Shells - A Buyer's Guide

Buying shells, online or otherwise, can prove a bit of a minefield at times. Many a crabber has ended up with shells arriving far smaller or larger than estimated, or a stack of a type that their crabs will not touch. This guide aims to provide a few tips and tricks for the search, alongside details of how we measure our shell stock.

 

 

So How Many Should I Get?
 

The answer to this question is not set by any means. Colony size, crab size and what you already have available all impact on this a lot. Even individual personality of each crab can! Some crabs will shell shop almost endlessly and prove very picky with their new home choice. Having at least a few is VITAL to prevent shell fights, promote good growth and generally keep them happy!

 

As a rough guide, we reccommend a minimum of five spare shells per crab, with more being better. This allows them to find the best fit possible and helps cut down the risk of shell fights as  much as possible. Buying new shells every now and then to give a change will also contribute to content crabs. If you end up with too many to fit in the tank setting up a second level or a specific shell basket (known as shell "pits" by many keepers), or rotating shells once a week, are options you can look at.

 

But What Size Do They Need?

 

A few things can be done to establish what sizes you need.

 

First, look at the shells your crabs are in now. Can they fit nicely into the shell and seal it with their BP (big pincher) and shield leg? Generally this indicates the best fit for a crab. When near a moult they may choose different sizes, and sometimes you get crabs that just prefer shells that look wrong or uncomfortable to us! Coenobita compressus is particularly noted for wearing tiny shells that almost seem like short-shorts to us, and will often cling to their carefully modified shell until it looks totally ridiculous. 

 The shell seal does however make a good foundation point for choosing new shells, if they can pull right back in until almost vanishing for example then adding some slightly smaller sizes as part of your new order is a good plan, if they seal perfectly then a couple that size and some bigger, and if they cannot pull right back in you should look at bigger sizes! 

 

Our shell measurements are taken from the opening.to give the best accuracy we can while shopping, and are obtained using vernier calipers. These are perfect for shell shopping as they allow you to get at least a rough measurement of a shell even while a crab is living in it (albeit with difficulty if they wriggle!).

 

What Types Should I Go For?

 

Shell type preferences vary from species to species and often crab to crab. Even shells of the right type may have issues so checking before buying from gift shops etc is important.

 

Things to be avoided in new shells

 

• Paints, varnishes and glued on decorations. These can flake off and be eaten by crabs, or picked off for nibbling, may leech harmful chemicals and generally are not comfortable for the crab. There is a common practice for crabs due to go to the pet trade to be forced from their shells into painted ones, a traumatic and often fatal due to stress or injury event. Cases of crabs literally glued into shells by the paint have been observed.

• Holes. Hermit crabs store water in their shells to help maintain humidity around their gills, holes can be uncomfortable and will hamper this.

• Damaged spirals. By this I do not mean deliberately modified shells with the internal spirals removed or smoothed, but jagged, cracked or dangerously damaged ones.

• Internal obstructions and protrusions. Parasitic calcium deposits, barnacles and natural buildup of very rough surfaces are uncomfortable or even harmful for the soft abdomen.

• Teeth at the shell lip. Some shells, such as some nerite species, have natural jagged growth at the entrance. While occasionally some crabs may ignore this it is often unfavourable. 

 

Shell type preferences by species are as follow, please note these are general guides and crabs are very much individuals, some may differ and that is prefectly normal! 

 

Coenobita clypeatus (Purple Pinchers)

Opening Shape Preference: Round

 PPs tend to prefer turbo shells as the all time favourite, with other similar shaped shells like magpies also being taken. When smaller moon shells are often well recieved, and they can go through a stage of liking conch shells, particularly before they hit medium. That said, they can and will take other species including babylonias, foxheads and frog shells, with frequent shell changes being very common. 

 

Coenobita compressus (E's)

 Opening shape preference: D shape

Es can be very picky and may hold onto a shell for weeks or months without changing, even if it seems far too small to our eyes. They are well known for heavily modifying them, often completely gutting the inner spiral to make a little empty cap that is as lightweight as possible. Babylonias and thais species are among those that may tempt them, but trying a variety in cluding turbos is best to give them the most choice and the best chance of changing happily. 

 

Coenobita violascens (violas)

Opening shape preference: D or ()

 Violas have a longer abdomen than most species and prefer a shell which allows for this, such as a foxhead or whelk. They also particularly love the long landsnail shells and others of that shape. Young violascens have been known to come from the wild in very very long spiral shells of various species, and will often choose these in captivity as well. 

 

Coenobita rugosus (Ruggies)

Opening shape preference: Round

 Ruggies enjoy changing shells almost as much as they bomb around in their tanks. Although the base favourite is turbos in my colony they will take moon shells when small, magpies, whelks, anything that takes their fancy. 

 

Coenobita perlatus (Strawberrys)

Opening shape preference: Round

 Most straws adore turbos, particularly the gorgeous tapestry turbo. Picking gorgeous polished shells that will bring out and enhance the deep red colouring is not vital but well worthwhile. 

 

Coenobita brevimanus (Indos, brevis)

Opening shape preference: Round

 Indos are the biggest of the captive species and as they grow to bigger sizes require very large shells that can prove expensive. Lightly polished or natural turbos and other similar shapes are best, those taken back to pure mother of pearl are thinner and more prone to damage from these giants moving around. 

 

 

 

Before Using and General Care

 

Before putting your new shells in the tank give them a check over for damage and then boil them for five minutes. You can do this in fresh or marine salt water, the crabs do however show a preference for shells done in salt so this is worth considering. You can offer them all piled in one area/basket or on a tray to help stop them getting buried or as dirty.

 

Periodically take out the empty shells for at least a shake or rinse to flush out built up dirt, check them for damage and take out the ones that have been completely outgrown. Sometimes crabs will use spare shells as a toilet dump or hide old food in them, others will bury the spares so these checks are an important part of tank maintinence. 

 

If a shell begins to lose its lustre you can gently polish them with a dab of pure olive oil and some clean kitchen roll.