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You are here: /main/research expeditions/CReefs 2006/features/true_crabs

True Crabs of French Frigate Shoals

Pseudoliomera Family: Xanthidae Photo: Gustav Paulay
FFS crab

Family: Xanthidae
Photo: Gustav Paulay

By Joel Martin

Phylum: Arthropoda.
Subphylum: Crustacea.
Class: Malacostraca.
Order: Decopoda.
Infraorder: Brachyura

Click here to see a colorful gallery of crabs found on this expedition!

Crabs are among the most easily recognized of all marine invertebrates.  Nearly everyone has an idea of what a crab looks like.  But actually, crabs come in a staggering variety of sizes, shapes and colors, making it hard at times to define what a crab really is.  Like lobsters, shrimps, hermit crabs, and other crustaceans with five pairs of legs, crabs are "decapods" (a name that means "ten legs").  The "true" crabs belong to a group of decapods called the Brachyura (meaning short-tailed), in recognition of the fact that their tail (abdomen) is reduced in size and is tucked under the body; the abdomen of a crab can only be seen from below (or by holding the crab upside down). The abdomen is wide in female crabs, in order for them to carry the egg mass after eggs have been produced, and it is narrow and straighter in male crabs.  All crabs have the first pair of legs modified into pinching claws, and usually these modified legs are heavier or longer than the remaining legs. This leaves four pairs of legs for walking or swimming. But several groups of crabs, including some that are found in the waters off French Frigate Shoals, also have claws on the back legs. These legs are specialized for holding pieces of sponges, sea fans, or other coverings over the crabs as camouflage. Some crabs even have three pairs of legs with claws on them. Most crabs are opportunistic feeders, which really means that they will eat just about anything they come across. But some are very specialized. For example, some of the species at French Frigate Shoals live only on live corals and are adapted to eating the mucous these corals produce; other species are found only on sea cucumbers; some species are adapted to opening snail shells; some eat algae; and on it goes. It is estimated that there are 5,000 different kinds (species) of crabs in the world, which is about a third of all known decapods. Only about 200 different species of crabs have been reported from Hawaiian waters, but we are adding to that number every day.


*All images and information from French Frigate Shoals are provided courtesy of the Northwestern Hawaiian Islands Marine National Monument, Hawaiian Islands National Wildlife Refuge, the Northwestern Hawaiian Islands State Marine Refuge, and NOAA's Pacific Islands Fisheries Science Center in accordance with permit numbers NWHIMNM-2006-015, 2006-01, 2006-017, and DLNR.NWHI06R021 and associated amendments.

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Ship Logs

Ship Logs:
Day-by-day activities of the ship: what research is being done that day, what the weather is like, what's for dinner, etc.

Daily or semi-daily personal journal entries by the particpants in the expedition. These journals do not necessarily reflect the positions of any of the agencies connected with this project.

Interviews with expedition participants, scientists, vessel crew, educators, etc.

Highlights or special information such as interesting discoveries or related research.

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