Reef temperature

[Reverend Reefer]

can someone explain how temp affects your reef tank?

i used to always keep my tank around 77-80, only because that's what seemed to be what everyone else would say it should be at.

after taking out a lot of my old powerheads (koralias and maxi-jets) and replacing with vortech, my temp has significantly dropped without me noticing (plus my thermometer broke). so for about 3 months now, i've been running my tank around 73-75 or so.

everything seems fine and quite happy?

i plan to add a heater in my sump when i get a chance but i'm just wondering now, where did this 77-80 benchmark come from and what happens as temp drops? i remember reading once that there's less oxygen at warmer temps somewhere but i just don't know if this is true? is there some function that dictates what happens as a function of temperature or is this just based on what temp the ocean reefs are?

i also remember from keeping seahorses that they are less prone to disease if you can keep the temp around 73 or at least below 75. not sure exactly why other than they generally inhabit cooler temp water.... are there maybe diseases that prefer hotter water? i know in lakes they say if the water gets warm there tends to be more bacterial blooms....? or is that a myth too?

[reefermad619]

From Randy Holmes article, Reef Parameters.


Temperature

Temperature impacts reef aquarium inhabitants in a variety of ways. First and foremost, the animals' metabolic rates rise as temperature rises. They may consequently use more oxygen, carbon dioxide, nutrients, calcium and alkalinity at higher temperatures. This higher metabolic rate can also increase both their growth rate and waste production at higher temperatures.

Another important impact of temperature is on the chemical aspects of the aquarium. The solubility of dissolved gases such as oxygen and carbon dioxide, for example, changes with temperature. Oxygen, in particular, can be a concern because it is less soluble at higher temperature.

So what does this imply for aquarists?

In most instances, trying to match the natural environment in a reef aquarium is a worthy goal. Temperature may, however, be a parameter that requires accounting for the practical considerations of a small closed system. Looking to the ocean as a guide for setting temperatures in reef aquaria may present complications, because corals grow in such a wide range of temperatures. Nevertheless, Ron Shimek has shown in a previous article that the greatest variety of corals are found in water whose average temperature is about 83-86° F.

Reef aquaria do, however, have limitations that may make their optimal temperature somewhat lower. During normal functioning of a reef aquarium, the oxygen level and the metabolic rate of the aquarium inhabitants are not often important issues. During a crisis such as a power failure, however, the dissolved oxygen can be rapidly used up. Lower temperatures not only allow a higher oxygen level before an emergency, but will also slow the consumption of that oxygen by slowing the metabolism of the aquarium's inhabitants. The production of ammonia as organisms begin to die may also be slower at lower temperatures. For reasons such as this, one may choose to strike a practical balance between temperatures that are too high (even if corals normally thrive in the ocean at those temperatures), and those that are too low. Although average reef temperatures in maximal diversity areas (i.e. coral triangle centered Indonesia,) these areas are also often subject to significant mixing. In fact, the cooler reefs, ( i..e. open Pacific reefs) are often more stable at lower temperatures due to oceanic exchange but are less tolerant to bleaching and other temperature related perturbations.

All things considered, those natural guidelines leave a fairly wide range of acceptable temperatures. I keep my aquarium at about 80-81° F year-round. I am actually more inclined to keep the aquarium cooler in the summer, when a power failure would most likely warm the aquarium, and higher in winter, when a power failure would most likely cool it.

All things considered, I recommend temperatures in the range of 76-83° F unless there is a very clear reason to keep it outside that range.

[Reverend Reefer]

this is very interesting. thank-you!

so in effect, having my tank at these lower temps has:

a) increased the oxygen levels of my system,
b) lowered waste production, and
c) slowed down metabolic rates

weird. i guess once i get the heater installed i can expect more growth as a function of temperature increase (along with more waste production).

[bertoni]

Some animals might start dying at those temperatures, but that's hard to predict. I lost a royal gramma once to a blown heater, but I have forgotten how far the temperature dropped. The yellow watchman goby pair in the tank didn't show any signs of distress.

[drew1]

I personally prefer about 78 for my tanks, but they seem to do equally well anywhere from 75 to 82. Much higher or lower and they start looking stresed. But I think that is more of a function of what they are used to. So if yours are used to 73-75 I would only very slowly and cautiously raise there temps. The change, if made to quickly will stres them out.

