep everything in the tank as happy and healthy as possible.

Conclusion

In this series we’ve examined temperatures on real coral reefs, coral bleaching, and high and low thermal limits of corals. In this article I gave my recommended target temperature range in reef aquaria, which is derived from what we know about the effects of temperature on corals, temperatures on reefs in nature, and personal experience. Next time we’ll discuss temperature variation in the aquarium (just how stable we do want temperature to be?), as well as what to do when equipment fails and our tanks are a lot hotter or cooler than we want them to be.

Previous Readings
The Great Temperature Debate Part I
The Great Temperature Debate Part II

Photos by Lissa Mann.