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Invertebrates: Hydroids

Hydroids (Staurocladia oahuensis): Reef Safe with Caution
These tiny hydroids are typically found on aquarium glass. Their population tends to wax and wane with tank nutrient levels. Although they do have some stinging potential, they do not appear to bother anything significantly.
Staghorn Crab (Manucomplanus varians): Not Reef Safe
This neat crab has a symbiotic relationship with its shell. The shell is composed of stinging creatures (similar to jellyfish) called hydroids and bryozoans. The shell of hyroids/bryozoans protects the crab from predators. When the crab eats, pieces of food remnants are often caught by the hydroids/bryozoans. As the shell grows, the crab trims the opening with its claws. Although this is a very unique crab, it is not recommended for most reef aquariums as the shell itself requires special care. The organisms on the shell may die if the aquarium is not mature and stable enough. Additionally, these stinging creatures may harm corals, or corals may harm it. Lastly, due to the unique shape of the shell, the crab may not be able to right itself if it falls over.

Digitate Hydroid:  Arguably Reef Safe
Usually digitate hydroids are seen in their fully extended 1-3″ length, but this is one unextended.  These are covered in nematocysts (stinging cells), so they could bother nearby corals as they attempt to catch food in the water column.  Although some reefkeepers have a problem with them, I have not noticed any detrimental effects from them.  They are difficult to remove, especially when they are hidden in the rocks.  However, their population waxes and wanes with the available food supply.  If they are getting to plague proportions, then I recommend decreasing feeding and increasing skimming/water changes.

Hydroid: Arguably Reef Safe
These tiny hydroids are typically found on aquarium rocks and sometimes even on Acropora corals. Their population tends to wax and wane with tank nutrient levels. Although they do have some stinging potential, they do not appear to bother anything significantly.

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