Population structure and growth of the hermit crab
Pagurus brevidactylus (Anomura: Paguridae) from the

northern coast of Sa‹ o Paulo, Brazil

F.L. Mantelatto*$, R.A. ChristofolettiO and W.C. ValentiP

*Departamento de Biologia, FFCLRP, Universidade de Sa‹ o Paulo (USP), Av. Bandeirantes, 3900, CEP 14.040-901, Ribeira‹ o Preto,
Sa‹ o Paulo, Brazil. OCampus do Litoral Paulista, Unidade Sa‹ oVicente, Universidade Estadual Paulista (UNESP), CEP 11330-900,

Sa‹ oVicente, Sa‹ o Paulo, Brazil. PDepartamento de Biologia Aplicada, FCAV, Universidade Estadual Paulista (UNESP),
CEP 14884-900, Jaboticabal, Sa‹ o Paulo, Brazil. $Corresponding author, e-mail: £mantel@usp.br

The hermit crab Pagurus brevidactylus (Crustacea: Anomura: Paguridea) from the infralittoral area of Anchieta Island,
Ubatuba, was characterized by population structure (size, sex ratio, reproduction and recruitment) and growth. Animals
were collected monthly during 1999 by SCUBA diving. A total of 1525 individuals was collected (633 males and 892 females),
695 of them were ovigerous females. Overall sex ratio was 0.7:1 in favour of females. The crabs showed a unimodal distribution
with males signi¢cantly larger than females. Ovigerous females were collected during all months and in high percentages from
1.0mm of shield length, demonstrating intense and continuous reproduction. The longevity was approximately 24 months for
males and 18 for females, which showed larger growth rate and reached sexual maturity earlier (two months) than males. The
low number of males in this population may be due to the longer life span. Moreover, the sexual dimorphism favours males
during the intra- and interspeci¢c ¢ghts by shell, food, reproduction and territory. Females demonstrated a short life cycle and
intense reproduction.

Pagurus brevidactylus (Stimpson, 1859) is a small abundant
hermit crab found in the infralittoral zone of Anchieta Island
(Mantelatto & Garcia, 2002), as well as in the northern coast of
Sa‹ o Paulo State. The biological aspects of this species in the
western Atlantic region are poorly known (Mantelatto &
Meireles, 2004). Therefore, the objective of the present study
was to describe the population structure (size, sex ratio, repro-
duction and recruitment) and the growth in the ¢eld of the
population inhabiting Anchieta Island, Brazil.

Hermit crabs were collected monthly from January to
December 1999 in the infralittoral area of Anchieta Island
(238320S 458040W), which is an ecological reserve of the northern
coast of Sa‹ o Paulo State, Brazil. Animals were collected in the
daytime by two people during 30 minutes of SCUBA diving.

Animals were frozen and transported to the laboratory where
they were removed from their shells and immediately weighed.
Sex was determined from the gonopore position; the presence of
eggs at the pleopods determined ovigerous females, being the
maturity of males and females based on the size of the smallest
ovigerous females. Shield length (SL, from the tip of the rostrum
to theV-shaped groove at the posterior edge), measured under a
compound light microscope equipped with a camera lucida
(50�), was the measurement of size of the hermit crabs.

Hermit crabs were grouped in size-classes of 0.2mm SL, and
the sex ratio for each was calculated. The reproductive period
was evaluated by percentage of ovigerous females relative to the
total number of females collected monthly. The Kolmogorov^
Smirnov test was used to verify the normality of the size^
frequency distribution of the individuals and the Pearson correla-
tion to test for a correlation between the water temperature and
the total of individuals collected monthly or the percentage of
ovigerous females. The w2-test was used to evaluate the sex ratio
and an unpaired t-test to compare the mean size of males and
females.

For growth analysis, the modal shield length of each cohort
was obtained from the size^frequency distribution at three
month intervals. The von Bertalan¡y model was used to deter-
mine growth for each sex by means of the Ford^Walford
method using CAJUS software. The growth rate equation (VT)
was derived by using the expression of von Bertalan¡y, resulting
in: VT¼k(L?�Lt) (Valenti et al., 1993). The weight growth curve
was ¢tted by a deductive method using von Bertalan¡y’s model
and the regression coe⁄cient of the weight/shield length relation-
ship (R.A. Christofoletti & F.L. Mantelatto, unpublished data).

A total of 1525 individuals was collected, 633 (41.5%) were
males, 892 (58.5%) were females and 695 (45.6%) of them
were ovigerous females. There was an unimodal size
distribution for each sex and only males had a normal distribu-
tion (P40.05). Sexual dimorphism was observed in that males
reached larger sizes than females. The mean size of males
(SL¼1.94+0.36mm) was signi¢cantly larger than ovigerous
females (SL¼1.74+0.26mm) and both were signi¢cantly larger
than non-ovigerous females (SL¼1.57+0.28mm) (P50.001).
Shield size varied between 0.91 and 3.16mm for males, 1.01 and
2.46mm for non-ovigerous females and 1.06 and 3.19mm for
ovigerous females (minimum and maximum SL, respectively).

