Abstract The aim of this study was to characterize the pattern of gastropod shell occupation in the field and selection of shell size and type under laboratory conditions by the hermit crab Pagurus criniticornis (Dana, 1852), inhabiting the infralittoral area of Anchieta Island, São Paulo, Brazil. Hermit crabs were obtained monthly during 1999 by SCUBA diving. For experiments under laboratory conditions, samplings were performed in 2002. The hermit crabs occupied 16 species of gastropods shells. Cerithium atratum (Born, 1778) was the most occupied shell (89.31%), followed by Morula nodulosa (4.73%) (Adams, 1845). No difference was observed in the pattern of occupation between males and females. The equations that best demonstrated the relationship between hermit crabs and their shells were those that involved Shell Wet Weight (SWW) and Shell Internal Volume (SIV). The laboratory experiments were in accordance to the pattern of occupation observed in the field; the mean value of SAI (Shell Adequacy Index) recorded to the population studied was 1.13 with a trend to increase this value in the last size classes. The results obtained corroborate with the hypothesis of the occupation process of shells governed not only by availability of shells, but also by its architecture. In addition, the shell stock in the area is one another important condition related to the exhibited pattern of shell occupation by P. criniticornis, and allows the stable coexistence among the island assemblage. The pattern of occupation observed promotes a high reproductive profile for the population studied, maximizing the populational growth.

A população do ermitão Pagurus criniticornis (Dana, 1872) foi avaliada com base na abundância sazonal, distribuição da freqüência de tamanho, razão sexual, período reprodutivo, fecundidade e relação com a concha ocupada. Os espécimes foram coletados mensalmente por meio de mergulho autônomo no infralitoral da Ilha Anchieta, Ubatuba. Um total de 1.017 indivíduos foi analisado. O tamanho dos animais (comprimento mínimo e máximo do escudo cefalotorácico, respectivamente) foi de 0,7 e 2,9 mm para os machos, 0,6 e 2,8 mm para as fêmeas não ovígeras e 1,0 e 2,5 mm para as fêmeas ovígeras. A razão sexual foi de 1:1,29. Foi observado dimorfismo sexual em função da presença de machos nas maiores classes de tamanho. Fêmeas ovígeras foram capturadas em todos os meses do ano, com a porcentagem variando de 8% (Julho) a 84,3% (Fevereiro) em relação ao total de fêmeas coletadas. A fecundidade média foi de 168 ± 125 ovos e tendeu a ser maior com o aumento do tamanho do ermitão. Fêmeas ovígeras de P. criniticornis foram encontradas ocupando quatro espécies de conchas de gastrópodes [Cerithium atratum (Born, 1778), Morula nodulosa (Adams, 1845), Anachis lyrata (Sowerby, 1832) e Modulus modulus (Linnaeus, 1758)] no entanto, a fecundidade não foi significativamente diferente em relação aos diferentes tipos de conchas. A população de P. criniticornis apresentou um perfil reprodutivo contínuo e intenso, possivelmente relacionado a estratégias de desenvolvimento que compensam a competição interespecífica existente na área de estudo.

The population of the hermit crab Pagurus criniticornis (Dana, 1852) was studied based on seasonal abundance, size frequency distribution, sex ratio, reproductive period, fecundity and shell relationship. Specimens were collected monthly by SCUBA diving in the infralittoral area of Anchieta Island, Ubatuba. A total of 1,017 individuals was analyzed. Animal size (minimum and maximum shield length, respectively) was 0.7 and 2.9 mm for males, 0.6 and 2.8 mm for non-ovigerous females, and 1.0 and 2.5 mm for ovigerous females. The sex ratio was 1:1.29. Sexual dimorphism was recorded by the presence of males in the largest size classes. Ovigerous females were captured during all months along the year, with percentages varying from 8% (July) to 84.3% (February) in relation to the total females collected. Mean ± SD fecundity was 168 ± 125 eggs and tended to increase with increasing hermit size. Shells of four gastropod species [Cerithium atratum (Born, 1778), Morula nodulosa (Adams, 1845), Anachis lyrata (Sowerby, 1832) and Modulus modulus (Linnaeus, 1758)] were occupied by ovigerous females of P. criniticornis but fecundity was not significantly different in relation to the different shell types. The profile showed continuous and intense reproduction of P. criniticornis probably related to strategies developed to compensate for interspecific competition in the studied insular area.

O objetivo do presente estudo foi caracterizar a composição dos caranguejos Majoidea habitantes do infralitoral rochoso/arenoso da Ilha Anchieta (Ubatuba), litoral norte do Estado de São Paulo. Os caranguejos foram coletados mensalmente durante 1999 e 2000, por mergulho autônomo e armadilhas. Foi registrada a ocorrência de oito espécies de caranguejos majóideos (Epialtus brasiliensis, Mithraculus forceps, Mithrax braziliensis, Mithrax hispidus, Mithrax tortugae, Pelia rotunda, Pitho lherminieri e Stenorhynchus seticornis) o que corresponde a 24,2% do total de majóideos registrados para o litoral do Estado de São Paulo. Mithraculus forceps, Mithrax tortugae e Stenorhynchus seticornis constituem populações frequentes na comunidade da área de estudo. A taxoeconose da área foi dominada por M. foreceps (57,62%). A ocorrência de Mithrax braziliensis no presente estudo amplia sua distribuição sul para o Estado de São Paulo.

The aim of the present study was to characterize the composition of Majoidea crabs inhabiting the infralittoral rocky/sandy bottom of Anchieta Island (Ubatuba), from north coast of São Paulo State. Crabs were collected monthly during 1999 and 2000 by using SCUBA and cages. The occurrence of eight species of majoids (Epialtus brasiliensis, Mithraculus forceps, Mithrax braziliensis, Mithrax hispidus, Mithrax tortugae, Pelia rotunda, Pitho lherminieri and Stenorhynchus seticornis) was registered and corresponded to 24.2% of total of majid crabs recorded in the coast of São Paulo State. Mithraculus forceps, Mithrax tortugae and Stenorhynchus seticornis were most abundant in samples and constituted populations that belonged to the brachyuran community of the studied area. The taxocoenosis of the area was dominated by M. forceps (57.62%). The occurrence of Mithrax braziliensis in the present study enlarged its south distribution to the São Paulo State.