Live rock hitchhikers guide – Saltwater Aquarium Blog

If you were to look up the entry for “live rock”, in The Hitchhikers Guide, you would find the following entry:

On earth, before it was destroyed to make way for a hyperspace bypass, monkey mammals called humans sometimes enjoyed collecting fish and corals in a glass box. Inside that box, they would pile a bunch of expensive rock that they called “live rock”.

The rock itself was not alive, but it was teeming with more biological activity than a Glugarian hyperspace rest stop toilet.

Don’t panic!

This guide will help you separate the good, from the bad and the very ugly hitchhikers you might find on your live rock.

See what I did there?

What you will find in this article

What is live rock?

Let’s start by answering the most basic of questions here—what is a live rock? Let’s start at the beginning. Coral reefs are diverse, underwater ecosystems where fishes, corals, and other invertebrates thrive and compete. They’ve been compared to underwater cities.

Live rocks are the buildings that give the city its structure and on and in which much of the life establishes a home.

Bacteria and invertebrates live in, around and on the rocks and corals and other sessile (non-mobile) invertebrates attach to the rock and carve out their own space on the reef. 

Why we use live rocks in our saltwater reef tanks

There are a few reasons we use live rocks in our saltwater reef tanks:

  • Create a natural look (replicates a natural reef)
  • Harbor beneficial bacteria that help purify the water
  • Provide a structure on which our corals can grow
    • By aquascaping the rocks (laying them out in a structured way) allows us to create a range of conditions, within the same tank that would be suitable to different aquatic species

But perhaps the most interesting reason why we add live rock to a reef aquarium is to:

Add to the biological diversity of the aquarium by transporting hitchhikers from the reef into our tank.

Live rock is sort of like opening up an invertebrate of the month club box and putting it in your tank. You never know what you’re going to get. The goal of this guide is to share some of the good, the interesting and the bad hitchhikers that relatively commonly stow away on live rock and make the journey into our tanks.

Guide to live rock hitchhikers

The intent of this article is to highlight a few of the most common and interesting hitchhikers. Since this list could be almost limitless, I could use your help. If there are interesting critters that you’ve seen that haven’t made this list, please leave a comment to add to the list.

Helpful and desirable hitchhikers

My family members think we are crazy for spending $50-100 for a rock that goes into our tanks.

To tell the truth, they are probably right, but we are crazy about our tanks and we know that creating biological diversity in our tanks is beneficial for the animals in our care and makes it even more interesting to watch. 

Here are a few of the helpful stow-aways we hope to get when we add some rocks to the tank.

Nitrifying bacteria

Nitrifying bacteria are the unsung heroes of live rock. You can’t see them, but they’re there, pulling toxic ammonia out of the water and turning into relatively harmless nitrate.

You don’t have to have a live rock to seed your biological filter with nitrifying bacteria, but the live rock does serve as a natural substrate to help support their growth.

Some people will actually start the cycling of their tank by adding uncured live rock to their tanks, thereby adding a ‘natural’ source of ammonia (organisms on the rock dying and decaying) and the nitrifying bacteria to convert that ammonia to nitrate.

Coralline algae

Natural reef live rock is so interesting to look at because of all the different textures and colors. Some of that texture and color comes from a type of algae called coralline algae. 

Coralline algae on glass

Coralline algae is a ‘good’ form of algae. Like the corals in our tanks, these algae use calcium to form a bit of a ‘backbone’.

The presence of coralline algae on your rocks (and glass) is a sign of a healthy tank, in much the same way that green grass (without a lot of weeds) is a sign of a healthy lawn.


‘Pods, as they’re so affectionately called, in our hobby, are some of the most sought after and desired of the live rock hitchhikers.

  1. Part clean-up crew.
  2. Part ant-farm (when you watch them crawl around at work).
  3. And perhaps most importantly, ‘pods ring the dinner bell for some of your fish and other invertebrates, as one of the most nutritious and natural foods. 

Attracting them to live in and populate our tanks is often a top goal when purchasing live rock. Scientifically speaking, there are approximately a gazillion different species of ‘pods in the world. For the sake of simplification, we reefers are generally interested in two types:


Amphipods are the larger variety. They are quite easy to see with the human eye. Digression—where did that expression come from—was there a point in time where people tried to use the eyes from other animals as an experiment to help them see things better? That’s weird. And Gross. They look a bit like the underwater, inside-the-aquarium love-child between a pill bug and a flea.


