Everything I’ve learned about hermit crabs

One day last year, my son came home from school and said he’d had a relief teacher who had told the class all about her pet hermit crabs. He proceeded to tell me all about pet hermit crabs, and showed me a list he’d written down of the things you need to house pet hermit crabs. I said that was all very interesting and if he still felt strongly about the matter at his next birthday, we could get some hermit crabs to keep in his old fish tank. Months passed, his level of enthusiasm remained high, and for his birthday we took him to the pet shop to buy two hermit crabs (as they like company), and various branded hermit crab accoutrements.

Hermit crabs sold as pets in Australia are Coenobita variabilis, a native Australian land hermit crab. They live in northern tropical parts of Australia, so they like a warm habitat. Generally they’re kept in dry fish tanks, with containers of fresh water for them to drink, and salt water to splash in.

Above is a picture of our tank right after we bought the two crabs, Nippy and Scratcher. This is what tanks look like if you buy the pet shop basics – it’s a bit of a grim sight. After I took this photo, we added sticks and rocks for climbing on, and I made the bedding a bit deeper using “play sand” purchased from Bunnings. Every time we added something new to the tank, the crabs came over and thoroughly inspected it – they really enjoy exploring new additions to their environment, and it’s fun to rearrange items and occasionally add new things in to give them a bit of a thrill.

Nippy and Scratcher were fascinating little creatures, and it was really fun watching them exploring around their tank, burrowing in the sand and climbing out precariously on sticks. However, about a month after we got them, I inspected the tank and made a rather horrifying discovery – a lone crab leg was lying on the sand and Nippy was wandering around the tank looking suspiciously well and happy. I felt that ominous sinking sensation that occurs when you realise you’re going to have to tell your child that one of their pets has died. I poked around in the sand, uncovering a mostly empty shell with the remainder of Scratcher in it, and tried to think about the nicest way to tell Edward “I think one of your crabs ate the other one.”

If you google “why did my hermit crab die”, you get any number of results assuring you that your hermit crab is probably not dead, it’s just moulting, as moulting hermit crabs don’t move and appear to be dead. You should never on any account actually inspect your hermit crab to check if it’s dead, the websites said sternly, because you might hurt it. Does anything smell like it’s rotting? No? Well your hermit crab is probably just fine, and you should treat it as if it’s still alive. I decided a dismembered leg was good enough evidence that Scratcher was no longer alive, and removed the remnants of his corpse from the tank.

I broke the news of Scratcher’s death, and spent a few days doing a lot more reading about hermit crabs and all the ways they can die. I decided I’d done two things wrong that probably led to Scratcher’s untimely death. Firstly, hermit crabs can only breathe in high humidity – that’s part of the reason why they need dishes of water and a heat pad under their tank. We didn’t have any indicator of the humidity level in the tank, hadn’t been spraying it with water, and it’d been winter (albeit a sub-tropical winter). It was possible the humidity in the tank had been too low. Secondly, when hermit crabs moult they can’t move (hence the whole “are they dead/are they alive” dilemma that hermit crab owners face) and are unable to defend themselves – they bury themselves to protect themselves during the moulting process. While I’d added enough substrate for the crabs to fully cover themselves, it wouldn’t have been enough to defend Scratcher from a curious Nippy.

A possible thirdly was the fact that the branded crab accessories we had were all peeling – the brightly coloured shells and water containers were shedding paint, and I wondered if this might have been poisonous if ingested. I don’t think this is actually the case, however in future I avoided brightly painted accessories and shells to avoid more peeling paint.

Back to the drawing board for the tank (or “crabitat” as hermit crab owners very cutely call it). I made a much deeper substrate/bedding area, coming about a third of the way up the tank – this is a combination of the “play sand” and coconut coir. Bunnings (and other gardening stores) stock bricks of pure coconut coir – both this and the sand are much more cost effective than the tiny bags of “hermit crab bedding” sold at pet shops. You just need to ensure that you are buying pure coconut coir with no added fertiliser. (Bunnings sell bricks labelled as “garden soil”, which are fine to use – the bricks labelled “feed and mulch” have additives.) I broke off part of the block of coconut coir, soak it in water, then mixed it with the sand until I had a nice consistency – around 2/3 sand and 1/3 coir.

On top of the bedding, I put some new netting for climbing, a coconut shell house for hiding in, some more spare shells (unpainted ones this time), two gauges for temperature and humidity, and finally – and a new crab, Outy. A bit later, I put up a “moss pit” – this is a suction cup soap holder stuck to the tank, and filled with sphagnum moss (you can buy blocks of this at gardening shops). Nippy and Outy were thrilled with their new tank decor and had a lovely time thoroughly exploring, and hurling moss about the tank. (Well I presume that’s what they were doing at night when I’d find the moss pit emptied in the morning, and moss strewn around everywhere).

Everything went well for a couple of weeks, and then first Nippy, then Outy, disappeared. I confidently announced that they were taking advantage of their lovely deep substrate and were moulting. I kept changing the water and food, awaiting their return. After a couple of weeks, Outy reemerged for a few days, then disappeared again. More weeks went by. “They’re not very interesting pets, are they?” my brother commented, surveying the empty tank. I became convinced that both crabs were dead – around two months had passed since I’d seen Nippy, and there were no signs of life. Refreshing the water bowls began to feel like a bit of a pointless endeavour.

As the end of the year approached, I told Edward that after Christmas we should probably have a dig around to try and find the crabs, as I suspected they might be dead. He solemnly agreed. Nippy and Outy were apparently listening to me, and accordingly on Boxing Day they triumphantly re-emerged, back from the dead, clambering around the tank as if nothing had happened.

I am hoping that soon they may like to shift out of their garish painted and branded shells and move into some of the slightly larger natural shells scattered temptingly around their tank. It is nice to have them back from their long absence, and according to The Internet, they will probably not moult again for at least another year. Soon we shall rearrange their tank to keep their minds – although surely hermit crabs must have rather small brains – happily occupied with interesting new things.

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