Crabs

Crabs

Crustaceans

Coral reefs are home to a variety of creatures, including: crabs, fish, turtles, eels, shrimps, sharks, urchins, sponges, and algae. These organisms like to live on coral reefs because they provide food and shelter from predators. Animals can hide in the cracks and crevices of the reef, created by corals.

Crabs help clean coral reefs by eating little pieces of food dropped by other creatures and eating dead animals. Many kinds of crabs are detritivores, which are organisms that feed on dead animals and plants.

Almost every living thing on a coral reef is food for another organism. This creates a complex food web.

There are bizarre shapes and adaptions in the world of crabs, also known as crustaceans. This sea animal group is best known for their hard outer shells and as it grows the shell must be removed. After this the new shell has to harden and during this period crabs are without its 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 the 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 found on coral reefs throughout the Virgin Islands:

Arrow crab

• Arrow crab: These are bizarre looking creatures who get their name from the triangular shape of their bodies. They have long legs that can reach a span of about 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 its shell (home) 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 and grows to be about 2 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.

The land crab is another common creature found on the islands. It is specifically known in St. John as the "large crab". These crabs are mostly gray, and are usually found burrowed in low areas along the coast.

Habitat and Biology of Island Crabs

Arrow 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

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.

Land Crabs

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. NOTE: Clarify, do the crabs have specific parts/chambers in their burrows where they keep food? Land crabs usually stay in their holes all day, and venture out at night to gather more food. 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.

Main Questions

1. How do crabs help clean coral reefs? By eating little pieces of food dropped by other creatures

2. What are detritivores? Organisms in coral reefs that feed on dead animals and plants

3. What are the 3 most common types of crab found on a coral reef? Red hermit crab, arrow crab, purple shore crab

4. What is the land crab known as in St. John? The “large crab”