Biodiversity of the Gammaridea and Corophiidea ( Crustacea : Amphipoda ) from the Beagle Channel and the Straits of Magellan : a preliminary comparison between their faunas

Gammaridea and Corophiidea amphipod species from the Beagle Channel and the Straits of Magellan were listed for the first time; their faunas were compared on the basis of bibliographic information and material collected in one locality at Beagle Channel (Isla Becasses). The species Schraderia serraticauda and Heterophoxus trichosus (collected at Isla Becasses) were cited for the first time for the Magellan region; Schraderia is the first generic record for this region. A total of 127 species were reported for the Beagle Channel and the Straits of Magellan. Sixty-two species were shared between both passages (71.3 % similarity). The amphipod species represented 34 families and 83 genera. The similarity at genus level was 86.4 %, whereas 23 of the 34 families were present in both areas. For all species, 86 had bathymetric ranges above 100 m and only 12 species ranged below 200 m depth. In the Beagle Channel, only one species had a depth record greater than 150 m, whereas in the Straits of Magellan, 15 had such a record. The Beagle Channel and the Straits of Magellan contain 57.5 % of the species, 71 % of the genera and 77.3 % of the families of Gammaridea and Corophiidea from the entire Magellan region. Rev. Biol. Trop. 55 (Suppl. 1): 103-112. Epub 2007 June, 29.

Biodiversity of the Gammaridea and Corophiidea (Crustacea: Amphipoda) from the Beagle Channel and the Straits of Magellan: a preliminary comparison between their faunas The Beagle Channel and the Straits of Magellan are two passages located in the southern tip of South America, connecting the Atlantic and Pacific Oceans (Fig. 1).Both the channel and the straits, including fjords, inner channels and inlets, are characterized by a complex coastline and different depths along their extensions (Hamamé andAntezana 1999, Pagès andOrejas 1999).
The Beagle Channel extends about 300 km; the eastern section has about 50 m depth and the western end 650 m (Colizza 1991); the bottoms of the channel are basically muddysandy, although in some sections predominate rocks, crushed shells and coarse sand (Brambati et al. 1991, Colizza 1991); the salinity reaches values not higher than 32 psu, or varies between 27 and 31 psu (Iturraspe et al. 1989); the surface temperature ranges between 3-10 ºC according to the season of the year (Iturraspe et al. 1989).The Straits of Magellan has an extension of 570 km and its depth increases from the east to the west, reaching in the western section 1 100 m; in the eastern side bottoms are mainly sandy, in the middle section these are pelitic and in the west side predominates muddy bottoms; the salinity varies between 28 and 32.5 psu; the surface temperature averages 7.3 ºC (Brambati et al. 1991, Panella et al. 1991).
The Beagle Channel and the Straits of Magellan are areas of great biogeographic and ecological importance due to their intermediate position between the Atlantic and Pacific Oceans, their proximity to the Antarctic and the influence of the sub-Antarctic currents (Knox 1994, Brandt et al. 1997, Gambi and Mariani 1999, Ríos et al. 2003).
From the biogeographic point of view, both passages are included in the Magellan region (Bastida et al. 1992, Lancellotti andVásquez 2000).De Broyer and Rauschert (1999) listed a total of 206 amphipod species for this region, and recently, Chiesa et al. (2005) increased the number of species to 219.
The study of the Gammaridea and Corophiidea amphipods (Crustacea: Amphipoda) from the Beagle Channel and the Straits of Magellan was initiated mainly by Stebbing (1888) and Schellenberg (1931) on the basis of material collected by the RV Challenger and the Swedish Antarctic Expedition, respectively.
The aims of this contribution are to ascertain the biodiversity of the Beagle Channel and the Straits of Magellan, the bathymetric ranges of the species and the faunistic affinity between the channel and the straits.

