Sponges?

[jabberwock]

The little yellow knobs...

 

[ebushrow]

Looks like it to me

 

[OldRed1]

These are rock boring sponges!

 

[jabberwock]

Rock boring sounds ominous... Googling now.

 

[jabberwock]

and dosing phytoplankton!

 

[WheatToast]

and dosing phytoplankton!

Phytoplankton (usually) won’t directly help sponges grow as the particle sizes are too large for them to consume. They instead primarily filter bacteria.
https://reefs.com/magazine/aquarium-invertebrates-a-look-at-the-sponges/

 

[ISpeakForTheSeas]

Phytoplankton (usually) won’t directly help sponges grow as the particle sizes are too large for them to consume. They instead primarily filter bacteria.
https://reefs.com/magazine/aquarium-invertebrates-a-look-at-the-sponges/

Yeah, most sponges that I’ve looked into primarily consume dissolved organic carbon (DOC) and specific bacterias, with a preference for algal-derived DOC (i.e. DOC produced by algae). Boring sponges primarily consume DOC (~80% of their total diet, with the remaining ~20% coming from particulate organic carbon (POC - primarily phytoplankton and bacteria). This ratio of approximately 80/20 seems to be pretty typical for sponges from what I can find.

Natural Diet of Coral-Excavating Sponges Consists Mainly of Dissolved Organic Carbon (DOC)

Coral-excavating sponges are the most important bioeroders on Caribbean reefs and increase in abundance throughout the region. This increase is commonly attributed to a concomitant increase in food availability due to eutrophication and pollution. We therefore investigated the uptake of organic...

journals.plos.org

Unfortunately, we don’t have an easy, hobbyist way to measure DOC that I’m aware of, but, there you have it.

Edit: meant to add on, most sponges - as WheatToast mentioned- don’t seem to eat much phytoplankton (they seem to basically just eat DOC and bacteria, though they may consume some small phytoplankton). The phyto may help by being eaten in small quantities, or by producing small quantities of DOC for the sponges to feed on though (so there may be a largely indirect benefit from it).