Faunal diversity of the benthic amphipods (Crustacea) of the Magellan region as compared to the Antarctic (preliminary results)*

To investigate the marine benthic ecosystems of the Magellan region and to compare them with the better known Antarctic systems, three campaigns were recently carried out in this area: the Joint Magellan “Victor Hensen” Campaign 1994, the “Polarstern” ANT XIII/4 cruise 1996, and the “Vidal Gormaz” CIMAR FIORDO II cruise 1996. Numerous and diverse zoobenthos samples were collected mostly with an Agassiz trawl and with a small dredge, an epibenthic sledge, with baited traps or by diving. All gears together gathered more than 132,000 specimens of gammaridean and caprellidean amphipods. 137 species of gammaridean amphipods have been identified from the material to date. About 20% of these species appear to be new for science. This taxonomic work takes place in the framework of a general revision of the Southern Ocean amphipod fauna undertaken by the”Antarctic Amphipodologists Network”. A complete list of the benthic species of gammaridean and caprellidean amphipods is presented, including the zoogeographical distribution and the new records. The new abundant material collected, still under study, will allow a comparison of faunal diversity, zoogeographical and ecological traits of the Magellan benthic amphipod taxocoenoses with those of the West and East Antarctic benthos.


INTRODUCTION
The marine benthic fauna and flora of the Magellan area as well as the structure and function of its benthic ecosystems remain poorly known in comparison with the more quickly developing knowledge of the contiguous Antarctic ocean.Three different campaigns were recently carried out to investigate the Magellan area: the Joint Magellan "Victor Hensen" Campaign 1994 (Arntz and Gorny, 1996), the "Polarstern" ANT XIII/4 cruise 1996 (Fahrbach and Gerdes, 1997;Arntz et al., 1997) and the "Vidal Gormaz" CIMAR FIORDO II cruise 1996 (Mutschke et al., 1995).Numerous and diverse zoobenthos samples were collected in the channels between Puerto Montt and Puerto Natales ("Vidal Gormaz"), in the Magellan Straits, in the Magdalena, Cockburn, Brecknock, Ballenero and Beagle Channels, in the region off the eastern entrance of the Beagle Channel down to Cape Horn ("Victor Hensen") and on the northern slope of the Drake Passage ("Polarstern").
The objectives of the amphipod study conducted in these three cruises were to compare faunal diversity, biogeographical traits and ecological roles of the benthic amphipod taxocoenoses of the Magellan region with the West and East Antarctic benthos (De Broyer and Rauschert, 1996).This paper presents some first results on the composition and affinities of the benthic amphipod fauna of the Magellan region.

MATERIAL AND METHODS
Most of the zoobenthos samples were collected with an Agassiz trawl (frame 150 x 50 cm, "Polarstern": 300 X 100 cm; mesh size in codend: 10 mm) and some by diving with hand nets.The bulk of the amphipod material studied here was sampled using a small dredge (frame of 48 x 18 cm, mesh size: 1.0 mm; "Victor Hensen" cruise: 0.3 mm).In addition, baited traps and a suprabenthic sledge (Brandt et al., 1996) were used during the "Victor Hensen" and the "Polarstern" campaigns.
Sampling equipment (including the suprabenthic sledge) together provided more than 132,000 specimens of gammaridean and caprellidean amphipods.Identification of the material is still under way.The systematic arrangement used here refers to De Broyer and Jazdzewski (1993;1996) as well as the zoogeographical scheme which is based on Hedgpeth (1969).

