Category Archives: Critters

A Simple DIY Crab Trap for Catching Aquarium Pests

This DIY crab trap is a quick and easy way to remove unwanted nuisance crabs from your aquarium.

Unwanted pests such as gorilla crabs and stone crabs often make their way into aquariums as hitchhikers on live rock.

At first that my not be a problem. They can even serve as a valuable member of a clean-up crew while they’re small. But as they get bigger and their appetites grow, they will often start to eat corals or other more desirable members of your clean-up crew such as porcelain crabs, snails, and even small fish.

Many aquarists will spend hours trying to spear them with a skewer, or a pair of large tweezers. And some will even resort to spending anywhere from $15 to $50 on a pre-made pest trap.

Before you spend that much time or money though, there is an old reef keepers’ trick you might want to try first.

You can make a very simple DIY crab trap using nothing more than a small glass.

Here you can see the quarter-sized stone crab I caught an ordinary shot glass. Two tries with shrimp didn’t work. But it took less than 30 minutes to catch when I used a little bay scallop as bait.

shot glass crab trap
Small stone crab caught in a DIY shot glass crab trap.

A shot glass will work for small crabs that are about the size of a quarter or smaller.  For larger crabs you can use a bigger glass; one that is just tall enough so that the crab can’t reach the lip if it extends its claw.

The only thing you need other than a glass is some bait. Some people have success with a piece or raw shrimp from the grocery store. I tried shrimp twice but didn’t have much luck with it. On the third attempt to catch the little stone crab that was terrorizing my tank, I tried a small piece of raw bay scallop and it worked like a charm.

Simply place the glass in the bottom of your aquarium about 30 minutes before the lights turn off for the night, and lean it at an angle so the lip of the glass is resting against a rock.

Then place a small piece of the bait in the bottom of the glass and let it sit overnight.

The crab will climb down into the glass to eat the bait. But it won’t be able to get back out because the walls of the glass are too smooth for it climb up.

It make take a few tries, but a lot of reef keepers have had great success using this quick and simple DIY crab trap.


Strange Blob in Your Aquarium? It Could be a Polyclad Flatworm

Have you seen a strange blob lurking in your aquarium when the lights go down? It may be a polyclad flatworm.

These unusual creatures are nocturnal, somewhat freaky, and NOT reef safe.

Those that find their way into reef tanks as uninvited hitchhikers on live rock.  They can distort their bodies to fit into the smallest of holes and are incredibly resilient – there have been posts on aquarist forums of polyclads surviving days and even weeks exposed to air… flatworm dips like Bayer and Flatworm Exit… and even boiling of the rock they were hiding in.

Meet the polyclad flatworm. Is this strange blob hiding in your tank?

The Polyclad Flatworm - is this strange blob hiding in your tank?
The PolyClad Flatword

While most of them encountered in the hobby range in size from a pin-head to a quarter, they can grow to over 6 inches in diameter.

Their primary food is clams – they’re even colored just like a clam’s mantle as camouflage.

However, they will also gladly and actively dine on worms, snails, crustaceans, and some corals.

If snails or other members of your clean-up crew seem to be disappearing at night, you may have one or polyclads in hiding in your tank.

I suspect anemones may be on their menu as well…

When I returned home from a recent trip I noticed my new rock flower anemone was missing. My wife was 100% certain it was still front-and-center at its spot in the sand the evening before. It was little guy, only about the diameter of a quarter in size, so I thought it might balled up and let the current carry it to a new spot. But after searching every cranny, crevice, and cave it was nowhere to be seen.

Then, a day later, I notice a freakish, quarter-sized shadow lurking on the sand in a multi-entrance cave. As soon as the light of the flashlight hit it, the strange blob glided across the sand down into a small crevice beneath the rock.

A day or two later I found the remains of my little rock anemone outside the den I’d seen the blob crawl into. The cause my rock anemone’s mysterious disappearance had been confirmed.

I began digging through online photos, trying to figure out what it was – and then I saw it, a picture of a weird creature that looked like the same strange blob lurking in my tank: a polyclad flatworm.

With a possible suspect ID’d I made a trap to try to catch it – here’s how…

  1. I took a small, 1/2 cup-sized Glad food container…
  2. cut 2-inch long slit in it that was about wide as a toothpick – wide enough for a flatworm to squeeze in but thin enough to keep my clean-up crew out…
  3. and put a canned clam in it (I would have cracked open a live one but they were out of season at the local grocery stores).

About half-an-hour after my tanks lights went down for the night, I went to put the trap in the tank. Before I even got it to the bottom of the tank, my wife spotted the blob crawling across the glass on the other side of the tank. It was in pursuit of a nassarius snail.

I grabbed a pair of 12-inch tweezers and pulled out of the tank. In hindsight this was a bad move, but I got lucky…

Polyclad flatworms are fragile. And if you tear them, even the tiniest piece left in the tank can grow into a new flatworm.  Fortunately, my tweezers gripped it near the center of its body where it is a little stronger and less likely to tear.

I’ve put the clam-baited trap in the tank for a few hours after sundown 3 times since then but haven’t caught or seen any others since then.  I intend to do it at least 3-4 more times over the next two weeks, just to be safe before I add a few new rock flower anemones to replace the one that it ate.

