Lighting the Reef Aquarium Part 5: Tridacna Clams

By Dana Riddle

In Part 4 of this series, we examined the lighting requirements of a popular aquarium coral, Acropora.

This time we’ll look at the light preferences of another popular reef aquarium inhabitant – the photosynthetic clam Tridacna.
According to World Registry of Marine Species (WORMS.org) there are 9 Tridacna species (T. maxima, T. squamosa, T. crocea, T. derasa, T. noae, T. squamosina, T. lorenzi, T. rosewateri and T. mbaluvana) all of which inhabit the Indo-Pacific Ocean. Their popularity is undeniable and is probably due to their colorful mantles (the fleshly tissues that extend over some of their calcium shells, or valves.)

To understand the lighting requirements of Tridacna (or more correctly their symbiotic alga -Symbiodinium), we should examine the types of zooxanthellae found within them. But first, some background on Symbiodinium. Years ago, it was thought there was only one zooxanthella species -Symbiodinium microadriaticum.

Today, there are 14 known species divided among clades (a clade is something that shares a common ancestor.). Some species/clades are ‘generalists’ meaning they are adaptable to a wide variety of conditions (such as tolerances of high or low light, temperature, etc.). Others are ‘niche’ species that have limited adaptive abilities. In addition, early attempts to genetically fingerprint zooxanthellae were crude and listed clades as only A, B, C, etc. Later analyses would refine these attempts, and found clades listed as, for example, A1, B3, C2, and so on.

Early research on Tridacna crocea, T. gigas, and T. squamosa found them to contain zooxanthellae described as simply Clade A (today, this might be found to be Clade A1.).
Researchers tell us that Tridacna maxima, T. gigas, and T. derasa can possibly possess the generalist Symbiodinium goreaui (also known as Clade C1.). Further research states specimens used in these analyses were collected at depths of 1 to 8 meters.
Other research found T. gigas and T. maxima could also have a symbiosis with Symbiodinium fitti (also known as Clade A3.) These were also collected at depths of 1 to 8 meters.

Tridacna species collected at depths of 1 to 10 meters near Okinawa, Japan had a relationship with Clade A6. We can assume this zooxanthella is tolerant of lower light and temperatures than their tropical counterparts.
So, what does this tell us? If we assume that few Tridacna clams are imported from Japan and that most are from the Indo-Pacific, those specimens in the aquarium trade contain zooxanthellae of Clades A and C (with the possibility of another, Clade D. Clade D is generally assumed to be an opportunistic clade that exploits stressed environments.). We can expect these clades to be tolerant of high light. Indeed, data I generated in Hawaii years ago (Riddle, 2007) is confirmed by recent research Jantzen et al., 2008 – See Figure 1.)

Figure 1. Photosynthetic performance of two Tridacnid clams.

An estimation of the maximum rate of photosynthesis is greater than 1,000 µmol·m²·sec.

From Jantzen et al, 2008.

The message here is clear – many, if not most, Tridacna clams contains zooxanthellae that are tolerant of high light. The thickness of these clams’ mantles also allows self-shading of zooxanthellae which, of course, increases the light requirements by increasing the Saturation Point. Jantzen et al. (2008) found T. maxima can survive if light intensity is enough. On the other hand, clams could benefit by feeding if light intensities are low.

References: Jantzen, Carin & Wild, Christian & Al-Zibdah, Mohammad & Ann Roa-Quiaoit, Hilly & Haacke, Christoph & Richter, Claudio. (2008). Photosynthetic performance of giant clams, Tridacna maxima and T. squamosa, Red Sea. Marine Biology. 155. 211-221. 10.1007/s00227-008-1019-7.
Riddle, D., 2007. How much light? https://www.advancedaquarist.com/2007/3/aafeature1

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Lighting the Reef Aquarium

Lighting the Reef Aquarium: Part II

The Myth of Corals Requiring Unlimited Amounts of Light

Lighting the Reef Aquarium Part 4

Light Requirements of a Coral Genus Often Found in Reef Aquaria

By Dana Riddle

In previous installments, we have discussed how to measure light intensity, reviewed light requirements of a Hawaiian coral, and discussed terminology. This time, we’ll look at light requirements of a coral genus often found in reef aquaria – Acropora.

Acropora is a pandemic genus and is found in Atlantic and Pacific Oceans. There are XXX species described and can occupy shallow to very deep waters. It is a common belief that small-polyp stony (SPS) corals’ zooxanthellae require high intensity light to survive. Is this true? To answer this question, we’ll revisit issues of photobiology.

As we discussed previously, Symbiodinium species and clades require a minimum amount of light, and this is called the Compensation Point. On the other hand, the Saturation Point is when the rate of photosynthesis does not increase with increasing light intensity. As reef hobbyists, it is our goal to exceed the amount of light required for the Compensation Point (necessary!) but not pass the Saturation Point (Note: This is for photosynthesis – the amount of light required for expression of color in corals often exceeds the Saturation Point. We’ll discuss this in the future.)

