Petrochirus diogenes is a large marine hermit crab, inhabiting a wide variety of shells. It occurs from North Carolina through the Gulf of Mexico to Brazil, including the West Indies (Perry and Larsen 2004).

The giant hermit crab, Petrochirus diogenes, is one of the most distinct hermit crab species in Florida. The portion of the body directly behind the eyes, called the anterior shield, is flattened, roughly square in shape, with tufts of hairs called setae scattered across the surface (Williams 1984). The front portion of the shield posterior to the eyestalks is trilobate (forms 3 lobes). The eyestalks are straight with setae above each cornea and along the length of each stalk. Claws are unequal in size (Ruppert & Fox 1988), a characteristic in crabs called heterochely (Bertini & Fransozo 1999). Although claw size varies with sex (see "Reproduction" below), the right claw is dominant and larger in both males and females. The two end segments (chelae) of the claw that together form the pincers, referred to as the 'hand', are roughened with coarse tubercles, or bumps, separated by setae across the surface with a row of spines lining the inner margin.Tubercles are also present on the inner crushing surface of the major chela and the minor chela is slightly concave, forming a spoon shape. The 2nd and 3rd pairs of appendages that serve as the walking legs are hairy underneath with a row of sharp spines down the two end segments, called the dactyl and propodus. The body of P. diogenes is generally red in color (Williams 1984; Ruppert & Fox 1988), with white spots on the carpus (3rd segment from the tip) of the walking legs, and red and white stripes at the base and tips of the antennae and antennules.

Gulf of Mexico

The range of the giant hermit crab extends from Cape Lookout, North Carolina to southern Florida on the east coast of the United States, throughout the Gulf of Mexico and Caribbean south to Brazil (Williams 1984). Juvenile P. diogenes can be found both inshore in estuaries, while adults generally occur offshore or around inlets and nearshore reefs (Ruppert & Fox 1988). Most individuals are associated with muddy to sandy or shell bottoms, and are found among beds of turtlegrass, Thalassia testudinum, in tropical and subtropical climate zones. The giant hermit is usually found subtidally from depths of 6 to 130 m (Williams 1984; Turra et al. 2002). However, some individuals have been collected around tidal flats at depths of only a few centimeters.Indian River Lagoon (India River Lagoon) Distribution:The distribution of P. diogenes within the India River Lagoon is undocumented. However, given its preferred depth and salinity (see 'Salinity' below) ranges, it is likely that most populations of the crab are located around the major inlets connecting the lagoon to the nearshore waters of the Atlantic Ocean.

Petrochirus diogenes occurs from North Carolina through the Gulf of Mexico to Brazil, including the West Indies (Perry and Larsen 2004).

Petrochirus diogenes occurs from North Carolina, Gulf of Mexico, and Antilles to Venezuela, Surinam, Brazil and Uruguay (Raz-Guzman et al. 2004).

Adult P. diogenes are the largest hermit crabs found in and around the IRL. Size of individuals is usually reported in the literature based on measurements of the anterior portion (anterior shield) of the body, directly behind the eyes. Some sexual dimorphism exists for the species with regard to size of adults, with males growing slightly larger than females (Williams 1984; Turra et al. 2002). Average (and maximum) lengths reported for the anterior shield are 36(40) mm and 20(32) mm for males and females, respectively (Williams 1984; Bernini & Fransozo 1999). However, P. diogenes are found living inside of large gastropod shells and are equipped with prominent claws that substantially increase the total size of these crabs from the measurements mentioned above. Males not only grow larger but are also heavier than their female counterparts, with average total-body wet weights of approximately 95 g and 50 g for males and females, respectively (Bernini & Fransozo 1999).As with most marine invertebrates, little is known about the lifespan or maximum age of wild populations, which varies substantially with food availability, predator abundance and environmental conditions. Hermit crabs, particularly juveniles, must select larger gastropod shells of the most favorable shape in which to live as they continue to grow. Some studies suggest that crab growth and reproduction may be tied to the availability of shells of the appropriate size and shape (Bertini & Fransozo 1999; Bertini & Fransozo 2000). See the 'Associated Species' section below for more information.

