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FAQs about Horseshoe Crabs, Selection

Related FAQs: Horseshoe Crabs 1Horseshoe Crabs 2, & FAQs on: Horseshoe Crab ID, Horseshoe Crab Behavior, Horseshoe Crab Compatibility, Horseshoe Crab Systems, Horseshoe Crab Feeding, Horseshoe Crab Disease, Horseshoe Crab Reproduction, & Crustaceans 1, Micro-Crustaceans, Amphipods, Copepods, Mysids, Hermit Crabs, Shrimps, Cleaner Shrimps, Banded Coral Shrimp, Mantis Shrimp, Anemone Eating Shrimp

Related Articles: Horseshoe Crabs: Latter Day Trilobites for Some Systems & CrustaceansMicro-Crustaceans, Amphipods, Copepods, Mysids, Isopods, Shrimps, Coral Banded Shrimp, Cleaner ShrimpP. holthuisi Pix, Mantis "Shrimp", Lobsters, Slipper Lobsters, Hermit Crabs, Squat Lobsters, Crabs, Arthropods, Pycnogonids (Sea Spiders),

About the only thing not likely eaten

horseshoe crab and pencil urchin... Mis-mixed mess  -- 10/22/11
So. I recently purchase a horseshoe crab for my 40 gallon tank,
<... a tropical species at least?>
I am aware that it is too small of a tank when it grows, but I have access to a 250 gallon when the time comes, I was reading your frequently asked questions and I saw one about a school's touch tank with the urchin eating the horseshoe crab.
well, I have a similar situation.
In my tank, I have a pencil urchin, two clownfish, a damsel, a cleaner shrimp, coral banded shrimp, sexy shrimp, scarlet and blue legged hermits, a flame scallop,
emerald crab and the horseshoe. Earlier today I caught the horseshoe crab swimming around the tank, which seemed strange to me as he was plenty happy to roam the sandy bed.
<... this is NOT a tropical species...>
My girlfriend came home a couple hours later and found the horseshoe crab stuck to the urchin, when she pried them apart, half the front of the crab's "face" was gone, however, the crab is still alive and doesn't seem much perturbed by the event, as far as I can tell, is there any hope for it, or is there anything I can do?
<Study ahead of purchasing livestock... This animal can't live in this setting... the Stenopid will likely eat the other crustaceans, the Pen shell starve...
What you've jammed together here won't work. Bob Fenner>
Re: horseshoe crab and pencil urchin
thanks for the advice, Buying livestock going by just the reef safe label was silly of me,
<Happens... all the time>
I'll be sure not to repeat the same mistake.
Thanks for helping out a newbie to the hobby.
<I assure you that you can count on what is archived on our site, in my written work as reliable. Cheers, BobF>

Horseshoe Crab... sys., sel.    2/21/11
I have spent a lot of time on your website and will not buy anything for my tank anymore without looking to see what you guys have to say about it first. I've spent a lot of time the last few days looking over all the info on Horseshoe crabs that you have, all the questions seem to be people who bought them and are now trying to figure out what to do with them, so thus I have not found all the info I am looking for. I think the horseshoe crab is an amazing animal and am very interested in setting up a tank for one. I was thinking to use a frag tank style tank, something at least 4 feet long and at least 2 feet wide, but only about 16 inches deep or so and setting up the whole thing as a sand bed for the horseshoe. I would set it up so it shared water with my reef tank
<... which is coldwater?>
so I could take advantage of the good water quality and filtration and chiller that I have on that, and add more total water volume to my system. The drawback of this is that I keep that system @ 76 degrees.
<This won't work>
I see that the commonly available Atlantic species of horseshoe prefer colder water than that, are the indo-pacific horseshoes better for warmer water,
<IF you could secure one of these it might work>
and if so are they ever available and how do i tell the difference between the species.
<Have never seen offered in the trade. ID info. you can find on the Net>
Also if I can find one that can handle my temperatures how deep of a sand bed should I set up.
<Four inches or so will likely do for the size of the specimen/s you'll be able to keep here>
And is there any other advice you would have for setting up a tank dedicated to a horseshoe?
<All I'm sure of is posted on WWM>
Lastly if this ends up happening, what quarantine recommendations do you have for horseshoes,
<Mmm, a couple weeks isolation from other systems. Do track metabolite accumulation in their quarantine system>
I am slightly paranoid about quarantine and have separate quarantine tanks for both my fish and my corals, nothing goes in without being in quarantine for a while.
Thank you,
<Welcome. Please do relate your experiences if you go forward w/ this project. Bob Fenner>

