Polychaetes of the soft bottoms of the Straits of Magellan collected during the Italian oceanographic cruise in February-March 1991 *

Species composition, distribution and biogeography of polychaetes collected from the soft bottoms of the Straits of Magellan (South America) in February-March 1991 are reported. In 16 benthic samples, collected with different tools (Charcot and triangular dredges, van Veen grab), a total of 1132 individuals belonging to 119 taxa of polychaetes were collected; only 49 of the taxa found have been determined at species level. Eighteen species were recorded for the first time in the Straits of Magellan. Species richness was relatively high considering both the number of individuals collected, and the taxa known from previous studies on the area (182 species). The biogeographical analysis, conducted only on the 49 taxa classified at the species level, showed the dominance of Magellan-Antarctic-Subantarctic species (M-An-S 50%), followed by Magellan-American (M-Am 14%), Magellan-Antarctic (M-An 12%), Magellan-Subantarctic (M-S 6%), Cosmopolitans (C 10%), and also by a few species with disjunct distribution (D 8%). Values of the Sørensen similarity index among stations were very low (below 0.30) with a few exceptions. Both number of species and of individuals were higher in the Atlantic sector of the Straits, especially at some stations characterized by heterogeneous mixed sediments (gravels and pebbles) and biogenic debris (mollusc thanatocoenosis). The high number of species recorded, coupled with low similarity values among stations, suggests that the soft bottoms of the Straits of Magellan show, at medium scale, a highly diversified mosaic of different biotopes. This can be due to various factors which are expected to vary along the wide geographic area investigated, such as the wide bathymetric range sampled, the type of sampling gears used, and last but not least the occurrence of many different environmental situations along the Straits.


INTRODUCTION
The Straits of Magellan (South America) (53°45'S Lat., 71°30'W Long.) represent one of the most interesting Subantarctic areas from both a biogeographical and an ecological point of view.The relative closeness of this area with the Antarctic continent, from which it is separated through the Scotia Arc, and the continuity with the American continent define the nature of biogeographic crossroads of this zone (Guzmán, 1992).This is well documented for both the benthic flora (Skottsberg, 1941) and the fauna (Brattström and Johanssen, 1983;Bastida et al., 1992).The peculiar hydrological conditions of the Straits, where different water bodies meet from the Atlantic and Pacific Oceans, that are also partially influenced by the Subantarctic currents of the Southern Ocean (Panella et al., 1991), contribute largely to its ecological importance as regards also the productivity of the pelagic system (Saggiomo et al., 1993;Magazzù et al., 1996).The hydrological regime of the Straits is a reflection of the complex geomorphological situation and bottom topography that also largely influence sediment features at large and medium scale (Brambati et al., 1991).These climatic, environmental and sedimentological features strongly influence the distribution and ecology of benthic organisms, and have great importance for the biogeographic and ecological comparison of bottom fauna with both the Antarctic and other Subantarctic areas.
In the framework of the Italian "Programma Nazionale di Ricerche in Antartide" (P.N.R.A.), the "Magellan Project" started in 1988 to promote geological, bio-ecological and oceanographic research in this important geographic area (Brambati, 1991).The first benthic survey within the Magellan Project was performed during the 2nd Italian Oceanographic cruise in February-March 1991(Guglielmo, 1991).In this work we discuss the biodiversity, distribution and biogeography of polychaetes collected during that cruise.Polychaetes resulted, in fact, one of the most abundant groups of infaunal organisms of the macrozoobenthos collected from the soft bottoms of the Straits of Magellan.A preliminary study on their distribution is available in Mariani et al. (1996).
Past data on polychaetes of the Magellan region derive mainly from historical expeditions (see Rozbaczylo, 1985 for review).Among the most recent and important studies available for taxonomy and distribution at class and family level it is worth mentioning Wesenberg-Lund (1962), Hartmann-Schröder and Hartmann (1962, 1965), Hartman (1964Hartman ( , 1966Hartman ( , 1967Hartman ( , 1978)), Orensanz (1990) for the order Eunicida, Blake (1983) for Spionidae, and Lana and Brémec (1994) for Sabellariidae.Some additional taxonomic and ecological information is available in Cantone (1990) and Sanfilippo (1994).A useful list of polychaetes from Chile, which also includes the Magellan region, is available in Rozbaczylo (1985), while for biogeographical comparisons the work of Knox and Lowry (1977) and Knox (1977) is still useful.Recently, Rozbaczylo et al. (1997), revising the past literature and adding data of a case study in the intertidal zones, gave a check list of 182 species of polychaetes reported up to date for the Magellan and Southern America Subantarctic regions.

