Species Description: Pagurus longicarpus, the long-armed hermit crab, is a small western Atlantic hermit crab. It belongs to the genus Pagurus, all members of which have unequal chelipeds (claws) in which the right is substantially larger than the left. In the case of P. longicarpus, the oversized claw is long and slender and approximately cylindrical in shape. Body color is highly variable, ranging from beige to off-white to greenish-grey to brown (Voss 1983, Rupert and Fox 1988).
Like all hermits, P. longicarpus protects its soft, asymmetrical abdomen by tucking it into and tightly curling it around the columella of the shell from a dead gastropod (Barnes 1987).
Potentially Misidentified Species: Pagurus longicarpus is one of at least 16 hermit crab species reported from the Indian River Lagoon, at least 6 of which belong to the genus Pagurus. The most abundant large IRL hermits, the striped hermit (Clibanarius vittatus) and the giant hermit (Petrochirus diogenes) are easily discernable from various species of Pagurus by their size. P. longicarpus can usually be distinguished from co-occurring congeners by the cylindrical shape of its enlarged cheliped (as compared to the flattened claw of P. pollicaris, for example).
Regional Occurrence: Pagurus longicarpus is a wide-ranging temperate species that can be found along the Atlantic coast from Nova Scotia south through Hutchinson Island, FL, and again along the Gulf coast from the Shark River in southwest Florida west to Galveston, Texas (Fotheringham 1976, Camp et al. 1977). The geographically disjointed distribution and discernable morphological differences between the Atlantic and Gulf populations suggests P. longicarpus may have been subject to past vicariance events whereby geographic or ecological barriers subdivided the ancestral population. Genetic analysis by Young et al. (2001) supports this hypothesis; mitochondrial DNA sequence data suggests the two populations diverged around 0.6 million years ago.
IRL Distribution: Pagurus longicarpus likely occurs throughout the IRL system, although the southern end of the estuary roughly coincides with the southern end of the range of the Atlantic population.
Age, Size, Lifespan: Pagurus longicarpus is a small hermit crab, with adult individuals attaining a length of around 2.5 cm or less (Rupert and Fox 1988).
Abundance: Williams (1984) reports that Pagurus longicarpus is one of the most common shallow-water decapods along the US east coast and Gulf of Mexico.
Reproduction: As is typical of decapod crustaceans, reproduction in Pagurus longicarpus is sexual, internal fertilization is employed, and the sexes are separate. Individuals must partially emerge from the protection of their gastropod shells and press their ventral surfaces together to allow copulation (Barnes 1987). Females extrude eggs into their shells, gather them via the pleopods, and then brood them in a manner similar to other crabs.
Wilber (1989) found a number of relationships between female P. longicarpus egg production and shell characteristics such as size and condition. Of particular interest, the author noted that medium-large and large females inhabiting seagrass beds who occupied severely damaged or fouled shells were only half as likely to be reproductive as females occupying shells of better quality. Reduced incidence of reproductive females in poor quality shells may be the result of poor nutrition due to the fact that relatively less protected individuals may spend more time buried than individuals occupying intact shells and, therefore, less time foraging. Wilbur (1989) also notes that seagrass-resident female crabs occupying moon snail (Polinices duplicatus) shells or shells larger than their predicted shell size exhibited enhanced clutch sizes compared to other individuals.
Based on the presence of large numbers (75%) of ovigerous female P. longicarpus during a three-month study near Alligator Harbor, Franklin County, FL, suggests that females typically produce more than one clutch per reproductive season (Wilber 1989).
Embryology: Roberts (1970b) indicates that brooding of eggs in P. longicarpus lasts only a few weeks. Roberts (1970a) identified and described four distinct planktonic zoeal larval stages and one megalops stage for P. longicarpus. This is a smaller number of planktonic stages than occurs for many decapods. The author suggested that except under suboptimal conditions, there is no planktonic prezoeal stage.
Roberts (1971) described some behavioral aspects of the megalops. At first, the megalops is an active swimming stage, but swimming activity declines with time. If individuals encounter suitable shells, they enter the shells and swimming activity ceases. If no suitable shell is found, swimming activity nevertheless becomes very infrequent after two days. The author found no evidence indicating that the species is capable of delaying metamorphosis until a suitable shell is located.
Fotheringham and Bagnall (1976) collected larval P. longicarpus from the water column in Christmas Bay, TX, from September through May, suggesting a long reproductive season for the species in the southern portion of its range.
Temperature: Pagurus longicarpus occurs as far north as Nova Scotia, indicating the species tolerates a wide range of temperatures. Examining the effects of extreme high temperatures, Fraenkel (1960) reports that 40°C is 100% lethal but that individuals exposed to 36°C for one hour survived as long as they were given 24 hours to recover at room temperature. Vernberg (1967) indicates that individuals acclimated to higher temperatures survived experimental temperature increases of the same magnitude (i.e., acclimation temperature + 5, 10, or 15°C) longer than individuals acclimated to lower temperatures.