[GT3]

78-85 is the ocean temperature in figi area where my live rocks from, so I'll keep the temp in that range, usually 79-81, never under 75.

[ezhoops]

I keep mine right around 71-72 with no problems, fish eat less grow slower, corals react fine, more oxygen, not to mention when I dove the GBR the water temp was a little lower than that.

[greenbean36191]

so in effect, having my tank at these lower temps has:

a) increased the oxygen levels of my system,
b) lowered waste production, and
c) slowed down metabolic rates

a) Well, at the temperatures we're talking about, the effect of temperature on oxygen saturation is pretty small. Going from 83 to 73 only increases the saturation by 8%. The effect is real, but that amount is essentially meaningless in the event of a power outage.

b&c) It really depends. In some species, metabolism and waste production are temperature dependent. Many other species' metabolisms compensate for temperature, so that there is little or no change in metabolic rate across normal temperatures and consequently no change in waste production or oxygen consumption.

not to mention when I dove the GBR the water temp was a little lower than that.

71 or 81? 71 is seriously cold for the GBR or reefs anywhere in the world and temps in that range have been associated with coldwater bleaching on parts of the GBR.

If you're interested, you can actually see real time and historical temp data at a variety of locations and depths on the GBR at this site: http://data.aims.gov.au/aimsrtds/fac...readings.xhtml

FWIW, the absolute coldest reefs have an average temperature of 70, but an average reef sees a low temp of about 77.

[greenbean36191]

I am actually more inclined to keep the aquarium cooler in the summer, when a power failure would most likely warm the aquarium, and higher in winter, when a power failure would most likely cool it.

This is just about the opposite of what you should be doing. Corals have a wide range of acceptable temperatures, but they can only tolerate temps within about 2-4 degrees of those that they've been acclimatized to. Keeping temps cooler in the summer and warmer in the winter maximizes the temp spike in the event of a power outage and makes it much more likely that the temp will exceed those limits.

[Reverend Reefer]

Originally Posted by greenbean36191

a) Well, at the temperatures we're talking about, the effect of temperature on oxygen saturation is pretty small. Going from 83 to 73 only increases the saturation by 8%. The effect is real, but that amount is essentially meaningless in the event of a power outage.

b&c) It really depends. In some species, metabolism and waste production are temperature dependent. Many other species' metabolisms compensate for temperature, so that there is little or no change in metabolic rate across normal temperatures and consequently no change in waste production or oxygen consumption.

hmmm, ok now i'm confused. if a=oxygen saturation, b=metabolism, and c=waste production, are somewhat irrelevant based on your arguments, then temp is not a factor when comparing the lower 70s range as compared to the higher 70s -low 80s range? if i understand what you're saying correctly?

[Reverend Reefer]

just from gathering opinions here, it seems the only argument for keeping our tanks in the higher temps is that natural reefs are at higher temps, although this also seems to be a point of contention? not that this isn't a good argument, just that there seems to be different opinions on what natural reefs seem to be at. on the other end of the scale, seems like some people are able to keep healthy reef tanks at lower temps with physiological-metabolic type arguments, although this also seems to be a point of contention as well. i wonder where the 77 degrees benchmark came from when i first started reef keeping? a quick google search also comes up with varying arguments on both ends of the scale....

now i don't know whether to put a heater in or not. i guess the other problem is instrumental, my heater's thermostat doesn't seem reliable. it was a pricey heater that eats up alot of power, but it doesn't seem to turn off at the right temperature that the dial is set at and i'm a little afraid i'd shock everything if it goes on too strong and for too long. seems that many people have this problem with many of the readily available heaters.

[drew1]

I believe the important point is to be within the acceptable norms AND be constant. The heater and/or chiller are there to prevent large swings in temp. A couple degrees due to lighting is ok (it happens on the shallow reefs too), but avoid the big swings.

[Reverend Reefer]

Originally Posted by drew1

I believe the important point is to be within the acceptable norms AND be constant. The heater and/or chiller are there to prevent large swings in temp. A couple degrees due to lighting is ok (it happens on the shallow reefs too), but avoid the big swings.

i guess all i'm trying to figure out is what are the acceptable norms and what are the caveats or benefits with being above or below this seemingly mythical 77 standard? i understand being constant and not having large swings. what i'm trying to figure out is if its worth my while to up my temp in the high 70s-low 80s or stick with my low 70s and sort of gauge from everyone here on RC what are the opinions, experiences or justifications for both ends of the scale....

