seaturtle.org : MTN : ARCHIVES : Sign In

To date, there is a paucity of data concerning the association between P. minutus and neritic-stage (subadult and adult) sea turtles. That is, such observations simply report the occurrence of P. minutus as an epibiont with no supporting biological or ecological information regarding the observed relationship. Here, we report the occurrence of P. minutus as an epibiont of large, immature and adult loggerhead turtles from St. Lucie County, Florida, USA. We also report behavioral, ecological and physical data from P. minutus and we discuss how the occurrence of P. minutus from this population of turtles can be used to elucidate some aspects of the behavior of loggerheads from Florida during the winter and early spring.

Collections for this study began in March of 2002 and ended in April 2003. Loggerheads that hosted P. minutus were encountered from March - April 2002, and from December 2002 – April 2003, after turtles were incidentally entrained into intake canals at the St. Lucie Nuclear Power Plant (SLNPP). The SLNPP is located on Hutchinson Island in St. Lucie County, Florida and draws cooling water from the Atlantic Ocean through three large diameter pipes (3.9 - 4.9 m). Cooling water then flows into a 1500 m long intake canal. The structures housing the intake pipes are 365 m offshore in approximately 7 m of water. The pipe openings are vertically oriented and are situated 3 m above the ocean floor. A large velocity cap sits approximately 2 m above each pipe opening and extends more than 3 m beyond the pipe diameter. Turtles encountering these structures are often transported into the intake canal where they are captured with tangle nets, dipnets and by SCUBA divers. Morphometric data are collected from each turtle, tags are applied to both front flippers and turtles are returned to the ocean approximately 750 m north of the canal.

From March 2002 through April 2003 594 loggerheads were captured at the SLNPP. Of these turtles, 22 individual loggerheads (mean straight carapace length (SCL) = 74.6 ± 3.4SD cm, range = 65 – 105.6 cm) hosted 28 crabs (3.7 % occurrence of P. minutus). It should be noted, however, that because collected turtles had been entrained and pulled through a network of pipes before entering holding lagoons, and turtles were subsequently captured using nets, the frequency of occurrence reported for P. minutus is likely much higher; as crabs can be dislodged or flee host turtles during entrainment and hand-capture.

Of the crabs collected, all were adults (15 males, 13 females). Six turtles hosted heterosexual crab pairs (27.3 %) and 16 turtles hosted crabs as singletons (72.7 %). Only one female crab was not ovigerous. Ovigerous crabs carried eggs pertaining to stages 1 (50 %), 2 (16.7 %) and 3 (33.3 %) as defined by Hartnoll (1963) where stage 1 eggs are eggs of uniform color and even yolk distribution, stage 2 eggs are eggs containing a clear area devoid of yolk and stage 3 eggs are eggs containing pigmented eyespots. Stage 4 eggs (hatching) and stage 5 eggs (hatched or empty eggs) were not observed from this group of crabs.

Male crabs and female crabs were similar in size. The average straight carapace widths (SCW) of males and females, including range, were 11.9 mm (9.1 – 16.5 mm) and 12.9 mm (9.8 – 18.9 mm), respectively. A single non-ovigerous crab, included within the above average for females, was 18.9 mm SCW.

Previous agonistic interactions were apparent in 7 specimens (3 males, 4 females; 25 % of the crabs encountered). Injuries resulting from these interactions included missing chelipeds and walking legs; however, none of these injuries were severe as all crabs had developed leg buds and no crab was missing more than a single appendage. Four injured crabs occurred as singletons upon their respective host turtle and three injured crabs were from heterosexual pairings. No heterosexual pair contained more than one injured crab.

A few points concerning aspects of the natural history of both host loggerheads and Columbus crabs can be gleaned from our data. The morphometric data we present for ovigerous P. minutus contain some of the smallest specimens reported for females with eggs. Data from other studies presenting similar information suggest that crabs approximately 11 mm SCW and smaller are juveniles and most often found as Sargassum associates (see Chace 1951). These crabs then emigrate to flotsam and turtles at the onset of sexual maturity. Although sexually immature P. minutus (9 – 13 mm SCW) can also be found as epibionts of turtles, albeit rarely (Dellinger et al. 1997; Frick et al. 2004), our data do confirm that some P. minutus are capable of attaining sexual maturity as small as 9.1 mm SCW. Moreover, it is possible that juvenile crabs collected from flotsam and turtles in other studies represent sub-adult specimens undergoing the initial stages of sexual maturity.

