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It's amazing that after so many years of diving your Science Instructor Development Course was able to completely change the way I look at coral reefs. Taking the 'Coral Reef Architecture & Organims' course was like switching on a light. I now even look at organisms like turf algae in a completely new way.

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Yellowline Arrow Crab - Stenorhynchus seticornis PDF Print E-mail




Family: Majidae
Species: Stenorhynchus seticornis
Species ID: M.SS

Description: A small, spider-like crab with a triangular body. This species has a serrated spine rising sharply up from between the eyes (1), long thin legs (2), and two small purple claws (3). The body is typically reddish brown and covered in fine white or yellow lines. Sexes appear similar, but females can be distinguished from males when carrying eggs under their body 

Maximum Size: 6 cm (2.5 in) not including the legs
Longevity: Up to 5 years 
Status: Not currently on the IUCN endangered species list
Yellowline Arrow Crab & People: These crabs are much too small to consume, but are widely collected for export due to their popularity in the aquarium trade


Geographical Range: Common throughout the shallow waters of the Caribbean
Coral Reef Zone: These crabs are found in all coral reef zones, except the reef crest zone
Favourite Habitat: Arrow crabs prefer rocky areas of reef with many hiding places, but can also be seen inhabiting sponges, and sheltering under anemones and within the spines of urchins
Depth: 3–40 m (10–130 ft)

A Day in The Life

Dawn: Arrow crabs return to their shelter from nearby foraging areas
Day: Arrow crabs remain hidden and inactive during the day, but will emerge to defend their shelter from intruders
Dusk: Arrow crabs leave their shelter for the evening
Night: These crabs are at their most active as they forage, defend territories and mate

Who Eats Who?

The arrow crab is an omnivore that combs the reef for algae, tube worms, bristleworms and dead organisms. This crab is eaten by a wide range of reef predators including grouper, puffers, triggerfishes, wrasses and grunts among others.

Scuba Diver & Snorkeler Best Practices

Refrain from feeding marine life Coral reef organisms should never be fed. Although this may seem like a harmless practice that allows you to get close to your favourite organisms, it actually disturbs normal feeding patterns and diets. Scientists have documented turtles being fed bread, dog food and even cheese—none of these foods are found naturally in the marine environment, and they can cause untold stress to the organisms that consume them. Conditioning wild animals to become comfortable with hand-feeding by humans alters their behaviour and makes them more vulnerable to capture, which directly affect their survival—this is particularly a concern for endangered sea turtles. 



Arrow crabs are easy to approach at night when they come out of hiding to feed, and are unafraid of divers. Although they remain hidden in crevices during the day, their natural curiosity means that they can sometimes be lured from their holes by waving your fingers at them.

Feeding Behaviour
Omnivorous arrow crabs emerge at night to search the areas near their shelter for food, which can consist of algae, detritus, and occasionally tube worms as well as bristleworms. When they find a tube worm, arrow crabs are known to use their narrow claws to reach into the tube and pull out the worm. Once a crab has captured prey, it is held up to the mouth while the crab feeds. Arrow crabs that cannot find large prey will pick at the surface of the reef with their claws to find detritus or bits of food discarded by other animals. While they forage, arrow crabs also tend to accumulate debris on their legs, and will periodically stop to scrape off this potentially nutritious debris with their claws and eat it.

Observe, record & share:

O M.SS-101 – Picking: Arrow crabs can be seen picking at the reef with their claws for small bits of food
O M.SS-102 – Grooming: Arrow crabs use their claws to scrape accumulated debris off the ends of their legs and feed it into the mouth
O M.SS-103 – Extracting tube worms: Arrow crabs pull tubeworms from their home
O M.SS-104 – Feeding on bristleworms: Arrow crabs use their claws to capture and consume bristleworms crawling on the reef

Attack & Defense
Arrow crabs are fiercely territorial and will defend their home from neighbouring arrow crabs and other reef inhabitants. Arrow crabs that approach another’s territory are greeted with a threatening display where the pinching claws are held up and away from the body. If the intruder does not retreat, the crabs will attack each other with their claws and grapple in a tangle of legs. If approached by a larger animal or predator, the arrow crab will escape to its shelter and use its spines and claws to defend itself.

Observe, record & share:

O M.SS-201 – Claw display: A threatened arrow crab will spread its claws and hold them stiffly away from the body while facing the threat
O M.SS-202 – Grappling: Crabs will grasp each other’s legs with their claws and fight
O M.SS-203 – Pinching: Arrow crabs that are molested will pinch with their sharp claws

Reproductive Behaviour
Arrow crabs reproduce sexually and do not undergo sex change during reproductive development. During reproduction either the male or female arrow crab must leave its territory to find a mate. This is the only time divers and snorkelers are likely to see two arrow crabs coexisting peacefully in the same territory. After a poorly understood courtship ritual, male crabs hold female crabs against their abdomen and transfer a small package of sperm which the female can store for later use. When the female’s eggs are ready for fertilization, they rupture this package to release the sperm. Eggs are carried for one to two weeks in a small scoop-like flap under the body, and females periodically pump this flap or stir their eggs with their claws to provide them with oxygen and clear away debris until they hatch. Arrow crabs reproduce year round with a peak in the spring from March to April.

Observe, record & share:

O M.SS-301 – Pair: Seeing two arrow crabs coexisting peacefully is a sign of mating activity
O M.SS-302 – Female carrying eggs: Eggs are cream-coloured and can be seen in an open flap beneath the female’s body
O M.SS-303 – Female cleaning eggs: Female picks away debris and dead eggs from abdominal flap with her claws
O M.SS-304 – Female mixing eggs: Female pumps the abdominal flap or mixes eggs with her claws
O M.SS-305 – Mating embrace: Mating arrow crabs press their abdomens together

Highlight Behaviours

Egg care: Females carry eggs in a scoop-like flap under the body (1), which allows them to keep a close watch over them and protect them from predators until they hatch. Female arrow crabs carrying eggs are excellent mothers and constantly fuss over the eggs in order to keep them clean and healthy. They pump the abdominal flap up and down to fan the eggs with oxygenated water and often mix them with their claws and pick away debris and dead eggs.

Did You Know?

• Like other crustaceans, young arrow crabs moult every few months as they grow to replace the old, tight shell with a new, larger one. This continues until the crab reaches its maximum size. Moulting occurs at night, when the pale, soft-bodied crab shrugs out of the old shell.
• The largest female arrow crabs can carry nearly 200 eggs at once.
• Arrow crabs occasionally “decorate” their body spines with bits of algae and sponge. This decoration helps camouflage the crab from predators.

What to do?

Share your observations today!: Discover your species of interest, observe its behaviour, and share your picturesand videos with friends and coral reef enthusiasts around the world! Upload media to the web, tagged with species common name (ex.: trumpetfish) and species ID code (ex.: A.AM) or species behaviour code (ex.: A.AM-101)

Observation Key

O Easy
O Not so easy
O Tough