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This thread is meant to provide data on whether or not the common tiny starfish known as "asterina" or aquilonastra are actually eating corals in reef tanks.
First, a comment on the name. All these types are in the family asterinidae, and within that family there is a genus asterina, and another genus aquilonastra. We believe that the tiny stars we refer to are in the genus aquilonastra. A number of these similar species got reclassified from genus asterina into aquilonastra (asterina burtoni got reclassified as aquilonastra burtoni for instance). My guess is that Aquilonastra conandae and A. burtoni are good candidates for most of the tiny stars in the hobby. Paper reference h/t @ISpeakForTheSeas and @Timfish is attached.
In my tank there have been phases when I grew a lot of zoas of various kinds, and I've also had phases of aquilonastra star introduction and rapid multiplication and observed what looked like feeding behavior on many zoas, and their decline.
There are many alternate explanations given to suggest these stars are not coral eaters: they are consuming biofilm, or the zoas are already diseased /dying, and the stars are consuming dead tissue, or consuming zoa tissue that is unhealthy in some way.
To try to address some of those and add a little bit of evidence to the question, I took some zoanthid frags and placed them on my sandbed or in an exclusion device of some sort.
The first exclusion device was simply to tie a frag to string hanging from the lid so it was suspended in the water. The second exclusion device was a water bottle that I cut out the sides of and replaced with net glued around it. The third exclusion device was simply a breeder box when I realized that's all I really wanted.
The effect of each of these exclusion methods was the same. In each one, zoa polyps were basically all open - while frags on the sand were often partially or totally closed.
So here's the actual data. What's plotted in this the data is the number of open polyps for three frags (cheap, likely nameless zoas) - I label them green, orange, and red. During this experiment one or two frags were always left exposed on the sand, and one or two were always protected by the exclusion devices. Zoas were rotated between protected and unprotected. The stars in the tank numbered in the hundreds.
The blue circles annotate the time when a frag was protected by an exclusion device. No frags decreased numbers of open polyps while protected.
Note that the green and orange zoas were much more quickly affected by being exposed, the red zoa - perhaps not at all. There is certainly a preference between different zoa types. Others have observed this also. No other corals seem to be eaten by the stars.
Below is an example of the polyp reduction and observed star behavior (green zoa).
Here are some pictures of the only type of star I observe in my system during this experiment.
The individuals are light or white with grey, brown/red, and orange speckles. I've seen 5 to 7 arms - most have 6.
It's been suggested (Dong Zou on reefbum 1:16:42) that ones with some red coloration are coral-eaters and my observations are consistent with that. I considered the possibility that the red coloration is a color that is created when the animal feeds on corals (nudibranch color is often coral-diet driven), but individuals from my sump with no access to corals also have the same red + orange spots. So this is not a coral diet-induced color.
This data presented here does not address the question of whether there is something else that might've been consuming the zoas (crab, fish etc) that might also have been kept away by the exclusion chamber. To address this aspect in Part 2 - I have a harlequin shrimp that has now removed all visible stars from my system. We'll see how the zoa frags do with the aquilonastra stars essentially wiped out from the system.
Bonus: here's a time-lapse I've posted before of seemingly predatory behavior from years ago when I had a lot of zoas and my star population rapidly grew for the first time.
First, a comment on the name. All these types are in the family asterinidae, and within that family there is a genus asterina, and another genus aquilonastra. We believe that the tiny stars we refer to are in the genus aquilonastra. A number of these similar species got reclassified from genus asterina into aquilonastra (asterina burtoni got reclassified as aquilonastra burtoni for instance). My guess is that Aquilonastra conandae and A. burtoni are good candidates for most of the tiny stars in the hobby. Paper reference h/t @ISpeakForTheSeas and @Timfish is attached.
In my tank there have been phases when I grew a lot of zoas of various kinds, and I've also had phases of aquilonastra star introduction and rapid multiplication and observed what looked like feeding behavior on many zoas, and their decline.
There are many alternate explanations given to suggest these stars are not coral eaters: they are consuming biofilm, or the zoas are already diseased /dying, and the stars are consuming dead tissue, or consuming zoa tissue that is unhealthy in some way.
To try to address some of those and add a little bit of evidence to the question, I took some zoanthid frags and placed them on my sandbed or in an exclusion device of some sort.
The first exclusion device was simply to tie a frag to string hanging from the lid so it was suspended in the water. The second exclusion device was a water bottle that I cut out the sides of and replaced with net glued around it. The third exclusion device was simply a breeder box when I realized that's all I really wanted.
The effect of each of these exclusion methods was the same. In each one, zoa polyps were basically all open - while frags on the sand were often partially or totally closed.
So here's the actual data. What's plotted in this the data is the number of open polyps for three frags (cheap, likely nameless zoas) - I label them green, orange, and red. During this experiment one or two frags were always left exposed on the sand, and one or two were always protected by the exclusion devices. Zoas were rotated between protected and unprotected. The stars in the tank numbered in the hundreds.
The blue circles annotate the time when a frag was protected by an exclusion device. No frags decreased numbers of open polyps while protected.
Note that the green and orange zoas were much more quickly affected by being exposed, the red zoa - perhaps not at all. There is certainly a preference between different zoa types. Others have observed this also. No other corals seem to be eaten by the stars.
Below is an example of the polyp reduction and observed star behavior (green zoa).
Here are some pictures of the only type of star I observe in my system during this experiment.
The individuals are light or white with grey, brown/red, and orange speckles. I've seen 5 to 7 arms - most have 6.
It's been suggested (Dong Zou on reefbum 1:16:42) that ones with some red coloration are coral-eaters and my observations are consistent with that. I considered the possibility that the red coloration is a color that is created when the animal feeds on corals (nudibranch color is often coral-diet driven), but individuals from my sump with no access to corals also have the same red + orange spots. So this is not a coral diet-induced color.
This data presented here does not address the question of whether there is something else that might've been consuming the zoas (crab, fish etc) that might also have been kept away by the exclusion chamber. To address this aspect in Part 2 - I have a harlequin shrimp that has now removed all visible stars from my system. We'll see how the zoa frags do with the aquilonastra stars essentially wiped out from the system.
Bonus: here's a time-lapse I've posted before of seemingly predatory behavior from years ago when I had a lot of zoas and my star population rapidly grew for the first time.