Petrochirus diogenes has the shield with scattered clumps of setae, the anterior margin trilobate. The chelipeds are massive and unequal, the right being slightly larger than the left. The hands and carpus of the chelae are roughened with grouped tubercles separated by appressed setae. The body is generally red in color. The chelipeds are reddish except between the fingers; there are white spots on carpal area. The walking legs have purple bands. (Perry and Larsen 2004)

Due to its large size and distinct red coloration, P. diogenes is unlikely to be confused with other species of hermit crabs found within its range.

Depth range based on 100 specimens in 1 taxon.
Water temperature and chemistry ranges based on 51 samples.

Environmental ranges
  Depth range (m): 5 - 100
  Temperature range (°C): 21.311 - 26.634
  Nitrate (umol/L): 0.289 - 2.951
  Salinity (PPS): 35.554 - 36.439
  Oxygen (ml/l): 4.223 - 4.999
  Phosphate (umol/l): 0.094 - 0.424
  Silicate (umol/l): 0.756 - 4.128

Graphical representation

Depth range (m): 5 - 100

Temperature range (°C): 21.311 - 26.634

Nitrate (umol/L): 0.289 - 2.951

Salinity (PPS): 35.554 - 36.439

Oxygen (ml/l): 4.223 - 4.999

Phosphate (umol/l): 0.094 - 0.424

Silicate (umol/l): 0.756 - 4.128
 
Note: this information has not been validated. Check this *note*. Your feedback is most welcome.

Petrochirus diogenes is found on mud, mud/shell, and sand bottoms from nearshore to 128 meters depth (Perry and Larsen 2004).

Petrochirus diogenes is found from 18 to 128 meters (Wenner and Read 1982, cited in Raz-Guzman et al. (2004).

Petrochirus diogenes is found on coastal jetties; muddy, sandy, and shelly bottoms; on shrimping grounds; and in turtlegrass (Thalassia testudinum) beds (Raz-Guzman et al. 2004 and references therein).

In a study of hermit crabs in southeastern Brazil, Leite et al. (1998) found P. diogenes only in the infralittoral zone (the region of shallow water closest to the shore, but beyond the intertidal zone), occasionally associated with soft substrata.

The giant hermit crab is somewhat opportunistic, preying on a variety of other invertebrates, as well as scavenging and feeding on macroalgae. Like most other marine organisms, feeding and prey location is tied to chemical signals detected from the surrounding water column. Hazlett (1971) found that by exposing the tips of the antennules of P. diogenes to fish extract, an immediate and pronounced feeding response was produced. Exposed crabs increased locomotion, digging behavior, and movement of the mouthparts and claws. Because of its unspecialized diet, P. diogenes is likely presented with continuous prey and feeding opportunities in most locations. However, it has been noted that individuals in captivity can persist for at least three weeks with no food (Hazlett 1971). In the field, this timetable is likely altered by factors such as water temperature and degree of movement, both of which can vary the metabolic rate of the individual.Predators: Few studies have documented common predators of the giant hermit crab. However, remains of P. diogenes have been found in the fecal pellets of the Kemp's Ridley sea turtle, Lepidochelys kempii (Witzell & Schmid 2005), and in gut contents of the Nassau grouper, Epinephelus striatus (Randall 1967). Crabs, especially juveniles, are likely preyed upon by a variety of large bony fishes, rays, octopi and other crustaceans.

Petrochirus diogenes is large enough that it can inhabit a fully-grown shell of the Queen Conch, Strombus (or Eustrombus) gigas. It can attack and eat a conch, thus obtaining both a meal and a shell (Iversen et al. 1986).