Copepods are much smaller, often tiny specks compared to the much larger amphipods.

feeding corals

image by Lycaon


Another group of helpful live rock hitchhikers is marine sponges.

cabbage leather coral and sponge

Check out the white sponges growing under this leather coral

They are filter feeders and come in a variety of colors and interesting growth formations. Sponges are susceptible to drying out when transported on un-cured live rock and can cause problems if placed in your tank, uncycled. But when cycled live rock (carrying sponges) is carefully transplanted to a tank, they can proliferate and thrive in typical reef tank water conditions.

Small Starfish

Another group of common hitchhikers on marine live rocks are starfishes. They come in a variety of shapes and sizes–correction–they all come in the shape of a star, but the size and shape of the arms will vary from species to species.

Asterina starfish

This little fellow, below, is an asterina starfish–note how one leg of the star is significantly longer than the other? Like your corals, these starfishes can reproduce asexually by breaking off legs and growing a new starfish.

asterina starfish is a saltwater aquarium hitchhiker

It’s a really cool thing to see–and you’ll feel great that they are flourishing in your tank–until you have a million of them all over the glass :).

Brittle or serpent starfish

This sea star group may or may not be bad news. Many species are super tiny and won’t cause any problems in your tank—but some of the larger individuals are opportunistic predators and when opportunistic predators live in a tank 24/7, they generally find an opportunity.Yoda wouldn’t be happy with us judging them by their size…but…that’s how I’d determine if it was a big deal or not. 


Stomatella snails

stomatella snail hitchhiker

Note the slug-like appearance, but with a very low profile shell.


You know you’ve hit the jackpot when the live rock you purchased has some corals attached. I can remember, when I first started in the hobby, obnoxiously picking through the live rock at my local fish store (shout out to The Hidden Reef, which was in Northeast Philadelphia at the time), to find pieces of live rock with interesting stuff growing on it–particularly coral polyps.

At the time, my tank sucked. Most of what I brought home didn’t make it, but one time, in particular, I did bring it home with some pink stuff on it, that eventually grew into this:

Zoanthid polyps

Green Zoanthids

Those green zoanthids hitchhiked their way into my tank and flourished. Just be careful what you wish for.

Interesting hitchhikers

Not known to be terrifically helpful or harmful, these critters find their ways into our tanks and hearts and add a little bit of texture and complexity to the ecosystems we have created.


Christmas tree worms or other fan worms

Fan worms are interesting little hitchhikers. They are delicate little filter feeders that look like fans. When all is calm in the tank, the fans come out and sift particles out of the water. When disturbed, the fans duck back down into their protective shells.

In a way, I think of fan worm hitchhikers as a sign of a really healthy tank. Their growth and reproduction in your tank means you’re doing something right.

Christmas tree worms are just one popular type of fan worm. 

If you see a picture of these cool hitchhikers from the right angle, they can look a bit like psychedelic miniature christmas tree forest. They probably look even more like it with the right psychedelic medication…

Since Christmas tree worms generally burrow into porites corals, it means you scored a 2-for-1 special on that rock.

christmas tree worm or fan worm

Spaghetti worms

A spaghetti worm is another polychaete, which makes them a relatively close relative of the fan worms and bristle worms.

Like the fan worms, they have a hard shell that holds most of their body safely away from predators and they extend their feeding tentacles out for respiration and feeding. 

But while fan worm appendages look like…fans…spaghetti worm appendages look like…any guesses?

Right. Milky/translucent weird string. 

I don’t think it looks much like spaghetti at all. That’s weird that you thought that ;).

Take a look.

Bristle worms

In my opinion, Bristle worms are one of the most under appreciated and (dare I say wrongfully hated) live rock hitchhikers that you will most likely run into. I’m not really sure why, other than that they’re ideally suited to live in most of our reef tanks, evading total annihilation.There are dangerous bristle worms that can be dangerous (if you get stuck on your bare hands), but those are extremely rare. I personally haven’t heard of a real, contemporary sighting in the home aquarium.The rest of them are better than harmless…they actually work on cleaning up your tank while you sleep. 

Learn more about bristle worms.

Problem Algae

For this section, I will use the phrase ‘problem algae’ to refer to the group of alga and non-alga species that can sometimes cause problems in our tanks.


