Abundance and behavior of the sally lightfoot crab (Grapsus grapsus) in the colony of the brown booby (Sula leucogaster) in the Sao Pedro and Sao Paulo Archipelago

Abstract

For nine days, the number of sally lightfoot crabs (Grapsus grapsus) in a 300 m² area within the brown booby (Sula leucogaster) colony and the activity of each crab were recorded in the morning (5:30-7:30 h), at midday (11:30-13:30 h), and in the afternoon (15:30-17:30 h). The average number of individuals was 80.2 ± 15.5 (59-111) in the morning, 3.1 ± 2.3 (0-7) at midday, and 65.3 ± 15 (49-92) in the afternoon. A total of 1338 activity records were made, with the crabs standing still or walking in 844 records and handling material or feeding in 494 records. In the latter records, 53% of the crabs were handling detritus (pieces of dry bird excrement and sediment), 12.2% were handling feathers, 12.8% were feed- ing on regurgitated fish, 8.9% were feeding on dead Sula leucogaster, 8.1% were feeding on crab, 4.7% were feeding on material adhered to egg shells, and 0.6% were feeding on live nestlings of S. leucogaster. The low frequency of crabs in the colony around noon shows that the crabs avoid insolation, hiding in shady places and tidal pools. In the Sao Pedro and Sao Paulo Archipelago, the presence of S. leucogaster represents an important food source for G. grapsus. This crab plays the role of a cleaner in the S. leucogaster colony, consuming food remains, broken eggs, and dead birds.