Planning on keeping a healthy adult horseshoe crab in the future, overview

owltower

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(sorry if this is a duplicate to the question post i made a little earlier, im not terribly experienced with a true forum format yet)
Hello! forgive my possible lack of candor, I'm totally inexperienced in the realm of saltwater enclosures, but I do have a crazy idea (ahhh yes, the classic clueless ideas layman i am, i'm sure, but that's why i'm here to learn! :)))) I seek an overview of what to expect, what to read up on, and some discussion as to how I would do such a thing with my partner later down the line (we adore benthic arthropods and a horseshoe crab is really high on the list)
So, i open with a bit of an esoteric and deceptively simple-sounding question:
How would one keep a horseshoe crab in a healthy environment as close as possible to its natural habitat?

I've browsed around the forums but it seems no one has put together a big or deeply informative thread for it. I see a lot of people trying to keep them in reefs or otherwise populated tanks, but often without the size or scope needed for growth beyond juvenile sizes, and it seems there's a stigma associated with them that they either die or wipe tanks then die. I seek to escape that. A large sepcialized tank is envisioned, with round corners (sharp corners are jarring to animals), probably a custom build all things considered, but that is a consideration for later.
i know this is very far down the road (me and her are still just aspirant collage students who are obsessed with ocean organisms, after all) but I want to aggregate as much information as i can now.

Do keep these broad goals in mind, ordered based on priority:
* ENVIRONMENTAL BEHAVIOUR: at minimum we want to allow them to sift the sand and dig as they would naturally, which probably requires a completely bio-active tank to facilitate algae, worms, debris, etc. and some brackish/open sea plants, as well as adequately deep sand. Reduces reliance on us for feeding and also results in what is probably overall more stimulated horseshoe crabs and other critters <3 how would this be done?
* LIFETIME: equivalent value to above, we want this crab to live healthily as long as possible, at least into maturity (9 or so years) and then sustain as long as possible after that. I've read the threads about them requiring a lot of small food as adults or even juveniles, but i feel it doable. thoughts?
* OTHER ENVIRONMENT: related to tank engineering, in an absolutely ideal tank we want to simulate both the North American or Asian littoral seabed AND a small section of coastal shore, so that their environment is further diversified. Alternatively, a mangrove environment is an idea since that environment's real depth is similar to a tank's is some areas, closer to what C. rotundicauda lives in). This would also probably require at least a part of the biodiversity present in the form of waste producing creatures and some very small coastal crustaceans, etc, regardless of the environmental type.
* BREEDING BEHAVIOUR: this is a totally crazy idea, i know, especially since captive breeding of HC's is a deeply complex and poorly understood subject. from what I've read this idea would require a permitted collection from the wild alongside at the very least some natal sands of the horseshoe crab in question, as L. polyphemus will only breed in sands close to their birthing areas among several other stipulations. but, wouldn't it be SO cool? The true ideal is to have at least a mating pair, possibly a second male to throw in some diversity and competition, though I worry that any more than two adults would be pushing the limits of a home aquarium. we'd be perfectly happy with one all things considered, but a breeding pair would be phenomenal both for conservation purposes and also for observation.
Of course such a feat would require extremely granular simulations of temperature, lighting, etc as well, hence its low position on the list.
*TIDAL/WAVE SIM: another crazy and probably unnecessary idea, but some form of tide depth or wave movement alongside other environment simulations would be awesome, but with it comes more complexity that may be undesirably taxing.

Thoughts? Please discuss. again i know it gets more crazy and ambitious as the list goes down, and even a bioactive tank is a wild idea as a beginner, but i do think keeping at least one healthy and stimulated horseshoe crab is possible with care towards its environment, and i want to make it work some day. I also may be way overthinking it but honestly any input is cool
Thank you!!!!
 

Jay Hemdal

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Welcome to Reef2Reef!

I've kept horseshoe crabs in home and public aquariums. Small ones in a home aquarium are not too difficult to manage. Large ones are a bit more difficult. One issue is that very large ones reach "terminal molt" where they won't molt again. They slowly go downhill from that point. Also, as they grow, the time between molts becomes longer. This means that any shell erosion or other damage, is not as quickly repaired as with smaller animals, so I've lost some crabs to that issue.

Jay
 

jda

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You are going to need a very well established tank with lots of things for the crab to eat. This is likely a multiyear journey just to get to the point to where you could house one with natural things for it to eat.
 

