How Many Urchins Are Too Many Urchins?

[vlangel]

No urchins for me. My tank is a high nutrient macro algae tank by design and can't have those little lawn mowers chomping down on my macros. The macro algae themself keep nuisance algae to a minimum. My tomini tang and orange spot blenny control the small amount of nuisance algae except bubble and that I periodically syphon out.

 

[Macdaddynick1]

pretty sure same urchin,just different colors

Oh no. Holloween urchins are a lot more efficient., they are also much bigger. They eat a lot more algae, they eat long hair algae, long ulva, etc. I have both kinds and the difference is huge. The only problem with holloween urchins is that they pick up a bunch of frags all the time.

 

[Hugh Mann]

They eat bryopsis too? Please tell me they eat bryopsis.

I had no idea urchins would eat meats and stuff, I thought they were algae only. If that's the case, I think food certainly won't be an issue. I love urchins, I think they're super cool. And since I don't have frags for it to disturb, even better.

Maybe I'll 3d print some tiny hats.

 

[Jon Fishman]

I have 3 (including a pencil) and I plan on about 4 more. I just need them all to be different though.

My pencil is pretty big...... so tell your friends

 

[burningmime]

I mean, it probably depends on how many dead urchins you want. You could theoretically add enough urchins to the tank so that it was all urchin and no water. But then they'd all be dead...amd you'd have to sell all the uni.

I'd add 2 more, wait until 4 months post move to 230, and evaluate then.

This got me curious. Let's assume that the average urchin is roughly 1.5in across when compressed and perfectly spherical. A gallon is 231 in³, therefore a 55 gallon tank is 12705 in³. According to the recently-proven Kepler Conjecture, the densest possible packing of spheres in 3-space is π/(3√2) roughly 74.05%, which means we have about 855 in³ worth of space to fill with urchins. That lets us provide space for roughly 1774 urchins.

However, urchins are not uniformly sized, meaning we have a Random Close Pack. That's rather unfortunate, since there will be more wasted space (perhaps taken up by boring stuff like rock/fish/coral) in between our urchins. We also have the sides/bottom of the tank to worry about. On the other hand, urchins are soft/compressible, not perfectly spherical, and some of their spines can overlap, all of which lets us fit a few more.

I'd say it would be safe to start with around 1500 urchins, which leaves some comfortable room to fill in the gaps with water. When you upgrade to your roomy 230 gallon, you can get 5000 more.

 

[ichthyogeek]

I love this. But also, you need to take into account the displacement of volume with live rock and such haha.

 

[Hugh Mann]

Just build the rock work out of urchins.

 

[X-37B]

My 120 has 2.
They only eat coralline.
I have no hair algae.
2 keeps the coralline in check.

 

[OldRed1]

This got me curious. Let's assume that the average urchin is roughly 1.5in across when compressed and perfectly spherical. A gallon is 231 in³, therefore a 55 gallon tank is 12705 in³. According to the recently-proven Kepler Conjecture, the densest possible packing of spheres in 3-space is π/(3√2) roughly 74.05%, which means we have about 855 in³ worth of space to fill with urchins. That lets us provide space for roughly 1774 urchins.

However, urchins are not uniformly sized, meaning we have a Random Close Pack. That's rather unfortunate, since there will be more wasted space (perhaps taken up by boring stuff like rock/fish/coral) in between our urchins. We also have the sides/bottom of the tank to worry about. On the other hand, urchins are soft/compressible, not perfectly spherical, and some of their spines can overlap, all of which lets us fit a few more.

I'd say it would be safe to start with around 1500 urchins, which leaves some comfortable room to fill in the gaps with water. When you upgrade to your roomy 230 gallon, you can get 5000 more.

I love that my Google search brought me to this comment. Thank you for the laugh this morning!