"It takes 9-12 years for a Horseshoe Crab to reach maturity, so all the Horseshoe Crabs you see on the beach are at least 9 years old and have a life span of 16-
40 years. The Crabs grow by molting or shedding and emerge 25 % larger with each molt. Molting occurs several time during the first two to three years and about once a year there after. After 16 molts (usually between 9 and 12 years) they will be fully grown adults."
Posted for my previous response in how slow these things grow. Taken from "http://www.thomaspointbeach.com/horseshoe.crab.fun.facts.pdf"
"The Horseshoe Crab can swim upside down in the
ocean." From the same page.
"On the underside of the opisthosoma is a series of six page-like structures called book gills. These are versatile organs used not only to breathe, just as fish use their gills for respiration, but are also used for swimming. Horseshoe crabs swim upside down by flapping the gills like wings. Swimming is an alternative mode of transportation used in emergencies, mainly to escape from predators or if the animal finds itself in rough surf."
From
Daily Kos: Marine Life Series: Horseshoe Crab Anatomy Article referring mainly to adult crabs.
They also swim ashore to mate.
From
Uncensored WildEyed: Horseshoe Crabs Mating! Nature Calendar
https://kb.osu.edu/dspace/bitstream/1811/5558/1/V70N05_276.pdf
^An academic article relating to the swimming habits of horseshoe crabs.
" They’re frequently offered in the trade and extremely hardy, given the proper care and system. Life span is 16 – 20 years in the wild, but unfortunately, many are inadequately housed in captivity (especially in reef tanks) where they quickly deplete the sandbed of edible fauna and slowly starve to death over a period of 6 to 12 months."
WWM simply backing up what I said in an earlier post in that they EAT like crazy and will need supplemental feeding the same as creatures like sand sifting stars.
"In the wild, the young stick to the shallows and intertidal zone."
"They also have a tendency to occasionally swim around upside-down – also normal so no worries."
WWM does say that they need tanks of several hundred gallons as adults. And I fully agree. However as babies as they're frequently sold in the hobby, it takes so long for them to outgrow a relatively normal sized tank, that it's not unheard of to keep juvenile crabs and rehome them when they get too big.
Also noting that WWM also states that they enjoy cooler waters so you if you run your tank on the upper end of the ideal reef temperature scale, you will decrease the lifespan of the crab.
Like any other creature, it requires proper research into diet and habitat requirements and will need rehoming when it gets older, but keeping a horseshoe for 2-3 years in a 55g aquarium (which is what my tank size is now) is not unheard of nor is it detrimental to the crab provided you pay attention to what it needs. Just because it requires special care doesn't mean it's completely unsuitable for the home aquarium.
Edit: I hope I'm not coming across as snarky or anything, I just disagree. I spent 3 months in college studying horseshoe crabs as part of a field assignment/project/internship. I just think they're an incredibly interesting animal that is virtually the same as they were in prehistory. They haven't evolved hardly at all in eons and yet they're full of personality, quirky, and were really fun to study. I bounced around from major to major and did so many different projects like these at university that while on the larger scale, I might not have a lot of broad spectrum knowledge, I do have a few deep pockets of information on specific things that actually do come in handy.