Preprint volume - SIBM
Preprint volume - SIBM
Preprint volume - SIBM
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Pre-print Volume - Posters<br />
BENTHOS COMMITEE<br />
F. BELCI, R. MUSSAT SARTOR, N. NURRA, D. PESSANI<br />
Laboratory of Zoology and Marine Biology, Department of Animal and Human Biology, Torino University, Via<br />
Accademia Albertina, 13 - 10123 Torino, Italia.<br />
fabio.belci@unito.it<br />
POPULATION BIOLOGY OF THE HERMIT CRAB<br />
CESTOPAGURUS TIMIDUS IN TWO POSIDONIA OCEANICA BEDS<br />
BIOLOGIA DI POPOLAZIONE DEL PAGURO CESTOPAGURUS TIMIDUS<br />
IN DUE PRATERIE DI POSIDONIA OCEANICA<br />
Abstract – Two populations of the hermit crabs C. timidus were analyzed as regards population structure,<br />
sex ratio, size, presence of ovigerous females and juveniles. The specimens were collected in two beds, one<br />
off Punta Manara (Ligurian sea), the other off Capo Stella (Elba Island), in three periods of the year and at<br />
seven depths. A direct connection between the status of the bed and the structure of the population is<br />
hypothesized. The hermit crabs live preferentially in the shallower stations and occupy Bittium latreillii<br />
shell.<br />
Key-words: decapods, seagrass bed, population structure, Ligurian Sea, Elba Island.<br />
Introduction - Inside the hermit crab group living associated with the foliar stratum of<br />
Posidonia oceanica bed, Cestopagurus timidus (Roux 1830) is considered the most<br />
abundant species (Borg & Schembri, 2000), even if this little crab lives also on rocky<br />
bottoms. The relationships between the hermit crab and the P. oceanica environment<br />
have been already studied by Zupo et al. (1989) and Belci et al. (2010). The aim of this<br />
work is to analyze the population biology of C. timidus in two posidonia beds,<br />
similarly exposed but in a different status of health and conservation.<br />
Materials and methods – The hermit crabs were collected in a P. oceanica bed off<br />
Punta Manara (P.M.) (Ligurian Sea) and in another off Capo Stella (C.S.) (Elba<br />
Island), by means of the hand-towed net semi-quantitative technique, in three periods<br />
(early spring, ES; late spring, LS; autumn, AU), at 7 different depths (from the upper to<br />
the lower limit). In each station, a sample and its replicate were collected.<br />
Results – Actually the P.M. bed extends from -5 to -21 m, showing a strong reduction<br />
of the surface and the withdrawal of the lower limit, now of regressive type. Shoot<br />
density values are low along the bed, which shows zones of dead matte and increasing<br />
turbidity. On the contrary, C.S. bed, extending from -5 m to -30 m, shows a progressive<br />
lower limit and good values of shoot density, especially at the upper limit and the<br />
intermediate zone. In P.M. bed, 738 specimens were collected, 354 adults (47%), and<br />
384 juveniles (sex not detectable, 53%). The adult population is made mainly by nonovigerous<br />
females which in AU reach the 87% of the collected hermit crabs; the<br />
ovigerous females are present in LS (41% of the hermit crabs) and AU and on the<br />
whole represent the 11% of all the specimens collected. The males (on the whole<br />
representing 7% of both communities) are almost absent in ES and LS. In respect to<br />
depth, the hermit crabs are numerous at -5, -9 (shallower stations) and -15 m<br />
(intermediate zone of the bed), almost absent near the lower limit (-21 m). The nonovigerous<br />
females have the least mean size (cephalotoracic shield length) (1.02 mm,<br />
n=291) compared with males (1.12 mm, n=25) and the ovigerous females (1.30 mm,<br />
41 st S.I.B.M. CONGRESS Rapallo (GE), 7-11 June 2010<br />
262