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Investigaciones marinas

versión On-line ISSN 0717-7178

Investig. mar. v.35 n.2 Valparaíso nov. 2007

http://dx.doi.org/10.4067/S0717-71782007000200012 

 

Invest. Mar., Valparaíso, 35(2): 121-125, 2007

Notas Científicas

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*

Abundancia y comportamiento del cangrejo costero (Grapsus grapsus) en la colonia de piquero café (Sula leucogaster) en el archipiélago de Sao Pedro y Sao Paulo*

 

Dimas Gianuca1 & Carolus Maria Vooren1

1Laboratorio de Elasmobránquios e Aves Marinhas, Departamento de Oceanografía, Universidade Federal de Río Grande (FURG), P.O.Box 474, 96201-900, Río Grande (RS), Brasil.


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 feeding 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.

Key words: Sao Pedro and Sao Paulo Archipelago, Sula leucogaster, Grapsus grapsus, Brazil.


RESUMEN. Durante nueve días se registró el número de cangrejos costeros (Grapsus grapsus) en una área de 300 m2 dentro de la colonia de piquero café (Sula leucogaster) y la actividad de cada uno de los individuos durante la mañana (5:30-7:30 h), mediodía (11:30-13:30 h) y tarde (15:30-17:30 h). La media de individuos en la mañana fue 80,2 ± 15,5 (59-111), en el mediodía 3,1 ± 2,3 (0-7) y en la tarde 65,3 ± 15 (49-92). Del total de 1.338 registros de comportamiento, 844 fueron de cangrejos detenidos o caminando y 494 manipulando algún material o alimento. De éstos, 53% fueron de manipulación de detrito (excremento seco de ave y sedimento); 12,2% de manipulación de plumas; 12,8% alimentándose de peces regurgitados; 8,9% de un Sula muerto; 8,1% de cangrejo; 4,7% de material adherido a las cascaras de huevos y 0,6% de cría de Sula viva. La baja frecuencia de los cangrejos en la colonia durante el mediodía es una evidencia que los mismos se protegen de la insolación, permaneciendo en la sombra y en pozas de marea. En el archipiélago de Sao Pedro y Sao Paulo la presencia de S. leucogaster representa una importante fuente de alimentación para G. grapsus, que desempeña un papel de limpiador en la colonia de esta ave, consumiendo restos de comida, huevos quebrados y aves muertas.

Palabras clave: archipiélago de Sao Pedro y Sao Paulo, Sula leucogaster, Grapsus grapsus, Brasil.


 

Grapus grapsus is a marine crab that inhabits rocky shores and islands of the Atlantic and Pacific coasts of the Americas. The species ranges from tropical latitudes in the Northern Hemisphere to temperate latitudes in the Southern Hemisphere (Rodriguez, 1980; Melo, 1996). Specimens can reach a total carapace length of 7.9 cm (Romero, 2003).

According to Romero (2003), in the bay of Ancón, Perú, G. grapsus feeds on cirripedes, mytilids, and porcelain crabs and occasionally acts as a scavenger. This crab also feeds on young sea turtles in the Brazilian Fernando de Noronha Islands (Bellini, 1996). In the Galapagos Islands, G. grapsus is a predator of newly hatched blue-footed boobies (Sula nebouxii) and masked boobies (S. dactylatra).

Three species of marine birds nest in the Sao Pedro and Sao Paulo Archipelago: the brown or common noddy (Anous stolidus), the black noddy (A. minutus, Family Laridae), and the brown booby {Sula leucogaster, Sulidae). G. grapsus is numerous in this archipelago, where it circulates and forages amongst the bird nests (Darwin, 1891; Murphy, 1936; Nelson, 1978; Both & Freitas, 2004; Cavalcanti, 2004). It also remains on the lookout, watching for opportunities to catch and devour newly hatched young (Both & Freitas, 2004). Murphy (1936) and Nelson (1978) indicate that this crab is a threat to the young of the birds that nest in the Sao Pedro and Sao Paulo Archipelago.

The Sao Pedro and Sao Paulo Archipelago (00°55'01"N, 29°20'44"W) is located 100 km north of the Equator and 1,010 km off the coast of Brazil (Fig. 1). It is made up of five small rocky islands with a total emerged area of 1.7 km2. The sun rises between 5:30 and 6:00 and sets between 17:45 and 18:15 local time (official time of Brasilia + lh).


The S. leucogaster individuals are concentrated on Belmonte Island, where there is a breeding colony of around 170 pairs (Cavalcanti, 2004). This island is 100 m long, 60 m wide, and reaches 17 m in altitude; there is one house (49 m2) on the island that is always occupied. The area selected for the present study was located near the house, where the S. leucogaster nests are particularly abundant (Edwards et al, 1981; Both & Thales, 2004). The nests are distributed on the rocky ground from approximately 2 to 16 m above the level of the high tide. The number of G. grapsus individuals and their activity were observed for nine days, from 17 to 25 August 2004, in an area of 300 m2 within the S. leucogaster colony, at three times: morning (5:30-7:30 h), midday (11:30-13:30 h), and afternoon (15:30-17:30 h). Abundance was measured as the average number of crabs at each sampling time. Variations in crab abundance between sampling times were studied using analysis of variance (ANOVA) and the Tukey test (Zar, 1999). The observations were done with the naked eye and the percentage of cloud cover was estimated daily.

The nests within the study area were counted and a sample set of 30 nests was selected at random to determine nest content and estimate the percentage of brown booby pairs with recently hatched young in the colony. These 30 nests were monitored daily to evaluate the loss of eggs and young.