Species richness and taxonomic diversity
According to De Broyer andJazdzewski (1993, 1996), the gammaridean amphipod fauna of the Magellan area comprises 174 benthic and pelagic species belonging to 104 genera and 36 families.The part of the new benthic material identified to date from all three cruises consists of 137 species (Table 1).About twenty percent (28 species) of all these species appear to be new for science (including 2 spp already described by Rauschert, 1996Rauschert, , 1998)).The new species belong to the families Cyproideidae, Eusiridae s.l., Gammaridae s.l., Liljeborgiidae, Lysianassidae s.l., Stegocephalidae and Stenothoidae.The new additions brought the Magellan gammaridean amphipod fauna to at least 206 spp ( including 2 spp described by Alonso de Pina, 1997), 113 genera and 42 families (Table 2).A list of the Magellan caprellidean amphipods is given in Table 1 but the new material is still under study.
The material from the "Vidal Gormaz" CIMAR FIORDO II cruise 1996, which is nearly completely identified, comprises 68 species, which seems relatively few in comparison with the number of species from the two other cruises.However, the channels between Puerto Montt and Puerto Natales -where this material was collected -lie mostly under the influence of large glaciers and high sedimentation rates have a negative impact on zoobenthos diversity.The amphipod fauna, and the benthic fauna in general, immediately in front of the glaciers is poor in species as well as in individuals.The bulk of the amphipod fauna that was collected from the front of glaciers is formed of species from the burrower families Oedicerotidae and Phoxocephalidae.On the other hand, the amphipod samples collected by "Vidal Gormaz" from open sea areas appear richer in species than the Beagle Channel and the Magellan Strait fauna.However, the species composition shows a slightly different picture.Detailed analysis and comparison of the faunal diversity within the different Magellan areas (and with the Antarctic sub-regions) await the completion of processing of the whole material of the three expeditions.Present results nevertheless allow a preliminary comparison of the species occurrence in the East and West Antarctic and the Magellan sub-regions (Table 1).The presence in the Magellan sub-region of at least 11 genera (Eusiroides, Lepidepecreoides, Melphidippa, Mesoproboloides, Oradarea, Platyischno- The species richness of the 17 most speciose gammaridean families in the different zoogeograph-ical sub-regions of the Southern Ocean is compared in Figure 1. With the discovery of the new species Victorhensenoides arntzi Rauschert, the family Cyproideidae was represented for the first time in the Magellan area by its collection during the "Victor Hensen" cruise 1994 (Rauschert, 1996).The MAGELLAN AMPHIPOD DIVERSITY 289 family was previously unknown from the Subantarctic and Antarctic regions.Related genera are known from Australia, New Zealand and the Mediterranean Sea.The family Stenothoidae, under revision by one of us (M.R.), received particular attention.Stenothoids are widespread in the Southern Ocean where they constitute the third most speciose gammaridean family.Representatives of this family are often overlooked or misidentified due to their small size.Their ecology remains poorly known.They could be associated with different sessile benthic organisms like algae, Hydrozoa, Ascidiacea, Porifera, or Octocorallia.Thirteen species were previously known from the Magellan area (De Broyer and Jazdzewski, 1993); 18 additional species were collected during the new campaigns.These new findings make the Stenothoidae the most speciose family for the Magellan area, along with the Lysianassidae s.l.(Fig. 1).Until now, the genus Scaphodactylus Rauschert and Andres appeared to be endemic to the South Shetland Islands but its occurrence is extended now into the Magellan area where it was detected for the first time during the "Vidal Gormaz" cruise 1996 (Estrecho Nelson,51°41,40'S 73°13,40'W,90m depth).

Zoogeographical affinities of Magellan amphipod fauna
Benthic gammaridean amphipods of the Magellan area exhibit different patterns of distribution in the Southern Ocean or the Southern Hemisphere (Fig. 2 and 3

