Coral reefs are home to a variety of creatures, including: crabs, many types of fish, shrimp, urchins, sponges, algae, etc. These organisms like to live on coral reefs because they are provided food from different organisms and are able to seek shelter from predators.  More specifically, there are crustaceans who dwell along corals for the same purposes, mainly to feed on bits and pieces of decaying substances or feeding on live organisms.
When it comes to describing crustaceans, the most common animals narrowed down into this category include crabs, lobsters, barnacles, and of course, shrimp. As mentioned before, crustaceans have a broad diet, and feed on many things, dead or living. Many kinds of crabs are omnivores feeding on a variety of organisms. Others are detritivores, which are organisms that feed on dead animals and plants.
There are bizarre shapes and adaptions in the world of crabs. This sea animal group is best known for their physical characteristics. What separates crustaceans from other species is their hard exoskeletons, which protect them from predators and hold water within the creatures shell. They also contain appendages which assist with locomotion, allowing them to gather food easily. Crabs, along with other crustaceans, outgrow their exoskeleton, and either mold into a new one, or like the hermit crab for example, must hunt for a new shell. Once crabs mold into a new shell, it has to harden. During the process of forming a new exoskeleton, crabs are without their main protection, making them vulnerable to other predators. A few of the main predators are the teeth of a triggerfish and the beak of an octopus, which can crack the toughest crab shells. But crabs still have their claw, which is capable of exerting hundreds of pounds of pressure. Some crabs can stun their prey by producing a deafening miniature sonic boom.
Common Crustaceans of the Virgin Islands
•Arrow crab: These bizarre looking creatures get their name from the triangular shape of their bodies. They have long legs that can reach a span of roughly six inches, giving them a spider-like appearance. They are known for pulling feather duster worms out of their tubes.
•Red hermit crab: This type of crab carries a shell on its back, and as it grows larger, it must continually find new shells. They are very good scavengers so it feeds on most anything it finds. The abdomen is soft and vulnerable, so it uses discarded shells to protect itself.
•Purple shore crab: This crab species is small and has a purple and red shell with a white underbelly. It grows to approximately two inches. It is commonly found on the open rocky seashores of the Pacific coast of North America and it can be seen scavenging the seashore for algae and dead animal matter to feed on.
•Land crab: This is another common creature found on the islands. These crabs are mostly gray, and are usually found burrowed in low areas along the coast.
Habitat and Biology of Island Crabs
Stenorhynchus seticornis, the arrow crab, is dispersed from the shores of North Carolina to Argentina. They typically thrive in locations in the Atlantic such as Florida and the Bahamas, as well as the coral reefs in the Caribbean. This crab species makes a long slow path across rock crevices, soft corals, and sponges along the reefs, as they cultivate themselves by feeding on small crustaceans and animals. Arrow crabs can also be seen on the deep sand floor, and shallower places containing and abundance of algae.
Circadian rhythms along with water temperature are very crucial to the breeding period, which occurs year round. The larvae of the S. seticornis differs from other crab species in that its anatomy is smaller compared to other types of larvae. The arrow crab doesn’t grow to be very big, and is composed of a thin reddish-brown platform with long, spiny, bright-red legs, limiting its speed. It is covered with a strongly calcified exoskeleton giving it protection.
Purple Shore Crabs
Purple Shore crabs can be found in shallow trenches along the shore and under large rocks. They are also high in certain estuaries. Their depth range is usually intertidal, and they are attracted to more sandy locations.
The Purple Shore crab has the scientific name Hemigrapsus nudus. This species ranges from purple to reddish brown, and its chelipeds (larger leg bearing) have purple spots as well as white tips. There is a size difference depending on gender, as males usually grow to be a little over 5 centimeters and females just over 3 centimeters. Purple shore crabs feed on different types of algae such as diatoms, which are stramenophila algae and desmids, which are green algae. These crabs are able to maintain constant osmotic pressure by varying salt and water concentrations, making them more specifically osmoregulators. These osmoregulaters can withstand hypo-osmotic and hyperosmotic environments. Females can carry up to 36,000 eggs, and are typically seen carrying them from January to early summer.
Hermit crabs are probably the most abundant species on land. They are found from mountains all the way to the tip of the shore. After growing out of their shells they temporarily head towards the ocean and head back towards the mountains at a given period. There can be thousands of hermit crabs, and they can cover almost an entire area of a beach.
While the adult hermits remain terrestrial, they must head toward the sea to release their larvae, waiting the baby hermits to migrate back the shore if they can survive in the ocean. These animals have very soft bodies which require a shell for protection. They are constantly finding new shells to replace their old one’s once they have grown out of them. They are typically a reddish color, while the shells they host can vary.
Recently, a new species of hermit crab called the candy cane crab was discovered near the island of Bonaire. Read more: http://www.treehugger.com/animals/candy-cane-crab-new-species-bonaire.html
This crab, called the "large crab" by islanders, inhabits swamps, wetlands, marshes, and other low-lying, secluded places across St. John. Although the entrances to their burrows may appear small, the holes get bigger as they get deeper. They store their food in the biggest chamber of their hole space, and keep it protected from the outside environment. Land crabs usually stay in their holes all day, and venture out at night to gather more food, returning before sunrise. Compared to other crab species along the islands, this particular crab is the most shy.
These tucked-away creatures, also known as Gecarcinus Lateralis, are gray, which gives them a camouflaged appearance on land. They typically grow to be four to five inches in length. They also have very bulky claws to protect themselves from predators.
Besides the danger of predators in the ocean, crabs along with other species face risk to natural destruction of coral reefs. Globally corals, known as the "rainforests of the sea" host many crustaceans, reptiles, seaweed, and bacteria, along with many more species. However, coral reefs are endangered by natural causes such as El Nino, which contributes to coral bleaching. Coral bleaching occurs when stress causes prolonged algae loss if the water temperatures are warmer than they should be. Local causes of stress include: overfishing, pollution, tourism, and increased levels of CO2 in the atmosphere. When things like this affect corals, that means it is affecting everything that usually thrives in them. Crabs are forced to relocate, and find it harder to gather food and feed off of organisms that were once available.
- Jones, Joshua. Marine Invertebrates of Bermuda. Article. http://www.thecephalopodpage.org/MarineInvertebrateZoology/Stenorhynchusseticornis.html
- Cowles, David. Hemigrapsus nudus. http://www.wallawalla.edu/academics/departments/biology/rosario/inverts/Arthropoda/Crustacea/Malacostraca/Eumalacostraca/Eucarida/Decapoda/Brachyura/Family_Grapsidae/Hemigrapsus_nudus.html
- Windspree Vacation Homes. Hermit Crab Migration on St John Island. Article. http://www.windspree.com/article/65-hermit-crab-migration-on-st-john-island
- Cho, Renee. Losing Our Coral Reefs. Article. http://blogs.ei.columbia.edu/2011/06/13/losing-our-coral-reefs/