What could this be growing near the zoas?

[fishmister]

Bought this Zoa rock months ago, noticed this after I’ve come back for a couple days away, any ideas what it could be? Thank you

 

[ISpeakForTheSeas]

Bought this Zoa rock months ago, noticed this after I’ve come back for a couple days away, any ideas what it could be? Thank you

Probably a sponge, and most likely harmless/beneficial.

 

[Lavey29]

A sponge surrounded by a huge amount of GHA.

 

[GSPClown94]

Probably a sponge, and most likely harmless/beneficial.

I don’t understand how this sponge growth could be beneficial in this situation? It looks like it’s growing pretty close to those zoas, will the zoas be able to expand its colony with that sponge there? I’ve encountered some sponges that have grown over zoas completely smothering them.

 

[ISpeakForTheSeas]

I don’t understand how this sponge growth could be beneficial in this situation? It looks like it’s growing pretty close to those zoas, will the zoas be able to expand its colony with that sponge there? I’ve encountered some sponges that have grown over zoas completely smothering them.

The zoas may or may not be able to expand with the sponge there (depends on the sponge and the tank conditions for the zoas) - that's definitely something to consider though, as I would expect it to probably keep the zoas from growing out toward the sponge in this case (they may still grow that way, just much more slowly, or they may not grow that way at all). It may be a worthwhile trade for anyone either 1 ) wanting to keep a sponge for biodiversity/aesthetic purposes, or 2 ) wanting to keep sponges for their ability to filter water, which may help to suppress certain pest algae (apparently not GHA) and/or disease:

I haven't been able to find any info indicating that they eat dinoflagellates (which is actually kind of surprising to me), but they do eat diatoms, bacteria, and even viruses.*

*Sources (for virus feeding/removal - the diatoms and bacteria I can show in other papers if wanted):

https://www.researchgate.net/publication/240845694_Virus_predation_by_sponges_is_a_new_nutrient-flow_pathway_in_coral_reef_food_webs

Marine virus predation by non-host organisms - Scientific Reports

Viruses are the most abundant biological entities in marine environments, however, despite its potential ecological implications, little is known about virus removal by ambient non-host organisms. Here, we examined the effects of a variety of non-host organisms on the removal of viruses. The...

www.nature.com

I say it's most likely harmless/beneficial because it doesn't seem to be chemically harming the zoas at all (even the smaller polyps closest to it at the edge of the colony are still open), and it seems to be growing around the base rather than up over the polyps themselves; so the zoas seem to be able to protect themselves from the sponge chemically.

That said, yes, some sponges definitely grow up and over corals and do smother them, but others grow up the "stems" and will stop at the polyps without over growing them, or will grow up and literally above them (not covering them, just sticking out over the top of them in the water) without harming them (I haven't seen any of this type grow enough to shade a colony to death yet; I could happen, but would be relatively easy to prevent by just trimming the growth back).

This is how I usually gauge if a sponge is harmful or not:

Most sponges are completely harmless/beneficial, but some can be invasive and/or harmful to corals (thankfully, these are rare).

To tell if a sponge is chemically harmful: if a healthy, established coral starts closing up or looking to be in bad shape on the side closest to the sponge as the sponge grows closer to it, and nothing else has happened (lighting changes, parameter swings, pests, etc.) that could explain it, then the sponge is probably chemically harmful.

Chemically harmful sponges are very rare.

For invasive sponges: unless it shows signs of being chemically harmful or starts actively growing over and smothering a coral's flesh/polyps, it's harmless. These can grow over the skeletons of corals, around the base/stalks of corals, even up into the water column above corals (where they're over the coral but not growing on the flesh or polyps themselves), etc. without harming the coral at all - as long as the coral flesh and polyps can get food, light, and flow, the sponge is harmless.

Invasive sponges are moderately rare.

Invasive and chemically harmful sponges are incredibly rare.

 

[Lavey29]

I don’t understand how this sponge growth could be beneficial in this situation? It looks like it’s growing pretty close to those zoas, will the zoas be able to expand its colony with that sponge there? I’ve encountered some sponges that have grown over zoas completely smothering them.

I've got zoas all over my tank and sponges too. The sponges tend to grow in the more shaded minimal light portions of the tank where zoas don't really thrive so in those instances sponge will outcompete for space but for the most part they stay in their preferred zones. Times like this, I enjoy watching nature do it's thing like it does naturally in the ocean. Zoas are very resilient and if they feel the sponge is a threat they invent a work around. I wouldn't worry about it and if you add an angel fish they eat sponge too.