[drew1]

If your tank is happy at 73-75 don't adjust it because others say you should. However you may find that some new corals don't appreciate the chill, especially if they are used to 80. But if you tank is established and happy don't change it. If down the road you decide 78 is a better temp then make the change slowly.

[PDAlber]

Originally Posted by greenbean36191

This is just about the opposite of what you should be doing. Corals have a wide range of acceptable temperatures, but they can only tolerate temps within about 2-4 degrees of those that they've been acclimatized to. Keeping temps cooler in the summer and warmer in the winter maximizes the temp spike in the event of a power outage and makes it much more likely that the temp will exceed those limits.

You ever do any diving off South Florida during the summer? There are upwellings that occur on a fairly regular basis, where the bottom temps can drop from the mid 80's to the low 60's in very short order. Last year, coral bleaching was reported when the temps stayed in the 50's for several days. I frequently swim through thermoclines where the water temp changes 5-10 degrees or more. As the currents move these layers of warmer and cooler water down the reef, the creatures experience temps that can change significantly in just minutes. Thats not to say someone would want to duplicate those swings in their aquarium, but I doubt it would be fatal should you have a power failure.

[bertoni]

Most of the animals we keep are from reefs at temperatures more like 82 F, which is what I targeted as the base temperature. Depending on what's actually in the tank, something in the low 70's might be appropriate, but that's not a tropical reef temperature. There are coral reefs living at a lot of temperatures around the world. For animals from tropical reefs, I'd expect them to be a lot healthier at normal temperatures for them, although they might survive in a larger range in a tank with no competitors from cooler areas.

[greenbean36191]

hmmm, ok now i'm confused. if a=oxygen saturation, b=metabolism, and c=waste production, are somewhat irrelevant based on your arguments, then temp is not a factor when comparing the lower 70s range as compared to the higher 70s -low 80s range? if i understand what you're saying correctly?

Well, it's a really complicated topic that doesn't lend itself to simple explanations, so don't feel too bad about being confused. The important part though is that metabolism and waste production are usually irrelevant at normal temperatures (~76-86 that most hobbyists run), at least as long as you stay within the part of that range that your animals have been acclimatized to. At temps above or below that, things get a lot more complicated, and how much difference temp makes will depend on the animals you keep and how extreme your temps are.

It helps to picture an idealized metabolic curve, which is basically a parabola. Near the top of the curve, or the temperature optimum, the rate of change is slow, or in temperature compensating species, almost flat. As you move farther out from the optimum, the curve steepens and the impact of temperature becomes more dramatic.

Obviously, there are absolute limits to how cold reef animals can get before metabolic stress begins. If you get too far away from the optimum, metabolism slows down so much that the biochemical reactions that need to happen, can't happen quickly enough to keep the animal functioning properly. The trouble is that since cold temps tend to be a much less frequent problem on reefs, the limits at the low end are much less well-understood than the limits at the upper end. We know that when these animals get cold, metabolism bogs down and things go bad, but we don't know where that cutoff begins. What we do know is that on a lot of reefs, once temps get below about 70-72, you tend to get bleaching, which is an obvious sign of acute stress, but generally one that occurs well after the actual stress threshold is reached. We also know that almost 85% of reefs occur in areas that never or only very rarely see temps below 73. Those that are colder than that also tend to have greatly diminished diversity- basically only the hardiest species live there. Taken together, that's a good indication that somewhere around 73-74 is about the lowest most corals do well at long-term. Again though, unlike the high-temp stress thresholds, that's based more on inference than actual measurements of the onset of physiological stress.

it seems the only argument for keeping our tanks in the higher temps is that natural reefs are at higher temps,

Not exactly, though that's part of it. For one, we know that temps in the 76-86 range are clearly acceptable for virtually all reef corals. On the other hand, we know that temps below that are harmful to at least some corals, but we don't know exactly how far below that the harm begins for each species. The idea is to shoot for what's acceptable for the weakest link. Likewise, many corals do fine at temps into the 90s, but many others don't, so it's generally recommended to keep temps well cooler than that.