Bertini and Fransozo (2000) studied the use of shells by Petrochirus diogenes in the Ubatuba region along the northern coast of São Paulo State, Brazil. The 634 P. diogenes specimens (405 males and 229 females) were using shells from a total of 12 different gastropod mollusk species, as follows (% of sampled crabs using each species in in parentheses): Tonnea galea (60.6), Zidona dufresnei (16.1), Strombus pugilis (7.4), Phalium granulatum (4.2), Cymatium parthenopeum (4.2), Stramonita haemastoma (2.2) Olivancillaria urceus (2.2), Adelomelon beckii (1.3), Buccinanops gradatus (1.3), Siratus tenvivaricosus (0.47), Fusinus marmoratus (0.31), and Cypraea zebra (0.16). Most shells had epibionts as well (barnacles, tube-dwelling polychaetes, bryozoans, sea anemones, seaweed, and bivalves). Tonnea galea shells, by far the most commonly used shells in this study, have a large aperture width (opening) and low average weight. It is not clear to what degree T. galea shells are actively favored by Petrochirus diogenes versus simply most available. Small hermit crabs inhabited a wide variety of gastropod shells due to their higher availability. However, the utilization of T. galea shells became predominant as the crabs attained larger sizes.(Bertini and Fransozo 2000)

In a study in Beaufort Harbor, North Carolina (U.S.A.), Kellogg (1977) reported that the small number of P. diogenes encountered inhabited 3 shells: Nassarius obsoletus (n=1), Polinices duplicatus (n=3), and Terebra dislocata (n=3). Bertini and Fransozo (2000) note that P. duplicatus was present in their study area in Brazil, but they found no instances of its use by P. diogenes.

None of the P. diogenes collected from Beaufort Harbor were sexually mature. Offshore, however, much larger specimens of P. diogenes were collected, many of which were sexually mature. Kellogg concluded that P. diogenes probably does not maintain a viable breeding population around the harbor, but that stray larvae may drift into the estuary from offshore. Some of these larvae apparently locate unused shells which occur in the channels. Shells inhabited by P. diogenes usually were heavily fouled, unlike those found in other areas, and unlike those inhabited by the other species. Petrochirus diogenes does not prefer shells in this condition. When placed in holding tanks containing a wide variety of shells (conditions, sizes, and species), the P. diogenes specimens readily abandoned their orginal shells for clean ones.

Perry and Larsen (2004) report that this species inhabits tun and murex shells offshore, with young specimens inhabiting Polinices, Busycon and Terebra dislocata shells.

