Tuesday, 27 September 2011

Yellowline arrow crab (Stenorhynchus seticornis)

Image showing an arrow crab (Stenorhynchus seticornis)
Yellowline Arrow Crab
Notice the distinguishing violet claws
Kingdom: Animalia
Phylum: Arthropoda
Class: Malacostraca
Order: Decapoda
Family: Inachidae
Genus: Stenorhynchus
Species: Stenorhynchus seticornisni
Conservation Status: Not assessed
Common Name(s): Yellowline arrow crab, arrow crab, spider crab

The yellowline arrow crab, is a strange, spider-like species of marine crab, occurring in the western and eastern Atlantic Ocean, from North Carolina and Bermuda to Brazil, including the Caribbean Sea, and in the coastal waters of Cape Verde. Individuals are commonly found on coral reefs, in depths ranging from 3 to 9 m (~10 to 30 ft).

The arrow crab has a triangular body with 8 thin and long spider-like legs and a pointed head. The legs are up to 10 cm (3.9 in) across, whereas the carapace is 3 to 6 cm (~1.1 to 2.5 in) long. The body and rostrum end up into a long point with serrate edges. The abnormally long legs have typically more than three times the length of the body.

There is some color variation between individuals. The body may be golden, yellow or cream, marked with brown, black or iridescent-blue lines, while the legs may be reddish or yellow with blue or violet claws. Their body is golden or yellow, accompanied by black, iridescent blue and white stripes.

The species is characterized by sexual dimorphism in regards to size, with females being smaller than males.

Like most crustaceans, they shed their exoskeleton as they age. The new skin hardens with calcium carbonate, which is acquired from sea water and by ingesting their old shell . You can see part of this process in the video below:

A pet arrow crab molting its shell

They are nocturnal animals that hide during the day under ledges, sponges and sea fans, coming out during the night to scavenge. They are also territorial and will respond aggressively to animals intruding their area. Some say they are docile towards divers, but this is purely anecdotal.

The species is commonly found living in association with Lebrunia danae, a sea anemone. It occurs frequently inside the anemone's pseudotentacles along with Pederson's cleaning shrimp (Ancylomenes pedersoni) and the spotted cleaner shrimp (Periclimenes yucatanicus).

Diet and Predators
It is mainly a scavenging animals, but sometimes exhibits carnivorous behavior, feeding on small feather duster worms (Sabellidae) and other small invertebrates of the coral reefs it inhabits. Some known arrow crab predators are the following:
  • Groupers
  • Puffers
  • Trigger fishes
  • Wrasses
  • Grunts
  • Sea stars

Sabellidae worm, most probably of the Sabellastarte genus
Sabellidae worm, probably Sabellastarte sp.

Reproduction & Life Cycle
During the mating period, the male places a spermatophore (a mass that contains the sperm) on the female, which will later fertilize her eggs. The female then carries the eggs under her abdomen until they hatch. The newborn hatchlings are zoea larvae and have a round transparent body.

The zoea immediately start swimming to the surface. There, they start feeding on plankton, until they reach their next developmental stage - by shedding their shell numerous times -, called megalops. It is in this form that they start to actually look like actual crabs. Still, they have to change their exoskeleton multiple times before they start looking like a mature arrow crab.

Close up video, showing an arrow crab eating hair algae from a rock

Conservation Status
There is little data regarding the conservation status of the arrow crab. However, it doesn't seem to be endangered or threatened, although theιr populations are presumably reduced, thanks to collection for aquarium trade.

Arrow Crabs as pets
The species is considered a relatively easy animal to care for, with many individuals keeping them as pets in aquariums. They are commonly kept in reef aquariums to control bristle worm populations.

The price for a live one apparently varies greatly, starting from $11 and up to $800!

Image of a Yellowline arrow crab

Interesting Facts About the Yellowline Arrow Crab
- The species was first described by German naturalist and entomologist Johann Friedrich Wilhelm Herbst (1743 -1807) in 1788, as Cancer seticornis.  It was also described as Cancer sagittarius by the Danish zoologist Johan Christian Fabricius (1745 - 1808) in 1793, a name which is now a junior synonym of the arrow crab. The genus Stenorhynchus was erected in 1818, by French zoologist, Pierre André Latreille (1762 - 1833).

References & Further Reading
- Giese C, Mebs D, & Werding B (1996). Resistance and vulnerability of crustaceans to cytolytic sea anemone toxins. Toxicon : official journal of the International Society on Toxinology, 34 (8), 955-8 PMID: 8875782
-  Melissa Block (2001). "Stenorhynchus seticornis, yellowline arrow crab". Animal Diversity Web. University of Michigan. Retrieved October 6, 2010.
-  "Opinion 763. Stenorhynchus Lamarck, 1818 (Crustacea, Decapoda): validated under the plenary powers with designation of Cancer seticornis Herbst, 1788, as type-species". Bulletin of Zoological Nomenclature 23 (1): 19–21. 1966.
- Peter K. L. Ng, Danièle Guinot & Peter J. F. Davie (2008). "Systema Brachyurorum: Part I. An annotated checklist of extant Brachyuran crabs of the world" (PDF). Raffles Bulletin of Zoology 17: 1–286.
- Humann, P. 1992. Reef Creature Indentification. New World Publications.
- Olhausen, P., R. Russo. 1981. Pacific Intertidal Life. Nature Study Guide.
- Snyderman, M., C. Wiseman. 1996. Guide to Marine Life. Aqua Quest Publication.

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