Zoogeographic relationships of Magellan Nudibranchia ( Mollusca : Opisthobranchia ) with particular reference to species from adjacent regions *

Extensive recent collections and taxonomical revisions provide a basis for the first zoogeographical analysis of Magellan Nudibranchia. Using 36 of 57 nominal species described in this area, the degree of endemism has been decreased from 70% to 31%. Very close faunistic relationships to northern Argentina are evident as five of six known species (83%) are shared with the Magellan Province. With a 43% overlap of Magellan species in the southern part of the Peruvian faunal Province (44 species), this area shows a transitional character between coldand warm-temperate waters. Only three of 36 Magellan species are found in common with the high Antarctic (ca. 30 species), revealing a low faunal similarity of about 10%. High Antarctic waters are characterized by high nudibranch endemism (about 80%). Of the ten nudibranch species reported from South Georgia, two are exclusively shared with Magellan, two with high Antarctic and three with both Magellan and Antarctic waters, indicating transitional, Subantarctic conditions. One species is endemic to South Georgia, two other species are insufficiently known and may or may not be endemic. Generally, the Antarctic Convergence appears to be a distributional barrier for Magellan nudibranchs. In contrast, some Antarctic nudibranch species extend far into the Atlantic, obviously with cold Antarctic deep water currents. Coastal, cold temperate Magellan nudibranchs have relatively low affinities to the deep water fauna off Argentina, which seems to be predominantly submerged Antarctic species, but this bathymetrical zonation needs to be further investigated.


INTRODUCTION
Nudibranchs are important members of many benthic communities due to their high species diversity, abundance and direct influences on community structure as selective predators of sessile filtering organisms (e.g.Clark, 1975;Todd, 1981).In the Magellan faunal Province, referring to the Patagonian shelf south of about 41°S on both the Pacific (Chiloé Island, Canal Calbuco) and Atlantic side (Península Valdés) including the Falkland Islands (see discussion in Brattström and Johanssen, 1983), nudibranchs were collected during several international expeditions during the last and at the beginning of this century.D' Orbigny (1835-46) was the first naturalist externally describing nominal nudibranch species during his "Voyage dans L'Amérique Méridionale".He was followed by Couthouy (in Gould, 1852Gould, , 1856)), Cunningham (1871), Abraham (1877), and Rochebrune and Mabille (1891) who added a large number of poorly described additional species.Bergh (1884Bergh ( , 1894;;1898), Eliot (1907) and Odhner (1926) gave more detail, examining internal organs, especially radular and genital features as they continued establishing new species.Studying opisthobranchs collected during the Lund University Chile Expedition, Marcus (1959) gave detailed descriptions of 25 species from southern Chile, nine of them new to science, and briefly discussed zoogeographic aspects.Later, Marcus and Marcus (1969) redescribed a few species collected from Argentina by the "Vema" Expedition.
In total, more than 50 nudibranch species have been reported from the Magellan Province, however, most of them are inadequately described or established on just a single or very few specimens from the type locality.As biogeography crucially depends on solid taxonomic and distributional knowledge (Briggs, 1987), no comprehensive zoogeographical studies have been undertaken on Magellan nudibranchs.
Recently, several scientists started to collect material and critically revise dubious Magellan nudibranch species (Fischer and Ortea, 1996;Millen and Schrödl, in review;Muniaín et al., 1991Muniaín et al., , 1996;;Muniaín and Ortea, 1998;Muñoz et al., 1996;Schrödl, 1996bSchrödl, , 1997aSchrödl, , 1997bSchrödl, , 1997c, in press a, in press b, in press c), in press a, in press b, in press c).Nudibranch collections during the Joint Magellan "Victor Hensen" Campaign 1994 yielded few specimens, but established the southernmost records and provided material of several poorly known species (Schrödl, 1996a).On this basis, the present study analyzes the biogeographic relations of Magellan Nudibranchia to adjacent regions, especially to the nearby Subantarctic and Antarctic waters.

