Species Description: Libinia dubia belongs to a group of brachyuran crabs commonly referred to as decorator crabs. Using hooked, Velcro-like setae on the surface of the carapace, the crabs attach bits of algae and invertebrates for camouflage. This behavior is most common in juveniles, and the shells of adult crabs are usually found clean. Under the decorative covering, the carapace of L. dubia is rounded, bearing approximately six spines down either side and along the median line on the dorsal surface (eg. Corrington 1927). A forked rostrum extends between the eyes, and the overall color of the body is yellowish to brown (Voss 1980). Long, thin walking legs originating from the rounded body give the crab the spidery appearance for which it is named. These legs culminate in curved points, allowing the crab to cling to various surfaces like rocks and jellyfishes (Ruppert & Fox 1988).
Potentially Misidentified Species: Three species of Libinia inhabit the coastal and estuarine waters of the Western Atlantic and Caribbean: L. dubia; the portly spider crab, L. emarginata; and the seagrass spider crab, L. erinacea.
The color and shape of all species are similar, and discrimination between juvenile specimens can be difficult. However, Libinia dubia and L. emarginata are distinguished by the number of dorsal median spines, bearing six and nine, respectively (Abele & Kim 1986). The maximum size of the portly spider crab is also slightly larger than that of L. dubia (Ruppert & Fox 1988). The rostrum of young seagrass spider crabs forks more deeply than L. dubia, and the horns curve toward one another (Abele & Kim 1986).
Regional Occurrence: The longnose spider crab is found in a variety of coastal and estuarine habitats to approximately 50 m depth (Williams 1984). The native range of L. dubia extends from Cape Cod to southern Texas, Bahamas and Cuba. However, in the past decade the species has been reported in the Mediterranean Sea off the Tunisian coast (Enzenross & Enzenross 2000), although the exact date and vector of introduction is unknown. In Florida, L. dubia has been documented as a common inhabitant of Florida Bay (Tabb & Manning 1961), and the most prevalent spider crab in Tampa Bay (Dragovich & Kelly 1964).
IRL Distribution: Both adult and juvenile L. dubia are found throughout the Indian River Lagoon. Juveniles are common in seagrass beds, and adults may inhabit more open sandy-bottom areas. Juvenile crabs can also be found attached to the cannonball jelly, Stomolophus meleagris, which occurs seasonally along the coast and throughout the IRL (Tunberg & Reed 2004).
Age, Size, Lifespan: Information on the lifespan and adult growth patterns of L. dubia is lacking. However, the average carapace diameter for mature crabs is 6 to 10 cm (Corrington 1927, Ruppert & Fox 1988), with the length of walking legs adding considerably to the total body size. As with most species, growth rates are likely dependent on food availability, environmental conditions and other factors.
Abundance: The longnose spider crab is a common inhabitant in seagrass beds and sandy areas, although the abundance of individuals is often quite low. Some seasonality occurs with the abundance of L. dubia in more temperate waters. In the northeast United States, both L. dubia and L. emarginata are found in greater numbers in fall and spring, becoming scarce in summer and rare in winter (O'Brien et al. 1999). Although little information has been collected on the spawning seasons of L. dubia, it is likely that food availability and/or water temperature plays a role in abundance of larvae and successful recruitment.
Reproduction: Like other brachyuran crabs, sex can be determined in Libinia dubia by examining the abdomen. In females, it is broader and can be tightly flexed to hold the egg mass, or sponge (eg. Ruppert et al. 2004). On average, females are also slightly smaller than males (O'Brien et al. 1999, Tunberg & Reed 2004). As with most decapod crustaceans, fertilization occurs during copulation. The male transfers sperm-filled cases, called spermatophores, to the female. After the eggs are fertilized, the female broods them on her abdomen until hatching.
Embryology: Once hatched, the larvae pass through three planktonic stages, two zoeae and one megalopa, lasting approximately nine days (Sandifer & Van Engel 1971). The zoeal stages are characterized by a long dorsal spine and a short rostral spine between the large eyes. These stages measure 2 to 3 mm in total length. Megalopae have lost the dorsal spine, the rostral spine is shortened and the legs are more prominent, creating a total body length of about 2 mm. When a suitable habitat is selected, megalopae swim to the benthos and metamorphose into juvenile crabs.
Temperature: Although few studies have been conducted on the physical tolerances of L. dubia, the prevalence of the species in tropical waters along with its seasonality in temperate regions, suggests that the crab has a preference toward warmer temperatures. Locally, crabs have been found in abundance in association with cannonball jellies in March (Tunberg & Reed 2004). In the laboratory, larvae have been successfully cultured between 25.5 and 28.5°C (Sandifer & Van Engel 1971).
Salinity: The longnose spider crab can be found in both coastal and estuarine waters, suggesting a large salinity tolerance for the species. In the Indian River Lagoon, salinities can fluctuate widely over 20 ppt or greater. In laboratory studies, no salinity preference was found for L. dubia (O'Brien et al. 1995), although it is not reported in freshwater. Culture of the crab has been successful at 22 ppt (Sandifer & Van Engel 1971).
Trophic Mode: Corrington (1927) defined L. dubia as a scavenger, feeding on easily procured plant and animal tissue and detritus. In seagrass beds, the spider crab consumes macroalgae as a portion of its diet, including Gracilaria tikvahiae and other algae of the genera Ulva, Hypnea, Chondria and Padina (Stachowicz & Hay 1999). When in association with various species of medusae, L. dubia has been found to feed on the mesoglea, the transparent body tissue of the jelly (Jachowski 1963, Phillips et al. 1969, Tunberg & Reed 2004).
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