Biogeographical distribution of the benthic thecate hydroids collected during the Spanish “Antártida 8611” expedition and comparison between Antarctic and Magellan benthic hydroid faunas*

During the Spanish Antarctic expedition “Antártida 8611” to the area of the Scotia Ridge islands, carried out in the austral summer of 1986-87 by the Instituto Español de Oceanografía, a large amount of benthic samples were obtained by trawling. Hydroids was one of the zoological groups best represented in the studied area. Forty-six species were found, of which ten were identifed to generic level only, in many cases because of the scarce material available (cf. Peña Cantero and García Carrascosa, 1995). In the present paper a study of the biogeographical distribution of the 36 species determined to speSCI. MAR., 63 (Supl. 1): 209-218 SCIENTIA MARINA 1999


INTRODUCTION
During the Spanish Antarctic expedition "Antártida 8611" to the area of the Scotia Ridge islands, carried out in the austral summer of 1986-87 by the Instituto Español de Oceanografía, a large amount of ben-thic samples were obtained by trawling.Hydroids was one of the zoological groups best represented in the studied area.Forty-six species were found, of which ten were identifed to generic level only, in many cases because of the scarce material available (cf.Peña Cantero and García Carrascosa, 1995).
In the present paper a study of the biogeographical distribution of the 36 species determined to spe-SCI.MAR., 63 (Supl.1): 209-218 SCIENTIA MARINA 1999 MAGELLAN-ANTARCTIC: ECOSYSTEMS THAT DRIFTED APART.W.E. ARNTZ and C. RÍOS (eds.)Biogeographical distribution of the benthic thecate hydroids collected during the Spanish "Antártida 8611" expedition and comparison between Antarctic and Magellan benthic hydroid faunas* cific level has been undertaken.Furthermore, an inventory of both the Antarctic and Magellan benthic thecate hydroid fauna has been carried out.This aimed at estimating the relationship between the Antarctic hydroid fauna and that present in the Magellan region.Nevertheless, it is necessary to take into account the limitations concerning our knowledge of the hydroid fauna in those areas.Thus, for example, athecate hydroids, which were not present in the "Antártida 8611" collection, are poorly known both in the Antarctic region and in the Magellan area, and for this reason are not considered in the present study.Moreover, the hydroid fauna is still unknown in large areas of the Antarctic and not even the sampling efforts in the area so far studied have been uniform.

MATERIAL AND METHODS
Figure 1 represents the itinerary followed during the "Antártida 8611" expedition.In Figure 2 are represented the different areas studied, along with the stations in which hydroid species were obtained.
In defining the limits of the Antarctic region the ideas stated by Hedgpeth (1969), and basically followed by Picken (1985) and Knox (1995), are considered.According to those authors the Antarctic region covers the whole of the area south of the Antarctic Convergence, whereas the Sub-Antarctic region includes the area between the Antarctic Convergence and the Sub-tropical Convergence.In this study, however, Andriashev (1964) and Picken (1985) have been followed in considering the northern limit of the pack-ice as the most valid limit for the Antarctic benthos.To the Magellan area the limits stated by Briggs (1974) have been followed; this author situated the northern Atlantic limit at the mouth of the Río de la Plata (Argentina) and the northern Pacific limit at the northern tip of Chiloé Island (Chile).
In this paper, Circum-Antarctic distribution indicates the distribution throughout the Antarctic region; Pan-Antarctic distribution also includes the distribution throughout the Sub-Antarctic region.Antarctic-Kerguelen and Antarctic-Patagonian distributions refer to the distribution throughout the Antarctic and either the Sub-Antarctic islands or the Magellan area, respectively.
Due to the absence of athecate hydroids in the "Antártida 8611" collection, and the scarce knowledge of this group both in the Antarctic region and the Magellan area, they are not included in this study.

