Correcting Previous Errors and Adding New Info on Fiddler Crabs

Steve

When i ventured into the world of fiddler crab care everything was new and i had searched far and wide for adequate information on their care.  While i found some sound information i also found some information that was less than ideal and some that was erroneous and i had posted some of that info my first blog post on fiddler care.  Not liking to be wrong but not liking to look like a dumbass either i must correct my errors for the good of the readers.

With experience i have learned quite a few things about the fiddlers and the first issue i must address is the misidentification of north american fiddler crabs sold in the pet trade.  Incorrectly identifying your north american fiddler likely won't interfere with how you care for it but it is important to know just which crabs you're caring for should you want to buy more of the same type. 

First things first, fiddler crabs fall under the genus of Uca.  The ones commonly captured and kept as pets in North America are part of the sub-genus minuca.  Uca pugilator (referred to as a sand fiddler), Uca pugnax (referred to as a marsh fiddler), and Uca minax (also referred to as a marsh fiddler but more commonly referred to as a red jointed fiddler) are common along the east coast of the US and are found as far north as Massachusetts.

With our first fiddler, Bob, i was unaware of the small differences between Uca pugnax and Uca minax.  Upon further research and closer inspection i can say now with confidence that Bob was in fact Uca minax, not Uca pugnax.  On the bright side, their care is the same.  They are similar in size and color (with uca minax getting a bit bigger with age) and are often found in the same habitats.

In captivity the red joints on the Uca minax may not be red, they might be orange or a ruddy brown.  No matter how hard we humans try we really cannot provide these animals with the adequate diet or space for them to be like they are in the wild, this can result in physical appearances that deviate a little from what is seen in wild fiddlers.  Often the crabs are transported in less than ideal and less than adequate conditions and will become "ill" during transport.  Being kept in freshwater conditions or (as wal-mart stores sell them) in little plastic dishes with barely any water can result in the crab having problems later on down the line.  Uca minax and Uca pugnax can both tolerate low salinity, but cannot molt or live long in a freshwater environment.  Uca minax can tolerate lower salinity than Uca pugnax.  Uca pugilator can tolerate higher salinity (reef aquarium levels even) as well as low salinity.  For minax, ideal salinity would be would be between 18 and 22 ppt.  Pugnax would do well between 20 and 28 ppt.  Pugilator can tolerate salinity up to 32 ppt, but with the fiddlers it is best to keep it under that.  32 ppt and beyond is reef salinity and brine salinity and fiddlers do not require those conditions and may not be able to come through a molt in such salinity.  Brine salinity is deadly to most fiddler crabs.  I've noticed that in my fiddlers, if the salinity rises due to water evaportion, they gain an orange color.  It is a natural response to rising salinity levels.  If you notice this issue in your crabs, test your salinity and reduce it if it reads high (past 28 ppt).

As time has progressed i have had 5 adult uca minax fiddlers.  Only one remains, his name is Steve and he is seen in the image above.  Bob passed away in early September 2014 from an unknown cause.  Bob lived alone in his tank, the salinity was great and the ph was perfect.  He had not been eating well for about a week and on his last day he had become sluggish and just wasn't himself.  He showed no outward signs of illness (no spots, no fungal growths, no missing limbs).  We figured it must have been age.  We had had him for 2 years and a couple months at that point. 
Both females i had passed away.  The first passed when i was ill in 2013 and unable to properly maintain the fiddler tanks.  Her name was Claudia and to this day i feel horrible because she passed due to my errors.  Our second female was murdered by Steve.  Her name was Fern, she was jolly and had a few egg sponges before the incident that caused her death.  I awoke one morning to Fern's molted shell and a dead Fern a few inches away near the filter.  She had a hole in her back and some of her claw tips were missing.  Steve had killed her and cannibalized her.  Then our most recent fiddler (purchased at wal-mart to save him from their negligence), Herb, we had for less than a month.  We purchased him at the beginning of January and he passed on a few days ago.  When we first brought him home he loved the tank, was eating good and was very active for the first couple of weeks.  Then he started slowing down, was hiding more.  We noticed his feeder claw had a strange black spot on it.  He was a young crab, his claw was small and he was about the size of a nickle.  There was hope he'd come out of whatever was wrong with him.  His condition deteriorated and he passed away.  After his death i did a bit of inspection, aside from the odd black spot on his feeder claw (which seemed to be part of the shell), he was missing a leg (which happened before we saw him at wal-mart but leg loss is usually not fatal in crabs), and on one of his leg joints there was a weird black spot in the tip joint.  The black spot made it impossible to bend the joint.  All his other legs bent just fine.  Just what these black spots were i don't know and it's incredibly difficult to find any information on fiddler crab diseases and disorders.  It's a mystery as to just what happened to him.  There's a little comfort in knowing he didn't die in the putrid centimeter of water in that awful wal-mart plastic dish, but due to his size i thought we'd have him for quite a while so his death was disappointing to say the least.

There's a myriad of problems these fiddlers can encounter in captivity.  Some of them we can control, most of them we cannot.  The problems we can prevent are chlorine/chloramine contamination, cyanobacteria (blue green algae) toxin contamination (occurs from overfeeding and too much light/sunlight after the bacteria has been introduced, which is usually by way of food), salinity issues, inadequate ph, ammonia contamination, and temperature changes.  Fiddlers are generally hardy little animals but they are arthropods and arthropods are sensitive to certain things other animals might not be sensitive to.  Avoid handling the fiddlers or putting your hands in the water if you just used soap, lotion, or hand sanitizer on your hands.

With some observation i discovered that fiddler crabs don't much care for loud music, it seems to scare them.  Fiddlers "hear" through the hairs on their body and some sound waves have negative effects on their behavior.  For that reason, it is wise to keep their aquarium away from loud noises and items such as TVs and stereos.

When i first started an aquarium with the fiddlers i thought they'd be fairly easy to care for.  In general they are.  What is difficult is when they die.  It's often for an unknown reason or may be the result of crab on crab violence.  With limited research existing for the public regarding diseases and disorders that may strike these animals, it leaves us humans with a lot of questions.

To any young people out there with a love for these creatures, please consider going into marine biology and specifically studying marine invertebrate diseases.  We need more scientists in the field.  So much is not yet understood about crabs and other marine animals, it would be a great benefit if more young people could pursue careers that would help these little creatures.

Hopefully i addressed all the errors i made previously.  Here's to hoping i didn't make any more in this post.  Further reading and information on the 3 most popular pet trade fiddler crabs in the US is below the picture of Herb.  There are also two links about hydrometers (one links directly to a hydrometer sale page). 

Herb (note the strange black spot on his feeder claw and then the strange black spot on the lower tip of his 2nd leg on the right side)

http://animaldiversity.org/accounts/Uca_minax/
http://animaldiversity.org/accounts/Uca_pugnax/
http://www.sms.si.edu/irlspec/Uca_pugila.htm
http://www.advancedaquarist.com/2002/1/chemistry
http://www.petsolutions.com/C/Aquarium-Hydrometers/I/Deep-Six-Hydrometer.aspx
http://www.geoexpro.com/articles/2012/04/marine-seismic-sources-part-xi-effect-of-seismic-on-crabs