Species Description: The giant hermit crab, Petrochirus diogenes, is one of the most distinctive 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 three 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”), 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 second and third 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.
Potentially Misidentified Species: 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.
Regional Occurrence: 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.
Age, Size, Lifespan: 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 for more information.
Abundance: 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).
Reproduction: 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.
Embryology: After fertilization is complete, the female clutches the eggs on her 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).
Temperature: Information concerning the temperature tolerances of P. diogenes is scarce. However, migrations and mass mortalities have been documented in colder months for populations residing in temperate zones (Turra et al. 2002), suggesting that the giant hermit crab requires and/or prefers warmer waters.
Salinity: Giant hermit crabs can be found in both marine and estuarine environments, although they are reported to prefer more saline conditions (Raz-Guzman 2004).
Trophic Mode: 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.
Bertini, G & A Fransozo. 1999. Relative growth of Petrochirus diogenes (Linnaeus, 1758) (Crustacea, Anomura, Diogenidae) in the Ubatuba region, São Paulo, Brazil. Rev. Brasil. Biol. 59: 617-625.
Bertini, G & A Fransozo. 2000. Patterns of shell utilization in Petrochirus diogenes (Decapoda, Anomura, Diogenidae) in the Ubatuba region, São Paulo, Brazil. J. Crust. Biol. 20: 468-473.
Caine, EA. 1976. Relationship between diet and the gland filter of the gastric mill in hermit crabs (Decapoda, Paguridea). Crustaceana 31: 312-313.
Fotheringham, N. 1980. Effects of shell utilization on reproductive patterns in tropical hermit crabs. Mar. Biol. 55: 287-293.
Hazlett, BA. Chemical and chemotactic stimulation of feeding behavior in the hermit crab Petrochirus diogenes. Comp. Biochem. Physiol. 39A: 665-670.
Leite, FPP, Turra, A & SM Gandolfi. 1998. Hermit crabs (Crustacea: Decapoda: Anomura), gastropod shells and environmental structure: their relationship in southeastern Brazil. J. Nat. Hist. 32: 1599-1608.
Lytwyn, MW & JJ McDermott. 1976. Incidence, reproduction and feeding of Stylochus zebra, a polyclad turbellarian symbiont of hermit crabs. Mar. Biol. 38: 365-372.
Provenzano Jr. AJ. 1968. The complete larval development of the West Indian hermit crab Petrochirus diogenes (L.) (Decapoda, Diogenidae) reared in the laboratory. Bull. Mar. Sci. 18: 143-181.
Raz-Guzman, A, Sanchez, AJ, Peralta, P & R Florido. 2004. Zoogeography of hermit crabs (Decapoda: Diogenidae, Paguridae) from four coastal lagoons in the Gulf of Mexico. J. Crust. Biol. 24: 625-636.
Telford, M & C Daxboeck. 1978. Porcellana sayana Leach (Crustacea: Anomura) symbiotic with Strombus gigas (Linnaeus) (Gastropoda: Strombidae) and with three species of hermit crabs (Anomura: Diogenidae) in Barbados. Bull. Mar. Sci. 28: 202-205.
Turra, A, Branco, JO & FX Souto. 2002. Population biology of the hermit crab Petrochirus diogenes (Linnaeus) (Crustacea, Decapoda) in southern Brazil. Revta. Bras. Zool. 19: 1043-1051.
Randall, JE. 1967. Food habits of reef fishes of the West Indies. Studies in tropical oceanography. Miami 5: 665-847
Ruppert, EE & RS Fox. 1988. Seashore Animals of the Southeast: A guide to common shallow-water invertebrates of the southeastern Atlantic coast. Univ. South Carolina Press. Columbia, SC. 429 pp.
Williams, AB. 1984. Shrimps, Lobsters and Crabs of the Atlantic Coast of the Eastern United States, Maine to Florida. Smithsonian Institution Press. Washington, DC. USA. 550 pp.
Witzell, WN & JR Schmid. 2005. Diet of immature Kemp’s Ridley turtles (Lepidochelys kempi) from Gullivan Bay, Ten Thousand Islands, southwest Florida. Bull. Mar. Sci. 77: 191-199.