Check out the short bit of video I shot to get really good look at this strange, carnivorous blob:

You can find great posts with even more information on polyclad flatworms at Reef2Reef, ReefCentral, and Chuck’s Addiction (a great all-round site for identifying hitchhikers).

And pictures of the many different color-combinations and camouflage patterns found in Google Images.


Pincushion Sea Urchin: Good or Bad for Your Tank…

Thinking of getting a pincushion sea urchin for your saltwater aquarium? It could be the coolest critter you add to your tank, or the most destructive…

I’d like you to meet Fred:

Fred, the pincushion sea urchin.
“Hi, I’m Fred!”

Fred is a white pincushion sea urchin. He is also a party crasher.

When I first set up my aquarium, I added 30 pounds of live sand straight from the Gulf of Mexico. While spreading the sand around the bottom of my tank, I found Fred hiding inside it. He was about the size of a grape.

He looked cool, survived a 900-mile journey as a stowaway, and was the very first creature (other than a few snails and tiny crabs) to inhabit my new 24g reef-in-a-box… albeit, unintentionally.

My wife and I quickly took a liking to him and she decided to name him Fred.

That was a little less than two months ago. Fred is now the size of a walnut. In other words, he’s a fast grower.

Let me share a few other things I’ve learned about pincushion sea urchins like Fred in the past seven weeks…

Pros of having a pincushion sea urchin in your tank:

  1. They are great algae eaters. As I said, Fred has gone from the size of a grape to the size of a walnut — about 1/5″ from tip to tip — in less than two months. He cruises the live rock and glass 24-7 in search of algae to chow down on. He’s not the greatest at eating film algae – so excel at keep the glass clean. From what I’ve read though, pincushion urchins are very good at mowing down hair algae. I don’t have hair algae in my tank (*knock on wood*), so I can’t confirm this firsthand. However, considering how quick he can eat a dime-sized piece of Nori seaweed, I’d say hair algae probably doesn’t stand a chance.
  2. They add a great touch of intrigue and uniqueness to your tank. Since sea urchins are very unique critters, far more unusual than the usual crabs, snails, shrimp and other invertebrates, they can make for a very intriguing focal point and source of conversation for people who view your tank.
  3. They add a lot of curiosity and comedy to a tank. As you can see from the pictures of Fred, pincushion sea urchins love to decorate themselves with camouflage – often with items that would seem to large or heavy for them to be able to hold on to. It can be very cute to watch and see what they are going to try to grab onto and haul around next. As you’re about to see though, it can also be very risky…
A pincushion sea urchin will carry anthing it can lift
Pincushion sea urchin: a comedian, weight lifter, and camouflage artist all in one prickly package…


Cons of having a pincushion sea urchin in your tank:

  1. They can get pretty big. In an aquarium environment they can grow as large as 8″.
  2. They will camouflage themselves with anything that isn’t nailed down, and even some things that are. Shells, rocks, loose coral frags, even snails and hermit crabs often get picked up and taken for an involuntary ride. This part of the comic charm of a pincushion urchin. Combine this with the fact that…
  3. They are very strong for their size. This means they can knock small coral frags loose and, as they get bigger, they may also inadvertently dislodge coral or live rock (Much like a turbo snail.). If you decide to add one to your tank, make sure all your rock and coral is stable and not easy to move – otherwise your cool little comedian may also behave like a prickly bulldozer. This picture should give you some idea of how strong pincushion sea urchins – and most other urchins – can be: Fred is only about 1.5″ in diameter and he is already easily lifting a rock his size, plus several shells and a few stems of sea mat…


Pincushion sea urchin carrying around a rock its size
Fred the strongman…


Pincushion sea urchin pulling a large piece of sea grass.
“Hmm… can I uproot this chunk of sea grass yet?

A few other things to be aware of if you’re thinking one:

  • If you’re tank is 20 – 30 gallons in size and algae is not prolific in your tank, you’ll need to feed it. A piece of Nori seaweed (you know, the stuff used to make sushi rolls – you can get it in most grocery stores for ~$5) will do just fine. Once a week, hold a piece that is about 1″ x 1″ gently against its spines long enough for it to feel what it is and start to take hold of it… hang it on the glass with a seaweed clip… or use a rubber band to hold the nori against a rock for the urchin to find. As the urchin grows, increase the feeding to a 2″ x 2″ piece of Nori.
  • Once the urchin grows to 5″, find it a new home that is at least 75 gallons. An urchin this size really can’t sustain itself well enough to survive, thrive, and continue to grow. As much as we love Fred, he was an accidental guest and we know that his stay with us is only temporary – he will be going to a new forever home with a larger tank soon…
  • If your pincushion urchin isn’t camouflaging itself with shells, rocks, or other debris, it may be a sign that it is sick or dying.
  • If you REALLY want to have and enjoy a sea urchin in a saltwater aquarium that is under 30 gallons, please consider a Tuxedo urchin instead. They exhibit the same comical ‘camouflage’ behavior as pincushion urchins and only grow to about 3″ in diameter.