Symbiodinium species can alter their Compensation and Saturation Points – this is called Photoadaptation. Photoadaptation allows zooxanthellae to lower their Compensation and Saturation Points as depth increases (and available light decreases) and in shaded areas. As such, a coral found in 40 meters of water sometimes requires less light than a coral found in a shallow tidepool. Photoadaptation is not unlimited, and some types of zooxanthellae can adapt better than others, and the type of zooxanthella plays an important part.

Figure One demonstrates the Saturation Point of various Acropora species.

Figure One. Saturation Points of Acropora species. Generally, the deeper the depth of the coral, the less light it requires. However, this is not the case.

The values shown in Figure One should not be considered as absolutes for many reasons. First, testing techniques (such as respirometry) can not distinguish light requirements of shaded or highly           illuminated areas – it reports only an average amount of light required. In addition, some corals (such as Acropora cervicornis) are known to host different Symbiodinium species throughout the year – zooxanthellae with tolerance of higher light (and heat) tolerances can be prevalent during the summer months. If light requirements are determined through PAM fluorometry, the results are germane only to the small area tested.

Whatever the caveats, the results of testing done by various researchers over the course of many years demonstrates that Acropora species do not need a great deal of light. Indeed, the most light used by an Acropora digitifera is about 20% of maximum sunlight at the water’s surface. These light intensities are easily achievable with today’s lighting systems.

Bear in mind that it is acceptable to expose a coral to light intensities higher than shown in Figure One, especially if expression of intense coloration is desired.

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Lighting the Reef Aquarium

Lighting the Reef Aquarium: Part II

The Myth of Corals Requiring Unlimited Amounts of Light

Orphek Atlantik V4 GEN2  Review and  time lapse video by Ryan

Would I recommend these lights to a friend?

Haha is that a joke?! Since buying these lights I have been actively trying to get every friend I know to switch over to these lights.

After first getting them I was just blown away by how they look in general, not even turned on, these lights are a work of true beauty.

When I mounted these lights to my hood and turned them on I was shocked to see how well two Atlantic v4s lit up my 180 gallon mixed reef tank 6x2x2 feet.

After playing with the lights for five hours and wondering at them in amazement was over the true test had begun.

WILL MY CORALS ALSO LIKE THESE LIGHTS?!

Atlantik V4 Reef Aquarium LED lighting

The Myth of Corals Requiring Unlimited Amounts of Light 

By Dana Riddle

I had the pleasure of living on the Big Island of Hawaii for almost two decades and took every opportunity to study the natural reefs there (See Figure 1.)  One of my projects involved the determination of light required by photosynthetic corals (note: required versus tolerated.)

Figure 1. This shallow tide pool, just inches deep, contains beautiful Hawaiian corals. Experiments were conducted on animals found here.

At the time, getting permits required by the State of Hawaii was relatively easy, and once these were in hand, the project could proceed. Determination of rates of photosynthesis requires all the equipment normally found in a wet lab, plus some highly specialized instruments.

The first of these is a quality Quantum (or PAR) meter (discussed in some detail in Part II of this series.) The second is a Pulse Amplitude Modulation (PAM) Fluorometer (See Figure 2.)

Figure 2. This instrument, called a fluorometer, can determine rates of photosynthesis in plants, algae, and zooxanthellae.

In a nutshell, this instrument monitors how light is used or disposed of during the process of photosynthesis and involves measuring chlorophyll fluorescence. It can determine if light is used in photochemistry, or shed as heat, or dissipated through natural protective cycles. It is well beyond the scope of this article to discuss these pathways.

Suffice it to say that, within the context here, we are interested in how much light is required to maximize photosynthesis within the zooxanthellae found within shallow water corals.

It is common among many to believe that zooxanthellae found in many shallow water corals have an unlimited ability to absorb light and use it in the photosynthetic process. After all, it is easy to find photos of corals fully emerged during low tide and being blasted by full strength sunlight. It leads us to believe that more light must be better!

When studying photosynthesis, there are a few things we need to know. Light (in the form of photons) can be absorbed by photopigments (such as types of chlorophyll, peridinin, etc.) and hence flow through the two Photosystems and beyond in the form of electrons. Other points to understand – there is a minimum amount of light required (called the Compensation Point, where oxygen production meets the respiratory needs of the coral animal and symbionts.)

The next milestone is the Saturation Point where the rate of photosynthesis is at a maximum and any increase in light intensity will not result in a higher rate. The third point is Dynamic Photoinhibition – it is protection to high light and involves the Xanthophyll Cycle. It is a natural process.

For the purposes of this article, we are most concerned with the number of photons (light intensity – PPFD or PAR) that exceeds the Compensation Point, meets and exceeds the Saturation Point and how Dynamic Photoinhibition affects the rate of photosynthesis.