MATERIAL AND METHODS
This study considers distributional data of 36 Magellan nudibranch species regarded to be trustworthy.The basis for this compilation are extensive personal collections carried out in coastal Magellan waters, mainly using SCUBA, from 1991 to 1996 (see Schrödl, 1996b).Additional specimens were dredged from coastal and deeper waters of the Magellan Strait and Beagle Channel area during the Joint Magellan "Victor Henson" Campaign 1994 using Agassiz trawls (see Schrödl, 1996a).Material collected by individual workgroups during the same expedition has not yet been studied.
The zoogeographical comparison (Table 2) shows a relatively high species number (44) and endemism (39 %) for the Peruvian faunal Province.The taxonomically better studied Magellan Province has fewer species (36) and a lower nudibranch endemism (31%).No endemic nudibranchs are known from the warm temperate waters of northern Argentina.The Subantarctic South Georgia Island possesses at least one endemic nudibranch species, while nudibranch endemism is highest (about 80%) in high Antarctic waters (Table 2).
The relations of Magellan species to adjacent areas is indicated by the number of shared species (Tab.2).Additionally, Table 2 gives the ratio number of common species to total species number of the respective area; this documents the relative influence of Magellan nudibranch species within the respective faunas.With 83%, Magellan species predominate in northern Argentina.The Peruvian fau-nal Province and South Georgia are both considerably influenced by Magellan species (Table 2).This is in clear contrast to the high Antarctic where only 10% of the known nudibranch species are shared with the Magellan faunal Province.
Only four species are reported from deeper waters of the Argentinian continental slope, of which one, Bathydoris patagonica Kaiser, 1980 seems to be endemic.Two species, T. vorax and A. kerguelenensis, are shared with cold temperate coastal, Magellan waters.Both species, however, only occur coastally on the southernmost tip of Patagonia (see Fig. 1D; Tab. 1).T. vorax has a Subantarctic, A. kerguelenensis a circumantarctic distribution, along with the remaining species Bathydoris clavigera Thiele, 1912.

