Hermit Crab

 

Sim B.  Janay W.  Cassie P.  Ashley T.

 

4th Period

 

 

I.          Scientific name /Common Name(s)

Cenobita (sp.)/ Hermit Crab

 

II.         Drawing / Picture with the anatomy labeled

     

     

III.       Description / Characteristics

     

            Hermit crabs are arthropods and like all other arthropods, they have jointed limbs and segmented bodies. Their bodies are separated into three regions: a head, thorax (mid-body), and abdomen. The head and thorax are covered with a very hard exoskeleton. The abdomen however is very soft and vulnerable and contains the liver and gonads. For protection the abdomen and tail can be flexed and twisted to fit into the burrows of a shell. Two pairs of small, hook-like limbs or legs make it possible for the hermit crab to anchor its body into the shell. A Hermit Crab's other two pairs of legs are used for walking and climbing. A hermit crab has two claws called chelipeds. The claws are the crab’s most important appendages. Crushing mouth parts called mandibles as well as the smaller claw, are used for eating. The much larger claw is used for protection. In case of danger, a hermit crab will retreat inside of his shell and use his big claw to block the entrance. The large claw is very strong and a hermit crab is capable of cracking open coconuts with it. The crabs have gills on their large claw in order to conserve moisture but these gills must stay wet in order for a hermit crab to survive. A hermit crab cannot survive for very long away from water.  If a hermit crab loses an appendage it is able to grow it back by regeneration.

      A hermit crab’s eyes are located on the ends of eyestalks that can move in all directions. They have compound eyes which means that they consist of many tiny lenses. As well as eyestalks the hermit crabs have 2 sets of feelers. The two longer feelers are called antennae and the two shorter feelers are called antennules. The antennae are located on the outside of the eyes and are used to touch objects. The antennules are located between the hermit crab’s eyes and are used to pick up odors and find food. Hermit crabs do not have ears but they can sense sounds using these feelers.

      Hermit crabs usually occupy the empty shells of periwinkles, whelks, and other gastropod mollusks. Often a colony of polyps will grow on the surface of the shell forming a rough, brown layer. Sometimes a hermit crab will have sponges or sea-anemones living on its shell that camouflage it and also eat scraps of the hermit crabs food.

      When a hermit crab outgrows his shell it is time to look for a new one. A crab is very selective in picking its home.  It will touch the new shell with its legs and antennae before trying it out. If it likes the shells, it will quickly and nervously move inside because this is the time when it is most vulnerable to predators. Hermit crabs may move into a shell and then later decide that they don't like it and will search for a new one. If a crab moves into a new shell it can move the sponges and sea anemones from the old shell and replant them on the new. Hermit crabs have been known to make their homes out of other objects besides shells such as empty tuna cans and sections of old bamboo cane.

            Hermit crabs are very social and active in the wild as well as in captivity. In the wild they normally travel in groups of up to 25 crabs. In captivity, hermit crabs liked to be played with frequently. They also enjoy bath and being dipped in the water. However, do not leave a crab submerged in water for more than 5 minutes because they can drown.       

       IV.      Natural Habitat

 

       Hermit crabs thrive in tropical regions such as the Caribbean, South and Central America and parts of Australia.  Although they are born in the ocean near the shore, hermit crabs migrate inland once they are adults. They dwell in rocks and crannies in beach side rocks, as well as in trees.  The name "hermit crab” is actually misleading since they live in colonies of up to 100 or more and are very social.

     Hermit crabs are nocturnal creatures.  The tropical sun as well as predators are dangers to them.  Hermit crabs comb the beaches en masse at night for remains of fish and vegetable matter.  They often live in symbiotic relationships with other denizens of the coast such as coral polyps, anemones, and sponges, which live on their shells and provide protection and camouflage in return for free meals gleaned from the crabs foraging.

     Hermit crabs can live up to thirty years in the wild.  They have fairly rudimentary lungs and can not survive long away from the water, so they are well suited to the rocky shores and tide pools they call home.  Hermit crabs often change shells in the process of growing.  In the wild, they have been known to use things as diverse as tuna cans, bottle caps, and pieces of plastic as substitute shells.