Horseshoe crab arriving today, sys.  (RMF, thoughts on releasing Limulus into the wild? I'm against...)<<Am totally opposed. RMF>> 11/17/09
I read with interest your FAQ section on Horseshoe crabs.
<Fascinating animals indeed.>
Unfortunately, I did so after ordering a 2-3" crab yesterday from a website that said they are easy to raise and will do well in reef tanks.
<Good heavens no!>
I have a 125-gallon reef tank and a 55-gallon FOWLR tank.
<Not a good home for this chap.>
However, after reading your FAQ's, it is clear this new arrival will not be happy in either place due to the water temp, among other things. I want to give him the best shot at survival until I can figure out what to do with him long term.
<Very good.>
I have a 100-gallon Rubbermaid tub, 80lbs of Caribbean Live sand still in the bags, plenty of "seasoned" salt water and cured live rock, powerheads, etc. I will use whichever of these things I need to for this crab and will purchase whatever else you think I need for the next year, short of a new aquarium.
<Actually, maintaining Limulus under lab conditions isn't especially difficult, and if you Google "Limulus polyphemus" (the Latin name) as well as Horseshoe Crab alongside "maintenance" and/or "aquarium" you'll find there are fairly reliable protocols for keeping these creatures for extended periods. While not precisely white mice, they are much used lab animals, particularly for medical research. See for example, here:
In short, they're kept reasonably cool, room temperature in an unheated room usually fine, and maintained in a clean, filtered aquarium. Feeding is done separately, with the animal turned upside down, food (chunks of fish, seafood, etc.) placed in its "claws", and then the animal replaced in a tub of seawater. It's left to feed for about an hour or until its defecated, and then returned to the maintenance tank. Repeat every couple of days. In the wild these animals hunt for small organisms within the sand bed, but this really isn't viable in captivity.>
Please let me know the best way to set up these things to make a suitable environment for him until he outgrows the 100 gallon Rubbermaid (by the way, how will I know when he has outgrown it?). Also, can I release him into the Long Island Sound once he grows and if so, what size should he be before release?
<I'm not wild about releasing captive animals into the wild. For one thing, at any number of points along the chain of handling it can be exposed to parasites, bacteria and viruses that don't exist in Long Island Sound.
There's also the issue that you may not even have Limulus polyphemus, with Asian sources certainly sending out Carcinoscorpius to pet shops in Europe, and therefore possibly elsewhere. Obviously releasing non-native species is a bad thing. Since these animals do reasonably well in public aquaria, you might send a couple emails out to such places, to see if they can home your beastie. Alternatively, humanely destroy the animal (again, info on this will be in the scientific, if not hobbyist, literature).>
Lastly, would it be okay to purchase minced clams (like they use for clam sauce) and chop them even finer for him?
<A variety of seafoods is best, since by themselves seafood items tend to be lacking in one or other regard. Unshelled shrimps have calcium but too much thiaminase, shelled cockles are low in thiaminase but not enough calcium, and so on. The more variety, the less chance of problems. Lab specimens take standard fish pellets readily enough, and while there's debate about their use as a staple, as an occasional vitamin top-up they may have their place. Alternatively, your usual marine vitamin supplement could be used.>
Thank you for any advice you may offer.
<Hope this helps. Cheers, Neale.>

Re: Horseshoe crab arriving today, more re sys.   11/17/09
Thank you for your prompt response.
<You're welcome.>
I will feed him/her as you propose.
I will set up the Rubbermaid home for him/her for now. According to the seller, this is an Atlantic Limulus.
<Good to know. Telling the species apart isn't difficult, and a cursory review of the literature should help if you're interested.>
I have an extra submerged pump/filter setup from a small koi pond which I can use.
Should I leave the bioballs in there or just use the sponge filter?
<Probably won't make much difference either way. These aren't "delicate" animals provided they aren't starving or overheated (which are the prime sources of mortality in captivity).>
Will the crab be okay with the water movement that comes from the top outlet (used to be a fountain) of the pump/filter setup?
<These animals are used to very strong water currents, but that does assume they're able to "dig in" a bit. I'd see how you go. In aquaria they often get pushed onto their backs, in which case you'll need to right them.>
Should I fill the tub to the maximum level? (100gal)
<More water the better of course, but 100 gallons for a 3-inch specimen might be overkill.>
The only room I can put the tub in is heated. I will put a thermometer in the water. What is my target water temp? Should I do periodic water changes?
<Wouldn't worry too much. Unless the water temperature is above 20 degrees C, these animals aren't stressed. Do read that article I linked for you; it contains details on the established protocols.>
Lastly, should I not bother opening the two 40lb bags of Live Sand and just leave the tank with not sand?
<In labs sand is rarely used because of the risk of the sand getting dirty and causing infections. Since "clean" crabs are essential for the work being done, this is helpful. But under aquarium conditions a few cm of
coral sand and crushed coral would be very useful. Live sand is obviously helpful in terms of water quality management, but the Limulus couldn't care less either way.>
I also have Live Rock that needs a place to go. Can I put it in the crabs tub or should I put it in a separate tub?
<Limulus don't like rocks. Plus, below 25 C, your expensive Fijian live rock is soon going to die back. Would concentrate on old-school canister-type filtration, ideally supplemented with some aeration to keep
the water nicely oxygenated.>
Over the next few days I will do the research you propose so I don't have to ask you so many questions!
<I'm no great expert on these beasts, though like Bob, I've bumped into them a few times in the scientific side of my life. They're virtually never kept properly by hobbyists, hence the need to concentrate on lab reports.>
Right now, I am in a hurry to set up the crab's new home before FedEx arrives :))
<You'll be fine. These are EXTREMELY hardy animals when given essentially correct conditions. There is an Asian species sold as a freshwater horseshoe crab that manages to cling onto life for months under such
conditions, even though it's a brackish/marine beastie. Don't panic, do your reading, and enjoy.>
Thanks again,
<Cheers, Neale.>
Re: Horseshoe crab arriving today
thanks again! I will give it my best shot!!!
<Good luck! Neale.>