Also, I think a major point of the argument is about whether the practical benefits to you as a reefkeeper are more important than the biological benefits to the animals we keep. From a biological perspective, depressed metabolism isn't a benefit- it's a form of chronic stress. The metabolic optimum is where the body functions most efficiently. It's where you get the fastest growth, most efficient use of food and oxygen, best reproductive potential, etc. On the other hand, from a practical perspective, a hobbyist might not want the fastest growth possible since it also comes with increased waste production and food consumption.

To reframe the same question though, what's the argument for keeping temps in the low 70s other than to depress metabolism or avoid buying or adjusting a heater?

there seems to be different opinions on what natural reefs seem to be at.

No need for opinions when there is actual data. This is a pretty well-studied subject and there are literally dozens of papers out there documenting the temperatures on reefs, as well as real-time data from several sources such as NOAA and AIMS. This is one of the seminal sources on the topic: http://icb.oxfordjournals.org/content/39/1/146.full.pdf

i wonder where the 77 degrees benchmark came from when i first started reef keeping?

Real world measurements of reefs, like the one above. Notice that in that paper the average low temperature is precisely 77 degrees. Again, the idea being that that temperature is acceptable for virtually all the corals in the hobby, while lower temps might not work as well for some of the less hardy corals.

Beware though that a lot of hobbyists make bad recommendations about temperature based on their poor understanding of how temp stress works in corals. You'll see lots of recommendations to keep temps towards the low end to increase your margin of error and that you should avoid temp swings, when in reality neither of these makes any sense based on what we know about stress in corals.

[greenbean36191]

You ever do any diving off South Florida during the summer? There are upwellings that occur on a fairly regular basis, where the bottom temps can drop from the mid 80's to the low 60's in very short order. Last year, coral bleaching was reported when the temps stayed in the 50's for several days. I frequently swim through thermoclines where the water temp changes 5-10 degrees or more. As the currents move these layers of warmer and cooler water down the reef, the creatures experience temps that can change significantly in just minutes. Thats not to say someone would want to duplicate those swings in their aquarium, but I doubt it would be fatal should you have a power failure.

I think you misunderstood my point.

I in no way think that temperature fluctuations are harmful to corals and are certainly not unnatural. In fact, the data on the issue shows quite clearly that variable temps are metabolically beneficial to corals and help protect them from temperature stress. You'll see me argue quite frequently that hobbyists should NOT try to avoid or minimize temperature variations as long as they stay within the range they have acclimated their corals to.

The recommendation I was responding to was about keeping the temps cooler in the summer to allow for a larger margin of error in case power went out and temp rose and vice versa during winter. That recommendation is based on the false presumption that the stress threshold is a fixed value, say 88 or so, and that by keeping the temp 2 degrees cooler, you give yourself an extra 2 degrees of wiggle room. We've known for a few decades now that that's not the case- that the stress threshold in corals is set by acclimatization to the average seasonal maximum temp. Regardless of whether that maximum is 86 or 82, you still have the same 2-4 degree F margin of error. You can literally have 15 degrees of variation during the day without issue, but only a 2 degree increase above the maximum can be catastrophic.

So consider what happens if the ambient temp is 86 and the power goes out long enough that the temp equalizes to it. If your tank's normal maximum temp is 84, you get a 2 degree excursion beyond the maximum, which is about the bare minimum amount it takes to get acute stress. If you keep the tank at 82, you get a 4 degree spike, which is more than enough to cause acute stress in most cases. Also, since the temperature differential between the tank and ambient temperature is greater, the temperature rises into the stress zone quicker. In the end, a misguided attempt to play it safe, actual ends up making the situation worse.

If anything, the temp should be slightly higher in summer and cooler in winter, to minimize the temp variation beyond the max and min which is what causes stress.

Also FWIW, the reefs in FL are a special case. The reefs north of Biscayne Bay have all been dead for the past 5,000 years, so don't tell us much of anything about temps. The reefs in the Keys are mostly marginal reefs (meaning conditions there are less than ideal for reef growth) and actual development only occurs in the lee of the islands where they're shielded from the water flowing out of Florida Bay. The type of damage that occurred this winter has happened throughout the history of the Keys and is the reason that there are no reefs between the islands in the Middle Keys.