Dinoflagellates are tiny, single-celled organisms that will look like ugly brown algae.They’re generally only problematic in a newly set up aquarium and are expected and ‘ordinary’, even though they are problematic. These tiny brown dots can cover large areas and clump, but they are distinguishable from other ‘problem algae’ types (not really algae, just called that) by their brown color and the lack of a continuous thick mat (see below for more about cyanobacteria, which looks like the thick mat I’m talking about) 

Cyanobacteria or slime mold

Cyanobacteria, or slime molds, are another problematic live rock hitchhiker than can plague a new, immature tank. 

There are two prominent types: blue-green and red-purple. As mentioned in the dinoflagellates paragraph above, one of the distinguishing characteristics of the cyanobacteria is that they form thick mats that are somehow dull and shiny at the same time with bubbles stuck in the mat (from respiration). Gross and a pain. Learn more about how to deal with cyanobacteria here.

saltwater aquarium learning experience

Red slime everywhere


Hair algae

Hair algae are green algae that look like a wild patch of grass on your rocks. Scrape your rocks clean, but it seems to pop up somewhere else. I’ve reached an age where the only hair (on my body) that grows as fast as hair algae is the hair on my ears and nose, it seems. Gross visual, I know. Unnecessary. I know. Funny? Marginally. Oh well.


Another problem group of live rock hitchhikers are the caulerpa algae. These frustrating stow aways can be a problem because they look pretty cool. They have interesting leaf-like structures (not really leaves) and spread by sending out runners. The runners attach to the rocks like their life depends on it (it does) and tiny fragments that remain attached to the rock just regrow. An amazing survival adaptation—but a pain if you’re trying to remove it from your tank.

caulerpa macro to remove nitrates and phosphates from a saltwater aquarium

Bubble algae

I feel like I need to admit something to you. I thought bubble algae were really cool the first time I saw them in my tank.

They look like emerald green colored pearls in your tank. 

When you have just a few, they add a little bit of interesting to your tank. But, like many of the other pesky hitchhikers in this article, they really enjoy living in our tanks and can get out of control quickly. 


Halimeda is a not-so-problematic algae hitchhiker. To me, they look like tiny little underwater cactuses (or is it cacti?) because they grow in little segments (check out the picture below).

In a pristine SPS set-up, they may look a bit out of place, but in a tank with some soft or LPS corals, they can look pretty cool. They will help remove nitrates and phosphates from the water. The only downside is that they will compete with your corals for calcium, so just watch your calcium levels (you should anyway), treat them like anything else in your tank you respect, and they should do fine.

Neomeris (spindle weed)

This is one of the strangest-looking hitchhikers I’ve had in my tank. They look like strange green cotton swabs or something. I don’t know how to describe them, just take a look. Not harmful, just from outer space. I’ve had them. Your tank will ignore them. 

Problem hitchhikers

Glass anemones and aiptasia

If you’re unlucky, you may end up with dreaded glass or aiptasia anemones. They might not look like much—one tiny little anemone stow away. At first glance, the first time I had one, I actually got excited (before I figured out what it was). The first raw emotion can be of joy and triumph that you managed to keep a polyp of something alive on the rock.

After proper identification, the next step is typically a Google search to learn more about the hitchhiker…followed by dread at what they find out.

Now, I think aiptasia anemones are also a bit misunderstood. They don’t really cause any problems in the tank. Their polyps can sting nearby corals (but so do many coral species). The only two things they are really guilty of are: 

  1. stealing a free meal intended for your fish or corals and
  2. growing to plague proportions in your tank

Glass or aiptasia anemones are certainly one of the bigger pests of the modern-day aquarium. One small anemone (or even the part of one small anemone) can become a plague in your tank before you know it.

aiptasia in between zoanthids

by Brian Jeffery Beggerly on Flickr

Mantis shrimp

Loved by the devoted hobbyist with the luxury of keeping it in a dedicated tank, the dreaded mantis shrimp is the bane of the hobby. This ambush predator loves to stow-away in the inner crevices of the live rock and dines on your favorite fish.

Gorilla and stone crabs

These guys can be surprising big and yet evade detection in your tank for a surprisingly long period of time. 

They’re bad news, because they will eat just about anything fleshing i your tank that you’re trying to keep alive and don’t want them to eat.


Buying live rock is like a box of chocolates. You never know what you’re going to get. Sometimes good, sometimes bad, almost always interesting and expensive, it’s one of the strange things we do.

Did you ever have any of the hitchhikers here? if so, please leave a comment and let us know.

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