 

V.        Life Cycle/ Reproduction

 

      The reproductive organs of hermit crabs are located near and just below the crab’s heart and open to the outside at the base of the last pair of walking legs in the male. In the female, they’re located at the base of the middle pair of walking legs. Female hermit crabs usually lay their eggs shortly after copulating,  however they can also store sperm for many months. The eggs are fertilized as they are laid by passing through the chamber holding the sperm. The eggs are carried and hatched in a mass attached to the abdomen inside the shell.

      The number of eggs is usually large, but depends on the crab’s size. The developing crabs go through four stages, two of which (the baupilus and protozoea) occur while still in the egg. Most crabs hatch at the third stage, the zoea. This is a larvae stage wherein the crab has several long spines, a long narrow abdomen, and large fringed antennae. The fourth stage of development is the magelops.

      Hermit crabs are usually born in the ocean, near the shore. Because of this, hermit crabs cannot reproduce in captivity. After the crabs are born, they move inland away from the water, where they search for abandoned shells to inhabit. Hermit crabs then begin growing and developing through a process call molting. In this process, the crabs sheds their exoskeleton. During this, the crabs are extremely vulnerable and inactive, and usually find protection by burrowing in the ground. It takes around 10 days for their new exoskeleton to harden, and during this period the crab is able to regenerate any lost or broken claws or legs. A hermit crab can molt as often as every other month when they’re young, or every 18 months when they’re older. The life span of the hermit crab in the wild is up to 30 years, while those in captivity live 15 years at the most.

 

VI.       Diet/Feeding Behavior

 

The diet of hermit crabs in the wild usually consists of either small fish or worms found on the sea shore or bottom, or scavenged dead or decaying material. Otherwise, household hermit crabs will pretty much eat whatever is given to them. You can buy commercial food such as crab cakes or pellets, but other foods are also healthy and enjoyable for the crabs. They enjoy peanut butter, pieces of fruit such as apples and bananas, lettuce, and bits of crackers and bread.

            Hermit crabs eat very little, and any uneaten food will begin to rot and smell inside the terrarium. Because of this, all food should be given fresh each day and all old food cleaned out. Water is best if put in a clam shell, because the calcium in the shell enriches the water and makes it healthier for the crabs. Hermit crabs can go up to 10 days without food, and usually need around 1/4”-1/2” of food every 2-3 days.

 

VII.      Special Requirements for classroom

 

      The temperature of the Hermit Crab’s environment must be between 70 and 85 degrees Farenheit, so either the thermostat in the classroom must read this or a heating pad needs to be placed under the terrarium. The sand in the tank must be cleaned once a month and kept moist at all times. Hermit crabs do not like sloppy wet tanks, but instead prefer a dry grouind and moist air. Hermit crabs are nocturnal animals, and they only come out and move around when it’s dark, so there must be a set period of darkness in the cage every 24 hours, preferably at nighttime. In other words, do not put a light in the terrarium. Make sure there is a sponge in their water shell so they won’t drown, and also an added feature could be a piece of wood for them to climb on.

 

VIII. Bibliography

 

Amanda's World of Hermit Crabs.  11 September 2000 http://members.aol.com/mspres2020/

 

Encarta Online Encyclopedia. 9 September 2000 http://encarta.msn.com/find/concise.asp?+i=06A24000

 

Franklin College. 9 September 2000 http://www.franklincoll.edu/tlcweb/crab/

 

Jeremy’s Hermit Crab Page. 9 September 2000 http://www.geocities.com/Heartland/Flats/5913/hermit/

 

McDonald, Meagan. Is This a House for A Hermit Crab. New York: Schindler, 1990.

 

McLeod, Lianne. How to keep Hermit Crabs. 8 September 2000 http://exoticpets.about.com

 

North. 7 September 2000 http://www.xs4all.nl/~pal/hermit.htm

 

Scott’s Hermit Crab Page. 12 September 2000 http://www.geocities.com/Heartland/Plains/9411/

 

Walls, J.G. (1982). Encyclopedia Of Marine Invertebrates. New York: Tfh Publications Inc., Ltd 2ND edition, 1982.