Ryan Photographic - Diogenidae - Left-handed hermit crabs
There are currently 429 species of diogenids distributed in 25 genera. They are called the left-handed hermit crabs because their left cheliped is enlarged, in contrast to most other hermit crabs in which it is the right cheliped that is biggest (but this generalization is not always true). There are usually two pairs of "walking legs". As decapod crustaceans they have 5 pairs of appendages, the others being the chelipeds (pincers) and two pairs of reduced hind legs which, along with a modified telson, grip the interior of their host shell.
As with most other hermit crabs (but see below) diogenids need to change shells as they grow. Battles over shells are not uncommon. Left-handed hermit crabs possess a keen sense of smell which enables them to locate food and evade predators.
As is the case with some pagurid hermit crab species, several diogenid hermit crabs eschew the normal molluscan sea shell and live in association with "walking corals". The story of this symbiosis is a fascinating one. The larval corals in question settle out on an empty mollusc shell that has been usurped by a sipunculid worm. The two typically grow together in the muddy and sandy areas they inhabit. If the coral is knocked over, the worm is able to help right it. In a recent paper, the authors describe a new species of hermit crab from the Amami Islands (near Okinawa in Japan) which has taken over the same mutualistic role with the walking coral. The scientists observed the hermit crab righting the host and digging the coral out after being buried in sediment.
While the sea anemone hermit crab, Dardanus pedunculatus (see below) is regularly seen on dives in the tropical Pacific, less widely known are the blanket hermit crabs which dispense with the shell entirely and wear a sea anemone like a blanket. In this extraordinary, and presumably obligatory, mutualism a threatened crab can pull the anemone over itself. The anemone gets access to an ongoing food supply and freedom from sediment burial as in the case of the walking corals mentioned above. But don't expect to see these on a dive, although widely distributed in the tropical Indo-Pacific they are normally found beyond scuba depth although one species has been collected as shallow as 30m, other species reach as deep as 1125m.
Dardanus guttatus Blue-knee hermit crab
Dardanus guttatus, Blue-knee hermit crab, Taveuni, Fiji-4978
Dardanus lagopodes Dark knee Hermit crab
Dardanus lagopodes Dark knee Hermit crab, Raja Ampat West Papua IMG_3336
Dardanus megistos White-spotted hermit crab
Dardanus megistos White-spotted hermit crab Puerto Galera, Philippines IMG_6048
Dardanus megistos, Kri, Raja Ampat, West Papua IMG_3886
Dardanus megistos, Kri, Raja Ampat, West Papua IMG_3887
Dardanus pedunculatus Anemone hermit crab
Dardanus pedunculatus Anemone hermit crab Puerto Galera, Philippines IMG_5955
Dardanus pedunculatus, hermit crab eye detail, Raja Ampat, West Papua IMG_0423
Dardanus pedunculatus, Kri, Raja Ampat, West Papua IMG_0418
Dardanus venosus (maybe)
Dardanus venosus (maybe), hermit crab, Glover's Reef, Belize - IMG_3413
Dardanus venosus, Stareye hermit, Glovers Reef, Belize Canon EOS 20D34