If you have ever set up, maintained or even researched marine aquaria, I am sure you are aware of live rock and its role in this hobby. Live rock is generally a piece of reef rock (ancient dead corals) that is pulled directly from wild reefs or culture facilities. It is then boxed and immediately shipped to your local fish store or a distributor. Live rock provides structure for aquascaping and biological filtration in your aquarium.

More often than not, live rock rock is still moist from being shipped right after it is collected. The reason for such quick shipment is to avoid the die off of beneficial bacteria and organisms that use the live rock as a host. This allows for a wide variety of beneficial bacteria to be introduced into a new aquarium for biological filtration processes. Along with introducing bacteria, oftentimes small animals and other organisms survive the trek. In the aquarium hobby, they are most commonly referred to as "live rock hitchhikers."

The list that follows includes some common and surprising hitchhikers hobbyists have discovered on their aquarium live rock. Our goal is to help you identify these organisms and teach you how they can affect your aquarium, for better or worse. We'll also include some tips on how to get rid of them if they pose a threat.

1. Anemones

Arguably the most common (and most problematic) live rock hitchhikers are anemones. They can be prolific and, with their potent sting, can have severe affects on corals and even fish or invertebrate if left uncontrolled.

Aiptasia (or glass anemones) are light brown/pink in color and semi-transparent. The tentacles are skinny with a pointed tip. They start out quite small, around ¼" to ½" in diameter, but can grow to 1 ½" to 2" if left uncontrolled. They are photosynthetic and will fully expand during daylight hours but quickly retract—almost disappear—when disturbed.

Aiptasia can reproduce and spread quickly throughout an aquarium. They seem to spread faster when agitated and new polyps often regenerate from their remnants. You will therefore want to kill them—and kill them fast.

There are several effective and affordable aiptasia treatments available, like Aqua Vision Aquatics Aiptasia Solution, Joe's Juice Aiptasia Eliminator and Red Sea Aiptasia-X, to name a few. We've included a video demonstration below to show you how to inject these sorts of chemical treatments. Some hobbyists create their own homemade paste using Kalkwasser and water to inject and kill anemones. Natural predators, such as peppermint shrimp, copperband butterfly fish and berghia nudibranchs, can also be very effective.

Majano anemones are generally green/brown in color. The tentacles are shorter than aiptasia with bulbous tips. They start out about the same size, roughly ¼", and can grow up to 2" if left untreated. They can be just as prolific as aiptasia in your aquarium and are hazardous to coral.

The aforementioned chemical solutions can also be used to control majano anemones. In fact, these treatments are more effective on majanos. They are easier to inject (or cover) with the solution and seem to die off quicker. Natural aiptasia predators, like those discussed earlier, are not effective for controlling majano anemones.


2. Bristle Worms

bristle wormBristle worms are orange, red or pink with small white bristles that protrude from the sides of their entire body. They are nocturnal creatures and can be very difficult to see. They seldom make an appearance outside of the sand bed and live rock crevices they live in. The worms' bristles can deliver a painful sting; larger specimens can deliver a piercing bite.

To the best of my knowledge, these common bristle worms are scavengers that feed on leftover food and detritus in your aquarium. Although some hobbyists have reported predatory nature in larger worms, I have yet to witness this in my own aquariums. They generally do not bother anything in your tank and increase the biodiversity feeding on detritus. If the population becomes out of control, I would speculate you have a waste problem. Change your water regularly and clean any mechanical filters to help cut down on debris/detritus.

The only method of control is to physically remove the worms. Since they are nocturnal and quick, it's very hard to grab them with just a pair of tweezers. I have attempted this myself, only to end up with wet shirt sleeves, beads of sweat pouring down my face and a waste basket full of empty beer cans. Needless to say, I recommend opting for an inexpensive bristle worm trap to save you time and stress.

3. Shrimp

Shrimp are seldom found in live rock because they are sensitive invertebrates that rarely survive the trek. Yet, to our amazement, there are a couple of different species that have reportedly been found hitchhiking on live rock into home aquariums.

mantis shrimpMantis Shrimp, aka "Tankbusters" or "Thumbsplitters" – Quite a few different species exist and some are even available for sale in the reef hobby. They range from rainbow to brown in color, depending on the species, and vary in size from 1" to 12". They can be easily identified by the orb-shaped eyes protruding from the top of their heads that always seem to be peering in opposite directions. They have a long, lobster-like tail and strong front appendages they use to kill prey and ward off assailants.

Mantis shrimp are very predatory toward fish and other invertebrates. Their powerful front appendages are either club or spear-shaped. Larger specimens have been known to crack aquariums or split open your thumb (hence the nicknames), so proceed with caution. Mantis shrimp are known for striking the tank wall and creating a loud clicking/popping noise when people walk close by.

If you find a mantis shrimp, lure it into a fish trap using a piece of squid or shrimp for bait. It may kill fish if you leave it in a stocked aquarium. You can then keep your feisty friend in a separate, dedicated tank if you'd like.

pistol shrimpPistol Shrimp – Pistol shrimp are one of the loudest, if not the loudest, living organisms in the ocean. They essentially create small sonic booms to kill prey underwater. Some species have a symbiotic relationship with gobies and are sold in pairs in the retail trade. The shrimp builds the burrow while the goby looks out for predators. Their color varies, from red and white strips to plain brown. They stay relatively small, measuring about ¾" to 2" max.

Pistol shrimps earn their namesake from a large front claw that makes a loud snapping noise when used. It can even punch a hole in the exoskeleton of a crustacean! Pistol shrimp are usually heard but rarely seen. They are elusive and typically stay hidden inside a burrow while a watchman goby scopes the scene for predators. If you hear a strange clicking noise coming from somewhere in your rockwork, you may have a pistol shrimp inside your tank.

