Distribution of land hermit crabs and gastropod shell usage in mainland Puerto Rico

Background/Question/Methods

Coenobita clypeatus is a land hermit crab found in coastal forest and rocky shores all over Puerto Rico. As an anatomical trait, its carapace covers only its cephalothoraxes, so it uses a wide variety of gastropod shells to protect their abdomen from dehydration and predations. Populations of land hermit crab C. clipeatus had been well studied in Mona Island, Puerto Rico, but no studies have been done in the mainland Puerto Rico. For years C. clypeatus’s populations had gone through drastic reduction caused by removal from their habitat for commercial purposes, such as fishing bait or as pets. Gastropod shells are important for the growth of the crabs.  There is evidence that shows that the removal of shells from the habitat had caused the populations size structure to decrease.  The objectives of this ongoing study are to describe the demography of C. clypeatus, to determine and compare densities of hermit crabs among rocky shores and coastal forest and to study gastropod shell use by hermit crabs.  Our methods consisted of collecting organisms along transects in the rocky shore and the coastal forest.  We measured the length of the large chelae, and the aperture, width and length of the gastropod shell. 

Results/Conclusions

We captured a total of 195 individuals. There is difference between the densities of hermit crabs in the rocky shore (1 individual /25 m²) and in the coastal forest (7 individual/ 25 m²).  Hermit crabs in the coastal forest had a larger chelae size (6.7 mm ± 2.84) than the ones in the rocky shores (2.4 ±9.59).  We identified two species of gastropod in the rocky shores and seven species in the coastal forest.  Shell aperture, width and length varied with gastropod species.  Preliminary results showed a difference in size of chelae and gastropod shell use between the two sampling sites.  Coastal forest offers a diverse environment for the hermit crabs in terms of availability of shells and protection against predators.  These factors have an effect on the growth and survival of the species.  Protection of coastal forests and control of beachcombers will benefit the populations of hermit crabs.