MATERIAL AND METHODS
The material analyzed in this study was obtained during the 2nd Italian oceanographic cruise (February-March 1991) on board the R/V "Cariboo" (Guglielmo, 1991).Samples for both hydrological and benthic studies were collected over a large spatial scale from 24 stations distributed along the whole Straits of Magellan, from the Pacific Ocean mouth (st.4) to the Atlantic Ocean (st.26) (Fig. 1).A distinction of the two oceanic sides of the Straits has been based on the location of stations with respect to Cape Froward (arrow in Fig. 1), which represents the southernmost point of continental South America.Stations located westward of this point are considered as belonging to the Pacific sector, while those located eastward as belonging to the Atlantic sector.This distinction has only an operational value and does not imply any biogeographical separation between the two ocean sectors.On the other hand, the two oceanic sides of the Straits show such strong geomorphological and hydrographical differences that their distinction has an ecological significance.
Benthic organisms were sampled by means of different gears: rectangular (Charcot-Picard type) and triangular dredges and a van Veen grab (65 l; sampling surface: ca.0.1 m 2 ), within a depth range varying from 30 m to 1100 m (Table 1) (Mazzella and Gambi 1993).The van Veen grab was mainly used in the Pacific sector due to the greater depths and more complex geomorphology occurring in this part of the Straits.The use of the grab and its effectiveness in sampling was limited by some navigation and logistic problems on board.Both kinds of dredges (triangular and rectangular) were used mainly in the Atlantic sector of the Straits (Table 1).A visual evaluation of the sediment type or other substrate features of each station was performed on board and reported in the station logs as a characterization of the bottom (Mazzella and Gambi, 1993;    collected with all the gears used was sieved with 1 mm mesh screen, fixed in 4% formaldehyde and then preserved in alcohol 70%.Polychaetes were counted and identified at the lowest taxonomic level possible.The classification at species level was often restricted by taxonomic problems within some families that need a thorough revision worldwide (e.g., Ampharetidae, Terebellidae), and in part by the poor condition of some of the collected specimens.For some of the Syllidae, the taxonomic analysis was limited to genus level due to their complex taxonomy and laborious processing.However, except for some Syllidae and a few other taxa, each taxon listed represents a single different species.Only the taxa classified at the species level were assigned to the biogeographical categories, according to the literature information (e.g., Knox, 1977;Rozbaczylo, 1985).Six main categories have been defined: Magellan-Subantarctic-Antarctic species (MSAn); Magellan-American (MAm), this category includes both the species endemic to the Magellan Straits and those distributed in the South American subcontinent; Magellan-Antarctic (MAn); Magellan-Subantarctic (MS), Cosmopolitans (C), and species with discontinuous distribution (D).
Due to the different sampling gears used along the Straits, the data did not allow a quantitative comparison of the stations.To study the qualitative faunistic similarity among stations, the Sørensen index was calculated.Stations 4 and 8 were excluded from the similarity analysis due to the occurrence of only 1 and 2 species, respectively.