Pistol shrimp can pose a threat to small reef fish, but are generally harmless. I see no reason to remove this shrimp from a reef aquarium. They are actually pretty cool when paired with a goby. If you are intent on removing one from your aquarium, I would recommend a fish trap. Good luck catching one, though. They are so small and elusive it will definitely be a challenge.

4. Crabs

crabsCrabs are one of the more pesky critters that can hitchhike on live rock. Many crabs are predatory toward reef fish and/or invertebrate and some will even eat your coral. Many crab species have been reported as live rock hitchhikers, including teddy bear crabs, decorator crabs, emerald crabs, acropora crabs and arrow crabs. Many of these species are available for sale in the hobby but are generally not recommended for reef aquariums.

Hitchhiking crabs come in a wide variety of shapes, sizes and colors. They may enter your tank very small, say ½" or so, and be hidden inside the crevice of a piece of live rock. Small crabs may, however, grow quite large depending on the species.

If you find a crab in your aquarium, the first thing you should do is identify it. If you're not sure what species it is, email us a digital photo. Knowing what type of crab you have will tell you a lot about its nature and what type of risk it poses, if any. Crabs may be harmless when they are small but can soon grow into a threat that puts every living thing in your tank at risk, especially fish and invertebrate.

You can safely remove most crabs from your tank by simply grabbing them with tongs or tweezersFish traps work, too. Comfortable crabs are bold and will be enticed by a meaty bit of squid or shrimp.

5. Snails

snailsSimilar to crabs, there are a wide range of snails and other shelled animals that commonly hitchhike on live rock into marine aquariums. Cowries, limpets, abalone, chitons and whelks have all been found hiding in live rock. Some of these organisms are harmless herbivores while others are coral and invertebrate predators that can consume clean-up crew critters and copepods.

Sundial and heliacus snails are two species to watch out for. They are easily identified by the distinct markings and cylindrical shape of their shells. Both are zoanthid predators that can be found hanging out on rock near a zoanthid colony. It is always a good idea to dip zooanthid colonies before placing them into your aquarium. Freshwater dips work quite well for these little buggers.

Cowries are another snail-like animal that can be disastrous if present in a reef tank. Some are herbivores and available for sale, like the popular Tiger Cowrie. They have a large mantle that expands to cover the entire shell, giving it a fleshy appearance often dark or black in color. If it feels threatened or if you touch the flesh, it will expose its shell. Cowrie shells are easy to identify because they are oval or egg-shaped and usually shiny, like a polished stone.

To find out what a shelled animal is eating, just observe it in your aquarium as it goes about its day. If you are unable to ID the animal, it is always best to err on the side of caution and remove it before it can harm prized coral.

6. Sponges

spongeSponges are a unique organism similar to, but not quite, coral. They filter food from water and are usually non-photosynthetic. They tend to grow then disappear for no apparent reason. This is due in part to the lack of suitable food sources in your tank.

Thousands of different types of sponges exist so they come in a variety of different colors and textures. Many reef aquarium hobbyists enjoy watching sponges grow because they are an oddity with cool coloration. Most sponges do not survive air exposure, but small buds inside moist pores of live rock may endure. Once the rock is established in your tank, the sponge may grow and even thrive as it consumes particulate matter in your aquarium water. Once the food source is depleted, the sponge will quickly die.

Some sponges carry toxins and can threaten corals by overgrowing and smothering them. A few ways hobbyists report sponge removal is by exposing them to air, scraping/brushing them away or covering them in reef glue or epoxy. Some aquarists remove them simply because they hog up prime coral real estate. A lot of hobbyists prefer to leave them alone so they can enjoy the biodiversity that comes with reef tank ownership.

7. Algae

algaeAlgae. The word alone makes saltwater aquarium hobbyists cringe. The irony is that the most common type of algae found in reef aquaria is also the most desirable: the colorful coralline variety. Coralline is a calcareous algae (it consumes calcium) that gives rock a sought-after purple, pink and/or red color. In addition to the biological filtration benefits, hobbyists often place live rock inside their reefs to seed their tanks with coralline algae so it will spread and decorate their aquascape.

Undesirable algae are usually categorized by color, such as green algae, brown algae and red algae. These types of algae are intrusive and can grow quickly. If introduced into a display tank, the algae can spread like wildfire and outcompete coral for space and lighting.

Green algae are frequent uninvited guests and can be difficult to control. Caulerpa, bubble algae, hair or turf algae, bryopsis and sea lettuce are all considered green algae. Algae is very hardy—it is prehistoric, after all—and will survive through just about anything. Algae spores on live rock often go unnoticed and begin to flourish once they have been introduced into an aquarium. That's one reason why curing live rock in a separate container before putting it in your tank is a good idea. Clean up crew critters are a great, natural way to control algae once introduced, but not all algae is edible by common reef aquarium species. If you're serious about aquariums and want to learn more about algae, we highly recommend Julian Sprung's book, Algae: A Problem Solver Guide. It's a terrific resource for identifying and controlling different types of algae.

Brown algae are less common than green algae but can still be an annoying eyesore. Dictyota, wafer algae and sargassum are resilient and the most frequently seen in reef aquaria. Brown algae are problematic because they are not usually eaten by regular reef tank inhabitants or clean up crews. The best way to get rid of brown algae is to physically remove it and perform regular aquarium maintenance.

Red alga includes the aforementioned coralline plus a few other types that are less common. Red dictyota, wire algae, red leaf algae and ogofall under the red algae umbrella. Some varieties of red algae are nutritious and can be used to feed herbivorous animals. Red is not as prolific as green algae and it can usually be consumed/controlled by clean up crews and herbivores when present in a reef aquarium.