MATERIALS AND METHODS
Part of the information on the species richness of Gammaridea and Corophiidea was obtained through specimens (a total of 2 977) collected at five localities along the Beagle Channel (Bahía Ushuaia, Isla Gable, Punta Moat, Cabo San Pío and Bahía Slogett) identified by Chiesa et al. (2005), and material from Isla Becasses (402 specimens), also located in the Beagle Channel (54º57'S 67º01'W; 30-40 m depth), analysed afterwards and here reported for the first time.
Other records from the Beagle Channel and those from the Straits of Magellan came from a personal compilation of previous literature, based on the following publications: Stebbing (1888), Schellenberg (1931), andDe Broyer andRauschert (1999).The catalogues by Lowry and Bullock (1976), Gonzalez (1991) andDe Broyer andJażdżewski (1993) -based on prior published data-were also reviewed for this contribution.Moreover, most of the original descriptions were consulted.The bathymetric ranges (minimum and maximum depths) of the species taken into account were detailed when possible.
Isla Picton, Isla Nueva, and Isla Lennox were considered as part of the Beagle Channel, and Cabo Vírgenes and Punta Dungeness as belonging to the Straits of Magellan (Fig. 1); the records from these areas were also based on the literature mentioned above.
The taxonomic classification of Gammaridea and Corophiidea follows Barnard and Karaman (1991) and Myers and Lowry (2003), respectively; the families Caprellidae and Cyamidae were not considered herein.The faunistic affinity between the Beagle Channel and the Straits of Magellan was estimated using Simpson's Coefficient of Similarity: C/N x 100, where C = number of shared species and N = total number of species reported from the smaller of the two groups being compared (Cheetham and Hazel 1969).

RESULTS
A total of 127 species of Gammaridea and Corophiidea were recognized in the Beagle Channel and the Straits of Magellan (Table 1).From these, 102 species were present in the Straits of Magellan and 87 in the Beagle Channel.Both areas shared 62 species, which represented 71.3 % of similarity.
The 127 amphipod species recorded from both areas were comprised by 34 families and 83 genera.A total of 25 families were reported for the Beagle Channel, whereas 32 families occurred in the Straits of Magellan.Twenty three families co-occurred in both passages.The families Ampeliscidae, Colomastigidae, Epimeriidae, Gammaridae s.l., Phoxocephalopsidae, Platyischnopidae, Podoceridae, Urohaustoriidae and Zobrachoidae were present in the Straits of Magellan but not in the Beagle Channel, whereas Cyproideidae and Eophliantidae were found only in the Beagle Channel (Table 1).Lysianassidae s.l., Phoxocephalidae, Eusiridae s.l. and Gammarellidae were the families with the highest number of species in both the Straits of Magellan (18, 9, 8 and 8 species, respectively) and the Beagle Channel (12, 8, 10 and 10 species, respectively).
In the Straits of Magellan, 75 genera were found, 24 of them were not present in the Beagle Channel.A total of 59 genera have been reported from the Beagle Channel, from which eight have not been recorded for the Straits of Magellan (Table 1).The Magellan Strait and the Beagle Channel shared 51 genera, representing 86.4 % of similarity.
With respect to the bathymetric range of the listed species, there was no documented depth data for 20 species present in either area or both together.For the Beagle Channel and the Straits of Magellan a total of 86 species were found in ranges shallower than 100 m depth.From these, 55 species had ranges from low tide down to 50 m depth, among which seven were recorded exclusively at low tide.Twelve species inhabited a depth range that exceeded 200 m, and only one of them (Victorhensenoides arntzi) was present in the Beagle Channel (Table 1).
From the total of species recorded for the Straits of Magellan, 26 had records exclusively between 0 and 50 m depth, and 30 species were found in depths below 80 m.In the Beagle Channel, 61 species were present exclusively in a range between 0 and 50 m depth, and 16 species were found in depths below 80 m.Only one species was reported from depths deeper than 150 m in the Beagle Channel, whereas in the Straits of Magellan 15 species were found below 150 m.

