This is a Photo of Arrow Crabs. Really!

arrow crabs
At first glance, this is a photo of long-spined sea urchins. But it also contains five, maybe six arrow crabs, embedded with  them. Also present: a reef scorpionfish (upper left), a juvenile cocoa damselfish (right center), Christmas tree worms and sponges.

IT MAKES SENSE THAT YELLOWLINE ARROW CRABS would want to hang out with long-spined sea urchins. After all, it should be a gimme that these crustaceans with 10 long, thin legs would seek to blend in with urchins whose most visible features are long, thin spines.

On the other hand, the sources seem to put Stenorhynchus seticornis all over the place – atop sponges, sea fans and other gorgonians, under and next to sea anemones. Anywhere but where I see them – with sea urchins (Diadema antillarum).

A SEA URCHIN PHOTO FILLED WITH ARROW CRABS

This is the kind of photo that becomes more interesting in retrospect than during shooting. At the time, it was a random photo of spiny sea urchins wedged into crevices in the rock substrate. Sometimes, that makes for a photographic effect. This was at Stetson Bank, a remarkable section of Flower Garden Banks National Marine Sanctuary, off the Texas coast.

From the first, it was apparent there were arrow crabs. But the closer I looked, the more it became apparent that the scene was loaded with them – at least a half-dozen.

arrow crabs
A yellowline arrow crab fully displayed in a tube sponge. Photographed at Roatan island, Honduras.

ARROW CRABS – COMPACT BODIES, VERY LONG LEGS

Arrow crabs get their common name from their small, arrowhead-shaped bodies. They have the body architecture of true crabs, with thorax and abdomen covered by a carapace – a dorsal shell.

But arrows arrange themselves more in the manner of daddy longlegs spiders, with compact, elongated torsos and legs more than three times the length of their bodies. Like other crabs, the first pair of their 10 legs have been developed into claws, with which they defend and handle food. The other four pairs provide locomotion, crab style – forwards, backwards, sideways.

ARROW CRAB FACTS

The little crabs’ bodies tend to be brown with white, brown or gold stripes along the upper side. Their claws are often violet in color.

Nocturnal and territorial, S. seticornis  is primarily a scavenger but also feeds on small feather duster worms and other invertebrates.

arrow crabs
A pair of yellowline arrow crabs where the sources say they are supposed to be – hanging out with a sea anemone.

HABITATS & RANGE

The arrows (and urchins) in this photo stand out so vividly because of the singular nature of Stetson Banks’ habitat. Although Flower Gardens’ East and West banks are conventional coral reefs, Stetson Bank, somewhat cooler in temperature, lacks extensive coral cover and is largely bare rock. Things stand out. All three banks are uplifted rock covering salt domes rather than conventional coral habits.

It should be mentioned that S. seticornis is found all over the place – so long as that place is in the Western Atlantic/Caribbean/Gulf of Mexico basin. There’s a similar, different species (S. lanceolatus) residing in the eastern Atlantic. There appears to be a candycane arrow crab (Latreillia valida) to be found in the western Pacific.

PRINCIPAL SOURCES: Reef Creature Identification, Florida, Caribbean, Bahamas, Reef Creature Identification, Tropical Pacific, Paul Humann, Ned DeLoach; Marine Life, Caribbean, Bahamas, Florida, Marty Snyderman & Clay Wiseman; Stenorhynchus seticornis –  yellowline arrow crab, Animal Diversity Web, University of Michigan Museum of Zoology; Stenorhynchus seticornis –  yellowline arrow crab, Encyclopedia of Life; Stenorhynchus seticornis, Wikipedia.org.

 

 

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