[Reverend Reefer]

ok greenbean, you've convinced me. i've installed the heater in the display and am slowly monitoring the temp increase. if i understand this correctly, given the wide range of corals i keep that certainly come from different varying regions of the world, the data suggests mean temps of 77. in keeping my temp lower, my hardy corals are fine, but are potentially being subjected to temp stress. for some reason, they are able to compensate and survive under these conditions, but if i up the temp slightly to the more optimal higher 70s range, they will likely be under less stress all things considered.

simply put of course, i'd be fine eating popsicles up north in an igloo, but i'd probably be ALOT more fine sitting on a beach drinking margueritas. got it! thanks.

[drew1]

Also FWIW, the reefs in FL are a special case. The reefs north of Biscayne Bay have all been dead for the past 5,000 years, so don't tell us much of anything about temps.

Greenbean - Not meaning to hijack the thread (it appears to be winding down) but I noticed you were from Florida. What did you mean by the quote above? Clearly when you dive off the coast of Florida you notice that the reef is anything but dead. Lot's of corals. Maybe not the elkhorns and staghorns you find further south but beautiful corals none the less. Also the deepwater Oculina reefs are clearly live coral reefs. Maybe a distinct species, but the area is a live coral reef. But I would suspect that the coral varieties available off the coast of FL actually tell you alot about temperature tolerance, rather than little. I know the fish I collected while diving always seemed a lot hardier than those that were imported. Many variables involved, but they were tough.

[Reverend Reefer]

haha drew1! are you trying to get me to change my mind again? just kiddin. so i did slowly up my temp in a day, is that too fast? anyways, i'm now averaging 75 (+/-2) degrees, compared to my averaging 73 (+/-2). not seeing much change in anything just yet. no specific responses from visual inspection. no signs of stress and no signs of happiness either from fish or corals. i saw more of the tiny collonista snails at night but could be unrelated.

[drew1]

Not at all. I think you're on the right track. Going from 73 to 75 in a day is nothing. I wouldn't expect you to see much difference. I don't think you'll much negative response from anyone in the tank as long as you stay in that range.
Just adding my own personal diving experiences: Temperatures do vary greatly on reefs around the world. When I dove in Australia I too remember it being much cooler than expected. We were cold at the end of a dive with a wetsuit. I would have said bottom temperatures were in the mid-70's and it was spring (November, I think). However, we also snorkled some classic shallow water reefs (probably 6-10 feet deep) that were FULL of beautiful branching corals that were toasty. I'd guess mid 80's. East coast of Florida temperatures range from 60's to 80's, although alot of my diving there was not traditional coral reefs but rather cocina and worm-rock that had corals (soft corals for the most part) scattered about. Palm beach temperatures were always close to 80 for me. And my experiences in the Carribean always seemed to be in the upper 70's to low 80's. Obviously all my experiences are strictly anecdotal, but they do seem to support the mid-70 to mid-80 range that is typically refenced.

[Reverend Reefer]

Originally Posted by drew1

Not at all. I think you're on the right track. Going from 73 to 75 in a day is nothing. I wouldn't expect you to see much difference. I don't think you'll much negative response from anyone in the tank as long as you stay in that range.
Just adding my own personal diving experiences: Temperatures do vary greatly on reefs around the world. When I dove in Australia I too remember it being much cooler than expected. We were cold at the end of a dive with a wetsuit. I would have said bottom temperatures were in the mid-70's and it was spring (November, I think). However, we also snorkled some classic shallow water reefs (probably 6-10 feet deep) that were FULL of beautiful branching corals that were toasty. I'd guess mid 80's. East coast of Florida temperatures range from 60's to 80's, although alot of my diving there was not traditional coral reefs but rather cocina and worm-rock that had corals (soft corals for the most part) scattered about. Palm beach temperatures were always close to 80 for me. And my experiences in the Carribean always seemed to be in the upper 70's to low 80's. Obviously all my experiences are strictly anecdotal, but they do seem to support the mid-70 to mid-80 range that is typically refenced.

very interesting! even if anecdotal, do you think you could summarize what you saw in what temperatures? just out of interest.... for example, in the cooler waters of Australia where you dove, did you see more of a distribution of X types of corals, as compared to the warmer Carribean waters, where you saw more of Y types of corals? i realize the Indo regions have the greatest diversity but just from your diving experiences what did you tend to find/see.

i understand that we are trying to do the impossible here given that tolerance levels of different types of coral seem to be varied with respect to temperature but its still very interesting.