Salinity: As an inhabitant of littoral estuarine habitats, Pagurus longicarpus is regularly exposed to measurable seasonal and tidal salinity fluctuations,
Roberts (1971b) reports a salinity range of 18-35.5 ppt as optimal for larval development through the megalopal stage. Biggs and McDermott (1973) suggest a slightly broader optimal range of 15-36 ppt for adult P. longicarpus collected from southern New Jersey.
Limb Generation: Pagurus longicarpus is capable of regenerating lost limbs, Weis (1982) noted rapid limb regeneration and molting after autotomy of 1-4 appendages in the laboratory.
Trophic Mode: Pagurus longicarpus is an omnivorous, generalist scavenger capable of consuming edible material from the surfaces of sand grains and also able to consume larger pieces of detrital plant and animal material (Roberts 1968, Caine 1975). In both cases, one (typically the smaller of the pair) or both of the chelipeds pick up particles and transfer them to the mouth. When feeding on enriched sand, the edible component is removed from grains by brushing activity of the setae-covered third maxilipeds. When larger detrital particles are consumed, considerably more complex mouthpart interactions are involved in the process of tearing off and ingesting small food fragments (Roberts 1968). Benthic diatoms form much of the diet, and small infauna (e.g., polychaetes) are often inadvertently consumed as well (Roberts 1968, Caine 1975). Many of the food items normally consumed are of comparatively low nutritional quality and so must be consumed in large quantities (Wilbur 1989).
Laboratory experiments by Whitman et al. (2001) demonstrate that P. longicarpus is also capable of facultative suspension feeding, but the authors concede that further study is required to evaluate the importance of this strategy in the wild. In addition to brine shrimp nauplii, laboratory-maintained P. longicarpus were capable of feeding on a variety of more ecologically relevant planktonic prey including first zoeal stages of Dyspanopeus sayi, Carcinus maenus, and Palaemonetes vulgaris, as well as newly hatched veligers of the gastropod Crepidula plana.
Laboratory studies reveal that P. longicarpus may resort to cannibalism if insufficient dietary resources are provided (Allee and Douglas 1945).
Planktonic larval P. longicarpus are likely to be opportunistic foragers. In the laboratory, they have been shown to be capable of consuming a variety of microalgal species, several types of microcrustaceans, and polychaete larvae (Roberts 1974). These dietary items were not equally capable of sustaining larval crabs through all larval stages.
Competitors: Competition among hermit crabs for dietary resources may occur despite the generalist foraging habits of Pagurus longicarpus and co-occurring species.
Interspecific and intraspecific competition among hermit crabs for a potentially limiting supply of gastropod shells is also believed to be typically severe. Vance (1972) noted that species-specific shell preference may allow the coexistence of similar species through resource partitioning. Allee and Douglas (1945) demonstrated that P. longicarpus lacking shells typically attack shell-bearing conspecifics without regard for the size of the competitor. They note, however, that attacks were only successful if the housed individual was smaller than the attacker.
Experiments by Wilber (1990) on P. longicarpus from Wakulla Beach, FL, indicated that behavioral shell selection exhibited a compromise between shell species and relative size and that animals avoided relatively large shells more than relatively small shells. This suggests that a larger-than-optimal shell has more negative attributes than an undersized shell.
Field studies conducted by Rittschoff et al. (1992) demonstrated that several hermit crabs, including P. longicarpus, exhibit behavioral responses to chemicals originating from crushed conspecifics. Looking further at the hermit crab Clibanarius vittatus, these authors report that behavioral responses are dependent on crab size, type of shell inhabited, and shell size and fit. Chemical cues originating from dead conspecifics typically elicited aggression/shell investigation responses in crabs occupying relatively small shells and alarm responses by crabs in relatively large shells. This suggests that the need to acquire limited high-value shell resources is sufficiently strong to preempt behavioral defense against predation even in the face of potentially imminent predation threat.
Tricarico and Gherardi (2007) conducted experiments to assess the factors that motivate P. longicarpus to switch shells. Offering test animals a choice of shells of varying quality, the study suggests motivation to acquire new shells was entirely dictated by the value and quality of the shell the animals currently inhabited rather than by the shell it is offered. Other studies (e.g., Pechenil and Lewis 2000, see below), however, suggest that selection may at times be based on the quality of the shell being offered.