RESULTS AND DISCUSSION
In Table 1 is shown the list of the 36 species collected during the "Antártida 8611" expedition and determined to specific level.The most important feature is the high degree of endemism at the specif- S. affinis (Jäderholm, 1904) Antarctic species, being present both in the Magellan and in the Kerguelen areas.At present, four species (A.elongatus, S. dichotoma, S. frigida and S. glacialis) are considered to have an Antarctic-Kerguelen distribution, their records are restricted to the Antarctic and Kerguelen areas (there are no records of these species in the Magellan area).Alternatively, two species (A.operculata and B. subrufa) might be considered to be Antarctic-Patagonian species, since their records are restricted to the Antarctic and the Magellan areas (they have never been recorded in the Kerguelen area).Symplectoscyphus subdichotomous has usually been recorded in Sub-Antarctic waters, though in a few occasions in Antarctic waters as well.Schizotricha multifurcata might be considered a Sub-Antarctic species, being present in the Magellan and in the Kerguelen areas with the most southern record at the South Sandwich islands.The remaining two species, H. delicatulum and L. dumosa, have an almost cosmopolitan distribution.
These results are similar to those obtained if all the known Antarctic thecate hydroid species are considered (Table 2).Thus, at specific level, the originality of the Antarctic benthic thecate hydroid fauna is remarkable, since 72 (69%) of the 104 species determined to specific level are endemic in the Antarctic region.Stepan'yants (1979), in a study on the Antarctic and Sub-Antarctic species of hydroids, found that of 91 species present in the Antarctic region, 44 species (48%) were endemic in that region.
At present, as is shown in Table 2, of the 72 endemic species, 21 (29%) may be considered to have a Circum-Antarctic distribution.The remaining endemic species are either endemic in East Antarctica (5 species or 7%) or in West Antarctica (46 species or 64%).These results contrast with those obtained by Stepan'yants (1979) who found that of 44 endemic species in the Antarctic region (see above), 13 (14%) were endemic in East Antarctica and 11 (12%) in West Antarctica.The much higher number of endemic species in West Antarcti-ca, at present, may be due to the unequal sampling and study efforts in East and West Antarctica since Stepan'yants paper.
The originality of the Antarctic hydroid fauna is greater if we consider the Antarctic species also present in Sub-Antarctic waters.Here are included the five species with Pan-Antarctic distribution, the two species with Sub-Antarctic distribution, the five species with Antarctic-Patagonian distribution and the nine species with an Antarctic-Kerguelen distribution.Thus, the number of Antarctic species either endemic in the Antarctic region or in Antarctic and Sub-Antarctic waters is raised to 93, representing ca 89% of the 104 species of thecate hydroids determined to specific level.
Only 11 Antarctic species of benthic thecate hydroids are also present outside the area comprised by Antarctic and Sub-Antarctic waters.These are four species with a multiaustral distribution, two bipolar species, one circumglobal species and four species with a worldwide distribution.
None of the 23 genera with representatives in the Antarctic region is endemic in this area.However, the genera Stegella, Staurotheca, Antarctoscyphus and Oswaldella are probably genera with the Antarctic region as centre of origin.Thus, of 23 species referred to Staurotheca [cf. Peña Cantero et al. (1997a) and present paper], 19 species are endemic in the Antarctic region, two have an Antarctic-Kerguelen distribution and the remaining two are endemic in the Magellan area.Of 22 species referred to Oswaldella (cf.Peña Cantero andVervoort, 1998 andPeña Cantero andGarcía Carrascosa, 1998) 21 are endemic in the Antarctic region, the majority being restricted to the continental Antarctic region (high Antarctica and Scotia Ridge), the remaining one is endemic in the Magellan region.Of eight known species of Antarctoscyphus [cf. Peña Cantero et al. (1997)], seven are endemic in the Antarctic region, whereas the remaining has an Antarctic-Kerguelen distribution.Finally, the single member of the genus Stegella (S. grandis) has an Antarctic-Kerguelen distribution.
The predominant life cycle (characterized by the suppression of the free-swimming stage) in the Antarctic thecate hydroid fauna might have favoured the high degree of endemism at specific level.In the Antarctic region primary production is highly seasonal and at most coastal sites is generally limited to the period of open water.However, a small fraction of nano-and picoplankton is available throughout the year (Froneman and Perissinoto, 1996).During the rest of the year redistribution of accumulated energy from one trophic level to another takes place (Picken, 1985).This fact might have favoured those groups of hydroids that have a reduced or even suppressed free-swimming stage.Thus, the majority of the known Antarctic thecate hydroid fauna has fixed gonophores and only a few species have medusae.In spite of the a priori lower dispersal potential of the fixed gonophores, there is a high number of Circum-Antarctic species which may have been favoured by the uniform general conditions present throughout the Antarctic Ocean during a long time.The high number of endemic species in West Antarctica might merely reflect which part of Antarctica has been better studied and certainly future scientific surveys will change the distribution of many of those species to Circum-Antarctic.

