Why are Acropora polyps less extended during the day, even though they extend at night?
From my own observations, because coral polyps are animals, nature is all about eating or being eaten. During the day, when the sunlight is out, the polyps may feel like it’s less safe to fully extend because they’re more visible and can become easier prey.
But if your water has plenty of available food, or if you regularly feed your corals while the lights are on, the polyps can start associating daytime with feeding. After repeated daytime feedings—especially if they aren’t getting nipped at by fish—they may start coming out more often because the reward of feeding becomes greater than the perceived risk.
At night, it’s a little different. Many corals evolved a nocturnal feeding rhythm because zooplankton availability in the water column often increases after dark. Some daytime coral predators may also be less active, which can make nighttime an even better opportunity for the polyps to fully extend and catch food.
So in my opinion, polyp extension is basically a balance between risk and reward. If a coral learns that food is regularly available during the day and nothing is bothering it while it’s extended, you may start seeing much better daytime polyp extension as well.
This is just my own observation why we tend to see stronger polyp extension at night, while some corals can almost be “trained” through regular daytime feeding to come out more during the day.
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I really appreciate your detailed explanation. It has been very helpful.
This aquarium has only been running for about three months.
I have not added any natural-origin filter media or plankton, so those elements are not really present in the system at all.
I had also wondered if this might be one of the reasons for the current situation. However, based on your explanation, I understand that the overall maturity and development of the aquarium system itself is an important factor, and that it tends to improve gradually as the system matures.
Thank you very much for taking the time to explain this so thoroughly.