The relatively low percentage of P. minutus from this study bearing injuries from agonistic interactions (25 %) is similar to data presented by Dellinger et al. (1997) and Frick et al. (2004) for crabs from loggerhead turtles. The same studies also found that P. minutus from turtles suffered markedly fewer instances of missing appendages when compared to collections obtained from inanimate flotsam and jetsam. Additionally, the large proportion of ovigerous crabs from our collection coincides with data from Dellinger et al. (1997) and Frick et al. (2004). Two large collections of P. minutus made by Dellinger et al. (1997), one from turtles and the other from inanimate flotsam, demonstrate that female crabs from turtles were brooding eggs more often (71 %) than female crabs from inanimate flotsam (21 %). Dellinger et al. (1997) suggest that, given their reported data, turtles appear to offer a substantially better platform for egg brooding and, likely, production than inanimate flotsam. These authors speculate that the higher instances of egg-brooding observed in P. minutus from turtles could be attributed to the fact that turtle crabs are likely exposed to better food or a larger variety of food items than those clinging to inanimate flotsam.

A study by Frick et al. (2004) determined that turtle crabs do feed upon a larger variety of food items than crabs inhabiting inanimate flotsam. However, the same study found that crabs inhabiting inanimate flotsam contained similar amounts of food, albeit a lower diversity of prey types, as crabs from loggerhead turtles. Moreover, crabs from inanimate flotsam produced similar sizes and numbers of eggs as those from turtles. Yet, Frick et al. (2004) did not examine a collection of crabs from inanimate flotsam as large as that examined by Dellinger et al. (1997). Therefore, the former study could not offer any supporting evidence that a higher proportion of turtle crabs were brooding eggs when compared to crabs inhabiting inanimate flotsam. Instead, a higher proportion of ovigerous crabs and, consequently, an increased chance of encountering ovigerous crabs from turtles than from inanimate flotsam suggest that multiple annual broods produced by crabs from turtles might be occurring with greater frequency than from crabs inhabiting inanimate flotsam. Because agonistic interactions between crabs on turtles are proportionately lower than agonistic interactions between crabs inhabiting inanimate flotsam (Dellinger et al. 1997, Frick et al. 2004), crabs from inanimate flotsam would likely experience decreased survival probabilities and reproductive output as energy is diverted from reproduction to repair injuries.

With regard to the life history of host turtles, the occurrence of P. minutus from neritic-stage loggerhead turtles suggests that these turtles were exhibiting a great deal of surface-oriented or pelagic behavior prior to their entrainment at the SLNPP; particularly during the winter and early spring. Loggerhead turtles are captured at the SLNPP year-round and turtles that hosted P. minutus were encountered in December, January February, March and April 2002 and 2003. As previously mentioned, Columbus crabs cling to most anything that floats. Physically and physiologically, Planes are surface-water specialists and these aspects are likely attributable to the colonization of other grapsid species into supra-littoral habitats and maritime forests (Williams 1984).

Although loggerhead turtles inhabiting the western North Atlantic, particularly the southeastern US, are most often referred to as largely benthic-associated animals, dietary studies reveal that some prey items consumed by neritic-stage loggerheads are actually largely pelagic species (i.e. Aurelia aurita and Physalia physalis; Bjorndal 1997; Frick et al. 2001). Moreover, a study by Frick et al. (1999) reports several accounts of pelagic foraging by loggerhead turtles during the winter and early spring in Florida, often alongside leatherback turtles (Dermochelys coriacea) – a turtle species that is well-documented to spend a great deal of time in the pelagic environment (Bolten 2003). Such observations might indicate why the turtles encountered during the present study hosted a pelagic species such as P. minutus. Additionally, surface-active behavior such as courtship and mating that occurs in Florida during the early spring (Frick et al. 2000) might explain why large adult-sized turtles (> 80 cm SCL for Atlantic U.S. C. caretta; Dodd 1988) sampled during this study (n = 5; 22.7 %) hosted P. minutus.

