Preprint volume - SIBM

Pre-print Volume - Posters
BENTHOS COMMITEE
F. BELCI, R. MUSSAT SARTOR, N. NURRA, D. PESSANI
Laboratory of Zoology and Marine Biology, Department of Animal and Human Biology, Torino University, Via
Accademia Albertina, 13 - 10123 Torino, Italia.
fabio.belci@unito.it
POPULATION BIOLOGY OF THE HERMIT CRAB
CESTOPAGURUS TIMIDUS IN TWO POSIDONIA OCEANICA BEDS
BIOLOGIA DI POPOLAZIONE DEL PAGURO CESTOPAGURUS TIMIDUS
IN DUE PRATERIE DI POSIDONIA OCEANICA
Abstract – Two populations of the hermit crabs C. timidus were analyzed as regards population structure,
sex ratio, size, presence of ovigerous females and juveniles. The specimens were collected in two beds, one
off Punta Manara (Ligurian sea), the other off Capo Stella (Elba Island), in three periods of the year and at
seven depths. A direct connection between the status of the bed and the structure of the population is
hypothesized. The hermit crabs live preferentially in the shallower stations and occupy Bittium latreillii
shell.
Key-words: decapods, seagrass bed, population structure, Ligurian Sea, Elba Island.
Introduction - Inside the hermit crab group living associated with the foliar stratum of
Posidonia oceanica bed, Cestopagurus timidus (Roux 1830) is considered the most
abundant species (Borg & Schembri, 2000), even if this little crab lives also on rocky
bottoms. The relationships between the hermit crab and the P. oceanica environment
have been already studied by Zupo et al. (1989) and Belci et al. (2010). The aim of this
work is to analyze the population biology of C. timidus in two posidonia beds,
similarly exposed but in a different status of health and conservation.
Materials and methods – The hermit crabs were collected in a P. oceanica bed off
Punta Manara (P.M.) (Ligurian Sea) and in another off Capo Stella (C.S.) (Elba
Island), by means of the hand-towed net semi-quantitative technique, in three periods
(early spring, ES; late spring, LS; autumn, AU), at 7 different depths (from the upper to
the lower limit). In each station, a sample and its replicate were collected.
Results – Actually the P.M. bed extends from -5 to -21 m, showing a strong reduction
of the surface and the withdrawal of the lower limit, now of regressive type. Shoot
density values are low along the bed, which shows zones of dead matte and increasing
turbidity. On the contrary, C.S. bed, extending from -5 m to -30 m, shows a progressive
lower limit and good values of shoot density, especially at the upper limit and the
intermediate zone. In P.M. bed, 738 specimens were collected, 354 adults (47%), and
384 juveniles (sex not detectable, 53%). The adult population is made mainly by nonovigerous
females which in AU reach the 87% of the collected hermit crabs; the
ovigerous females are present in LS (41% of the hermit crabs) and AU and on the
whole represent the 11% of all the specimens collected. The males (on the whole
representing 7% of both communities) are almost absent in ES and LS. In respect to
depth, the hermit crabs are numerous at -5, -9 (shallower stations) and -15 m
(intermediate zone of the bed), almost absent near the lower limit (-21 m). The nonovigerous
females have the least mean size (cephalotoracic shield length) (1.02 mm,
n=291) compared with males (1.12 mm, n=25) and the ovigerous females (1.30 mm,
41 st S.I.B.M. CONGRESS Rapallo (GE), 7-11 June 2010
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