Lighting

Giant clams, host anemones and reef corals all carry single-celled algae called zooxanthellae in their tissue (Module 3). The algae photosynthesize and pass sugars to their host. When the algae leave, the animal turns pale or white: bleaching.

Giant clams get most of their energy from light; filter feeding matters less as they grow. Giant clams need strong light (Michael Maddox, President of IMARCS). Host anemones such as carpet anemones can't survive on feeding alone. For many corals the algae are the main food source and much of their color. So for these animals, light isn't decoration: it's how they eat. Much of the published light research, including most of Dana Riddle's, measured corals; this module says which animal each finding comes from. Sections 8 and 9 cover clams and anemones in detail.

PAR (photosynthetically active radiation) is light from 400 to 700 nm, the range photosynthesis uses. It's measured as PPFD: the number of photons in that range landing on a square meter each second, in µmol/m²/s. In this course, "PAR" numbers are PPFD.

Lux and lumens are weighted to what human eyes see, which is mostly green, so they're the wrong tool for corals (Dana Riddle). Watts per liter or gallon tells you nothing about an LED (Dana Riddle), and different lighting types give very different output for the same watts. Eyes are poor judges of blue light. Copying another tank's settings is no substitute for measuring your own (Dana Riddle). For scale: full tropical sun at noon is about 2,000 PPFD.

Chlorophyll a absorbs most strongly near 440 nm (violet-blue) and 675 nm (red). In sea water, red is absorbed first (Dana Riddle). Reef lights are rich in blue and violet; most fish look their best with some full-spectrum white added.

Dana Riddle's measurements found photosynthesis about the same under 4,000 K and 12,000 K metal halide lamps at the same PPFD: spectrum came second to intensity. In a 75-day trial, a stony coral grew under four quite different LED spectra with no significant difference. The Kelvin rating (color temperature) is a poor guide to a lamp's real spectrum. PUR means the part of PAR a particular organism can actually use, but there's no meter for it; measure PAR.

Many corals make fluorescent proteins that glow under UV, violet and blue light. Blue light drives their production far better than red. Each pigment has its own best light level, and that range can be narrow, so one setting won't show every color at its best (Dana Riddle).

Chromoproteins, the non-glowing purples and blues, can need much more light: in Dana Riddle's report, a purple Acropora lost its color at 138 PPFD and kept it at 875. Color can need more light than growth does. UV isn't required for color; blue light keeps it (Dana Riddle). Most plastic splash covers let some UV-A through.

As light rises, photosynthesis rises, then levels off: the coral is saturated. Push further and it falls: photoinhibition. Dana Riddle measured these points in living corals: most corals saturate at about 200 to 300 PPFD. One Porites lobata photosynthesized at 350 PPFD about as much as at 75. A soft coral, Sinularia, saturated higher than some stony corals, against common belief.

Corals protect themselves from extra light within minutes, by shedding it as heat, but only up to a point; beyond that comes lasting damage and, at the extreme, losing their zooxanthellae. "More light is better" isn't supported (Dana Riddle). Too little light starves photosynthetic animals; for corals, about 100 PPFD has been a working minimum, lower if the light is mostly violet and blue. Giant clams are different: their thick mantles shade their own algae, so they saturate much higher (section 8).

The daily light integral (DLI) is the total light received over a day, in moles of photons per square meter. Work it out as PPFD x hours x 0.0036. Dana Riddle measured natural reefs in Hawaii: a winter tidepool 5 cm (2 inches) deep peaked near 1,000 PPFD and received about 22 mol/m²/day. A reef 3 m (10 ft) deep reached about 780 PPFD on a sunny day, about 34 mol/m²/day.

An aquarium at 500 PPFD for 12 hours gives about 21.6, as much as that tidepool. A well-lit aquarium can match or exceed the light a natural reef receives. Copying reef intensity isn't the goal, since much of it's above what corals can use (Dana Riddle).

IMARCS observations (Michael Maddox, President of IMARCS): Tridacna maxima and T. crocea grow faster as light increases, up to about 500 PPFD. Stichodactyla gigantea and Heteractis magnifica thrive at 300 to 450. S. haddoni thrives at 200 to 350.