ISpeakForTheSeas

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Welcome to Reef2Reef!

I think I've covered just about everything below, but if I missed something or you have other questions, please ask!
I've read the threads about them requiring a lot of small food as adults or even juveniles, but i feel it doable. thoughts?
It is possible. Jay brings up some very good points to consider. Beyond that, though, from what I've been able to find, keeping the horseshoe crab healthy may actually be easier than emulating its natural habitat like you're hoping to.

With that in mind, let's address the health/lifespan and breeding first (they're actually relatively easy to breed compared to a lot of other species, and not quite as difficult as their wild breeding behaviors would lead you to believe - the diet really seems to be the key; chances are extremely high that you will need to feed them daily even with things for them to eat growing in the tank with them):
Fair question- looking at it, they are slow growers (the PDF with a graph detailing average growth per year in mm won’t let me select the graph, but I’ll link it below). They take ~9-11 years to reach maturity, and they molt ~16-17 times over that timeframe (molting less frequently the larger they get). They hit ~2 inches wide at 4 years old, ~6 inches wide by 9 years, ~9” by 15 years, and - for the females, which grow larger - ~12” by 18 years (though their growth does vary depending on their location), so, OP, you may have a lot more time I would have guessed.

Breeding specific info:
The gelatin was to keep the food (the Skretting aquaculture feed I mentioned) stable in the water. Some of them have been successful with the larval rearing, and more research has been/is being done to make egg production in captivity more efficient (to the point where it is seen as a viable method of collecting the blood needed):

With regards to the parameters (in case the studies already listed aren't proof enough that these things are basically bullet proof):
Also, the Skretting feed used in one of the studies:
(This link is the study that had the gelatin feed discussed above.)
Now for the environment:
-20 to 30 ppm brackish salinity is ideal
-Deep sandy/muddy bottom
how would this be done?
For the bioactive elements (i.e. the plants/algae and other critters in the tank with the horseshoe crabs), you can find a bunch of macroalgae and a handful of seagrasses for the tank relatively easily - other plants that could potentially be grown in this setting are a lot harder to find, but you might be able to find a couple that can handle the temps, wetness, and salinity alright. The quote below may help with that:
The database below definitely isn't perfect, so I'd verify that any plant on the list you're interested in can handle both submersion and 35ppt salinity (a lot of plants can handle one or the other; relatively few can handle both), but it should be a good place to get some ideas from at least:
Currently, the only non-mangrove/seagrass plant that I've verified can handle submersion and 35 ppt conditions is Sarcocornia perennis - but it grows best at lower salinities and when kept moist rather than submerged (it was submerged at 5cm under the water in the study below, and that was the condition that correlated with the highest mortality rates in the plant).

As a note here, the depth the plant is going to be submerged at under the water is important (as a general rule of thumb, the less water that covers the roots, the better, though are definitely exceptions to this rule; verifying how much water the plant you're interested in can handle is a good idea).
For creatures, you can find a number of different critters (hermit crabs, small crabs, shrimp, brittle starfish, Aquilonastra starfish [known in the hobby as Asterina starfish], some kinds of bivalves [these are harder to keep though], sand-sifting sea cucumbers, etc.) to put in there as well (most places don't purposefully offer bristleworms and the like, but they're not really hard to find - you should be able to find them in an LFS or from a local hobbyist, or you can order them from IPSF). Live rock from Florida or some live sand/mud like you can get from Indo-Pacific Sea Farms (IPSF) would be great for adding some biodiversity to the tank and helping out with this.
in an absolutely ideal tank we want to simulate both the North American or Asian littoral seabed AND a small section of coastal shore, so that their environment is further diversified.
Honestly, emulating the habitat (the type of substrates, creatures, and plants) wouldn't be too difficult, but figuring out how to make a proper shoreline in the tank that can handle the horseshoe crab crawling around on it may be tough without an insanely long tank (see the wavebox quote below for some idea there).
*TIDAL/WAVE SIM: another crazy and probably unnecessary idea, but some form of tide depth or wave movement alongside other environment simulations would be awesome, but with it comes more complexity that may be undesirably taxing.
This one would actually be relatively easy - for the tide (high tide/low tide) simulation, you would just need a sump and two timer-controlled pumps (one to pump water from the display/horseshoe crab tank to the sump at a specific time for a specific length of time - this would bring the low tide - and one to pump water from the sump to the display tank at a specific time for a specific length of time - this would bring the high tide).