Taxonomic and systematic considerations
Polychaetes were collected at 16 stations of the 24 sampled (Table 1).A total of 1132 individuals (representing 35% of the infaunal abundance), belonging to 35 families and 119 taxa, were collected (Table 2).Only 49 taxa have been classified at the species level; the other taxa were identified at genus (39) or family (29) levels (Table 2).For 26 specimens, also the attribution at family level was also impossible.Except for the Syllidae, that still need a more detailed taxonomical analysis (see Methods), for most of the other undetermined species the systematic attribution was restricted by the fact that the collected specimens were incomplete or badly dam-aged, or belonged to poorly known genera that need a world-wide revision, to be properly classified.However, we provide indications related to the taxonomic attribution of some taxa that may be useful for future systematic analyses of polychaetes in the area.
Cossura sp. 1 The two specimens collected had only the first 20 chaetigers.A long palp originates from the 3rd chaetiger; this character, coupled with the chaetal morphology, suggests a close affinity with Cossura chilensis, a species already reported for this area (Rozbaczylo, 1985).
Cossura sp. 2 The only specimen collected is incomplete (19 setigers only); the palp originates from the 2nd chaetiger, the anterior notopodia have large acicular hairy chaetae.These features suggest an affinity with Cossura heterochaeta, species known for the Argentine continental shelf.
Prionospio (Prionospio) sp.The loss of the 4th pair of branchiae prevents species identification.The lack of lateral wings in the peristomium restricts the possibilities to P. elhersi or P. orensanzi.
Spiophanes sp.The specimens lack the posterior parapodia, impeding distintion between S. soederstroemi and S. kroyeri, while for the other features we could exclude S. tcherniai and S. bombyx.
Harmothoe spp.The analysis of the elitrae, which were all separated from the specimens found, indicates the presence of at least 3 different species.
Harmothoe sp. 1 The specimens found have some of the dorsal elitrae with a series of large vesicles on the outer rim; this feature suggests an affinity with the species H. spinosa.
Ancistrosyllis sp.T he only specimen found differs from A. groenlandica for the colour pattern, the beginning of the notopodial uncini and the shape of the prostomium.The other species of the genus reported for Chile is A. quellina.
Aglaophamus sp. 2 The specimens have the post-setal lobes very well developed, symmetrical and with an oval shape; the proboscis has 12 rows of papillae.
Drilonereis sp.The only specimen found is very close to the description of Drilonereis sp. in Orensanz (1990).
Schistomeringos spp.The three specimens found, even though incomplete, belong to this genus that according to Orensanz (1990) needs a general revision in the Southern hemisphere.
Anobothrus sp.The presence of 15 thoracic and 12 abdominal segments, suggests an affinity with A. antarcticus.However, all members of Ampharetidae need a thorough revision.
Melinna sp.Two specimens found in station 18 are surely different from both M. cristata and M. cristata australis (Hartmann-Schröder and Hartmann, 1965).
Neosabellides sp.The only specimen found differs from the unique species of the genus known in Chile, N. elongatus.Samithella sp.The genus is known for the Magellan region, however as all its members need a revision we preferred not to attribute the species name.
Sosanopsis sp.The only specimen found shares many characters with the periantarctic species S. kerguelensis (Monro, 1939): absence of paleae, 12 uncinigerous segments, smooth tentacles.However, the branchial distribution pattern is different from that reported by Hartman (1966).The genus is new for the Straits of Magellan.
Pista sp.The spiralled branchiae are similar to that of P. cristata.However, this species according to Hutchings P. (pers.comm.) has a boreal distribution, and the past attributions of austral specimens to this taxon probably refer to new species.
Thelepus sp.The only specimen found is similar to the description of T. cincinnatus.However, T. cincinnatus, as P. cristata, is a species probably restricted to the boreal hemisphere and the past attributions of austral specimens to this taxon probably refer to new species (Hutchings P., pers.comm.).
The maximum number of individuals and of taxa (81% of the total) has been observed in some stations located in the Atlantic sector of the Straits (st.17, 18, 21 e 26), while in the Pacific sector both species richness and abundance were very low (Fig. 2a and 2b), especially in stations 4 and 8 which were represented only by 1 and 2 species, respectively.Among the most diverse stations of the Atlantic sector, both stations 17 and 18 were characterized by a muddy sand mixed with gravel and pebbles, the most abundant species were represented by Prionospio (Minuspio) sp., Spiophanes sp., Paramphinome australis, Onuphis pseudoiridescens, Ninoe falklandica, Neosabellides sp., and Samithella sp.Station 21 was characterized by a mixture of sand, gravel and pebbles and by the presence of biogenic debris deriving from a conspicuous thanatocoenosis of the bivalve Zygochlamys patagonica (Di Geronimo et al., 1991, 1992).In these stations the polychaete assemblage was composed of species typical of soft sediments, as well as of epibenthic (e.g., Harmothoe spp., Idanthyrsus armatus), and interstitial (e.g., Exogone spp., Sphaerosyllis spp.) forms.Furthermore, a few other polychaete species belonging -Trends of the total number of polychaete taxa (above) and of individuals (below) collected from the studied stations along the Straits of Magellan (stations where no benthos or polychaetes were found are also reported in the X-axis).