Published ranges, in PPFD: T. maxima about 150 to 350 and T. crocea about 200 to 400; most other giant clams about 150 to 350, and below 100 a clam may survive for a while but won't thrive; bubble tip anemones about 150 to 350; SPS corals about 200 to 350; soft corals and LPS about 75 to 150. Sources differ, especially for LPS. A coral from deep or shaded water can need less light than one from a shallow tidepool (Dana Riddle). Measure at the animal, bring it up to its range slowly (section 15), and watch how it responds.

A giant clam's zooxanthellae live in a branching tube system that runs from its stomach into the mantle. The clam takes sugars from them, and digests some of the algae too. Photosynthesis is a clam's main source of energy, and filter feeding matters less as it grows: Klumpp and Griffiths found filter feeding supplied most of a tiny T. gigas's needs but only about 18% at 100 g. T. maxima lives on light alone; T. squamosa feeds more. Young clams kept in darkness die within days.

Clams handle far more light than corals. Their thick mantles shade their own algae, which raises their saturation point (Dana Riddle). T. maxima showed no saturation near 600 PPFD, and 200 to 300 is generally sufficient (Dana Riddle); IMARCS sees T. maxima and T. crocea grow faster as light rises to about 500 (Michael Maddox, President of IMARCS). T. maxima and T. crocea need the most light; T. derasa, T. squamosa and T. gigas a little less. A clam's colors aren't pigments: reflective cells called iridophores in the mantle scatter light deeper and sideways, spreading it evenly through the algae.

Placing a clam: T. crocea and T. maxima on rock or hard ground where they can attach; T. derasa, T. squamosa and T. gigas on sand. Use a level spot that exposes the whole mantle to the light, and choose it by the PAR reading. A healthy clam opens fully by day with an evenly colored mantle. A pale or white mantle is bleaching: from too much light, heat, a bulb change, too little light, or a sudden jump in light, including the clearer water after big water changes or fresh carbon. Most bleached clams can recover over months with stable conditions and feeding.

Host anemones carry zooxanthellae, and carpet anemones can't survive on feeding alone. Prolonged bleaching can starve them. Their clownfish help: nutrients from the fish reach both the anemone and its algae. Pryor and colleagues found that bleached bubble tip anemones living with clownfish regained their algae in about 2 months and their color in about 3.5 months; those without fish didn't recover.

IMARCS observations (Michael Maddox, President of IMARCS): S. gigantea and H. magnifica thrive at 300 to 450 PPFD, and S. haddoni at 200 to 350. In nature, S. gigantea lives on very shallow sand flats and seagrass beds, H. magnifica in clear water with strong current, and S. haddoni on sand.

An anemone that isn't happy with its light or flow wanders until it finds a better spot; once settled, most stay put. Don't glue it in place. Guard every pump intake, because an anemone that wanders into a pump will likely die, and give carpet anemones a sand bed deep enough to bury their foot. A pale or white anemone is bleaching; measure the light at the anemone and check the temperature (Module 14). During some ciprofloxacin treatments, IMARCS leaves an anemone with no light for 24 to 48 hours, because light breaks the drug down (Michael Maddox, President of IMARCS).

LEDs are programmable and dimmable, use less energy, and lose little intensity over their life; Dana Riddle found LEDs that rival or beat metal halide. Their focused beams need good optics, or they make a hot spot. T5 fluorescent tubes give an even spread; they lose intensity and blue as they age, and are usually replaced every 9 to 12 months. Heat cuts their output: about 1% for every 1.1 C (2 F) above 25 C (77 F). Metal halide lamps are hot point sources, usually mounted at least 30 cm (1 ft) above the water, and also fade with age.

Point sources (pendant LEDs, metal halides) make strong shimmer; panels, strips and T5s spread light evenly. A hybrid adds T5 or LED strips to a main light. Aging lamps and yellow water both cut violet and blue first (Dana Riddle).

Mounting height changes spread and PAR, not spectrum. Lower, the light is narrower and brighter, with a hot spot; raised, it spreads wider and dimmer, with fewer hot spots but more light spilling into the room. Mount at the maker's height (for many LEDs, 20 to 30 cm, 8 to 12 in, above the water), then measure. PAR falls the farther you go from the light, so the sand gets less than the top of the rock.

