What The Heck Is That In My Aquarium?

Why Strange Things Seem To Appear Out of Nowhere

Saltwater clownfish in an anemone

What is that white fuzzy looking thing growing on my live rock? What are those little white dots growing on my aquarium glass? What are those little white bug-like things crawling around in my tank? Something really weird showed up in my aquarium. Is it going to hurt anything?

Sound familiar? These and similar questions are not odd, but what is odd is how many aquarists don't realize that when it comes to keeping a saltwater system, it's not unusual for strange things to show up. We don't understand why someone is so surprised when it happens because it's all part of a marine environment!

Reasons Organisms Appear

Although organisms are usually introduced into an aquarium by way of newly added live rock and live sand, there are other reasons why things appear out of what "seems" like nowhere.

They Grew Larger

Many organisms are microscopic or plankton-sized when they start out, so until they grow large enough to be seen with the naked eye, you don't know they are there.

Some organisms that can be seen with the naked eye, no matter how large or small, are experts at hiding. They hitchhike in on live rock and sand, and it is only after you have placed it into your aquarium that these organisms then crawl out and make themselves at home.

They Are Hitchhikers

Often corals, anemones, urchins, sea cucumbers, starfishes, nudibranchs and other forms of marine life play host to other, much smaller hitchhikers on the reef. Just a few examples of the many types of symbiotic relationships that occur in nature are the Pacific Clown Anemone Shrimp (Periclimenes brevicarpalis) and Anemone Crab (Neopetrolisthes ohshimai) that live with various types of sea anemones. The Sea Cucumber Crab (Lissocarcinus orbicularis) lives commensally on the body, but most often among the tentacles or in the mouth, as well as the anus of several species of sea cucumbers.

They Are Eggs

Marine animals can have babies. As examples, some crabs and shrimps can hatch out live fry when mated pairs are present, snails reproduce by laying weird looking egg sacs, and corals spawn or split.

For these reasons, organisms that suddenly show up in an aquarium were most likely there all along or were spawned by other tank inhabitants. You just couldn't or didn't see them yet.

Types of Organisms

Here are just some of the strange things one might find showing up all the sudden in a saltwater aquarium.


  • All Types - Micro, Macro, and Coralline
Extreme Close-up of Red Coralline Algae
Red coralline algae Pam Susemiehl / Getty Images

Arthropodas - Crustaceans

  • Copepods & Amphipods, Barnacles, Crabs, Hermit Crabs & Shrimp, including the unwanted Mantis Shrimp.
Mantis shrimp Andrea Izzotti / Getty Images

Cnidarians - Anthozoans - Hexacorallians

The Order Actiniaria in this taxonomy line contains Suborders and Families of Sea Anemones, which includes the unwanted species. The Subclass Hexacorallia also contains Tube Anemones (Order Ceriantharia), Stony Corals (Order Scleractinia), Zoanthids (Order Zoanthinaria), and even Black and Wire Corals (Order Antipatharia), which are rarely found in saltwater aquariums.

Cnidarians - Anthozoans - Octocorallians

The Subclass Octocorallia taxonomy line contains Soft Corals & Gorgonians.

Cnidarians - Hydrozoans

The Class Hydrozoa contains Hydroids. The typical Hydroid is a polyp colony on a flexible featherlike or fernlike skeleton consisting of a main stem and numerous side branches.

Bryozoans (or Ectoprocta)

Tiny colonial animals. Some species look like algae, while others resemble small branching or encrusting corals.

Chordata - Urocordata (or Tunicata)

Tunicates & Sea Squirts


Starfishes, Sea Urchins & Sea Cucumbers

Marine Worms

Flat (Platyhelminthes), Ribbon (Nemertina), Acorn (Hemichordata - Enteropneusta) and many other types of worms, including the unwanted Annelida - Polycheata - Bristle & Fire (Amphinomidae) worms, and the desirable Feather Duster (Sabellidae) and Christmas Tree (Serpulidae) species.

Detail of a feather duster worm growing on a coral reef.
Feather duster worm Ethan Daniels/Stocktrek Images / Getty Images

Molluscs - Bivalves

Clams, Mussels, Oysters & Scallops

Molluscs - Gastropods

Snails, Sea Slugs/Hares & Nudibranchs



Are These Safe Organisms?

Not having purposely put some of these things in your aquarium, the first question you ask yourself when you see something new is, "What the heck is that?", and the second, "Is it safe for my aquarium?" You won't know the answers to these questions until you have properly identified whatever it is, and this is most times easier said than done.

The taxanomy of the animal kingdom is vast and complicated, which makes trying to identify something that suddenly appears in your aquarium very difficult to pinpoint.

With new born organisms this is particularly true, because during an organism's life cycle it can go through many different shapes and color changes, morphing into something that looks totally different from what it started out as. Another problem is that an organism may be so small, you can't even begin to guess what it might be.

What to Do About Organisms

Here is what you can do to help identify strange things that show up in your aquarium:

  • Buy some good invertebrate identification books for your saltwater reference library.
  • Refer to marine invertebrate database and profile information, as well as photo galleries online.
  • If you have a personal saltwater Website, create something like a "Can You Help Identify This?" page. You can display photos here and allow visitors to leave comments or email back to you about them.
  • Post a message in a Saltwater Aquariums Community Forum asking for help with identification. If possible include a photo of good clarity, or provide a link to a Web page you may have created as described above.
  • Photo Emailing Tip: When you email a photo to another aquarist asking for help with identification on something, be kind. Only send an image that is reasonably sized, and is clear enough to tell what you want identified including a "brief" description.
  • Check with your local aquarium store to see if they can help identify the organisms, especially if it is the store from where you acquired live rock, as they may have seen those organisms before.

If you cannot identify what you have, you basically have two choices of what to do:

  • Leave it alone and see what happens.
  • Remove it, if you are worried that it may harm your aquarium.