Overall sex ratio was 0.7:1 in favour of females and was signi¢-
cantly di¡erent from the expected 1:1 (P50.01). Females were
dominant in the smaller size-classes (approximately 70%), while
the percentage of males became progressively larger after 1.8mm
of SL.

Ovigerous females were collected during all months and in
high percentages (470% in all months), demonstrating intense
and continuous reproduction in this region. No signi¢cant corre-
lation (P40.05) was observed between water temperature and
percentage of ovigerous females. The smallest female (1.01mm
SL) belonged to the second size-class (1.0^1.2mm of SL),
whereas approximately 50% of the females were ovigerous and

Journal of the Marine Biological Association of the United Kingdom (2005)

J. Mar. Biol. Ass. U.K. (2005), 85, 127^128
Printed in the United Kingdom



with an increase of this percentage in the next size-classes. Avery
low number of immature individuals was found, although there
was a greater abundance of small specimens every three months
which showed a continuous recruitment.

In relation to growth, seven and ¢ve cohorts coexisting at the
same time were observed for males and females, respectively.
Males presented larger asymptotic length (L?¼2.87mm) and
weight (W?¼0.188 g) and smaller growth constant (K¼0.11)
than females (L?=2.18mm;W?¼0.075 g; K¼0.29) (Figure 1).

The individuals attained the morphological sexual maturity
(based on the smallest ovigerous female, SL¼1.06mm) at
approximately 2 and 3.5 months for females and males, respec-
tively, and from this age there was a marked weight gain for both
sexes (Figure 1). Female growth was faster than males at the start
of the life span, and reached the asymptotic length faster than
males (approximately 18 months for females and 24 for males).

The mean size of the individuals in the present study was
smaller than the size observed for the same species by
Negreiros-Fransozo et al. (1991). The di¡erence among mean
size of individuals for the same species, from di¡erent areas, has
been reported for other hermit crabs (Manjo¤ n-Cabeza & Garc|¤ a
Raso, 1994) and can be in£uenced by utilization of shells, preda-
tion, diet, competition and parasitism. Furthermore, the high
density and abundance of this and other hermit crab species
coexistent in the same area (Mantelatto & Garcia, 2002) may
promote competition by resources (Mantelatto & Meireles,
2004) and in£uence the size of individuals of this population
(Manjo¤ n-Cabeza & Garc|¤ a Raso, 1994).

Pagurus brevidactylus exhibited sexual dimorphism that is
important in selective pressure as intra- and interspeci¢c ¢ghts
for food, copulation, territory and shell. The females reached
sexual maturity earlier than males (approximately 2 and 3.5

months, respectively) in the life span. This fact emphasizes the
intense reproduction of P. brevidactylus, starting within the ¢rst
months of life. Females were captured in larger numbers,
reached smaller maximum size and showed larger growth rates
than males during the initial stages of the life cycle. Therefore a
larger number of females growing faster in a shorter time than
males, promoted an accumulation of females in the initial size-
classes and consequently the variation of sex ratio from 1:1 in
these classes. The proportion of males clearly increased with
larger sizes and in£uenced the sex ratio in function at size.

Pagurus brevidactylus exhibited longevity of approximately 24
months for males and 18 months for females, which is similar to
observed populations of other hermit crabs at similar sizes, such
as 24 to 27 months for the diogenid Diogenes pugilator (Manjo¤ n-
Cabeza & Garc|¤ a Raso, 1998) and 21 months for the pagurid
Cestopagurus timidus (Manjo¤ n-Cabeza & Garc|¤ a Raso, 1994).
Many di¡erences observed in life phases for males and females
can be related to habitat, pollution, food, shell utilization, stresses
and size of animals. However, the present study demonstrated a
well established population in Anchieta Island.

This work is part of a MSc thesis (Curso de Po¤ s-Graduac� a‹ o
em Cie“ ncias, Biologia Comparada da FFCLRP/USP) by
R.A.C. and was supported by a scholarship (Proc. 99/11680-3)
and research grants to F.L.M. (Proc. 98/07454-5) from FAPESP
and CNPq (Research Scholarship). We thank the Secretaria do
Meio Ambiente do Estado de Sa‹ o Paulo, IBAMA and Parque
Estadual da Ilha Anchieta for permission (Proc. 42358/98). Spe-
cial thanks are due to many colleagues from the Laboratory of
Bioecology and Systematic of Crustaceans, FFCLRP/USP who
helped with sampling and laboratory analysis. We also thank R.
Shimizu (USP), G. Bond-Buckup (UFRGS), and anonymous
referees for helpful criticism and suggestions during the ¢rst ver-
sion of this manuscript.

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Submitted 8 October 2003. Accepted 8 December 2004.

128 F.L. Mantelatto et al. Population structure of Pagarus brevidactylus

Journal of the Marine Biological Association of the United Kingdom (2005)

Figure 1. Pagurus brevidactylus. Curves of (A) growth in length
(SL¼2.87[17e70.11(t+1.64)] formalesandSL¼2.18[17e70.29(t+1.22)] for
females); (B) growth in weight (W¼0.188[17e70.11(t+1.64)]3.34 for males
and W¼0.075[17e70.29(t+1.22)]3.42 for females); and (C) growth rate
(VT) of the total of individuals collected from Anchieta Island.