DISCUSSION
Extensive recent collections and taxonomical revisions enable a first zoogeographic account of Magellan nudibranchs, regarding their endemism and faunal relationships.In former studies (Carcelles and Williamson, 1951;Marcus, 1959) 32 of 46 species (70%) appeared to be endemic.Based on the species list given in Table 1, only 11 of 36 species (31%) exclusively occur within the Magellan faunal province.Five of them are still only known from their type localities (Tab.1), indicating collecting bias.Four species with more than a single record appear to be endemic within the Atlantic area, including the Falklands (Malvinas), eastern Magellan Straits and Beagle Channel, one to the Atlantic side of Tierra del Fuego, and two in Chilean Patagonia (see Fig. 1A; Tab. 1).This indicates a certain faunal separation into Pacific and Atlantic elements.Interestingly, not a single nudibranch species fits within the limits often proposed for the Magellan faunal Province that is, exclusively occurring south from 41°S on both the Atlantic and Pacific coasts of South America (see discussion in Brattström and Johanssen, 1983).
Instead, many species formerly considered to be endemic to Magellan waters or known from a single locality were recently found to have a much wider range: a total of 10 species and Aeolidia papillosa var.serotina Bergh, 1873 occur throughout the Magellan Province.On the Pacific side, they range northwards with the cold-temperate Humboldt Current waters, favoured by areas of strong upwelling, into central and even northern Chile, the southern part of the Peruvian faunal Province (Schrödl, 1996b;1997a; this study, see Fig. 1B).These species, as well as Ancula fuegiensis and Thecacera darwini which appear to be limited to Pacific waters (Fischer and Ortea, 1996;Schrödl, 1996b), confirm the general tendency of Magellan marine organisms to extend into a transitional area off central Chile (Brattström and Johanssen, 1983).
On the Atlantic side a similar phenomenon occurs: several wide-ranging Magellan species, Anisodoris fontaini, Gargamella immaculata, Holoplocamus papposus, and Tritonia challengeriana extend north of the Valdés Península to northern Argentina and the La Plata river (see Fig. 1B).Polycera marplatensis (as P. quadrilineata var.marplatensis) occurs from Argentinian Patagonia (Schrödl, 1996b: 31) to Brazil (see Muniaín and Ortea, 1998).From northern Argentina, no endemic species and only a single member of the Atlantic warm water fauna, Mariona cucullata Gould, 1852 (= T. occidentalis Bergh, 1884;see Odhner, 1926see Odhner, , 1934)), is known.With a total of only six species, northern Argentina is still very poorly studied.As five of these six species (83%) are shared with the Magellan region and at least four species show an extended Magellan distribution, however, this area is clearly predominated by Magellan nudibranchs.
The Antarctic nudibranch fauna is comprised of about 30 species from south of the Antarctic Convergence excluding South Georgia and other Subantarctic islands (Wägele, 1993, modified).The short geographical distance between South America and the Antarctic Peninsula and the potentially high dispersal ability of planktonic nudibranch larvae would suggest close faunal affinities.There are, however, only three species (Table 1; Fig. 1D) in common between cold temperate (=coastal) Magellan (36 species) and Antarctic waters (30 species): while Austrodoris kerguelenensis was critically revised recently (Wägele, 1990), taxonomic studies remain to be done on T. challengeriana and Cuthona georgiana (see Cattaneo-Vietti, 1991;Schrödl, 1996 b).This results in a strikingly low nudibranch similarity of 10%, coinciding with values for shelled gastropod species given by Linse (1997).On generic level, Millen and Schrödl (in review) showed Magellan nudibranch genera to be even closer related to the Aleutian fauna than to that of Antarctica.The Antarctic Convergence thus has been a rather strict distributional boundary for temperate nudibranchs.
The molluscan fauna of South Georgia was characterized to have 1) low affinities to temperate faunas (Powell, 1951) and 2) high rates of endemism (Dell, 1972).However, this does not hold true for nudibranchs.Of a total of ten nudibranch species reported from South Georgia, two species are common with the Magellan province, but not with the Antarctic: Flabellina falklandica was indicated to have northern Pacific ancestors (Millen and Schrödl, in review), but recently shows a wide Magellan distribution from Chiloé Island over southern Patagonia and the Falklands, and additionally occurs in South Georgia and Crozet Island (see Fig. 1C).It has obviously spread there with the help of circumantarctic currents (Marcus, 1959).Dispersal by rafting seems possible since F. falklandica was found in abundance on kelp feeding hydroids which grow on the fronds (see Millen and Schrödl, in review).The second species, Tritonia vorax, occurs in deeper waters (100m-360m) off the Beagle Channel, the Burdwood Bank and South Georgia, but is not reported from other locations south of the Antarctic Convergence.Three species from South Georgia (A. kerguelenensis, T. challengeriana, C. georgiana), are shared with both Magellan and Antarctic waters (see Fig. 1D), and two, Bathydoris clavigera and Tritoniella belli Eliot, 1907 are circumantarctic species.This confirms the transitional, Subantarctic character of the nudibranch fauna of South Georgia suspected by Marcus (1959), with close relations to both Magellan and Antarctic waters.As Wägele (1991) pointed out, there is no or only a low nudibranch endemism in South Georgia.Cuthona antarctica Pfeffer in Martens and Pfeffer, 1886 may be endemic (Tab.2), but, together with specimens of Cuthona schraderi (see Cattaneo-Vietti, 1991) from South Georgia, requires taxonomical revision.Austrodoris georgiensis established on a single specimen from South Georgia by García et al. (1993) is here considered to belong to the circumantarctic and very variable A. kerguelenensis as redescribed by Wägele (1990Wägele ( , 1993)).A specimen previously assigned to Cadlina falklandica Odhner, 1926 differs from all other southern cadlinids and thus is described as a new species (Schrödl, in press c); this is the only nudibranch species so far known to be endemic to South Georgia.
Distributional studies on Magellan and Antarctic nudibranchs not only have to consider latitudinal but also bathymetrical aspects.It seems possible for cold adapted, Antarctic nudibranch species to submerge in lower geographical latitudes and inhabit the South American continental slope, probably due to intermediate Antarctic cold water currents flowing into the Atlantic.This is indicated by the occurrence of the circumantarctic Tritoniella belli (as T. sinuata Eliot, 1907;see Wägele, 1989 b) north to the Shag Rock Bank (53°S; 160m depth; Odhner, 1926).The circumantarctic and eurybathic, eye possessing Bathydoris clavigera (as B. argentina Kaiser, 1980;see Wägele, 1989 a) extends north to a latitude of about 44°S off Argentina, where it was collected between 400-1200 m depth (Kaiser, 1980).Austrodoris kerguelenensis probably even occurs far north, off Mar del Plata (37°36'S), but in 740 m depth (Bouchet, 1977: as A. macmurdensis; see Wägele, 1991;1993; this study, see Fig. 1D).Until now, nothing is known about the extension and history of these deep water populations.
Interestingly, only two of 36 Magellan species were found in deeper waters off Argentina (Table 1).Although representing 50% of the four species known from the Argentinian slope (Table 2), neither are exclusively shared with coastal Magellan waters.Instead, all three non-endemic species show clear relationships to Subantarctic or Antarctic regions (Table 1).Thus the nudibranch fauna of the Patagonian shelf appears to have relatively low affinities to that of deeper areas which are strongly influenced by Antarctic species.The eye-lacking Bathydoris patagonica is apparently endemic to the Argentinian slope (see Kaiser, 1980), and may indicate the existence of a unique Argentinian deep-sea fauna.However, greater depths off South America are far too poorly known to draw final conclusions.

FIG. 1
FIG. 1. -Distribution of Magellan Nudibranchia.A. Three apparently endemic species: Polycera priva seems limited to the Chilean fjord region, Eubranchus fuegiensis is only known from Tierra del Fuego and the Beagle Channel, and Geitodoris patagonica occurs in the Atlantic sector of the Magellan Province.B. Gargamella immaculata, having a wide Magellan distribution with extensions into central Chile and northern Argentina.C. Magellan and Subantarctic distribution of Flabellina falklandica (record from Crozet Island omitted).D. The circumantarctic (not all records indicated) Austrodoris kerguelenensis only occurs coastally in southernmost Patagonia.In lower latitudes it submerges, being reported from 740 m depth off Mar del Plata.

TABLE 2 .
-Zoogeographic relationships of Nudibranchia of the Magellan Province (MP) to adjacent regions.