to Serpulidae and Spirorbidae have been reported by Sanfilippo (1994) as epibionts on the valves of the Zygochlamys.Finally, station 26 characterized by a muddy sediment mixed with gravel and pebbles, showed an assemblage very similar to that of st.21, but with the abundance also of the burrower maldanid Axiothella minor, probably linked to the fine fraction.Some species, even though not very abundant and locally distributed, seemed related to the occurrence of a typical sediment structure, such as the two closely related maldanids Maldane sarsi and M. sarsi antarctica at the muddy stations 10 and 15 (Table 2).
On the whole, the polychaete species richness recorded during this survey along the Straits was relatively high, considering the low number of stations studied and of the individuals collected.Furthermore, compared to previous knowledge about polychaetes of this Subantarctic area, that reported a total of 182 species (Rozbaczylo, 1985;Rozbaczylo et al., 1997), our results are remarkable, and they contribute to increase the number of species known for this zone to more than 200 taxa.
The biogeographical analysis showed a low degree of endemism for polychaetes in the Magellan region, compared to isopods (Mariani et al., 1996), and a higher affinity with the truly Antarctic Province, as observed for sponges (Sarà, 1992).However, this picture may be biased by the relatively low number of species determined (49), from a total of 119 taxa found.
The trends observed for both number of species and individuals along the Straits (Pacific versus Atlantic sector) seem due to the bias introduced by the different sampling gears used in the two sectors of the Straits (see Methods).Similar trends have been observed also for the isopods collected at the same stations (Lorenti and Mariani, 1997), and which are more mobile than polychaetes.
The high species richness found, coupled with the low values of faunistic similarity among the stations, suggests that the soft bottoms of the Straits of Magellan show, at the investigated spatial scale, a mosaic of highly diversified biotopes (Mariani et al., 1996;Lorenti and Mariani, 1997).This may be due to various factors such as the wide geographic area investigated, the wide bathymetric range sampled, and the different sampling gears used.This situation is well synthesized in st.21 where the presence of a coarse, biodetritic component (Zygochlamys thanatocoenosis) in the sediment favours the occurrence of a rich and diversified polychaete fauna.A similar pattern of species diversification at this scale has also been recorded with the bryozoan (Rosso and Sanfilippo, 1991;Moyano, 1992), molluscan (Di Geronimo et al., 1992) and isopod assemblages (Lorenti and Mariani, 1997).
The pattern of species richness herein reported, even though mainly recognizable at the moment only along the Atlantic sector of the Straits, confirms that both composition and diversification of the polychaete assemblages are related to sediment heterogeneity and bottom structural complexity.This is a quite common feature for polychaete populations, and has been recorded also in high Antarctic soft bottoms (Gambi et al., 1997).

FIG. 1
FIG. 1. -Map of the Straits of Magellan with the location of the sampling stations during the 2nd Italian Oceanographic cruise in March-April 1991.The distinction between Pacific and Atlantic sector is based on Cape Froward (arrow; see text).

Table 1 )
. The biological material SOFT BOTTOM POLYCHAETES OF THE STRAITS OF MAGELLAN 235

TABLE 2 .
-List of the polychaete taxa found at the studied stations along the Straits of Magellan.

TABLE 2 .
(Cont.)-List of the polychaete taxa found at the studied stations along the Straits of Magellan.

TABLE 2 .
(Cont.)-List of the polychaete taxa found at the studied stations along the Straits of Magellan.Laena collaris, Euchone analis, have been recorded for the first time in the Straits of Magellan during this survey.Taking in to account the collection of three potential new species (see above systematic considerations) and of a new genus (Sosanopsis), a total of 21 taxa can be considered as new records for the studied area.

TABLE 3 .
-Values of the Sørensen similarity index among the studied stations along the Straits of Magellan.Stations 4 and 8 have been excluded from the analysis due to the occurrence of only 1 and 2 species, respectively.Values higher than 0.50 are indicated in bold.