Training … On the Sun

About a week ago, I mentioned my new Hawaiian sun coral and the fact that I was training it to open diurnally rather than at night.

In the wild, sun corals lead a mostly nocturnal existence, in part because of the tides and in part because it’s safer to extend their tendriled polyps when the fish are mostly sleeping.

13G10 sun open dark

A friend asked how I go about training corals, so I thought I’d share that process here today.

Sun corals eat by opening their mouths to catch and absorb plankton and other tiny dissolved particles from the water and by extending their tentacles to grasp larger pieces of food, which the corals then shove into their waiting mouths.

Most sun corals spend the daylight hours closed up tightly.

13E Sun coral (Maui)

At night, they open to feed.

However, suns can sense high levels of food in the water. They “taste” high levels of plankton and other foods, and (once they know the water isn’t teeming with predators eager to nip the coral’s tentacles off) instinctively open to receive them.

I feed my tank at roughly the same times every day: 4:30pm and 9:30pm. That’s accurate to within about half an hour in either direction. The lights go on around 4pm and go off at 11 (timers are wonderful things). The 4:30 feeding consists of defrosted frozen mysis shrimp and a small amount of liquid food – usually a combination of “oyster feast” and live phytoplankton. (A piece of advice: don’t assume everything in my refrigerator is meant for human consumption.)

The later feeding consists of mysis and frozen enriched brine shrimp only.

When I buy a new sun coral, I swap the feedings until the training is over – the liquid food goes in with the later feeding instead of the earlier one, to ensure some microscopic food remains in the water after the lights go out. This prevents the coral from starving while it learns to open with the light.

13E29 New Sun Open

The “training” itself involves placing little pieces of mysis onto the coral’s polyps at feeding time even though the coral is closed. Within a couple of days, the coral senses the food on top of the polyp and opens its mouths to suck in the mysis. Gradually, the coral comes to expect food at regular times, and slowly starts to open its mouths at feeding time. That’s when the liquid food goes back to the early feeding.

Within a few more weeks (or, in the case of smarter corals, just a couple of days) the coral not only opens its mouths to feed but extends its polyps also. The smartest ones eventually learn that “feeding time” comes shortly after the lights turn on, and open as soon as the tank lights go on overhead.

13G10 flappy and new sun

Some corals learn faster than others, and not all of  them eat “meaty foods” in a way that permits me to train them, but sun corals learn pretty quickly, and their lovely display is well worth the extra effort it takes to teach them to open in daylight.