Acanthastrea-Eating Spiders

Here’s another one for your nightmares!  For years I had heard rumors of a stealthy Acanthastrea predator, but I had yet to find one (to my relief.)  A while back, I attended a coral fragging demonstration, and along with my frag freebie, I also received these lil buggers.

Acanthastrea-Eating Spider

Acanthastrea-Eating Spider (missing a leg)

Rumor has it that these spiders tend to prefer  Acanthastrea (typically called “acans”) corals, but they have also been found on other large polyp stony corals (LPS).  Thankfully I eradicated these spiders before I had a chance to find out if they would infect my other LPS corals as well.

This Acan-eating spider (hereafter referred to as AES) has (or had, I should say) eight legs.  This is likely a female as the males have a specially modified leg (called an oviger) for carrying eggs (unless I knocked that leg off accidentally too) (Cowles, 2009).  As you can see, there is practically no body – it is mostly legs.  On the left side of the AES in the photo above are the chelicerae (mouth parts) and the proboscis (tubular mouth part for ingestion).  Also note – these are not true spiders!  (Although they still give me the heebie jeebies!)

Phylum:  Arthropoda (joint-legged invertebrates)

Subphylum:  Chelicerata

Class: Pycnogonida (sea spiders)

Order: Pantopoda

Acan-Eating Spider

Acan-Eating Spider

Note how different the AES above is from the Zoanthid-Eating Spider (ZES) shown below.  The AES is much larger and has a much smaller (proportionally) body.  Both have eight legs.

Zoanthid-Eating Spider

Zoanthid-Eating Spider

How to Identify:

AES are nearly colorless, so they blend in quite well with the coral.  Even after a thorough visual inspection of the coral prior to dipping, I only found these after they fell off in the dip.  The ones I found were about 1 cm in diameter (nearly half an inch).  So, I recommend a good coral dip prior to introducing the coral into your tank.

How to Treat:

Hydrogen peroxide dip:  This is my preferred method for AES.  This dip creates bubbles that will actually lift the AES off the coral – they usually can’t hold on!  Use this dip with caution as overdoing it can cause serious coral damage.  Also, only dip LPS in it.  Soft corals and small polyp stony (SPS) corals do not typically respond well.  Mix a 10:1 tank water to hydrogen peroxide (typical 3% stuff at your local pharmacy) ratio dip.  Dip the coral for 20 seconds, and lightly blow water around the coral with a pipette.  The AES should bubble up to the surface.  I recommend dipping once weekly for a month in case any eggs were left behind.

Bayer Insecticide dip:  This is my second-favorite dip.  If the hydrogen peroxide dip doesn’t work or you don’t feel comfortable with it, this one will actually kill the spiders.  Full instructions are here.  Again, I recommend dipping once weekly for a month in case any eggs were left behind.

CoralRx and other commercially-available dips:  Although I haven’t used these dips on AES, they should work…possibly with varying levels of effectiveness.  Several brands tout effectiveness against pycnogonida (sea spiders).  Again, I recommend dipping once weekly for a month in case any eggs were left behind.

Natural predators:  Certain wrasses and other typical pod-eating-vertebrates may eat AES, but I’m not aware of any definite natural controls.

Manual removal:  If your eyes are good enough (or are helped with a magnifying glass), you may be able to manually remove AES with tweezers or a pick.  However, you may miss some and end up with a worse problem down the road.


Although Acanthastrea-Eating Spiders are quite creepy-looking, they are not a horrible pest to eradicate.  They are highly susceptible to coral dips and can even be manually removed.  If you know you have AES, please be a responsible reefkeeper and do not pass your coral on without at least notifying the new owner.


Cowles, Dave, link, 2009.



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