Hermit crab

The hermit crab is a small crustacean belonging to the taxonomic superfamily Paguroidea. They are best known for their reclusive behavior from which they get their name. Although in possession of an exoskeleton similar to other arthropods, the hermit obtains additional protection and solitude by hiding in the shell of a sea snail (marine gastropods). When they grow too large for their current home, they search for another, often testing several for until they obtain the proper fit.

Anatomy
Antennae: The Hermit crab has two pairs of sensory organs used to get an idea of its surroundings. Antenna - the longer outer pair often called “feelers” used to touch and feel what is in the crabs path. Antennule - the inner “bent” pair which enables the crab to taste and smell (this is the pair that you touch gently with food when beginning to hand feed your crab).

Compound Eyes: Their eyes comprised of faceted lenses, which are especially adept at picking up small movements. Eyes are located on movable stalks.

Maxillipeds: Also referred to as mouthparts, are 3 pairs of appendages used to move food into the crabs mouth as well as for grooming.

Exoskeleton: This is the hard outer, protective layer which must be shed as the crab grows.

Chelipeds: (kēlu-ped) These are claws used to grasp and pinch. Large claw - the larger left claw which is mostly used for defense and to effectively seal off the opening in a properly fitting shell. Small claw - the smaller right claw is most often used in feeding and scooping water up to the crabs maxillipeds.

Pereiopods: Technically hermit crabs have 10 legs including the chelipeds. The 2nd and 3rd pair are walking legs used for movement. Gonopores are at the base of the 3rd pair of legs on a female. The 4th pair is stubby looking legs used to move in and out of shell. The 5th pair is gill grooming appendages that end in pinchers for cleaning the gills and removing waste from the shell. Male crabs have tufts of hair that hide openings at the base of the 5th pair of legs.

Cephalothorax: The head and back region on a hermit crab.

Carapace: The hard outer shield consisting of an exoskeleton covering the cephalothorax.

Setae: Sensory hairs found on the exoskeleton.

Gills: Organs which enables the crab to breath. They have advanced gills located between the 4th and 5th periopods housed in the branchial chamber which must be kept moist to function.

Abdomen: Or tail as it is commonly called. This is the soft portion of the crab protected by a shell and contains the digestive and reproductive organs.

Pleopods: Feathery looking appendages located on the left side of a female’s abdomen that are used for carrying eggs.

Uropods: Appendages located at the tip of the abdomen that are used to secure the crab in its shell.

Telson: The tip of the tail ending in the anus.

Reproduction
Hermit crabs tend to reproduce many in order to maintain their population. The female protects her fertilized eggs underneath her "abdominal flap" or "apron". This is a V-shaped "hatch" that is on the underside of the crab. Males have one too, but theirs is much narrower because he doesn't need to hold thousands of eggs.

Crabs, in general, are sexually dimorphic with the females growing larger than the males. Hermit crabs are an exception, because the males tend to grow larger than the females. The male has one larger claw that is used for competing for a female with other males. When a male chooses him mate, he grabs her by the shell, guarding her from other potential mates.

During the mating ritual the female's eggs mature and eventually are laid through a duct deep inside the shell called a gonopore. As they are laid they are glued to the soft abdomen of the mother. The male hands her a sperm packet and she tucks it in her shell with her claw. After the eggs are fertilized the male releases her and leaves to find another mate.

Once the eggs are ready to hatch, the mother pulls herself out of her shell as far as possible and, with her claw, fans out the larvae into the water. The larvae are planktonic and spend up to two weeks metamorphosing into the adult form. Once they settle to the bottom they must quickly find a tiny, empty snail shell to live in or they will die.

Ecology
Feeding: Hermit crabs will basically eat anything they can get their claws on, or they will starve. Anything you will eat they will eat, minus the sugary and highly-processed foods. They like meat, vegetables, fish, and fruit. Even citrus fruits! 

Hermit crabs are usually found in tropical coasts, like in Southern Florida and land masses in California. The majority of hermit crabs live in the sea, but some have adapted to living on land.

The hermit crabs as a transportable "home", also known as its shell, which provides shelter and protection from predators. It has five sets of legs and two claws. One of these claws is bigger than the other and is used to seal up the opening after backing in to it's shell. The other smaller claw is used to eat by carrying small pieces of food to it's mouth. 

The Shell
Hermit crabs choose their habitat by the variety of criteria is expected to match effective selection patterns. Their exoskeleton is not enough protection from all the animals that would gladly enjoy a hermit crab as its next meal. It then inserts is abdomen into a suitable, empty gastropod shell. The suitability of the shell depends mostly on the size. It needs to be able to get it's whole body into it so that when a predator comes along, it can put all parts of its body in the shell and seal it with its larger claw.

Hermit crabs have specific preferences on which type of habitat suits it best. Within that habitat, the crab searches for the correct gastropod shell to occupy. They are very picky and compare their shell they are in now and the one they are examining to determine if it is just as good or better. Then it crawls out of it's shell and into the new one. Since large shells are hard to find  the competition for the shells are fierce. They try and pull each other out of the shell they are in, because generally one crab is bigger than the other so it wants to bigger shell, but then the bigger crab is completely without a shell making itself an "easy meal".