Black spongy thing on rocks

[reeferab]

Hi could someone please help identify what is the black spongy things growing on my rock?

Almost all started off as circular discs and have started to encrust the rock. They don’t go away at night and don’t blow over like cyano does. Does not appear to have hair like hair algae.

I would guess it’s a black sponge of some sort but haven’t found much info on black sponges (if that’s what this is).

Tank is 13.5G/52L approx 4months old. Only a clown and jester goby as fish. Cuc is various snails, cleaner shrimp. No crabs. Mixed reef tank.

Also have pineapple sponges in the tank. In fact a turbo snail has them growing in this shell and deposits around the tank.

Using Fluval sea marine 3 light in case that’s of importance.

 

[smitten with ocean life]

@ISpeakForTheSeas

 

[ISpeakForTheSeas]

@ISpeakForTheSeas

From a glance, some parts do look spongy, but looking at the growth patterns, I'm inclined to think it's an encrusting algae of some variety.

That said, there are a few black encrusting sponges, most are totally harmless/beneficial, but at least one of the species is known to be both invasive and chemically harmful to corals (not a common combination for sponges).

Closer pics of the texture of one or two of the specimens under white lighting may be helpful for an ID. Also, if you have a decent microscope and can get a sample of one, you might be able to tell if it's a sponge or algae with that (sponges generally look basically like a solid wall of color with some spiky shapes mixed in - spongin and spicules - while algae looks more like plant cells).