Shadows under ledges can cut PAR by about 75%. A coral moved from the open into a shadow, or shaded as its neighbor grows, gets far less light than the map says. Measure where the animal actually sits.

A quantum meter (for example, an Apogee MQ-510) has a sensor that goes under water and a meter that stays dry. Hold the sensor level, facing up toward the light; a rod marked with depths helps you place it the same way each time. Readings move as the water ripples: watch for a few seconds and take the average. The MQ-510 corrects for being under water itself; a sensor calibrated in air reads low under water unless it's corrected.

Know the meter's limits. Most meters under-read violet and UV LEDs: as much as 40% of a UV LED's output can fall outside a meter's range (Dana Riddle). Two meters can differ by about 15%. A meter can max out (1,999 on Apogee) under strong light. The largest source of error is usually the user (Dana Riddle). Clean the sensor with water or a mild detergent and a soft cloth; never abrasives. Salt won't come off with alcohol.

Take readings in a grid across the tank at three heights: the sand, halfway up the rock, and the top of the rock. Write them on a photo of the tank. The map shows the hot spots, the dim corners and the shadows.

Place each animal where the reading matches its range (section 7), then watch it. Map again after any change: a new fixture, a new height, new tubes, a new program, or rock moved. When changing intensity, change every channel by the same proportion to keep the spectrum.

Run 8 to 12 hours of light a day, with a ramp up and a ramp down; a common pattern is 1 hour up, 8 hours at full, 1 hour down. Leave at least 10 hours of darkness. Moonlight doesn't count toward the photoperiod. In Hawaii, natural day length is about 13 hours in summer and 10.75 in winter (Dana Riddle).

Natural full moonlight is very dim: Dana Riddle measured under 0.05 PPFD. One blue LED can be brighter than that. Temperature has stronger control over coral spawning than moonlight (Dana Riddle), but light at night matters: on reefs exposed to light pollution, corals spawn one to three days closer to the full moon, and Craggs and colleagues recorded aquarium spawning up to nine nights late, possibly from outside light. Keep the night dark.

A new fixture, new tubes or a brighter program is a big change. Start lower than you think you need, then raise it in small steps, weeks apart, watching the animals at each step. Red Sea's manual starts new LEDs at 50% of the target program and takes 50 days to reach it; Orphek suggests steps of about 5%, 1 to 2 weeks apart. New T5 tubes can give about twice the PAR of old ones, so change 1 or 2 a week rather than all at once.

A coral or clam moved to a brighter spot is the same: give it time, or start it lower and move it up. A clam used to low light can bleach or close up when suddenly exposed to high PAR. To bring a clam up to brighter light, lay layers of shade cloth or screen over the clam only and remove one layer at a time over a couple of weeks; do the same after a bulb change. Avoid moving a clam around again and again.

Too much light: polyps hidden, soft corals shrunken, corals that retract in the middle of the day, and paling that can end in bleaching. Too little light: photosynthetic corals turn darker brown, holding more zooxanthellae, soft corals stretch toward the light, anemones wander looking for light, and SPS lose color. Giant clams bleach pale or white with too much, too little or a sudden change in light; anemones bleach or wander.

To fix it: measure first, at the animal. Then change one thing at a time (Module 1): move the animal, change the intensity in small steps, adjust the height, or replace aging tubes. Check the water too: yellow water strongly absorbs violet and blue (Dana Riddle), so run carbon (Module 17) and do water changes. Give each change time before the next.

Clean the lens or splash cover when salt, dust or film builds up, following the maker's instructions; Red Sea specifies isopropyl alcohol for its lens. Salt creep conducts electricity, raising shock and fire risk. Clean fans with compressed air, about every 1 to 2 years. Keep the room cool: Red Sea specifies below 25 C (77 F) for its lights. A noisy fan that cleaning doesn't fix needs replacing.

Most aquarium lights aren't waterproof. Make a drip loop in every cord, a U-shaped dip below the outlet, so water runs off instead of into the socket. Mount fixtures firmly, never resting on a glass lid where they could fall in. Keep power strips away from splashes, and dry your hands before touching plugs (Module 2). Don't look directly into the LEDs at any intensity, and don't touch the lens while it's on: it can burn.