Tank Size and Crabs Per Tank

This is a tricky and much debated subject within the hermit crab world. There is no 100% fits all set of numbers, a lot of outdated information floating around and it is not an easy thing to lay down guidelines for. Over time the care standard of land hermit crabs within captivity has been improving and with it the practice of very small tanks and overcrowding is slowly dying out. Not providing enough space for your crabs, aside from the moral aspects of it (after all, we all know an animal should be able to move freely and express itself), can cause issues such as stress, increased fights, difficulty navigating and in the case of wildly undersized tanks difficulty maintaining optimal conditions.

 

 Here I will discuss ways to help work out the best minimum tank size for your colony, but as always this is the minimum suggested and providing the maximum space you can will always be the preferable route to take. Always keep in mind that however small they are land hermit crabs will grow, and with good care you could end up with a whole colony of jumbos to house! 

 

Things To Consider

 

When working out a tank size, looking at more crabs or simply looking at guidelines to decide which to follow, there are several important things to keep in mind. It is very easy to follow the train of, "well it is only a small crab", or to assume they don't need that much space, especially if you become more and more interested in them as pets and wish to branch into other species or simply greater numbers. Sometimes, no matter how much we love them, it can be easy to lose sight of what is best for the ones we already have, and many a keeper has ended up with sleepless nights worrying about upgrading, moving or just simply how they will house so many jumbos!

These are some things to think about;

 

• Crab Size - Body and Shell

Those adorable little crabs you picked up earlier seem tiny in their shells, hiding away and nervous, but what can pop out may have a bigger leg span than you think, and their shells can take up a lot of space too! Some species, like violascens, may even favour shells with protrusions and much more bulk than a neat little turbo shell, meaning even getting into a hide can be difficult without a larger one provided and they may get stuck without enough space between wall and decor. They will usually work themselves free but it is may cause needless stress and is easily avoided. Other crabs, such as compressus, may wear shells that are far too small and thus make them seem smaller than they actually are. 

 

•  Crab Growth

Your land hermit crabs should be moulting, and thus growing, regularly. One of the biggest pitfalls for new owners can be that they buy very small crabs, anticipating years before they even double in size, then find that they shoot up to much greater sizes in months. Larger crabs will generally moult less (and for longer) and often have less dramatic size gain each time, but babies will moult faster, more frequently and may have greater comparable gain. Holding off moults, which occurs with incorrect care and may happen sometimes if overcrowded, causes health issues and may prove fatal, so allow them plenty of space!

 

•  Crab Species

Some species, such as Coenobita compressus, are more active either generally or in one aspect of their lives (such as digging) and thus should be provided an extra space allowance wherever possible.

 

•  Tank "Footprint"

A suitable land hermit crab tank should have enough space for substrate depth, climbing room, the essentials and general exploration. A more arboreal tank, while deep, won't provide the floor space needed. A very wide but shallow tank won't provide the depth needed. Standard fish tank sizes will often provide the width/height/depth ratio needed.

 

•  Substrate depth

Depending on the size of your land hermit crabs, a minimum of 7 inches of substrate should be available. It is recommended to put in an extra inch or so to start, as this allows for natural settling/compacting of the substrate. For larger crabs at least 3x the height of the largest crab should be provided. Plenty of space should still be available above ground.

 

•  Water Pools, Decor, Shells etc

Fitting in two (potentially large) water dishes, a food dish, hides, branches to climb on, cover, spare shells/a shell shop and anything else your crabs might need can quickly eat up space. If the tank is too small it may simply not be possible to fit everything in. Shells in particular when you have larger crabs or a larger colony can quickly take up more and more space, with larger replacing smaller as the crabs grow.

 

•  "Free" Space and Exercise 

Once all the needed items are in, empty space on the substrate for the crabs to burrow down from and to generally move around freely should be available. Aiming for a sense of security (from leafy cover) while still providing spacious areas should be the goal. The more tightly packed in items are the less chance to avoid digging under your dishes the crabs have!

