MTN 90:11-13 Additional Evidence Supporting a Cleaning Association between Epibiotic Crabs and Sea Turtles: How will the Harvest of Sargassum Seaweed Impact this Relationship?

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Additional Evidence Supporting a Cleaning Association between Epibiotic Crabs and Sea Turtles: How will the Harvest of Sargassum Seaweed Impact this Relationship?

Michael G. Frick, Kristina L. Williams & David Veljacic
Caretta Research Project, P.O. Box 9841, Savannah, Georgia 31412, USA (E-mail: caretta05@aol.com)

Several studies have documented the occurrence of the Columbus crab (Planes minutus) as an epibiont of sea turtles in the Atlantic Ocean (Caine 1986; Chace 1951; Crane 1937; Davenport 1994; Dellinger et al. 1997; Frick et al. 1998; Roux 1828). Currently, three species of sea turtles in the Atlantic are reported as hosts of P. minutus: the loggerhead (Caretta caretta; Dodd 1988), green (Chelonia mydas; Crane 1937), and hawksbill (Eretmochelys imbricata; Witzell 1983) turtles. On sea turtles, P. minutus is commonly found clinging, with the walking legs, onto the underside of the turtle around the tail, cloaca, and hind limbs (Chace 1951; Crane 1937; Davenport 1994). This common attachment site led early reports to suggest that epibiotic P. minutus were possibly coprophagus, feeding upon the feces of the host turtle (Chace 1951; Crane 1937). Crane (1937) reported the stomach contents of one P. minutus, removed from the cloacal region of an adult green turtle. She found that the stomach of the crab contained finely digested animal material and suggested that it probably represented the excrement of the turtle. However, Davenport (1994) suggested that if P. minutus is in fact coprophagus then one would expect a P. minutus removed from a C. mydas to have stomach contents comprised mainly of plant matter since the green turtle is largely herbivorous (see Bjorndal 1985). Instead, Davenport (1994) reports that P. minutus apparently feeds within and upon a turtle’s epibiotic community. His findings were based upon stomach contents of one adult female P. minutus that was removed from a juvenile loggerhead captured off the coast of Madeira. He found that the crab had eaten several items that have been commonly reported as epibionts of Caretta (e.g. Lepas barnacle cyprids, commensal amphipods, and other P. minutus). Additionally, Davenport (1994) suggests that P. minutus may also utilize the host turtle as a platform for capturing night-time neuston, since the aforementioned crab had also fed upon the eyes and antennae of a nocturnal euphausid. Davenport (1994) also proposed that the postero-ventral positioning of P. minutus upon a host turtle is probably a reflection of the available hiding / grasping sites on the turtle as well as a refuge from turbidity created by the hydrodynamics of the turtle carapace.

A study by Dellinger et al. (1997) reports that P. minutus from juvenile and subadult loggerheads off the coast of Madeira were larger than P. minutus collected from flotsam in the same area. Additionally, heterosexual crab pairs and ovigerous females were also common on turtles but uncommon on local flotsam. These findings suggest that turtles are the most prolific habitat for reproductive efforts by P. minutus. Dellinger et al. (1997) also noted a higher occurrence of juvenile P. minutus on local flotsam than on turtles. It is possible that such habitats are important nursery grounds for young Columbus crabs and they may also represent areas where turtles initially acquire P. minutus.

Caine (1986) and Frick et al. (1998) report three additional epibiotic crab species associated with loggerheads: mottled shore crabs (Pachygrapsus sp.), common mud crabs (Panopeus herbstii), and Say’s mud crabs (Neopanope sayi). During the 1998 nesting season in Georgia, USA (May-August) we collected epibiotic crabs (N. sayi, P. herbstii, and P. minutus) from nesting loggerheads (see Frick et al. 2000 for barrier island study site localities). Twenty-two crabs, from 21 individual turtles, were collected. Stomach contents were obtained from 4 P. minutus, 5 N. sayi, and 2 P. herbstii. Eleven P. minutus were sent to Dr. Raymond Manning at the Smithsonian Institute in Washington, D.C. for deposition into the museum’s specimen collection. Epibiotic crabs were collected from the postero-dorsal region of sampled turtles (see Frick et al. 1998), although we did not look for crabs in the postero-ventral area. However, one individual P. minutus was collected from a loggerhead nest during nest relocation. It is assumed that the crab was displaced from a postero-ventral position while the turtle was nesting. Future investigations of nesting loggerheads should include the postero-ventral area.

Stomach contents from P. minutus yielded brown algae, amphipods (Podocerus chelonophilus, Dulichiella appendiculata, and Caprella andrea), and barnacle cyprids (Balanidae). Say’s mud crabs (N. sayi) had consumed P. minutus, amphipods (Caprella sp. and Corophium sp.), leeches (Ozobranchus margoi), brown algae, and green algae. Common mud crabs (P. herbstii) had consumed brown algae and O. margoi.

Our findings support Davenport’s (1994) conclusion that P. minutus, as well as other epibiotic crabs, aid in cleaning epibiota from loggerheads. It is also possible that epibiotic crabs glean epibiota from other sea turtle species that have been reported to host epibiotic crabs. More research is needed to determine the frequency of this observed relationship.

In addition to their residency on nesting loggerheads, mud crabs commonly occur in a variety of habitats along the East Coast of the United States where they feed upon oyster spat, young oysters, periwinkle snails, and other crabs (Heard 1982). Their presence on nesting loggerheads is probably attributed to the coastal residency of turtles during the nesting season, as they have not been reported on non-nesting turtles. Columbus crabs are reliant on buoyant material (including turtles) for their survival and are most often associated with the floating brown alga, Sargassum, hence their other common name: the gulf-weed crab (Davenport 1994). Currently, it is believed that Sargassum mats in the Sargasso Sea support the most extensive breeding congregations of P. minutus anywhere in the North Atlantic Ocean. However, additional evidence suggests that turtles also offer P. minutus exceptional breeding substratum (Dellinger et al. 1997). Davenport (1992) provides the most detailed description of the ecology of P. minutus.