Gherardi et al. (2005) examined the role of odor in individual recognition by P. longicarpus and discovered that individual crabs are capable of discriminating between larger conspecifics inhabiting high-quality shells and smaller conspecifics inhabiting low-quality shells, provided the crabs are familiar with one another. The authors conclude that crabs appear capable of associating odor information from other individuals with memories of past interactions.
Predators: Tunberg et al. (1994) lists a number of predators of hermit crabs, including fish, gastropods, crabs, and octopods. Many such species, although not all, can extract hermit crabs from their protective shells without inflicting any shell damage (Bertness 1981). Laboratory studies of blue crab (Callinectes sapidus) predation on the seagrass-associated Pagurus maclaughlinae reveal an alternate strategy, in which blue crabs secured the shell of a prey hermit with one cheliped and progressively crushed the outer lip of the shell (rolling it as it does so) until the hermit could no longer retract into the spire of the shell (Tunberg et al. 1994).
P. longicarpus reportedly will not feed if it is not safely occupying a shell, suggesting that perceived vulnerability to predation is sufficient to alter crab behavior. Allee and Douglis (1945) report that isolated shell-less P. longicarpus maintained in the lab die sooner than shelled animals, due in part to the fact that exposed animals cease eating.
A study by Kuhlmann (1992) on the predation of Pagurus longicarpus showed that predation rate was independent of shell species, shell size, and shell damage. Experiments by Heck and Wilson (1987) employing tethered predatory decapods in seagrass beds similarly revealed a lack of correlation between hermit crab predation rate and the thickness or ornamentation of their gastropod shell homes. Other studies, however, suggest P. longicarpus shell selection behavior may be mediated by predator presence. Rotjan et al (2004), for example, demonstrate that the presence of chemical cues from the predatory green crab Carcinus maenus alters P. longicarpus shell choice behavior in favor of intact shells.
Parasites: The polychaetes Lepidonotus sublevis and Dipolydora (=Polydora) commensalis may take up residence in the lumen of the shell and may feed on developing crab embryos (Fotheringham 1976, McDermott 1999). The turbellarian Stylochus zebra can also be found in P. longicarpus shells (Lytwyn 1979).
Co-Occurring Species: A number of colonial epifaunal hydroids are known to occur on the shells of Pagurus longicarpus and other pagurid hermit crabs, although the literature indicates that preference for hydroid-colonized versus -uncolonized shells varies among and within crab species. Brooks and Mariscal (1985), for example, report that north Florida Gulf coast P. longicarpus initially selected for hydroid-colonized (Hydractinia echinata, Podocoryne selena) shells but subsequently switched to bare shells even when predators were present. Although emphasis has been placed on the potential benefits of crab-hydroid symbiosis (e.g., protection for crabs to predators), Weissberger (1995) demonstrated that shell colonization by the hydroid Hydractinia symbiolongicarpus offered New England P. longicarpus no protection from predation by the American lobster Homarus americanus. Moreover, Damiani (2003) reports that female P. longicarpus occupying shells colonized by H. symbiolongicarpus experience reproductive costs including depressed ovigery, smaller clutch size, and increased frequency of clutch failure.
Wilber and Herrnkind (1994) report that the predatory crown conch (Melongena corona) can be important as a source of new shells for co-occurring P. longicarpus populations. Working at Wakulla Beach on the northern Gulf coast of Florida, these authors reported average rates of new shell (Littorina irrorata) acquisition ranging from 4 to more than 23 new shells per day in P. longicarpus subpopulations surveyed within 40 meter squared salt marsh plots. The number of newly acquired shells by the crabs varied directly with M. corona, supporting the contention that predation events constituted a major source of shells.
Tricario and Ghererdi (2006) indicate that simulated gastropod predation attracted P. longicarpus, and concluded that nondestructive gastropod predator density ultimately regulates the supply of high-quality shells for this hermit crab.
Studies conducted by Pechenil and Lewis (2000), however, indicate that not all shells made available by co-occurring predators are equally valued by crabs. They showed that shells that have been drilled by natacid gastropods are avoided by P. longicarpus, even as shells with other forms of damage were deemed suitable by crabs. The authors suggested that strong behavioral avoidance of drilled shells may indicate such shells would expose resident crabs to increased predation, osmotic stress, and eviction by competing hermits.
Habitats: Pagurus longicarpus occurs in a variety of estuarine and coastal habitats from the intertidal to as deep as 45 m (Gosner 1979). Heck and Spitzer (undated), describing the fauna of the northern Gulf of Mexico, listed seagrass meadows, mud and sand bottoms, and beach surf zones as likely inshore habitats in which to encounter this species. The species is also a common inhabitant of oyster reef habitats.
Activity Time: Pagurus longicarpus is active primarily during daylight hours.
Economic Importance: Pagurus longicarpus has some economic importance as a bioassay organism (Eisler and Hennekey 1977).
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