One of my favorite mobile invertebrates of all time is the Tuxedo Urchin (Mespilia globulus).
Tuxedo Urchins are small sea urchins that have broad bands of color in between their spine rows. Most tuxedo urchins are blue or black and occasionally red. Their spines are short which is nice as there is less risk of you getting speared by a spine like with a long spine urchin. Besides small spines, tuxedo urchins don’t get big as their maximum size is about 3 inches (~8cm).
Like most urchins, Tuxedos graze on algae. They are voracious eaters that will almost constantly cruise your tank eating any algae they can find. They’ll also climb on your glass and they’ll even clean up equipment (power heads, pump intakes, etc). Unlike other urchins, Tuxedos won’t bulldoze your rock work or coral although don’t be surprised if small frags or empty hermit crab shells become hitchhikers on the urchin’s back. Tuxedos use these items for camouflage against predators.
While other mobile invertebrates like fromia or linckia starfish are very sensitive creatures that usually don’t fare well in aquariums, Tuxedo urchins are hardy and require minimal care as long as they have algae to eat. If your aquarium doesn’t have any algae for the Tuxedo to eat, you can hand feed the Tuxedo small pieces of sheet algae (nori) to satisfy their dietary needs. Just make sure you keep your fish away as the fish will often swipe the algae from the urchin before the urchin has a chance to ingest it!
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