DISCUSSION
The knowledge of the amphipod fauna from both passages is dispersed in several taxonomic papers (Schellenberg 1931, Lowry and Bullock 1976, Gonzalez 1991, De Broyer and Jażdżewski 1993and De Broyer and Rauschert 1999).The present contribution provides the first comprehensive list of the gammaridean and corophiidean amphipod species from the Beagle Channel and the Straits of Magellan, and the first preliminary comparison between their faunas.
The study of the material from Isla Becasses in the Beagle Channel yielded two new records of species for the entire Magellan region, Schraderia serraticauda and Heterophosus trichosus; these records increase the number of species from 219 (Chiesa et al. 2005) to 221 in the Magellan region.S. serraticauda was previously known from the sub-Antarctic islands (Auckland and Macquarie islands) and H. trichosus from the West Antarctic (Bransfield Strait and South Shetland islands); moreover, Schraderia represents the first record of the genus for the Magellan region.
The species reported as Uristes serratus (Schellenberg, 1931) for the Argentine sector of the Beagle Channel (Bahia Ushuaia) by Chiesa et al. (2005) is a new undescribed species.This is the first record of Uristes for the Beagle Channel.
All the amphipod families lacking in the Straits of Magellan and the Beagle Channel, but present in the rest of the Magellan region, were characterized by their low diversity in this region, since they only comprised one or two species (De Broyer and Rauschert 1999).On the contrary, Lysianassidae s.l., one of the most specious families in the entire Magellan region (De Broyer and Rauschert 1999), was also well represented in the areas analysed by us.The absence of Ampeliscidae in the Beagle Channel is relative, since Chiesa et al. (2005) found one species (afterwards it was concluded that they could be two) in a locality in the vicinity of the Beagle Channel (Bahía Aguirre).
The higher number of species reported only from shallow waters of the Beagle Channel and the Straits of Magellan and the low number found in deeper waters indicate the necessity of collecting material at depths below 100 m in both passages, especially in the Beagle Channel, where only one species (Victorhensenoides arntzi) was found at a depth about 150 m.In general, more extensive sampling in the channel and the straits would allow a more accurate estimation of the bathymetric ranges of the species.
Although the Magellan region amphipod fauna of Gammaridea and Corophiidea is far from being well known, it is remarkable that 57.5 % of the species, 71 % of the genera and 77.3 % of the families from the Magellan region (De Broyer andRauschert 1999, Chiesa et al. 2005) are present in both the Beagle Channel and the Straits of Magellan.These comparatively high percentages, estimated for the southern tip of South America, could be due to the fact that this area has been more intensively sampled than the northern part of the Magellan region (Atlantic and Pacific Oceans).
It is expected that the results on species richness and faunistic affinities obtained here will be modified as new samples from the Beagle Channel and the Straits of Magellan are studied; the existence of several still undescribed taxa (new genera and species) and new records listed by De Broyer and Rauschert (1999) for the Beagle Channel, the Straits of Magellan and adjacent magellanic waters, also support this hypothesis.

ACKNOWLEDGMENTS
We thank the staff of the Base Naval Ushuaia for their logistic support; Gustavo Lovrich for his kind help with the survey arrangements and Diego Zelaya for his comments on the manuscript.This study was partially supported by the Agencia Nacional de Promoción Científica y Tecnológica (PICT No. 11.180).

TABLE 1
List of gammaridean and corophiidean amphipods present in the Straits of Magellan (SM) and the Beagle Channel (BC), including their local bathymetric ranges

TABLE 1 (
Continued) List of gammaridean and corophiidean amphipods present in the Straits of Magellan (SM) and the Beagle Channel (BC),including their local bathymetric ranges identified; from these, Curidia magellanica is a new record for the Beagle Channel, and two species (Schraderia serraticauda and Heterophoxus trichosus) are cited for the first time for the Magellan region (Table