Two stony coral species were chosen for these experiments. The first was Porites lobata (commonly called the Lobe Coral) and the ‘SPS’ coral Pocillopora damicornis (the Wart Coral.) Results were comparable, and we’ll discuss only those in the procedure involving P. lobata.

Figure 3 demonstrates the results.

Figure 3. Maximum photosynthesis in the zooxanthellae in this Porites stony coral was achieved at about 200 µmol·m²·sec. At 350 µmol·m²·sec the rate of photosynthesis was about that seen at 75 µmol·m²·sec!

A close examination of Figure 3 reveals the maximum rate of photosynthesis (Saturation) was achieved at a light intensity of ~200 µmol·m²·sec and Dynamic Photoinhibition came into play shortly thereafter.

The take home message is this: These Porites corals living in waters only a few centimeters deep required only a fraction of the light intensity they will experience. Their zooxanthellae will be at a maximum rate of photosynthesis early in the morning, and their natural protective devices (the Xanthophyll Cycle) comes into play during mid-morning and protects them until light intensity drops in the late afternoon (assuming it is sunny all day.)

Other experiments suggest that most corals require lower light intensities to achieve maximum rates of photosynthesis.

Next time, we’ll look at the light requirements of a coral often found in reef aquaria – Acropora species.

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Measuring Light Intensity

By Dana Riddle

If I were to ask reef aquarium hobbyists these questions, their answers would likely be as follows:

Q: What is the Specific Gravity of the water in your tank?

A: 1.025

Q: What is your calcium concentration?

A: 400 milligrams per liter (or parts per million)

Q: What is the highest light level in your tank?

A: I likely won’t get an answer.

Why? It is probably due to several thing:

1. Lighting can be a complicated issue, and there isn’t a universal correct answer.

2. Measuring light intensity requires a meter, and they are sometimes expensive.

3. Measuring light intensity isn’t very exciting!

I’ll try to ‘un-complicate’ the issue of lighting in this and future articles.

As for the expense of a meter, a fish club could pool members’ resources and purchase one.

Not much we can do about making light measurements exciting – but they are no more boring than calcium or alkalinity tests.

Many light sources (such as fluorescent, metal halide and mercury vapor lamps) can lose intensity over a period of months.

Monitoring their output is important and can be used to time lamp replacement.)

Generally, light-emitting diodes (LEDs) do not lose a great deal of intensity over their life spans (years under best of conditions.)

In this installment, we’ll examine the different ways to measure light in an aquarium, and the pros and cons of each.

These devices are called Lux Meters and Quantum (or PAR) Meters.

Figure 1. A few of the light measuring devices in the author’s laboratory.

Lux Meters

Pro: Relatively inexpensive.

Cons: Sensor is most sensitive to green light, and ‘sees’ light that is important to

photosynthesis poorly (such as violet/blue and red light.)

Lux meters can be the least expensive method of measuring light in an aquarium, but the sensor responds best to green light (its response is photometric and corresponds to those light wavelengths that the human eye perceives best but are least effect in promoting photosynthesis.) A lux meter’s sensor must be waterproof, and preferably cosine corrected (meaning the measurement is not greatly affected by the angle of the sensor in relation to the lighting source) and have the ability to read to at least 100,000 lux (the intensity of full-strength sunlight.) Measuring light with a lux meter is better than no measurements at all. Conversion of lux to Photosynthetically Active Radiation (our next subject) is possible if conversion factors are known.

Quantum or PAR Meters

Pro: Used in peer-reviewed scientific journals and by serious hobbyists, making comparisons easy.

Most sensors are cosine-corrected, waterproof and designed to deliver accurate results when submerged in water.

Some have good responses to LED lights.

Con: More expensive than lux meters.

Quantum meters were used mostly by researchers, and professional aquarists until the late 1990’s when one costing about $200 US was introduced. Since then, PAR measurements (correctly called PPFD for Photosynthetic Photon Flux Density) have become the standard for reporting light intensity in an aquarium.  Since the energy level of a photon is not important in photosynthesis (a blue photon promotes photosynthesis as well as a red one once it is absorbed by photopigments such as chlorophyll) quantum meter reports the number of photons across a bandwidth of about 400 nanometers to 700 nanometers (a nanometer is one-billionth of a meter) without regard to their energy levels.

PPFD is reported in units of microMol per square meter per second, or µmol·m²·sec. A quantum meter should report light intensity to at least 2,000 µmol·m²·sec (which is about the intensity of full-strength sunlight.)

There are other ways to measure light intensity, such as a radiometer (which reports in radiometric units of watts or micro-watts.)

There are very few scientific papers concerning corals that use radiometric units, and none in hobbyist literature, so we can disregard this method.

Now that we know about methods of measuring light, we’ll next look at light intensities on a real reef.

One might ask – “Why bother measuring light, since corals can use all the light they can get!

I see photos of corals exposed to air and intense light at low tide and they’re doing fine!” We’ll examine why this is wrong in the next installment.

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