Subject: another photo, please: blue-legged hermit?   8/28/09
> Hi Bob,
> Do you have this beastie? Clibanarius tricolor, or similar?
> As a swap, I can offer a photo of Carcinoscorpius rotundicauda, a euryhaline Limulus doing the rounds as the "freshwater horseshoe crab" even though it doesn't last long in freshwater (no surprise).
> Thanks!
> Neale
Will either of the attached do? B

Re: another photo, please: blue-legged hermit?
Yes, perfectly well! Thanks Bob.
Attached, a "freshwater" horseshoe crab.
Cheers, Neale
<Will post w/ your prev. comment with credit to you. BobF>

FW Limulus, not!  8/29/2009
Hi Bob,
Here's a better caption:
"Carcinoscorpius rotundicauda, one of several Southeast Asian Limulus relatives. Basic care similar to Limulus polyphemus, except that Carcinoscorpius rotundicauda is truly euryhaline and tolerant of a broad salinity (and temperature) range. Sometimes sold as a freshwater invertebrate, as it does sometimes occur in freshwater habitats, having been recorded from the Hooghly River 90 km (56 miles) upstream.
However, it does not do well in freshwater aquaria, but can be maintained in brackish or marine aquaria from SG 1.010 upwards, assuming other factors (substrate, diet) are appropriate."
Cheers, Neale
Will append. B

Horseshoe crab, leave at the beach  8/9/09
I have questions because of a very stupid situation, as probably evidenced by the title, that I need to resolve.
Recently my family brought home a live horseshoe crab this past Friday (along with about a dozen horseshoe crabs which died upon the touch of chlorinated water-- the horseshoe crab I managed to get some spring water for which I added salt).
<They are very sensitive to water chemistry, unless the salt levels are at the appropriate level, 35PPM, pH, and composition (marine salt not table salt) then they will quickly die.>
He is literally surviving in a clear plastic storage tub over this weekend, and we attempted to dump some sardine in there and now a piece of chicken, as well as one of the dead hermit crabs.
<I would not try to feed him, more likely to just foul the water.>
There is no sand of which to speak and I have changed the water twice since then. We are hoping to get him back to the beach today but we are wondering if we will end up shocking the crab, as many marine animals get easily shocked by temperature changes.
<Getting him back to the beach is his best and probably only chance, a difficult species to keep even or dedicated hobbyists.>
I've put the crab out on the fire escape in the hope that the water will more closely match the outside (I realize this is unrealistic but I still feel it's better than my guesswork).
<Temperature is probably the least of his concerns to be honest.>
Since I am assuming he ate neither the sardine or the chicken, we are also wondering how long he can go without food, and have we inadvertently debilitated him too long to survive upon his return.
<He can go several days at least without food, best to just get him back to the ocean as quickly as possible.>
Re: Horseshoe crab, 8/10/09
Thank you very much for your advice. He was returned today, and very quickly burrowed himself into the sand of the wet shore, which I assume means he is healthy enough to recover speedily.
<A good sign.>

