Tips for Identifying Unknown Critters

We all get them occasionally, those little unidentified critters that magically appear in our tank even after we’ve been so careful to source good quality, clean live rock (sometimes even starting with dry rock) and dipping our corals on top of carefully inspecting everything going into our tanks. But even so, they always appear at the most inconvenient time when we can barely catch a glimpse of them before they disappear into a crevice or hole.

Most of us aren’t marine biologists, so we are often lost on where to start with identifying unknown critters (UC). We also often do not have a lot of information to provide to others or use to look up, so the task of identifying the UC can be daunting to say the least.

Personally, I’m a bit impatient when it comes to identifying UC’s. I often get the feeling that if I don’t figure out what this thing is that is swimming/crawling around in my tank, it may cause unrepairable damage before I realize it’s bad. Often, however, this is not the case and I should really calm down and take things slower. Even though I’ve recognized that my research methods may be fear driven, they are often successful, and I have even started using them to help other hobbyists identify their UC’s. So, without further delay, here are my methods for identifying a UC:

Defining Features

Try to take note of every defining feature you can, they are often tags or important aspects of a scientific description which can lead to a successful identification of the UC via species information sites. Googling things such as colors, patterns, and body shape can often lead to a clue or series of clues that will help you go in the right direction. If you have a list of defining features, you can google image search them and oftentimes get really close to the final identification.


Knowing just a few defining features of this Machine Gun Shrimp (Coralliocaris graminea) helped me provide positive ID to a Reef2Reef user via a Google Image search, such as “two pistol claws”, “green shrimp with blue eyes”, and “Acropora dwelling shrimp”.

Knowing the Source of your Livestock or Liverock

If you have an idea about where some of the stuff comes from in your tank, you may be able to figure out the geographical range of your UC, which can help identify it overall. A lot of LFS will often be able to tell you where their livestock was sourced from and often they will make an order from a specific geographical area such as an Australia Order, or Indonesian Maricultured Order. I have been able to identify some commensal coral crabs based only on the fact that I knew where the mini colony of Acropora I bought came from. So, ask your LFS before you buy something, it may make a potential UC identification process much easier.

Identification Books

Identification books are super helpful in helping not only identify a UC, but also find out a lot of other useful information about it. Such as what it eats, where it is found, and what it may have rode into your aquarium, whether it be anemone, coral, or rocks. My favorite series of books are the Reef Set books written by Paul Humann, Ned Deloach, and Les Welk, but there are several identification books out there. Shell identification books are also a great thing to have, since a large amount of the UC’s I help identify have a hard outer shell or exoskeleton that is often sought after or found by beachcombers.


This book in particular can offer a wealth of knowledge about identifying unknown hitchhiking critters since most of our corals come from the Indo-Pacific Region. Even if you don’t use these books for identification purposes all the time, they still make great books to flip through and learn about all the various critters found on natural reefs.

Other Hobbyists

By far the easiest way to identify a UC is to ask fellow hobbyists here on Reef2Reef or perhaps another online reefing forum such as your local club forum. Odds are, at least someone else has had the same UC pop up in their own tank, or at least something similar that could point you in the right direction. In this instance, its best to capture a video or at least a picture of the UC, this will greatly increase the likelihood of getting a positive ID. Most forums even have a dedicated thread section for identifying UC’s, which would be a great place to start.


The Reef2Reef Hitchiker & Critter ID Forum is a wealth of information and a great resource if you find yourself with a unknown critter. Several active Reef2Reef members frequent this thread and can often give you a positive ID or point you in the right direction pretty quickly.

Trapping for Closer Inspection

UC’s are often unlikely to stay in one easy to observe spot, so I generally recommend to folks to try and remove it from the aquarium and place it in an observation container, which can pretty much be anything that will hold water. I will say that the easiest thing to take pictures of with the UC inside is probably a plain plastic cup that isn’t see-through. This container will provide two benefits, easier observation and immediate quarantine from the aquarium in the event that the UC turns out to be non-reef safe.

It may be a good idea to keep some one of these small “Critter Carriers” on hand for unidentified critters and also quickly quarantining other things such as Bubble Algae, Aiptasia/Majano, or even some surprise baby fish.

Identification Sites

Some specialized websites have been created to help reef hobbyists identify UC’s. These sites are good for commonly found invertebrates and algae, but some of them show off a few rarer critters and can also help point you in the right direction. One of my favorite sites is LionFish Lair’s ( but there are also some “mega threads” on Reef2Reef and other reef forums that are dedicated to helping hobbyists identify UC’s.

So, if you ever find yourself in a situation where you need to identify some strange looking worm, or an ominous pair of eye stalks staring at you from beneath a coral, maybe try giving some of these tips a try. If all else fails, it is always best practice to remove the critter and wait for a positive ID. Some folks banish their UC’s to be “sump monsters”, so that’s always an option as well!

About author
Nathaniel Holley is a relatively new hobbyist in the world of reefkeeping, having only been keeping and maintaining reef aquariums since 2015. However, since he first started wanting a reef tank, Nathaniel has been learning as much as he can about the hobby.

Nathaniel's main passion for the hobby stems from being able to successfully create and maintain a mini-ecosystem in the comforts of his home. He also enjoys helping other hobbyists and is a member of the Reef Squad Leadership team on Reef2Reef as such. Currently Nathaniel keeps a Innovative Marine Nuvo Fusion 20 mixed reef aquarium but has already put together some plans for a much a larger aquarium that will be built as soon as he becomes a permanent homeowner.

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