Think Tank's hermit crab exhibit and the associated panel "Shell Seekers" provide visitors with an opportunity to explore the definition of tools and the evidence of thinking during tool use.
Distribution and Habitat
Land hermit crabs of many different species are found in tropical areas of the Indo-region, the western Atlantic, and the western Caribbean. They live close to the shoreline and must have access to land and water. There are also entirely aquatic species of hermit crabs.
The front half of a hermit crab is covered with a hard exoskeleton (hard outer skin). The long abdomen has a softer exoskeleton, that can adjust to fit into a spiraled snail shell. A crab presses its abdomen against the shell's inner wall by contracting its longitudinal muscles. The large left claw is used for defense, for holding onto tree limbs, and for balance. The smaller right claw and the next pair of appendages are used for collecting and passing food and water to the mouth. Hermit crabs have stalked eyes with acute vision and two pairs of antennae. The longer pair of antennae is used for feeling, the shorter for smelling and tasting.
As a hermit crab grows, it molts, shedding its exoskeleton
and creating a new, larger one. A crab builds up enough
water pressure in its body to split the old skin. A
crab may eat its molted skin, possibly for its calcium,
vitamins, and minerals.
Hermit crabs use shells for protection from predators and
dehydration. A hermit crab has a ranked hierarchy of fixed
motor patterns that it goes through when choosing a new shell.
Image and intention may be part of the pattern, but that does
not mean thinking is occurring. The steps are always the same,
in the same order, and show no flexibility.
In the wild, hermit crabs are omnivorous scavengers, meaning
they eat whatever they can find. At the Zoo, they are fed
fish food, crab food, and various fruits and vegetables.
Land hermit crabs live about ten years.
Hermit crabs mate in saltwater. Both males and females partially
emerge from their shells to mate. After the eggs hatch, the
larvae go through several aquatic life stages and molts. When
they reach adulthood, the crabs migrate to begin their terrestrial
Land hermit crabs are good climbers and should be kept in
a covered container. They need a good supply of empty shells
to try on after molting. Hermit crabs also need high humidity
and should be able to go into water. At the Zoo, petroleum
jelly is used around the edges of the tank to keep ants out.
Hermit crabs are not aggressive, but the large, purple claw
can give a nasty pinch — so zookeepers handle them with
Caring for Hermit Crabs
The National Zoo does not encourage purchasing any type of exotic pets. However, if you do own hermit crabs, we encourage you to read through the following information to best care.
Providing hermit crabs with the right type of housing and food can help them live for several years.
Here are several guidelines to keep a hermit crab happy and healthy.
- Housing: Glass aquariums (10 gallon size minimum) with mesh and glass lids are the very best homes for hermit crabs. Plastic containers commonly marketed for hermit crabs are not good because they cannot hold in heat or humidity like glass can. Aquariums and mesh lids are available in most pet stores, and you can have glass cut to fit the top at most home improvement or hardware stores.
- Substrate: Substrate is the hermit crab equivalent of bedding. Hermit crabs need to be able to tunnel under the surface in order to find a safe place to grow (this is known as molting). The best substrate for doing this is actually a mix of common play sand (found in home improvement stores) and cocofiber (sold in most pet stores). Use about four times as much sand as cocofiber, and wet the mixture with marine salt water (discussed below) until it is sand-castle consistency. At least four inches of substrate is necessary, with more needed if you have really big crabs.
- Heating: The best type of heating for hermit crabs are heat pads (commonly sold in the reptile aisle in pet stores) that are ideally attached to the side or back of the glass aquarium.
- Gauges: You will need both a temperature gauge and a humidity gauge. Both can be bought at most pet stores. The temperature needs to be kept between 73 and 80 degrees Fahrenheit at all times, although it can fluctuate within the range throughout the day. The humidity needs to be at least 75 degrees, because unlike humans, hermit crabs breathe through modified gills, and these gills can’t work if they are not moist.
- Water: Hermit crabs need access to two types of water to thrive. One is fresh water, but if you are using chlorinated tap water, you will need to buy a dechlorinating product at a pet store to take out the harmful trace metals. The second is marine salt water, made with either “Instant Ocean” or “Oceanic.” DO NOT use table salt. This is poisonous to hermit crabs. Provide both waters separately, in dishes deep enough so that your biggest crab can fully submerge, but with an exit, such as a branch or rock, so that your smallest crab can get out.
- Food: Commercial pet foods that pet stores will provide basic nutrition, but avoid products with preservatives, such as BHA and BHT. Hermit crabs can eat many of the same foods that humans do, but it is important that their food never have salt or seasoning in it. For a well rounded diet, they need to be regularly supplied with a variety of fresh (or dehydrated) fruits or vegetables, grains (like plain oats, cooked wheat pasta, whole grain rice), meat (small pieces of unseasoned or salted human grade fish, poultry, or beef), other proteins (freshly dead crickets, mealworms, or krill from a pet store), dried or fresh leaves and flowers (that have never been exposed to pesticides), unsalted nuts, and seaweed. You can experiment with extras to feed your hermit crabs, just be sure that whatever you try it is all-natural, preservative, pesticide and salt/seasoning free.
- Shells: Hermit crabs need extra shells to change into and bigger shells to row into. It is easiest to find proper hermit crab shells online.
- Friends: Despite their name, hermit crabs are social animals! They love having friends are to hang out with, so never have only one.