Size structure and sex composition of Eurypodius latreillii Guérin , 1828 ( Brachyura ) and Pagurus comptus White , 1847 ( Anomura ) in southern Chile *

The decapod crustaceans Eurypodius latreillii Guérin 1828 (Brachyura) and Pagurus comptus White, 1847 (Paguridae) were the most frequently occurring species in Agassiz trawl samples obtained during the Chilean-German-Italian “Victor Hensen” Expedition (October / November 1994) in the Straits of Magellan and south of the Beagle Channel (Arntz et al., 1996a; Gorny et al., 1996). Both species were known previously from southern Chile, covering a wide range of geographical distribution along the SCI. MAR., 63 (Supl. 1): 339-345 SCIENTIA MARINA 1999


INTRODUCTION
The decapod crustaceans Eurypodius latreillii Guérin 1828 (Brachyura) and Pagurus comptus White, 1847 (Paguridae) were the most frequently occurring species in Agassiz trawl samples obtained during the Chilean-German-Italian "Victor Hensen" Expedition (October / November 1994) in the Straits of Magellan and south of the Beagle Channel (Arntz et al., 1996a;Gorny et al., 1996).Both species were known previously from southern Chile, covering a wide range of geographical distribution along the SCI.MAR., 63 (Supl.1): 339-345 SCIENTIA MARINA 1999 MAGELLAN-ANTARCTIC: ECOSYSTEMS THAT DRIFTED APART.W.E. ARNTZ and C. RÍOS (eds.)Size structure and sex composition of Eurypodius latreillii Guérin, 1828 (Brachyura) and Pagurus comptus White, 1847 (Anomura) in southern Chile* South American coasts, including the Magellan region in Chile and the Patagonian region in Argentina, as well as the Falkland Islands.The northern distributional limit of E. latreillii is Independencia Bay in Peru (Pacific), and the Gulf of San Matías (Atlantic), whereas the most northern range of P. comptus is Coquimbo (Chile) in the Pacific and Uruguay in the Atlantic Ocean.Knowledge of the biology of these species is scarce and restricted to populations north of the Magellan region.Retamal (1974) studied the presence of epizoites like bryozoans, sponges and seaweed on the caparace of E. latreillii, and Soto and George-Nascimiento (1991) analyzed the use of gastropod shells by P. comptus in the intertidal zone off central Chile.
The main objective of the present study was to increase the knowledge on population structures and sex composition for both species in the southernmost part of Chilean waters.

MATERIAL AND METHODS
The analyzed material was collected in October and November 1994 during the Joint Chilean-German-Italian Magellan "Victor Hensen" Campaign in southern Chile by means of an Agassiz trawl (AGT) with a mouth width of 1.5 m and 10 mm mesh.A detailed description of the sampling procedure has been presented by Arntz et al. (1996b).
Specimens used in the present study were caught at 15 stations in the Straits of Magellan and south of the Beagle Channel (Table 1 and 2).The material comprised 509 specimens of E. latreillii, collected at various locations in the Straits of Magellan, another 264 individuals caught south of the Beagle, 420 P. comptus specimens from the Straits of Magellan and 227 from south of the Beagle.We measured carapace length (CL), carapace width (CW), and determined wet weight (WW) without shells for P. comptus, and without epizoites for E. latreillii.
The size structure was analyzed by means of histograms of size distribution at 5 mm caparace length (CL) intervals for E. latreillii and at 1 mm (CL) intervals for P. comptus .
The sex proportion (males/females) was analyzed according to Bustos and Retamal (1985).Analyses of variance ANOVA (Sokal and Rohlf, 1987) were performed to estimate significant differences between the two areas (Straits of Magellan and south of the Beagle Channel) and two depth ranges (shallow water ≤ 75 m; deeper water > 75 m).

RESULTS
The sex composition of E. latreillii in the Straits of Magellan and south of the Beagle revealed similar characteristics, whereas the sex proportion (males: females) varied between 0.21 and 1.88 (Table 2).No significant differences between areas were found, whereas low values (from 0.21 to 0.39) were observed in deeper waters, indicating a dominance of females (Table 2).In the Straits of Magellan high percentages of ovigerous females were found toward deep waters (Table 2).
The males of E. latreillii varied in mean size between 24.7 and 38.8 mm CL (3.02-17.15g WW), and females between 25.1 and 39.0 mm CL (5.20-19.17g WW) (Table 3).The size frequency peaks of males and females in the Straits of Magellan were between 34.5 and 39.5 mm CL; males collected south of the Beagle presented the major frequencies between 29.5 and 44.5 mm CL, whereas for the females particularly high values were observed between 29.5 and 34.5 mm CL (Fig. 1).The size structure varied significantly with depth, large-sized males and females (38.7 and 37.9 mm average CL, respectively) were observed in deep waters.The smallest individuals (< 20 mm CL) were collected in the area south of the Beagle and in the Straits of Magellan at stations 812 (115 m), 926 (49 m) and 960 (36 m); large-sized ovigerous females occurred 340 R. SOTO et al.The carapace of E. latreillii was covered frequently by epizoites such as green and red seaweeds, poriferans, bryozoans, foraminiferans, polychaetes (Sabellidae and Serpulidae), molluscs (Aulacomya ater, Hiatella solida), ascidians, nematodes, pycnogonids, crustacean decapods (Liopetrolisthes patagonicus), amphipods and isopods.
The sex proportion of P. comptus in the Straits of Magellan and south of the Beagle Channel was similar, fluctuating from 0.79 to 3.31, but no significant differences were found between areas and depths (Table 2).
The males of P. comptus in the Straits of Magellan varied in average size between 5.5 and 7.6 mm CL (0.27-0.65 g WW), and females between 5.1 and 7.3 mm CL (0.21-0.46 g WW) (  presented the major frequencies between 4.50 and 6.50 mm CL (Fig. 2).The size structure of males and females varied with depth, with the largest mean sizes [individuals of 7.6 mm CL (males) and 7.2 mm CL (females)], occurring at intermediate depths.Significant differences were observed in the Straits of Magellan (Table 5).Individuals of P. comptus smaller than 7 mm CL were collected in the area south of the Beagle at depths between 15 and 30 m (stations 1149, 1162 and 1228), and in the Straits of Magellan at 36 m (station 960).Larger-sized specimens (> 10 mm CL) occurred in the Straits of Magellan at station 812 (115 m) and 926 (49 m).

