Taxonomic study of the isopod crustaceans from the Spanish expedition “ Antártida 1986-11 ” in the Scotia Sea , South Atlantic *

This paper describes the Crustacea Isopoda collected during the “Antártida 1986-11” expedition in the Scotia Sea between November 1986 and February 1987. This expedition was performed aboard the B/F “Pescapuerta Cuarto”, a commercial fishing vessel equipped to evaluate Antarctic fishing resources. Qualitative and quantitative observations were made of species of commercial interest (for the expedition objectives see Balguerías et al., 1989). The material collected that did not belong to species of commercial interest was separated according to taxonomic groups and sent to specialists at universities throughout Spain. Although the expedition was not performed with the specific objective of sampling isopods, following an initial evaluation of the material collected, this was considered a matter of interest. The results obtained from this study are reported here. SCI. MAR., 63 (Supl. 1): 275-279 SCIENTIA MARINA 1999


MATERIAL AND METHODS
The sampling area included the continental shelf of the Scotia Sea archipelagos (Shag Rocks, South Georgias, South Sandwich, South Orkneys, Elephant, South Shetland), to a depth of 500 m.The area was contained within the meridians 62º22'50" and 26º14'20" of longitude W and the parallels 53º18'40" and 63º02'20" of latitude S. Table 1 shows the material examined and the sampling data.Furthermore, in Table 2, the trawling activity in each archipelago is indicated.
Comparative sampling was performed using semipelagic gear with a nylon sleeve and with a 68 mm mesh size in the cod-end.For selectivity experiments, an additional 40 mm mesh size cod-end was added to the gear, completely covering the original but spaced at a distance of 2 m (Balguerías et al., 1989).
At each fishing station a 30 minute trawl haul was taken, using a stratified random sampling method.The shelf of each island was divided in strata of 150 m (100 m at Elephant island).The number of trawls depended on the area of the strata.The total number of samples obtained from each archipelago, the number containing isopods, and the percentage of samples with isopods with respect to the total are indicated in Table 2.
The material captured was frozen and maintained at a temperature between -30 and -40ºC awaiting transfer to the laboratory.After de-icing, the specimens were fixed in 70% ethyl alcohol.In studying  the specimens, dissections were unnecessary.They were sufficiently large to enable taxonomically distinctive characters to be observed with the naked eye or with the aid of a stereoscopic microscope.Serolidae species were identified using the keys of Bastida and Torti (1973), Brandt (1988), and also the notes of Wägele (1994), while Arcturidae species were recognized using the keys of Wägele (1991).

RESULTS
The number of trawls carried out was 345, from 50 of which isopods were collected.A total of 123 specimens were examined, of which 99 were adult or juveniles and 24 were postmancas (the latter are not considered in Table 3).
The number in which species appeared varied among geographical areas.Antarcturus hempeli was  3).Only 21% of Serolidae and 12.5% of Arcturidae species were collected with respect to the species previously found in the archipelagos of the Scotia Arc (19 species of Serolidae and 32 species of Arcturidae (Winkler, 1994)).Several species have been recorded for the first time in some archipelagos: Antarcturus hempeli, in the Shag Rocks and the South Georgias; Antarcturus spinacoronatus, in the South Georgias and the South Orkneys; Chaetarcturus longispinosus, in the South Orkneys; Natatolana meridionalis, in the South Georgias; and Iathrippa trilobatus, in the South Shetlands (Table 3).

DISCUSSION
The presence of only 123 specimens in 345 trawl hauls is very low.However, a semipelagic trawl was used, and the fact that trawling was not conducted with the purpose of capturing isopods, meant that small specimens were not collected with the meshes employed (40 mm).Only the largest species (Glyptonotus antarcticus, Ceratoserolis trilobitoides, Antarcturus hempeli, A. spinacoronatus) were captured in higher numbers with this mesh size (Table 3).A more accurate sampling procedure would have resulted in the collection of many more specimens, especially Asellota.
Furthermore, the same number of trawls was not performed in each archipelago, therefore the number cannot be simply compared.Yet, the ratio between number of samples and number of samples containing isopods (Table 2) seems to confirm that with more intense sampling the number of species would most likely be higher.
Species biogeography in the Subantarctic region suggests two subregions (Magellan and Scotia Arc), probably due to the Antarctic Convergence (Winkler, 1994).In the species cited by Brandt (1991) as being exclusive to either the Magellan or the Scotia Sea subregions, Iathrippa trilobatus and Ceratoserolis trilobitoides were the only two species recorded here to be cited in the Magellan subregion.Therefore, our results confirm the marked distinction between the fauna of both subregions.
All the 11 species sampled have previously been mentioned as pertaining to the Scotia Sea (Brandt, 1991;Winkler, 1994).

TABLE 1 .
-List of trawl stations and material examined.

TABLE 2 .
-Number of samples obtained at each Scotia Arc archipelago: SHR, Shag Rocks; SG, South Georgia; SS, South Sandwich Islands; SO, South Orkneys; E, Elephant Island; SSH, South Shetlands.

Table 3 .
-Number of specimens collected and distribution of species at each Scotia Arc archipelago (abbreviations as in Table2).Glyptonotus antarcticus was the most abundant species in the South Sandwich Islands and the South Orkneys, while Ceratoserolis trilobitoides was characteristic of Elephant Island and South Shetland (Table