11.+Arthropoda+-crustacea

=__Hermit Crabs__=

By Rithvik Pottepalem


1. Classification/Diagnostic characteristics


 * Phylum : Arthropoda
 * Subphylum: Crustacea
 * Class: Malacostraca
 * Order: Decapoda

Arthropods (and their relatives) are ecdysozans that have paired appendages as shown by the picture above. This characteristic gave them the clade its name from the greek word arthron, which means joint and pods means "foot" or "limb". Crustaceans bodies are divided into three parts: head, thorax, and abdomen. Segments of the head are fused together, and the head bears five pairs of appendages. Each other part of the body bears one pair (usually) and crustaceans can have multiple thoracic and abdominal segments. Additionally, in many species, a fold of the exoskeleton, the carapace, extends dorsally and laterally back from the head to cover and protect some of the body segments. Crustaceans are the most dominant population wise of the marine arthropods that exist. [|https://www.britannica.com/animal/crustacean#toc33800]

2. Relationship to humans Many humans have hermit crabs that they keep as pets. Hermit Crabs are also used for research. At Tufts University, it was found that hermit crabs can locate new and housing using previously unknown social networking skills. Hermit carbs are able to find the best shells when they need a new home. Real Estate agents believe that they will be able to use the same skills that hermit carbs use to find better home for their customers. @https://now.tufts.edu/news-releases/social-networking-hermit

3. Habitat and niche Hermit crabs tend to live in shallow water such as tide pools and salt marshes when long-clawed, while broad-clawed crabs usually live in deeper water. They are omnivores and in bottom-dwelling communities act as scavengers, by adding in the process of recycling energy back into their ecosystem. Hermit crabs, when they mature into an adult, tend to live on land. As it becomes an adult, it slowly loses the ability to breathe underwater. [|http://www.cell.com/current-biology/fulltext/S0960-9822(07)02354-8 (1)]

4. Predator avoidance Hermit crabs in suboptimal shells often have to resort to initiating a ‘shell fight’ in an attempt to take a better shell from another crab. Hermit crabs have to size up the quality of the opponent's shell and have to assess the fighting ability of their opponent. The encounter involves two distinct roles, ‘attacker’ and ‘defender’. An attacker in a suboptimal shell will initiate a fight against a usually smaller defender in possession of a better shell. Because shells are critically important for both sexes, males and females engage in agonistic encounters over their ownership.

- Fights usually last for between eight and ten bouts. The encounter ends when a decision is made from the defender to relinquish its shell, allowing the attacker to evict it by pulling it out through the aperture; or the attacker may give up, and release the defender, without effecting an eviction. If the attacker is successful, it will perform the usual investigatory activities on the vacated shell and decide whether to occupy it permanently.

- Hermit crabs living on land are nocturnal. Since their enemies are all active during the day, they are active during the night to avoid them.

In order to protect its soft body, Hermit crabs wear the discarded shells of other animals. However, this is not exclusive to a shell. They have been found wearing everything from a plastic bottle cap to an airline liquor bottle. []

5. Nutrient acquisition A hermit carb's appendages can be used to capture food. They often grasp their prey with their claws, tear, grind, and shear with their mouth. Hermit carbs eat their prey alive or scavenge. @https://www.sciencedaily.com/releases/2010/04/100426141458.htm

6. Reproduction and life cycle A hermit crab's life cycle begins with the release of of eggs into the ocean tidal pool. Then, the hermit crab larvae (aka the zoea) attaches to a plankton until it develops into a form that visibly resembles an adult hermit crab. Once mature, the zoea journey to land in search for a shell to protect their soft, vulnerable abdomen. When the hermit crabs have reached land they reside in areas about 1-2 miles away from the shore, and develop gills, which allow them to breathe air, through metamorphosis. Hermit crabs have the ability to regenerate limbs during the molting process-which is when shed their exoskeleton and infuse new tissues with moisture, thus forming a new, bigger exoskeleton. After each molt the hermit crab must find a larger shell to protect themselves.