 

•  Enrichment

Adding in extra items beyond the necessary, either as temporary additions or permanent, helps to keep the crabs active and can sometimes aid in stopping them fixating on destroying the fake plants or other items in tank. Moss pits for example aid humidity levels and are popular to sleep in, leaf litter adds to substrate and serves as a food, branches or sprays of fresh or dried plants/trees serve as food and new spaces to explore. 

 

•  Maintaining Tank Conditions

Some crabbers find that smaller tanks (particularly the 10 gallons, which are really too small for all but an ISO) can be harder to stabilise with proper humidity and temperatures available. Overheating can be an issue, particularly in the summer months. No matter what the size of the tank using properly sized heat mats, insulation and a generally good setup will assist with this.

 

•  Tank Toppers

The addition of a second smaller tank on top the first, a "topper" that the crabs can climb into, is a method some use to allow for extra space. This does not however increase the available moulting space so should be considered a possible way to give existing crabs extra climbing space more than a way to fit more in. 

 

•  Keeping Track

Simply put, the more crabs you have in a tank the harder it may be to keep track of things or fix issues that crop up. This applies even when the tank size is large enough to support them all. Individual shell swaps and favoured foods are fairly harmless, but moults, inactivity or even death may go missed in particularly large colonies where it becomes impossible to track which crab is which. Splitting colonies between tanks, keeping species only tanks and simply sticking to more manageable numbers all help with this, and proper inter-tank hygiene will help contain outbreaks of issues like mites should they occur. 

 

Working out crab sizes

 

The general rule of thumb for working out the size of your crabs that has been in use for many years now is as follows, taken by measuring their approximate leg span while standing. While there is no universal standard, one crabbers idea of a jumbo often being far off potential size and another perhaps overestimating, this acts as a base template that many are familiar with.

 

Crab Size Band                                         Legspan

Jumbo                                               Over 3 inches / 7.6cm

Large                                         Approximately 3 inches / 7.6cm

Medium                                   Approximately 2.5 inches / 6.4cm

Small                                       Approximately 1.5 inches / 3.8cm

Teeny                                         Approximately 1 inch / 2.5cm

Micro                                       Approximately 0.5 inches / 1.3cm

 

Some species may come up larger or smaller than others, or simply grow much faster or more slowly, meaning their jumbo size may be different to that of others. Others may be much "skinnier" individuals vs a more bulky crab of the same leg span. For the purposes of tank sizing however going by leg span is one of the easiest methods.

 

Base Tank Sizes

 

Unlike the outdated x gallons per crab guidelines, starting off with a base minimum size ensures you don't end up with crabs in a tank which is far too small for the inhabitants, or in one which simply does not allow any real space to explore or fit everything in! This is a rough outline of the bare minimum size recommended as the base for each size band, and aims to allow for some growth (which should be regular and in a smaller crab's case fairly fast). If you have a mix of crab sizes then always size for the largest member, and try to get the biggest tank possible to avoid an urgent upgrade in the near future.

   

   Crab Size                                 Base  Minimum Tank Size                       Max crabs this size

Micro/Tiny                                        75L / 20 US gallons                                                5

Small                                               75L / 20 US gallons                                                4

Medium                                           115L / 30  US gallons                                             4

Large                                              190L / 50 US gallons                                               4

Jumbo                                             285L / 75 US gallons                           1 - with smaller companions

Multiple Jumbos                              380L / 100 US gallons                                            4

 

Allowing for additional tank mates

 

If you have your base tank size and either already, or do, have more than the max crabs for it in that size band, or have a mix of sizes, then working out a rough additional space allowance per crab is important. As stated before, this will vary based on individual crab (personality, species and setup can all impact). Aim to allow more wherever possible.

 

Crab Size                                            Additional Space Per Crab                    

Micro/Tiny                                                   18L / 5 US gallons

Small                                                          18L / 5 US gallons

Medium                                                       30L / 8 US gallons

Large                                                          38L / 10 US gallons

Jumbos                                                57 - 75L / 15 - 20 US gallons

Written by Rachel Turner for Crabby Cravings, not to be reproduced or reused without permission.