Experts-What is this black stuff?

[old salt reefer]

So this black algae took over my tank(rocks).

No snails would touch it, in fact they avoided it and quit even going on the rocks at all. Tangs wouldn't eat either. It was even killing my corals (acropora) from the base up. After searching every LFS for months(winter time) I found a urchin(have 2 now) that in a couple of months has eaten 90% of it except in the cracks of the rocks it can't get to .It really is black(or very very dark dark green) under white light. Since I can't take a good picture of my tanks no matter what I do, here are some pictures of other peoples tanks with said black algae. It's on the magnet in the first picture and on the right side of the tile in the second picture.

 

[Timfish]

WHen you say "white light" is that white aquarium lights white room lights" I ask because the pictures look like they were taken with aquarium lighting which even with a "white" spectrum still have a lot of blue which can skew colors. I'm curious what it would look like under 5500K to 6500K white lights. It does looks to me like a mix of some cyano and a small read truf algae, maybe Ceramium spp.

 

[GarrettT]

I have the same thing. Will post an image when the 10k halides come on. I’ve been curious about this as well.

 

[Aqua Man]

Black Sponge

 

[Timfish]

Black Sponge

Looks like Terpios hoshinota, a black cyanobacteriosponge. This is why pictures under different spectrum is important as many red algae and rad algae/cyano can look black under high K color temp lighting. And FWIW not all balck sponges are Terpois, I hae two black sponges in my tanks that won't tolerate the high light Terpois does and are not invasive.

 

[old salt reefer]

By white light I mean a warm light bulb above my sink.
Would urchins eat sponge?

Some days when it was really bad(all my rocks covered in it) it looked(under AB+ light spectrum LEDs) a very short haired brown and if you touched it the very short haired brown would suck in( inches around where you touched it) and become black like shown in the pictures I posted.
Now that the urchins have eaten most of it you can get under the edges of it and it pulls off in small sheets leaving pristine bare rock.

 

[old salt reefer]

Found some pictures I took back on 07/22/22 of the black algae in my tank.

 

[old salt reefer]

And today after the urchins have done their thing

 

[old salt reefer]

Looking at the pictures, look at how much the Magic Wand Tenius has grown in 2 months. Nice to have my corals growing again instead of slowly being killed by this black stuff.

 

[ISpeakForTheSeas]

Would urchins eat sponge?

They might, yes - as long as the sponge lacks the proper defensive chemicals to combat them (either through killing them or through just being unpalatable to them). Some sponges have toxins, some don't, some toxins are effective against echinoderms (such as urchins), some toxins are not.

Terpios hoshinota contains a couple different cytotoxins which may be a potential deterrent, but at least some sea urchins also have cytotoxic coelomocytes, which to me would seem indicate a chance that the toxins of the above sponge are not toxic to or are only mildly toxic to urchins (I'm neither a toxicologist nor a marine biologist, so I don't know based on the chemistry of the toxin or the biology of urchins if they would be impacted by these specific toxins or not). So, if what you're dealing with is Terpios hoshinota, that would be very interesting to see sea urchin predation on it.