Antarctic hydroid fauna vs Magellan hydroid fauna
As shown in Table 2, the Magellan area is richer than the Antarctic region concerning the biodiversity of thecate hydroids.At family level, it is remarkable that whereas the nine families of thecate hydroids with members in the Antarctic region also have representatives in the Magellan area, there are three families of thecate hydroids (Syntheciidae, Plumulariidae and Aglaopheniidae) with members in the Magellan area, but without representatives in the Antarctic region; the members of these three families are typically distributed in temperate and warm waters.
In the Magellan area, as in the Antarctic region, Sertulariidae is the dominant family.Thus, of 126 species (cf.Table 2), 57 species (45%) belong to this family.However, whereas in the Antarctic region the following family is Kirchenpaueriidae with ca a 20% of the species (see above), in the Magellan area it is the family Campanulariidae with 22 species (ca 17%), followed by Lafoeidae with 13 species (ca 10%).
The differences at generic level between both areas are greater than at family level.The two regions differ in the number of genera, in the presence/absence of certain genera and in the number of species of genera common to both.
The much higher diversity at generic level in the Magellan area is remarkable.Thus, as shown in Table 2, in the Magellan area species of 40 genera of thecate hydroids have been reported, whereas in the Antarctic region species of only 23 genera are known, representing ca 58%.Consequently, there is a high number of Magellan genera without representatives in the Antarctic region.In contrast, with the exception of four genera (Lafoeina, Stegella, Halisiphonia and Antarctoscyphus), the remaining genera present in the Antarctic region also have representatives in the Magellan area.Stegella and Antarctoscyphus are genera that probably originated in the Antarctic region, being composed of either species endemic in that region or Pan-Antarctic species.Thus, as stated above, the single member of the genus Stegella has an Antarctic-Kerguelen distribution and of eight known species of Antarctoscyphus, seven are endemic in the Antarctic region, whereas the remaining one has an Antarctic-Kerguelen distribution.
There are two common genera that probably originated in the Antarctic region and that differ greatly in the number of species present both in the Antarctic region and in the Magellan area.Thus, as was stated above, of 23 species referred to Staurotheca, 19 species are endemic in the Antarctic region, two have an Antarctic-Kerguelen distribution and the remaining two are endemic in the Magellan area.Of 22 species referred to Oswaldella, 21 are endemic in the Antarctic region, the remaining one being endemic in the Magellan area.There are also large differences concerning the genus Schizotricha.Thus, of nine species recorded from Antarctic and Sub-Antarctic waters (cf.Peña Cantero, 1998), a single species is present in the Magellan area, the remaining species being endemic in the Antarctic region.
On the other hand, there are genera in which the proportion is clearly contrary, having a larger number of species in the Magellan area.Thus, El Beshbeeshy (1991), in the Magellan area, referred 26 species to the genus Sertularella and 21 to the genus Symplectoscyphus, whereas in the Antarctic region two and 13 species, respectively, have been recorded.
The genus Abietinella, with its only known member (A.operculata), is shared both by the Antarctic region and the Magellan area, and might be considered to have an Antarctic-Patagonian distribution.
At specific level, though the diversity is higher in the Magellan area, the degree of endemism is lower in that area.As was stated above, of 104 species determined to specific level, 72 (69%) are endemic in the Antarctic region.In the Magellan area, however, of 126 species, 49 (39%) may be considered endemic.
At present, it is difficult to know which species inhabiting the Antarctic region are of Magellan origin and migrated to Antarctica via the Scotia Ridge and, inversely, which species of Antarctic origin followed the opposite way.Before determining the relative importance of the contingent of species the Antarctic region and the Magellan areas have in common, it would be necessary to improve scientific knowledge concerning the hydroid fauna of both areas, since they are still poorly known, existing for instance vast Antarctic areas in which the hydroid fauna is completely unknown.It would also be necessary to know the relationships between the Antarctic hydroid fauna and that of other areas, mainly that of the Sub-Antarctic islands, but also that of New Zealand, South Australia and South Africa.