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Contents
#1 - Arthropods (Bugs and shrimp)
#2 - Annelids (Segmented Worms)
#3 - Molluscs (Snails and Clams)
#4 - Cnidarians (Anemones and Corals)
#5 - Algae and Bacteria
#6 - Miscellaneous

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Legend
- Organism is potentially toxic or venomous to humans. Research is recommended.
- Organism is a threat to other reef life through predation or aggression. Generally not reef safe.
- Organism is a pest that may overrun a tank. May not be reef safe.
- Organism is beneficial to the reef tank. Benefits the ecosystem or is a sign of favourable water conditions.
- Organism is a food source for other reef inhabitants. Will provide beneficial live food for tank inhabitants.

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External links and further reading

*** RTAW has an excellent guide to live rock hitchhikers
*** Chuck's Addiction has a comprehensive list of live rock hitchhikers
*** WWR has a reasonable hitchhiker identification guide
*** Saltwaterfish has an identification system for live rock inhabitants

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1. Arthropods


Amphipods
A very common scavenger. A sign of a healthy reef system and a source of food for anything that feeds on small crustacea. If nothing is eating them they may breed profusely but generally don't cause damage even if they do multiply.


Copepods
Very small free-swimming crustaceans that move with a jerky motion. They are a sign of a healthy reef and provide food for many inhabitants.


Isopods
Insect-like crustacea. Though some are benign algae-grazers, a majority are predatory and this extends to parasitism of fish and shrimp. They may be mobile and active, or stationary on their preferred food source. Size ranges from roughly the size of a sand grain up to 5cm species. Care must be taken if physically removing larger specimens as they can deliver a painful bite.


Mysis
Very fast, small and shrimp-like creatures that suggest a healthy tank and provide a nutritious live food for fish and some invertebrates. Hard to see as they move fast and are almost completely transparent. Look for the reflective eyes with a light at night time.


Sea Spiders
Most sea spiders are very small, unwanted guests in reef tanks. Predation or parasitism by the sea spider can damage corals, soft corals, and sea anemones. Large forms are generally slow but smaller species can hide effectively in small coral polyps.

2. Annelids


Bristleworms
While small, the majority of bristle worms are beneficial scavengers of leftover food and also aid in sand beds. Larger species may be predatory opportunistic feeders, and the bristles can produce an irritating sting when handled.


Eunicid Worms
Small individuals are scavengers that are unlikely to harm your tank. They are the most common worm found in live rock tunnels. These worms have five tentacles on the head and an impressive set of jaws. They are generally predators and can grow over a metre in length.


Featherduster Worms
An extremely common, small worm living in a tube. They are beneficial filter feeders that indicate a healthy reef. They will retract into the calciferous tube when startled.


Fireworms
Somewhat similar looking to a large bristleworm. These pests feed on corals and inflict a painful sting when touched.


Spaghetti Worms
Spaghetti worms live in burrows and search for food by extending their very long tentacles. Desirable in a reef aquarium, with their presence indicating a healthy reef system and their scavenging of uneaten food. However, the long tentacles may bother corals that they pass over.


Tube Worms
Similar to the feather dusters these are beneficial scavenging worms that live in a tube. They will not filter but use their tentacles in a similar manner to the spaghetti worm to grab food.

3. Molluscs


Chitons
Peaceful grazers on algae and the biofilm found on live rock. The most problem they might cause is eating prized coralline algae.


Limpets
The vast majority are harmless algae grazers. Easily identified by their pointed shells.


Nudibranchs
The most common species are small but will eat coral and zoanthid polyps. Can be hard to see, and larger types may have a serious sting.


Stomatella Snails
Outwardly similar to a slug with a small fingernail-like shell on the back. They are consummate algae eaters, even tackling cyanobacteria, and will reproduce in the home aquarium.


Vermetid Snails
Tube-dwelling snails that emit a mucus web to catch food. Their presence is a sign of a healthy system but their mucus net may irritate corals. Can be distinguished from tube worms by the shiny lustre inside the tube and an additional tube layer.

4. Cnidarians


Aiptasia Anemone
Very common and unattractive anemones that will move around your tank often unseen. They multiply rapidly and will sting corals and other reef inhabitants. Can overtake a tank if ignored.


Mushrooms
These hardy corals occasionally hitchhike on live rock as they can survive extreme conditions. Not necessarily bad and they are generally sought after but nevertheless they can produce a nasty sting to other corals.


Polyps
Polyps such as the clove polyp can occasionally enter via live rock, and is generally an appreciated hitchhiker. However, it can look superficialy similar to jellyfish polyps or bryozoans. Can spread quickly.


Zoanthids
Generally brown of green in colour, zoas and other button-like polyps are for the most part appreciated as guests on live rock. However they may be undesirable to some as they contain potent poisons if cut or killed and can quickly overtake a tank with their fast growth.

5. Algae and Bacteria


Cyanobacteria
Nuisance "algae" that is slimy to touch present in nearly any healthy reef system. Can become a problem when it blooms and coats sand, rock and corals.


Diatoms
Common in new or re-cycling tanks and produces an unsightly brown or rust-coloured dusting over the sand and glass. Generally doesn't stay in bloom proportions for extended periods.


Filamentous Algae
One of the most common nuisance algaes. It is quick-growing and can take over fast, and is very difficult to remove once established.


Macroaglae
Various types of macroalgae have benefits or drawbacks to a marine system. They do sequester many undesirable pollutants but several may cause tank crashes by "going sexual" and their rapid growth means they can easily overtake a tank. Best utilised in a separated refugium.

6. Miscellaneous


Asterina Stars
Odd looking white or light brown starfish with mismatched leg sizes. Most species are harmless algae grazers and will generally be found on glass. Any on rock should be observed as a few species have been recorded eating coral. They will multiply in the aquarium.


Brittle Stars
Harmless scavengers and a welcome part of the clean-up crew. Usually only visible by their arms extended out of holes in the rockwork. Smaller species may even reproduce.


Flatworms
Small, toxic worm-like creatures that can quickly overwhelm an aquarium. Some types may damage corals and generally speaking coloured species are undesriable compared to transparent or white types.


Peanut Worms
Nocturnal scavenger of leftovers and detritus. Generally a good critter to have, but you are only likely to see the proboscis-like extension coming from a hole in the rock or under the substrate.


Sea Urchins
Most are harmless and graze on the biofilm on rock. Some are venomous, and most types are hard to keep as they will quickly eat all available food in a smaller tank.

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Sources:

Images used from:


http://www.aims.gov.au
http://www.aquayee.com
http://www.arcodiv.org/
http://changingoceans2012.blogspot.com.au/
http://www.liveaquaria.com
http://www.marineaquariumsusa.org
http://www.marinelifephotography.com
http://www.nanoreef.com/
http://www.reefcorner.com/
http://www.reefguide.org
http://www.saltwaterfish.com/
http://www.sciencephoto.com/
http://sydney.edu.au/
http://www.thereeftank.com/
http://www.wetwebmedia.com