The presence of G. grapsus was verified in all the morning and afternoon observations, with respective averages of 80.2 + 15 (59-111) and 65.3 + 15.5 (49-92) individuals; the highest density recorded was 0.37 crabsm"2. The average number of specimens at midday, 3.1 + 2.3 (0-7), differed significantly (p < 0.001) from the morning and afternoon abundances (Fig. 2). The average daily cloud cover was 26.4% and the greatest crab abundance at midday was recorded on the cloudiest day of the study period (Fig. 3). Of a total of 1,338 behavior records, 63.1% were of crabs standing still or walking and 36.9% were of crabs manipulating some material or feeding. Seven items were identified that were explored by the crabs: detritus (dry bird excrement, sediment), S. leucogaster egg shells, S. leucogaster feathers, fish regurgitated by S. leucogaster, dead S. leucogaster, S. leucogaster young (recently hatched, live), and crabs (parts of G. grapsus) (Table 1). During the nine days of observations the crabs killed only one S. leucogaster chick, and this occurred outside the nest where this chick had hatched (Fig. 4).





A total of 115 nests were found in the study area. At the beginning of the observations, 16 (33.3%) of the 30 nests monitored contained eggs and/or young: four nests had one egg and one young, five had one young, six had two eggs, and one nest had only one egg. Considering the ratio of 0.3 recently hatched young per nest, it was estimated that there were 34.5 recently hatched young in the study area when the observations began. The eggs in the nests with an egg and a nestling hatched during the study period. The youngest offspring survived between one and four days. No mortality was observed in nestlings without siblings. In two of the nests that contained two eggs, one of these was lost.

The scarcity of crabs in the colony at midday indicates that the crabs protect themselves from insolation at this time of day, hiding in shady areas and tidal pools. This hypothesis is supported by the abiotic data obtained in April 2001 by Medevedovski (2001); this author reported temperatures of the rocky soil in the study area that were 26-36°C in the morning (6-9 h), 33-40°C at midday (11-14 h), and 26-34°C in the afternoon (15-18 h). The number of G. grapsus was also reduced at midday in emerged sites along the Peruvian coast (Romero, 2003). The present study was carried out during the dry season (Innocentini et ah, 2000); it is likely that reduced G. grapsus abundance at midday is less marked during the rainy season (February and March) when cloudy skies are predominant. S. leucogaster reproduces throughout the year (Nelson, 1978; Martins & Dias, 2003; Branco, 2003; Both & Freitas, 2004). During storms, waves reach the emerged part of Belmonte Island, destroying S. leucogaster nests and eggs and killing the young (Both & Freitas, 2004; Cavalcanti, 2004). Such events provide the sally lightfoot crabs with abundant food.

Only one chick of S. leucogaster was killed by crabs during the present study. This number is very low in view of the fact that 34 recently hatched chick where estimated in the study area. S. leucogaster usually lays two eggs and the two chicks hatch a few days apart. The second to be born is always expelled from the nest by the firstborn, characterizing this species as one of obligatory fratricide (Nelson, 1978; Both & Freitas, 2004; Cavalcanti,2004).Murphy(1936)andNelson(1978) state that G. grapsus causes important mortality of S. leucogaster chicks. However the present study is evidence that on Belmonte Island the predation by G. grapsus occurs only on chicks expelled from the nest and therefore already eliminated from the reproductive process. Moreover, no mortality of nestlings without siblings was observed in the monitored nests. G. grapsus predation on chicks of the boobies S. dactylatra and S. nebouxii expelled from the nests was also reported in the Galapagos Islands (Lougheed & Anderson, 1999). In spite of potential predators on these islands other than crabs (e.g., birds and snakes), the only proven cause of mortality in recently hatched S. dactylatra and S. nebouxii was fratricide due to the efficient parental care of the surviving nestling (Anderson & Hodum, 1993; Lougheed & Anderson, 1999). On the other hand, on Belmonte Island, crabs may steal eggs and young from the nests when the colony is disturbed by humans (Darwin, 1891; Nelson, 1978).

The S. leucogaster colony on Belmonte Island represents a important food source for G. grapsus and may contribute to the abundance of these crabs in the area (up to 37 individuals-100 m2), which is greater than the abundance found on Mexican islands (maximum 12 individuals-100 m2) (Mille-Pagaza et ah, 2003) and along the coast of Peru (up to 70 individuals along 100 m of rocky coastline) (Soto et ah, 2002). This crustacean plays a cleaning role in the S. leucogaster colony, feeding on the organic matter that otherwise would accumulate in the area. Experiments of G. grapsus exclusion from nesting areas would allow us to study up to what point this crustacean has a positive or negative influence on the reproductive success of S. leucogaster on Belmonte Island.

ACKNOWLEDGEMENTS

The authors thank the "Secretaria da Comissao Interministerial para os Recursos do Mar (SECIRM)" for logistic and financial support of the project "Monitoramento das aves do Arquipélago de Sao Pedro e Sao Paulo", with which this work is connected. They are also grateful to Paula Laporta and Silvina Botta for help with the Spanish language.

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Received: 16 May 2005; Accepted: 17 August 2007

Corresponding author: Dimas Giamuca (dmsgianuca@hotmail.com)

*This work was presented at the XXV Congreso de Ciencias del Mar de Chile and the XI Congreso Latinoamericano de Ciencias del Mar (COLACMAR), in Viña del Mar, 16-20 May 2005.