polyp extension day versus night

[CTaylor]

Hi,
I'm pretty sure when I have gone scuba diving I've much more polyp extension at night than at day time. And from what I remember in marine bio, coral polyps extend at night.
My acro polyps all extend quite a bit at night, not much day time. I've seen many acros that have big PE daytime in aquariums. I've done a search on this on this site, and it seems reefers seem to think it's due to fish in the tank that nip at SPS polyps. I have a yellow candy hogfish, blue damsel and several neon gobies. I'e seen the damsel occasionally pick at my birdsnest (but this is the one sps I have that has PE both day and night). The damsel doesnt bother the acros.

Are there just certain species of acro that extend at night and day nearly equally? And by chance none of my three are of that variety?

My tank is new, but even at this point I'd have to say is doing 'better' overall than my previous tank (which was successful, but killed by hurricane irma power outtage). I do have very low phosphorous (5 ppB -- on Hanna checker), but I dont have a Phosph remover, it's just my chaeto I guess. My nitrates are 3-5 ppm (not 0), which I think is ideal. Alk 10 dkh. I havent tested Ca, but I believe it's 450 ppm, will test. But it goes hand in hand with alk (I have a Ca reactor). Mg should be 1350... will check that. But that is what my readings were on my last tank, and that one also had 90% PE at night.
My lights over my 108 (48x24x22): Kessil AP700 at max 30%, and 2 a360 on low, just for even light coverage. Acros are at very bottom of tank. Will increase light intensity super slow over time.

So why am I seeing acros (not mine) with big PE in daytime? Are they stressed or need to look for food at all times?

 

[NS Mike D]

It's my understanding that polyp extension has two roles, one to capture food and nutrients from the water column and the other to increase surface area for photosynthesis. I understand that much zooplankton vertically migrate at night, so it makes sense that corals will send out their sweepers at night.

My guess is you are correct in that your sps polyps are pre-dispositioned to filter feeding over photosynthesis, whereas corals usually open up with daylight much like plants.

there are a number or corals that will extend polyps during the day and then send out sweepers at night

 

[Tripod1404]

IF you dont have enough light, or if the flow is to high, There will be minimal polyp extension. Polyp extension cause self-shading. If light i not strong enough or if the flow is too strong to catch anything, there will be minimal polyp extension.

 

[CTaylor]

there is no light at night. So how does that go along with higher polyp extension at night? And if that's the case of higher light = more PE, and if sps extend more at night in nature (which I THINK is true), then why do corals extend in the actual ocean more at night? Yes, I know that some LPS extend their sweepers sometimes at night.
*In my last tank I had a few acros right under the A360, A360 was 5" above water, acros about 6" below it. They had same high flow night and day. Big extension at night, tiny bit at day.

 

[DesertReefT4r]

Many corals feed on phyto and zoa planktons at night so there will be increased poly extention. Most of my sps had good PE with lights on. My LPS Awould get crazy PE at night though.

 

[Tripod1404]

there is no light at night. So how does that go along with higher polyp extension at night? And if that's the case of higher light = more PE, and if sps extend more at night in nature (which I THINK is true), then why do corals extend in the actual ocean more at night? Yes, I know that some LPS extend their sweepers sometimes at night.
*In my last tank I had a few acros right under the A360, A360 was 5" above water, acros about 6" below it. They had same high flow night and day. Big extension at night, tiny bit at day.

You are comparing two completely different conditions. Corals behave drastically different between the day and the night, and night is not equal to low light. Most LPS corals also inflate to increase their surface area and get more light, but they deflate during the night. With you logic they should inflate during the night since there is low light.

For SPS corals, there is already no light and photosynthesis during the night and therefore polyps cannot cause shading. If light is low, they cannot afford to have their polyps out and cause shading. Even more so if the flow is already to high to catch anything.

At least with acros, sometimes the most shaded parts (like the base or the underside of the branches) of an otherwise well illuminated coral can have more more PE during the day compared to the parts that get more light. In this case, since those parts already get very little light, shading is not an issue and they try to help colony by hunting.