For crustacea in general, most species include two sexes, but many individuals, especially of barnacles, can possess both sets of reproductive systems and transition into fertile females, even if they start as men. There are wide differences between the structures of males and females in crustacea which can lead to complications during reproduction. Most crustacea reproduce by fertilizing eggs with sperm, but some can produce eggs that develop without the need to be fertilized. Fertilization can also occur outside of the body, in which female crustacea release their eggs directly into the water and are fertilized by sperm outside of the body. [|crabstreetjournal.org/blog/2013/02/21/what-is-the-life-cycle-of-a-land-hermit-crab/]

7. Growth and development
 * At hatching, the young of some species are born as larvae, but there are also species that release their young as juveniles that are similar to adults in form
 * Others release their eggs into the water or attach them to an object in the environment
 * Despite being crabs, hermit crabs do not have their own protective shell or other form of protection and thus must inhabit the empty shells of snails. As hermit crabs grow, they often outgrow the shell they are in and need to abandon that shell for a larger "home". Scientists have observed hermit crabs participating in collaborative behavior where they will group together and take over the shell of one crab as it abandons it for a larger crab in an orderly line and process.

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8. Integument []
 * Rigid exoskeleton known as a cuticle.
 * contains stiffening materials everywhere except at the joints
 * However, this exoskeleton cannot expand
 * They also have a shell, which they will often trade with other hermit crabs. This process is extremely interesting because the way the hermit crabs communicate and socialize is very similar to human activity.

9. Movement Using their legs, hermit crabs have the ability to move forward and backward in contrast to the side-to-side motion of other crabs. For burrowing, climbing, digging, and walking, they hold their abdomens up as they travel in search of good hiding spots or sniffing out other dead crabs to loot the abandoned shells. During molts, they can appear to be immobile for weeks on end, and they are actually immobile when aquatic temperatures get too cold, recalling their ectothermic nature. Hermit crabs are also nocturnal, so it is rare to see or hear much movement during the day, lending to the somewhat characteristic clickety-clack up and down the beaches in tropical areas at nighttime. http://animals.mom.me/hermit-crab-move-1375.html [|https://www.britannica.com/animal/crustacean/Form-and-function-of-internal-features#toc33812]

10. Sensing the environment []
 * crustaceans heavily rely on their various appendages for sensing their surroundings.
 * They have eyes which are attached to eye stalks that can move. The eyes have many lenses which allow the hermit crab to see many pieces of an image.
 * They also have antennae to gather information for the surroundings.
 * Hermit crabs have an excellent sense of taste and smell

11. Gas exchange []
 * Crustaceans can exchange gas via certain appendages
 * Hermit crabs have gills but are terrestrial creatures, so as long as their gills are moist, they can breathe out of the water. Their articulating plates seal their gills to keep them moist. Scientists think that when air flows over a terrestrial crab's gills, it forms bubbles, which the crab then blows out.

12. Waste removal [|https://www.britannica.com/animal/crustacean/Form-and-function-of-internal-features#toc33812]
 * waste products are excreted via urine through the antennal glands, or via feaces through their anus into shells
 * In order to move the feaces outside, hermit crabs bump their abdomen around their shell

13. Environmental Physiology []
 * Dwell in water but the exoskeleton allows for terrestrial invasion
 * If the hermit crab is terrestrial and adult, it has gills that allow them them to breathe in moist air. These gills can no longer allow them to breathe in oxygen from the water.

14. Internal Circulation []
 * Internal body cavity is a hemocoel, or "blood chamber," in which fluid from an open circulatory system bathes the internal organs before returning to blood vessels
 * Hermit crabs lack a heart

15. Chemical control (i.e. endocrine system) Hormones produced in the X-Organ, in the eye stalks, influence blood glucose levels, gonad development, and moulting. [|Source] []
 * Food is digesting internally
 * Endocrine System: molting cycle that occurs the entire life of hermit crabs
 * in order to increase in size, they need to shed their exoskeletons. The molting process occurs immediately under the molting hormone: 20-hydroxyecdysone (20-HE)
 * This process starts in vivo and in vitro

**__Review Question__** 1. What is the natural habitat of hermit crabs? Hermit crabs tend to live in shallow water such as tide pools and salt marshes when long-clawed, while broad-clawed crabs usually live in deeper water.

2. How do hermit crabs avoid predators? Hermit Crabs often use their shells to protect themselves from predators. They hide in their hard shells to keep predators from getting to the more vulnerable parts of their body.

3. How do Hermit Crabs remove waste? Waste products are excreted via urine through the antennal glands, or via feces through their anus into shells

4. What part of the body do hermit crabs rely on the most? Hermit Carbs rely on their appendages mostly to sense their environment.

5. How do hermit carbs exchange gases? Crustaceans can exchange gas via certain appendages. They also have gills and can use them to exchange gases as long as they are moist.