During their first decade of life loggerheads are highly pelagic, particularly within oceanic habitats (water masses away from the continental shelf ≥ 200 m deep; Hedgpeth 1957; Bolten 2003). Such behavior decreases in frequency as turtles migrate from their nursery habitats to the neritic habitats that will support them for the remainder of their life (Musick & Limpus 1997). However, it appears from data collected during epibiont and dietary studies that, while benthic habitats are heavily utilized by neritic stage loggerheads, the pelagic environment still represents an important habitat for loggerheads as they mature, even into adulthood (Frick et al. 2001; present study).

Acknowledgments: We thank the PADI foundation for providing the microscopy equipment used in this study and the Florida Power and Light Company for continuing to support sea turtle research at the St. Lucie power plant. We also thank M. Godfrey, L. Campbell and two anonymous reviewers for comments that greatly improved the manuscript.

BJORNDAL, K.A. 1997. Foraging ecology and nutrition in sea turtles. In: Lutz, P.L. & J.A. Musick (Eds.) The Biology of Sea Turtles. CRC Press, Boca Raton, pp. 199-232.

BOLTEN, A.B. 2003. Variation in sea turtle life history patterns: neritic vs. oceanic developmental stages. In: Lutz, P.L., J.A. Musick & J. Wyneken (Eds.) The Biology of Sea Turtles Vol. II. CRC Press, Boca Raton, pp. 243-257.

CARRANZA, A., A. DOMINGO, A. VERDI, R. FORSELLEDO & A. ESTRADES. 2003. First report of an association between Planes cyaneus (Decapoda: Grapsidae) and loggerhead sea turtles in the southwestern Atlantic Ocean. Marine Turtle Newsletter 102:5-7.

CHACE, F.A. 1951. The oceanic crabs of the genera Planes and Pachygrapsus. Proceedings of the United States National Museum 101: 65-103.

DELLINGER, T., J.A. DAVENPORT & P. WIRTZ. 1997. Comparisons of social structure of Columbus crabs living on loggerhead sea turtles and inanimate flotsam. Journal of the Marine Biological Association of the United Kingdom 77: 185-194.

DODD, C.K, JR. 1988. Synopsis of the biological data on the loggerhead sea turtle Caretta caretta (Linnaeus 1758). U.S. Fish and Wildlife Service Biological Report 88(14). 110 p.

FRICK, M.G., C.A. QUINN & C.K. SLAY. 1999. Dermochelys coriacea (leatherback sea turtle), Lepidochelys kempi (Kemp’s ridley sea turtle), and Caretta caretta (loggerhead sea turtle). Pelagic foraging. Herpetological Review 30: 165.

FRICK, M.G., C.K. SLAY, C.A. QUINN, A. WINDHAM-REID, P.A. DULEY, C.M. RYDER & L.J. MORSE. 2000. Aerial observations of courtship behavior in loggerhead sea turtles (Caretta caretta) from southeastern Georgia and northeastern Florida. Journal of Herpetology 34: 153-158.

FRICK, M.G., K.L. WILLIAMS & L. PIERRARD. 2001. Summertime foraging and feeding by immature loggerhead sea turtles (Caretta caretta) from Georgia. Chelonian Conservation and Biology 4: 178-181.

FRICK, M.G., K.L. WILLIAMS, A.B. BOLTEN, K.A. BJORNDAL & H.R. MARTINS. 2004. Diet and fecundity of Columbus crabs, Planes minutus, associated with oceanic-stage loggerhead sea turtles, Caretta caretta, and inanimate flotsam. Journal of Crustacean Biology 24: 350-355.

HARTNOLL, R.G. 1963. The biology of manx spider crabs. Proceedings of the Zoological Society of London 141: 423-496.

HEDGPETH, J. 1957. Classification of marine environments and concepts of marine ecology. In: Hedgpeth, J. (Ed.) The Treatise on Marine Ecology and Paleontology Vol. I, Ecology. Memoir No. 67 of The Geological Society of America. Geological Society of America, NY. 550 pp.

MUSICK, J.A. & C.J. LIMPUS. 1997. Habitat utilization and migration in juvenile sea turtles. In: Lutz, P.L. & J.A. Musick (Eds.) The Biology of Sea Turtles. CRC Press, Boca Raton, pp. 137-163.

WILLIAMS, A.B. 1984. Shrimps, lobsters and crabs of the Atlantic coast of the eastern United States, Maine to Florida. Smithsonian Institute Press, Washington, DC, 550 pp.