Taxonomical and zoogeographical preliminary results
In addition to an important catch of unknown species and new records for the area, the abundance of material collected will allow revision of a great part of the Magellan fauna.This taxonomic work takes place in the framework of a general revision of the Southern Ocean amphipod fauna, undertaken by the "Antarctic Amphipodologists Network" (Andres, Bellan-Santini, Berge, Coleman, Conlan, De Broyer (coord.),Hendrycks, Jazdzewski, Rauschert, Takeuchi, Thurston) South America and Southern Hemisphere.
ferent Southern Ocean faunules (Fig. 1) showed that the Magellan gammarideans are dominated by lysianassoids (which are however about half the number of Antarctic species), stenothoids and eusiroids.The high number of stenothoid species -as in the West Antarctic-is probably a result of the relative importance of the study effort.Gammarellids (which here comprise a significant part of species formerly attributed to Pontogeneiidae) appear the most diverse in the Magellan area.Thirty-three (16 %) of the benthic species of the Magellan subregion co-occur in the Antarctic region (12 spp or 6 % extend only to South Georgia): they belong mostly to the free-living Eusiridae (10 spp) and Lysiannassoidea (8 spp), to Stenothoidea living in association with different benthic animals or algae (8 spp), and to the burrowing Phoxocephalidae (4 spp).
The new material should also allow reevaluation of the zoogeographical affinities and tracing of the origin(s) and dispersal of the Magellan amphipod fauna in comparison with the traits of the West-and East Antarctic faunas.This preliminary zoogeographical analysis showed than an important part (not less than 29 spp) of the species occurring in the Magellan region are distributed also along the South American shelf, in the Atlantic ocean sometimes to Brazil (see for instance Wakabara et al., 1991) or to Peru on the Pacific side.The species with extended distribution, in particular the so-called "cosmopolitans" or the australasian species recorded in the southernmost part of South America, most often demand careful re-examination and confirmation of their identity.

Scavenger fauna
Trap samples usually provide scavenging species which are not commonly taken by trawls, sledge and dredge.They also give indication of the relative importance of the scavenger component in benthic communities.The most successful sample was taken at "Victor Hensen" station 1147 (72h at 115m) and is probably due to the longer residence time on the bottom.The transect off Isla Picton ("Victor Hensen" stations 1147, 1171, 1198) indicates the probable preference of Orchomenella chilensis and Tryphosites chevreuxi for depths below 100m and of Tryphosella schellenbergi for depths shallower than 60m.Except detailed analysis of population structure or stomach contents of the samples, more precise interpretation of trap results (e.g.selective attractivity to different baits) seems difficult due to the small number of operations, the different residence times on the bottom and the use of non standard baits.At station 1198, traps contained a small number of amphipods belonging to species or families not usually known as scavengers and their attraction to bait should be confirmed.Some samples from stations shallower than 50m i.e. sta.1226 (49h at 24m) and sta.1230 (11h at 50m) were partly spoiled due to significant presence of Myxinidae in the traps which provided abundant mucus and could have ingested trapped amphipods.
FIG. 3. -Zoogeographical affinities of the Magellan gammaridean amphipods.Legend: E: East Antarctic sub-region; W: West Antarctic subregion; G: South Georgia district; S: Subantarctic Islands sub-region; M: Magellan sub-region; A: South America (north to M); Austral:South America and Southern Hemisphere.

TABLE 1 .
-List of benthic gammaridean and caprellidean amphipod species known from the Magellan sub-region (including the provisional list of new records) and their occurrence in the East and West Antarctic sub-regions, the Subantarctic Islands sub-region and outside the Southern Ocean.Legend: E: East Antarctic sub-region; W: West Antarctic sub-region; G: South Georgia district; S: Subantarctic Islands subregion; M: Magellan sub-region; A: South America (north to M); O: outside Southern Ocean and South America; XX: new records for the Magellan sub-region.New material listed in bold face.

TABLE 2 .
-Taxonomic diversity of the benthic gammaridean amphipods of the Southern Ocean (updated from De Broyer and Jazdzewski, 1996, for the Magellan region; provisional identifications excluded) , Prometopa, Scaphodactylus, Synopia, Victorhensenoides, and a new genus of Gammaridae s.l.) is recorded for the first time or confirmed in case of previous doubtful records (Eusiroides and Platyischnopus).

TABLE 3 .
-Scavenger amphipods and other organisms collected in baited traps in the area of the eastern entrance of the Beagle Channel(Joint  Magellan "Victor Hensen" Campaign 1994)