To get a wave going, there are a few options - a wavebox (the most conventional option at the moment), a motorized paddle, and/or air blowers. I'd recommend the wavebox/paddle just for efficiency's sake.

For the wavebox:
I've heard wave/surge boxes can give a proper wave, or there are a few other DIY solutions that you could use to make a wave too (though adapting some of these to aquariums may be a bit difficult). If you want the waves to break (i.e. if you want them white cap in the tank), that's a lot harder to setup properly and would take a lot of playing around to get just right, but it's also possible and could give you a nice swash zone (the area where the water from breaking waves that is above the actual waterline of the tide runs over the beach). Anyway, here's a DIY wave box:
Mudskipper aquariums are a great place to start looking for a "traditional" sand/beach slope, but I'm not sure how well they would hold up against any sort of actual wave:

For the paddles/blowers:
 

flashsmith

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Unless they are housed in a public aquarium they should be left in the wild. They all seem to eventually slowly starve in home systems.
 

ISpeakForTheSeas

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Forgot to add the feeding info:
It seems to just be the diet/feeding.

Horseshoe crabs are actually aquacultured by a few biomedical companies, and they seem to be basically bulletproof as far as water parameters go (they thrive in conditions that would likely kill half the stuff in our tanks), but they do have a reputation for being impossible to keep the hobby, and it’s speculated to be a nutrition deficiency- from what I’ve seen it’s probably a mix of both too little food (the groups culturing them feed like 5% of the crab’s body weight per day, IIRC, so these things eat a ton) and food that doesn’t meet their nutritional needs.

Edit: It was 3% of the body weight per day.
Honestly, I’m not sure - one of the groups listed what specific feed they were using, but it’s a commercial feed from a company (Skretting) in Utah that doesn’t list their ingredients. From what I’ve been able to find, though, it seems to just be a relatively standard, high quality aquaculture feed (good protein and fat contents, a wide range of food items in it to ensure a good spectrum nutritionally, etc.), so I’d assume pretty much any good feed (like LRS, Hikari Mega Marine, or Rod’s, for frozen examples; TDO Chromaboost & NLS Marine or Otohime & NLS Marine for pellet examples) should work.
 

Jonathan6402

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I have a young horseshoe crab of 15cm that lives in a 10 gallon fish tank with a marine rock and it has black spots. It has not changed its shell for more than 2 years. I don't know what is happening. And what food should I give my horseshoe crab? Can you help me?
 

Jay Hemdal

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I have a young horseshoe crab of 15cm that lives in a 10 gallon fish tank with a marine rock and it has black spots. It has not changed its shell for more than 2 years. I don't know what is happening. And what food should I give my horseshoe crab? Can you help me?

Welcome to Reef2Reef!

Can you post a picture of the crab?

Black spots on horseshoe crab shells are a result of physical damage that may then become infected with bacteria. fungus and algae. That in turn may worsen the issue. Horseshoe crabs cannot repair their shells except when they molt. I'm not sure why your crab has not molted in two years, it should have molted at least twice during that time. It may be that the tank is too small for it.

Feeding horseshoe crabs is basically just placing some seafood items on the bottom of the tank (shrimp pieces, squid, even small fish pieces). The crab will then scavenge them. Any food not eaten after about 6 hours should be removed.

Jay
 

ISpeakForTheSeas

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I have a young horseshoe crab of 15cm that lives in a 10 gallon fish tank with a marine rock and it has black spots. It has not changed its shell for more than 2 years. I don't know what is happening. And what food should I give my horseshoe crab? Can you help me?
Welcome to Reef2Reef!

Yeah, it definitely should have molted, so pics would be helpful there.

For feeding, my recommendation for premade feed at this point would be Larry's Reef Services Reef Frenzy (LRS Reef Frenzy) as the primary feed supplemented with New Life Spectrum Marine Fish Pellets (NLS Marine Pellets).

Hikari Mega Marine or Rod’s Original would be possible alternatives for a frozen feed, and TDO Chroma Boost or Otohime would be possible alternatives for pellet feeds - I'd still recommend supplementing any of these with the NLS Marine Pellets to ensure a good mix of algae to try and ensure a proper nutrition.

As mentioned above, in horseshoe crab aquaculture, they typically feed 3% of the horseshoe crab's body weight per day, so if you want to make sure you're feeding a proper amount, you can weigh it and find out.
 

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