Horseshoe crab  7/4/08 This is for Bob Fenner, or if Bob is not there, then hello to whomever is helping me today. I have had my horseshoe crab, "Dozer", short for bull dozer, for several years now. He is getting kinda big for my 55 gallon salt water aquarium. He is about 4 inches wide. I only have one rock and some barnacle shells in the bottom. The bottom is mostly sand. I have been thinking about letting him go in the wild where I have seen several adult horseshoe crabs. Is this a good idea? <Mmm, no> The last time I returned a fish to my LFS it died there so I don't want dozer to have the same fate. I figure he would have a better chance in the wild. What do you think? <That in all (not just the majority) cases, returning any life to the wild is a poor idea> Secondly, I am wondering what is the best creature to get to stir up my sand substrate? I have three percula clown fish, two of them are breeding. I have two light blue damsel fish, a cleaner shrimp, a Brownbarred / Banded Goby, a small crab not sure what type, and some feather dusters. Thank you, and I look forward to your response. Kathy <Please read here: http://wetwebmedia.com/marine/maintenance/index.htm the third tray down. Bob Fenner> Nature is like a spider web - if you pull on one string, you affect everything else in it P Please consider the environment before printing this e-mail <Good spiels... I'd add, "think about the environment before you reproduce">

Re: horseshoe crab  7/4/08 Ok, I am wondering then what can I do with dozer, he is disturbing my clownfish, knocking everything over in the tank. I, like many other people, was told that dozer would only grow to a certain size and then stop growing (probably they meant that he would just die, but did not want to tell me that). <Mmm, yes... a fallacy that seems to be perpetuated forever> Now the poor guy is stuck in a 55 gallon tank, and I am sure that he is going to shed again soon, and that is when the real trouble will begin. Should I find a LFS that will take him, I know of one about an hour away that has some very big tanks. I am sure that he would be happy for awhile there, if they feed him. <Yes... Craigslist as well is worth listing this animal in> I donated one horseshoe crab that got large, like this one, to the Miami sea aquarium. But they kept him in the same size tank and used him for educating children about SeaLife. Does not sound like the kind of life I want for this guy. <Alternatives?> I realize now that buying a horseshoe crab was a mistake, but I need to find a better place for dozer and need advice on what is the best thing to do for Dozer. Thank you for your help. Kathy <Thank you for sharing. It's not just/only non-indigenous species that I and others are and should be concerned with... even returning native species to environments they hail from entails risks... principally of introduction of pathogens, vectors. Bob Fenner>
Re: horseshoe crab
7/4/08 Well we will see, I have posted Dozer on Craig's list, I have no idea if anyone serious will be interested, so we will see. I will only give him to someone who has a large established tank, and who knows how to care for him. <Good> If I don't get anyone interested I will take my chances and try to find a suitable place in the wild for him <Please... don't do this... see my prev. email> as I think that would be better than being cooped up and starving in a small tank. If I did not have any fish I would keep him, but I am afraid that he will eat my fish one day. He eats my snails that I get to control the algae, so I don't buy them anymore. He is just too big for me to keep. I certainly will not get another, even though I love to watch him and find the species very interesting. Kathy <Ah yes. B>

Horseshoe Crab selection, systems   1/17/06           Ok, first off I have to say I did not do an extensive amount of research on horseshoe crabs before I bought one (and now I regret that very much). I went to one site that said they were easy to keep and ate matter in the substrate.  With this lazy-man information I went to my local fish store and picked one up.  In my obsession with my new horseshoe crab I decided to do more research and came upon this site (which I might add is very helpful).  I feel really bad for my horseshoe crab, I fear he will starve. <Most do> I have only a 55 gallon tank, and about half the sand bed is covered with rock.  The temp is usually kept at 78 degrees Fahrenheit, I'm aiming to lower it (would 75-76 degrees be good or still too warm?). <If this is a/the (typical) coldwater species, (the Atlantic U.S....) yes>   His tank mates are one scooter blenny, one royal Gramma, two peppermint shrimp, one emerald crab, and about a dozen types of snails and maybe ten scarlet reef hermits.  As I now see I believe I cannot support one of these creatures with my sand bed alone like the site said (oh yeah, my sand is sugar fine and about 2"-3" deep, as of now the horseshoe crab is about 1.5" across and .5" tall)  I'm also worried about his health in the present, let alone starvation in the future.  When I first introduced him he had trouble getting under the sand, he would go in half way, and then come out again. <Good description> He was also very clumsy, crashing into walls and the rock and sometimes flips himself over.  I woke up this morning, watched him a bit, and he was finally able to get under the sand, but he's not moving, I can see the lump where he is and he hasn't moved for about 2 hours.  Is this normal? <Do have periods of long senescence> Any way, if he does survive *fingers crossed*, is there a way I can possibly feed him manually by putting food under his shell? <Can be tried> Any other suggestions on feeding are welcome, and what can I feed him to supplement his diet of worms other things in the sand that will be healthy? Thanks for your help! <What little I know re the captive husbandry of these ancient arthropods is posted on WWM. Bob Fenner>  

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