when i went to the Toronto zoo, i found that their reef tanks didn't look as beautiful as many of the reef tanks posted here on RC, and even my tank looked more colorful. however, after thinking about it more, i think for educational purposes, they limited the tank's inhabitants to what would naturally be found in X region that the zoo was trying to mimic. meanwhile, our tanks have a variety of corals that come from many different parts of the world. i wonder if there are optimal temperatures for different types of coral or if they just happen to tolerate the temps of region Y or Z but would actually be happier in some optimal temp.

i guess those questions might be too big for any of us to answer... i mean just because a particular species is evolutionarily adapted to cooler temps, does this necessarily mean that they would not be "happier" in warmer ones? bah, sorry for draggin this thread along... somebody shut me up please. i guess the other thing that got me interested was the TOTM a couple months ago where someone has a cool deep water reef and it got me thinking about temperature again.

[drew1]

When I did most of my diving I was focussed on photography and simple life diversity. Corals were something you didn't touch, and maybe oohed and ahhed over after the dive, but really I didn't focus on the corals. I kept marine aquariums that had fish and inverts, but never corals. You could dive a 10 foot by 10 foot square for 30-60 minutes and not run out of stuff to see. Nudibranchs, bristle worms, polyps, sponges, crabs, shrimp, etc, etc within any given point on the reef. It's amazing how much divers overlook because they are in a hurry to go from point A to point B.
Another interesting point is that when diving I used to take a lot of pictures. When you look around during the dive eveything looks kind of brown and dingy. However, when the pictures come back (before the days of digital) they'd be full of reds and oranges! Lots of really pretty colors - sponges, encrusting corals, etc. Without a light (or flash) you'd never see the colors.
Anyway, back to the question. The only thing I can really point to was the shallow reefs in Australia. Lots of branching SPS type corals and very warm temps. Do they like the temperature more, or is it the light? Or maybe the turbulance from the shallow water? Or maybe a specific combination. Deeper dives (less light, cooler temps - generally) tended to see more coral "heads" rather than branchy corals. In addition to the large brain corals were lots of encrusting corals, smaller soft corals, sponges, gorgonia, etc. But this is also the area I saw a greater diversity of life. Every coral head or basket sponge had it's own little micro-cosm.
Anyway, again just my observations.

[Reverend Reefer]

Originally Posted by drew1

When I did most of my diving I was focussed on photography and simple life diversity. Corals were something you didn't touch, and maybe oohed and ahhed over after the dive, but really I didn't focus on the corals. I kept marine aquariums that had fish and inverts, but never corals. You could dive a 10 foot by 10 foot square for 30-60 minutes and not run out of stuff to see. Nudibranchs, bristle worms, polyps, sponges, crabs, shrimp, etc, etc within any given point on the reef. It's amazing how much divers overlook because they are in a hurry to go from point A to point B.
Another interesting point is that when diving I used to take a lot of pictures. When you look around during the dive eveything looks kind of brown and dingy. However, when the pictures come back (before the days of digital) they'd be full of reds and oranges! Lots of really pretty colors - sponges, encrusting corals, etc. Without a light (or flash) you'd never see the colors.
Anyway, back to the question. The only thing I can really point to was the shallow reefs in Australia. Lots of branching SPS type corals and very warm temps. Do they like the temperature more, or is it the light? Or maybe the turbulance from the shallow water? Or maybe a specific combination. Deeper dives (less light, cooler temps - generally) tended to see more coral "heads" rather than branchy corals. In addition to the large brain corals were lots of encrusting corals, smaller soft corals, sponges, gorgonia, etc. But this is also the area I saw a greater diversity of life. Every coral head or basket sponge had it's own little micro-cosm.
Anyway, again just my observations.

that is really really interesting! one day, when i finish skool, i would love to learn how to dive. a friend of mine mentioned that about the coloration as well - they're not so 'vibrant' to the naked eye while diving. i always wondered about that, especially when you see the deep deep sea creatures with magnificent coloration. seems unnecessary, from an evolution point, to have these vibrant colors without much light to showcase them. on land, it seems that creatures that live in the dark tend to lack coloration and are often albino. in the ocean, it doesn't seem to be the case necessarily.

i guess in the end if you consider that most of the time, the coloration isn't from the corals themselves but the zooxanthellae they host. i wonder what non-photosynthetic corals have that cause them to color up? man am i ever long winded.... stop encouraging me drew1 !!!