DISCUSSION
The sex composition of E. latreillii in the Straits of Magellan and the area of south of the Beagle Channel was characterized by high proportions of females, especially ovigerous females in deep waters, and smaller females in shallow waters (< 25 m depth).The sex composition of P. comptus varied with depth and between areas, particulary at station 926 (Estrecho Laredo) of the Straits of Magellan (m/f = 3.31).The variation of the sex composition may be related with ontogenetic migrations, because female crabs have been observed close to the shore during austral spring (Gorny, pers. comm.), probably to release the larvae in shallow water.This may explain the low presence of females at greater depth and displacing of large ovigerous females of E. latreillii toward shallow waters.
The length structure distributions within both species were similar in both areas, but characterized by the presence of larger-sized individuals (males and females) toward deep waters, and of larger-sized ovigerous females toward shallow water in the Straits of Magellan area (Table 6) which may be related to their life cycle.The habitat of benthic early stages is unknown, but there exists some evidence for Paralomis granulosa (Lithodidae) (see Castilla, 1985;Lovrich, 1997) of differential distribution in the adult populations with smaller individuals (5-50 mm CL) living in holdfasts of Macrocystis pyrifera and rifts, moving into deeper water as they grow.
The sex composition and length frequency of P. comptus are regulated by disponibility, diversity and amount of vacant gastropod shells in the environment, due to the intra-specific competition by size and behaviour in pagurids (Abrams, 1980;Camare-na and Carvacho, 1987).Regarding diversity of gastropod shells observed in our study, specimens were found to inhabit shells of Polinices uber, Photimula caerulescens, Pareuthria plumbea, Adelomelon sp.Margarella violacea and Priene rude, species that vary in size and form.In the intertidal zone of central Chile (Soto and George-Nascimento, 1991), P. comptus used shells of Tegula atra, Tegula tridentata, Prisogaster niger, Crassilabrum crassilabrum and Nucella calcar.
Based on our results, it seems plausible that the population structure of both species is related to geographical patterns of the region, as well as to the life cycle of the species.
Information about populational aspects of decapods inhabiting the southernmost part of Chilean waters is scarce and restricted to the northern area of the Magellan region.Exceptions are the studies on Paralomis granulosa and Lithodes santolla (Molina 1782) published by Campodónico (1977), Guzmán and Ríos (1986, 1987), and Lovrich (1997).These authors noted differences in population parameters, and related them to characteristics of coast line morphology, sediment composition (Brambati, 1991;Colizza, 1991) and oceanographical patterns in the region.
Our findings concerning the epizoites on the carapace of E. latreillii are in agreement with the results of other studies: Retamal and Yañez (1973) found poriferans and hydrozoans on the carapace and the pereiopods of E. latreillii collected in Bahía Concepción.In the Magellan region (Bahía Inútil, Seno Otway, La Madre de Dios and Canal Trinidad), 344 R. SOTO et al.Retamal (1974) reported bryozoans, poriferans and seaweeds as epizoites on the carapace of E. latreillii.We assume that size distribution patterns of E. latreillii may be related to the presence of epizoites, as high biomasses of ascidians and molluscs (Aulacomya ater) may affect the motility and the behaviour of these crabs.
in shallow waters of the area south of the Beagle (station 1149: 15 m) and the Straits of Magellan at station 816 (57 m), 861 (25 m) and 949 (24 m).

TABLE 1 .
-Station list (AGT), location and depth (m) in the Strait of Magellan (SM) and south of the Beagle Channel (SB).

TABLE 2 .
-Number of individuals per station and sex proportions (m/f) ) in the Straits of Magellan (SM) and south of the Beagle Channel (SB).Below, table (Ftab) and calculated (Fcalc) values from variance analysis (ANOVA) are given for the sex proportion (m/f) comparing the Straits of Magellan (SM) with the area south of the Beagle Channel (SB), and different depth (depth limit shallow/deeper at 75 m).(**) Significant differences (p > 0.05); ns: not significant (p < 0.05).
FIG. 1. -Length structure (CL mm) of Eurypodius latreillii in the Straits of Magellan and south of the Beagle Channel.

Table 4 )
. Males and females collected south of the Beagle 342 R. SOTO et al.

TABLE 3 .
-Size and weight ranks, average sizes and average weights of Eurypodius latreillii per station in the Straits of Magellan (SM) and south of the Beagle Channel (SB).CL = carapace length (mm), CW = carapace width (mm), and WW = wet weight (g).