Logan and Morreale (1994) reported the adverse effects of epibiont colonization on sea turtles. Epibiotic drag drastically alters the hydrodynamic characteristics of the turtle carapace and in turn increases the amount of energy needed for the turtle to swim. Thus, it is assumed that any activity or organism that rids a turtle of its epibiota would be beneficial to the host turtle. By removing small epibionts and epibiont larvae, epibiotic crabs could be considered ‘beneficial symbionts’ of the sea turtles they are associated with. Yet, the importance of their occurrence relative to the particular life stages of sea turtles needs further elucidation. For example, relatively large subadult and adult sea turtles would undoubtedly have an easier time compensating for increased drag as a result of a heavy epibiont load than would smaller pelagic stage juvenile sea turtles. Thus, their importance in removing epibionts from turtles might decrease as the turtles become larger. However, since small pelagic stage sea turtles do not have the means by which to scrape away epibiota (i.e. on benthic substrates) nor the strength to compensate for a heavy epibiont load, during this life stage an epibiotic crab may be more important. With this in mind, special attention is needed to collect information on epibiotic crabs associated with pelagic stage juvenile sea turtles. If P. minutus routinely gleans epibiota from juvenile sea turtles than their presence and abundance could ultimately influence the overall health of sea turtle populations that have an oceanic pelagic stage. Moreover, the possibility exists that habitats that support the reproductive efforts of P. minutus (i.e Sargassum mats) could be just as essential for the survival of juvenile sea turtles. In light of the aforementioned possibilities and the need for further research on epibiotic crabs, it seems that the recent decision by the National Marine Fisheries Service to allow for the harvest of Sargassum seaweed (see Evans 2000) may have more deleterious effects than the accidental capture of an occasional juvenile sea turtle.

Acknowledgements: We would like to thank Randy Isbister, Robert A. Moulis, Mark Dodd, Adam MacKinnon, Barb Zoodsma, John Robinette, Charles Warnock, Andrew Jackson, Stacie Knight, Lianne Pierrard, Peter Range, Wayne Witzell, the Wassaw Island Trust, the Turner Foundation, and the Courtney Knight-Gaines Foundation. Additionally, we would like to thank John Davenport for providing helpful comments on our manuscript as well as pertinent literature. Finally, we would like to give our most sincere farewells and thanks to the late Dr. Raymond Manning who aided in the stomach content analysis of epibiotic crabs and provided us with much of the literature cited herein.

BJORNDAL, K.A. 1985. Nutritional ecology of sea turtles. Copeia 1985: 736-751.

CAINE, E.A. 1986. Carapace epibionts of nesting loggerhead sea turtles: Atlantic coast of U.S.A. Journal of Experimental Marine Biology and Ecology 95: 15-26.

CHACE, F.A. 1951. The oceanic crabs of the genera Planes and Pachygrapsus. Proceedings of the United States National Museum, 101, No. 3272: 65-103.

CRANE, J. 1937. The Templeton Crocker expedition. III. Brachygnathous crabs from the Gulf of California and the west coast of lower California. Zoologica New York 22: 47-78.

DAVENPORT, J. 1992. Observations on the ecology, behaviour, swimming mechanism and energetics of the neustonic grapsid crab, Planes minutus. Journal of the Marine Biological Association of the United Kingdom 72: 611-620.

DAVENPORT, J. 1994. A cleaning association between the oceanic crab Planes minutus and the loggerhead sea turtle Caretta caretta. Journal of the Marine Biological Association of the United Kingdom 74: 735-737.

DELLINGER, T., J. 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. 1988. Synopsis of the biological data on the loggerhead sea turtle Caretta caretta (Linnaeus 1758). Biological Report of the U.S. Fish and Wildlife Service 88(14), 110 pp.

EVANS, D. 2000. US NMFS rejects proposed ban on the harvest of Sargassum seaweed. Marine Turtle Newsletter 87: 3.

FRICK, M.G., K. L. WILLIAMS & M. ROBINSON. 1998. Epibionts associated with nesting loggerhead sea turtles (Caretta caretta) in Georgia, USA. Herpetological Review 29(4): 211-214.

FRICK, M. G., K. L. WILLIAMS, D. VELJACIC, L. PIERRARD, J. A. JACKSON & S. E. KNIGHT. 2000. Newly documented epibiont species from nesting loggerhead sea turtles (Caretta caretta) in Georgia, USA. Marine Turtle Newsletter 88: 3-5.

HEARD, R.W. 1983. Guide to common tidal marsh invertebrates of the northeastern Gulf of Mexico. Mississippi-Alabama Sea Grant Consortium MASGP-79-004, 82 pp.

LOGAN, P. & S. J. MORREALE. 1994. Hydrodynamic drag characteristics of juvenile L. kempi, C. mydas, and C. caretta. In: B. A. Schroeder and B. E. Witherington (Eds.). Proceedings of the thirteenth annual symposium on sea turtle biology and conservation. NOAA Technical Memorandum NMFS-SEFSC-341, pp. 248-252.

ROUX, P. 1828. Crustaces de la Mediterranee de son littoral. Paris, 176 pp.

WITZELL, W. N. 1983. Synopsis of biological data on the hawksbill turtle, Eretmochelys imbricata (Linnaeus, 1766). FAO Fisheries Synopsis 137, 78 pp.