The shallow-water Asellota ( Crustacea : Isopoda ) from the Beagle Channel : Preliminary taxonomic and zoogeographical results *

The Beagle Channel is a narrow, elongated sound located at the southern tip of South America. Brandt et al. (1997) stated that this channel is one of the key areas for taxonomic, ecological and biogeographic research, and they suggested that it should be investigated faunistically in more detail. Many isopod Asellota have been reported from the Magellan region, mainly by Nordenstam (1933), Menzies (1962), Winkler and Brandt (1993) and Winkler (1994a). Despite this, only seven species are known from the Beagle Channel up to now (Monod, 1926; Nordenstam, 1933; Brandt, 1999). The aims of this study are to describe the taxonomic composition and abundance of the shallowwater Asellota from the Beagle Channel, and to anaSCI. MAR., 69 (Suppl. 2): 159-166 SCIENTIA MARINA 2005


INTRODUCTION
The Beagle Channel is a narrow, elongated sound located at the southern tip of South America.Brandt et al. (1997) stated that this channel is one of the key areas for taxonomic, ecological and biogeographic research, and they suggested that it should be investigated faunistically in more detail.*Received May 10, 2004. Accepted April 21, 2005.Many isopod Asellota have been reported from the Magellan region, mainly by Nordenstam (1933), Menzies (1962), Winkler and Brandt (1993) and Winkler (1994a).Despite this, only seven species are known from the Beagle Channel up to now (Monod, 1926;Nordenstam, 1933;Brandt, 1999).
The aims of this study are to describe the taxonomic composition and abundance of the shallowwater Asellota from the Beagle Channel, and to ana- lyze the faunistic affinities between this channel and the surrounding areas.

MATERIAL AND METHODS
The specimens studied were collected at four shallow-water localities, one in the Beagle Channel and the other three in front of its eastern mouth (Fig. 1, Table 1).For the sake of simplicity, all these localities are regarded as belonging to the Beagle Channel.Samples were dredged from the Argentine navy vessel "Alferez Sobral" at 15-35 m depth in February and May 2001 and September 2002.A 45 x 50 cm trawl fitted with a net of 2 mm mesh size was used.The material collected was fixed in 10% buffered formaldehyde solution and transferred to 70% ethanol.The Asellota were picked from the bulk samples and identified to species (or genus) level.
In this contribution the Magellan region has been defined as the Patagonian shelf south of 41°S on both the Pacific and Atlantic coasts (including the Malvinas/Falkland Islands); the Scotia Arc comprises the South Georgia, South Sandwich and South Orkney Islands; and the Antarctic Peninsula includes the South Shetland Islands.
The biogeographic analysis deals with the Asellota whose vertical distribution is restricted to depths < 200 m and those which were found at greater depths but also at < 200 m.In order to facilitate comparisons between areas, a list of all the Asellota collected in shallow waters from the Magellan region (including our data from the Beagle Channel), the Scotia Arc and the Antarctic Peninsula is provided (Table 3).Papers which were consulted to prepare this list are mentioned in Winkler (1994b).Other papers utilised include the following:   Kussakin (1965), Richardson and Hedgpeth (1977), Branch et al. (1991), Teodorczyk and Wagele (1994), Wilson and Wagele (1994), Serov and Wilson (1995), Mariani et al. (1996), Lorenti and Mariani (1997), Brandt (1999), Castello (1999Castello ( , 2004)), Cariceo et al. (2002), Lorz and Brandt (2003), and Just and Wilson (2004).For the faunistic analysis, the localities marked with one or two asterisks and the specimens identified as "Paramunna cf.menziesi", "Sporonana sp.", "Austrofilius sp.A", "Ianiropsis sp.A", "Caecianiropsis cf.ectiformis", "Joeropsis sp.A", "Munna sp." and "Munna spec." in Table 3 were excluded.Junior synonyms in Table 3 are mentioned only when these names were used for specimens collected in the Magellan/Antarctic/Subantarctic areas.Affinities between areas were 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 (see Cheetham and Hazel, 1969).The term "diversity" refers to the total number of species, and the term "abundance" to the total number of specimens.
Among the Asellota, 25 species belonging to six families were identified (Table 2).Three of these species are new to science and 12 are new records for the Beagle Channel.All these new records belong to species formerly found in the Magellan Strait or other localities of the Magellan region (Table 3).
At family level, the Paramunnidae showed the highest diversity (11 species) and abundance (1,463 specimens).Janiridae was the second family in diversity (7 species) and Joeropsidae the second family in abundance (852 specimens) (Table 2).

Faunistic affinities between the Beagle Channel and the Magellan Straits
Including the data herein reported, the number of shallow-water asellotes in the Beagle Channel and the Magellan Straits was 23 and 34 respectively (Table 3).Sixteen of the species of the Beagle Channel co-occur in the Magellan Straits, which represents 69% of similarity (Table 3).
The Beagle Channel and the Magellan Straits shared six of the eight families reported from the Magellan region: Paramunnidae, Janiridae, Joeropsidae, Munnidae, Santiidae and Acanthaspidiidae (Table 3).One species of Stenetriidae has been reported from the Magellan Straits but no specimens were found in our samples.No member of Desmosomatidae has been reported from the Magellan Straits, and our samples did not contain any species of this family either.
For both areas the Paramunnidae and Janiridae were the most diverse families (Table 3).For the Paramunnidae 11 species were found in the Beagle Channel and 14 in the Magellan Straits.Of these, only five species co-occur in both areas.For the Janiridae six species were found in the Beagle Channel and eight in the Magellan Straits, all the species recorded from the Beagle Channel being also present in the Magellan Straits.