Hermit crabs could be considered one of the best examples of obligatorily associated marine organisms. Following settlement and metamorphosis from a planktonic larva to a benthic juvenile, each crab must locate and crawl into an appropriately-sized gastropod snail shell. Failure to find a shell usually results in death from predation or other environmental hazards. If no suitable vacant shells are available, crabs may fight each other or kill and remove living snails from their shells to obtain optimal housing (e.g. Williams 1984). Shell selection can vary substantially among species, size classes and habitat. Giant hermit crabs have been found to inhabit shells of several marine gastropods throughout their range and lifetime, including: the giant tun, Tonna galea; the fine snail, Zidona dufresnei; the West Indian fighting conch, Strombus pugilis; the queen conch, Strombus gigas; the scotch bonnet, Phalium granulatum; the giant triton, Cymatium parthenopeum; the Florida rocksnail, Stramonita haemastoma; the bear ancilla, Olivancillaria urceus; the volute snail, Adelomelon beckii; the nassariid snail, Buccinanops gradatus; the knobbed whelk, Busycon carica; and the eastern auger Terebra dislocata (Fotheringham 1980; Williams 1984; Bertini & Fransozo 2000). The giant hermit is also commonly, but not obligatorily, associated with a few additional invertebrates, including: the hermit crab anemone, Calliactis tricolor; the porcelain crab, Porcellana sayana; and the zebra flatworm, Stylochus zebra (Williams 1994; Ruppert & Fox 1988). The anemone, C. tricolor, is often found attached to the shells of several species of hermit crabs and occasionally to the carapaces of true crabs (Ruppert & Fox 1988). The body of the anemone is dull brown to pink with cream stripes, bearing up to 200 short tentacles that may be white, pink or orange. The mouth of the anemone, located in the center of the tentacles, is marked with yellow, red and pinkish-purple bands (Ruppert & Fox 1988).Anemones are usually quite small, but can grow up to a few centimeters in diameter. The relationship between P. diogenes and C. tricolor is thought to be mutualistic. The anemone gains mobile shelter, food and reduced competition from other anemones. In turn, it has been suggested that the anemone affords the crab camouflage and some degree of protection, via its stinging tentacles, from potential predators such as octopus (Ruppert & Fox 1988). Although the anemones can and do relocate independently, crabs also actively collect anemones from nearby rocks and/or transfer them as they move into new shells. As its name indicates, the zebra flatworm, S. zebra, is striped with white and black markings. It reaches lengths of approximately 5 cm and usually inhabits the shells of the giant hermit at densities of 1-2 per crab (Ruppert & Fox 1988). Worms are generally found on the interior wall of the largest body whorl of the crab's shell, although they may crawl across the interior of the aperture or along the exterior surface of the shell in search of attached slipper snails, Crepidula plana or C. fornicata, on which they feed (Ruppert & Fox 1988). S. zebra may also feed on fecal pellets produced by the host crab or scavenge on bits of food in and on the shell caught by the crab or the attached anemones (Lytwyn & McDermott 1976). Eggs masses of the worm are often found affixed to the interior surface of the shell as well. Because of these characteristics, the relationship between S. zebra and P. diogenes is thought to be commensal (Lytwyn & McDermott 1976), with the worm benefiting from increased food resources and mobile shelter. However, some speculation exists that the symbiosis between these two species may be somewhat parasitic in nature, as S. zebra is also known to feed of the developing eggs of its host (Lytwyn & McDermott 1976; Ruppert & Fox 1988).The porcelain crab, P. sayana, is associated with a few species of large hermit crabs and gastropods. When associated with the giant hermit crab, P. sayana generally lives inside the shell at recorded densities of up to 11 individuals per hermit crab host (Telford & Daxboeck 1978). The carapace length for adult porcelain crabs ranges from 5 to 14 mm. Background body color varies from red to rusty brown and is overlaid with numerous yellowish, bluish-white or purplish-white spots outlined in bright red (Telford & Daxboeck 1978; Williams 1984). Like the zebra flatworm, this relationship is thought to be an example of commensalism, with the porcelain crab benefiting from the hermit crab by receiving shelter and food scraps. However, the hosts have been known to carry their associated P. sayana with them as they transfer into new shells, suggesting the relationship may be somewhat mutualistic (Telford & Daxboeck 1978).

The abundance of P. diogenes in the IRL has not been documented, although it is likely that numbers are low and centered around inlets where crabs enter from nearshore environments. However, populations in other parts of the crab's range can be quite large. For example, P, diogenes is one of the dominant hermit crab species in the southwestern Gulf of Mexico (Raz-Guzman et al. 2004).

Aside from overall body size and weight, P. diogenes exhibits sexual dimorphism based on claw size. Although the right claw is dominant in both sexes, it is greatly enlarged in males (Williams 1984; Bertini & Fransozo 1999). It has been suggested that male giant hermits use their large claws for defense and in battles for territory and mates (Bertini & Fransozo 1999). As with other crustaceans, P. diogenes reproduces sexually via copulation and the transfer of a spermatophore from the male to the female.

After fertilization is complete, the female clutches the eggs on her a abdomen during development. Hatched larvae enter the water column and pass through 5-6 zoeal stages and one glaucothoe before metamorphosing into juvenile crabs (Williams 1984). The total duration for this planktonic cycle can range between about 31 to 84 days, depending on food availability and water temperature (Provenzano 1968). Ovigerous (egg-bearing) females are reportedly most abundant in Florida during the month of August (Provenzano 1968). Based on the absence of ovigerous females under 10 mm, it is believed that this is the transitional size from juvenile to adult females (Bertini & Fransozo 1999).