Faunistic affinities between the Magellan region and the Scotia Arc and the Antarctic Peninsula
Table 3 lists all the Asellota recorded at depths < 200 m from the Magellan region, the Scotia Arc and the Antarctic Peninsula.Of the 89 species reported for the entire area, 58 were found in the Magellan region, 23 in the Scotia Arc, and 50 in the Antarctic Peninsula.The Magellan region shared 7 species with the Scotia Arc (30% of similarity) and 13 species with the Antarctic Peninsula (26% of similarity).
The Paramunnidae showed the highest diversity in the Magellan region and the Antarctic Peninsula, while the Janiridae was the most diverse family in the Scotia Arc (Table 3).

DISCUSSION
The examination of just a few samples yielded three new species and 12 new records for the Beagle Channel, which suggests that many species of Asellota still remain undiscovered in this coastal area.Winkler (1994b) mentioned that the Paramunnidae was the most diverse family in the Magellan region, followed by the "Janiridae/Joeropsidae", a fact that is consistent with our results in the Beagle Channel.Nordenstam (1933) erected the genus Antennulosignum (Paramunnidae) to include A. elegans from the Malvinas Islands.This species is reported herein for the second time after its original description (Table 2).Brandt (1999) described some specimens from the Beagle Channel as Pleurosignum chilense Menzies, 1962.However, it is likely that because of their peculiar antennule the specimens studied by Brandt belong to the genus Antennulosignum.
The genus Caecianiropsis (Janiridae) includes three species.Of these, only C. ectiformis (Vanhoffen, 1914) is found in the southern hemisphere.This species was previously known from the following Subantarctic islands: Kerguelen, St. Paul, and Marion/Prince Edward (Vanhoffen, 1914;Kensley, 1976;Branch et al., 1991).Our finding of C. cf.ectiformis in the Beagle Channel widely extends the range of distribution of the genus Caecianiropsis in the southern hemisphere.
The genus Munna (Munnidae) is well represented in the Magellan region (Winkler, 1994b), as well as in Antarctic and Subantarctic waters (Teodorczyk and Wagele, 1994).Although six species were reported from the Magellan region (Nordenstam, 1933;Menzies, 1962;Winkler, 1992Winkler, , 1994b;;Teodorczyk and Wagele, 1994;Lorenti and Mariani, 1997) only one, Munna gallardoi Winkler, 1992, was identified from the Beagle Channel (Table 3).Most probably, among the specimens herein reported as "Munna sp." more than one species were included, but the poor condition of the available material has prevented us from identifying them.
The family Santiidae has a pronounced southern distribution, and its species occur at depths of less than 100 m (Wolff, 1989).Five species of Santia have been reported from the Magellan region (Winkler, 1994b;Lorenti and Mariani, 1997); in contrast, only one has been found in the Beagle Channel so far (Table 3).
Two species belonging to the family Acanthaspidiidae have been recorded from the Magellan region.Ianthopsis laevis Menzies, 1962, endemic for this region, was recorded herein from the Beagle Channel.On the other hand, Ianthopsis bovallis (Studer, 1884) was not reported from the Beagle Channel, despite its wide distribution in the southern seas (see Winkler, 1994b;Lorenti and Mariani, 1997).
The species of the genus Tenupedunculus (Stenetriidae) and those of the families Desmosomatidae and Munnopsidae are distributed mainly in deep-sea waters (Kussakin, 1973;Serov and Wilson, 1995).However, emergence has been postulated for many deep-sea Asellota, especially at higher latitudes (see Brandt et al., 2004).This could explain the larger number of species found in the Scotia Arc/Antarctic Peninsula in comparison with the Magellan region (Table 3).Although no species of the three families mentioned above have been recorded from the Beagle Channel, Eugerdella falklandica (Nordenstam, 1933) and some species of Tenupedunculus, a genus that is distributed from around the southern tip of South America to the Antarctic, are likely to be found in future surveys carried out in the Beagle Channel.Menzies (1962) recorded four species from Chile, viz., Uromunna schauinslandi (G.O. Sars, 1905) near 41°S, Joeropsis bidens Menzies, 1962 between 42 and 20°S, Santia dimorphis (Menzies, 1962) near 44°S and Santia laevifrons (Menzies, 1962) between 44 and 32°S.All these species were found slightly below 41°S and thus were listed in Table 3.Because zoogeographic regions cannot be neatly divided, the addition of these species to the Magellan fauna must be taken with caution.
Knowledge of the Magellan Asellota is still scarce and further taxonomic studies are strongly required.Although the results presented herein are preliminary, they will hopefully contribute to a better understanding of this faunal group.
FIG. 1. -Locations of the sampling stations in the Beagle Channel.
not in Paramunna afterJust and Wilson (2004).SHALLOW-WATER ASELLOTA FROM THE BEAGLE CHANNEL 161

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Localities sampled in the Beagle Channel.

TABLE 2 .
-Number of Asellota isopods collected in the Beagle Channel during this study.(*) First record for the area.

TABLE 3 .
-Asellota collected in the Magellan region, the Scotia Arc and the Antarctic Peninsula at depths < 200 m.The species shared between the Beagle Channel and the Magellan Strait are in bold.Depths refer to the entire bathymetrical range of the species.BC, Beagle Channel; MS, Magellan Strait; OL, other localities.

TABLE 3 (
Cont.).-Asellota collected in the Magellan region, the Scotia Arc and the Antarctic Peninsula at depths < 200 m.The species shared between the Beagle Channel and the Magellan Strait are in bold.Depths refer to the entire bathymetrical range of the species.BC, Beagle Channel; MS, Magellan Strait; OL, other localities.