The occurrence of pycnogonids associated with the volcanic structures of Bransfield Strait central basin ( Antarctica )

The most recent reports on pycnogonids from Antarctic and sub-Antarctic waters are those of Arntz et al. (2006), Bamber (1995, Falkland Islands and South Shetland Islands), Child (1994, 1995 diverse zones), Chimenz and Gravina (2001, Ross Sea), Munilla (1991, 2000, 2001a, 2002, Scotia Sea and Antarctic Peninsula), Munilla and Ramos (2005, Antarctic Peninsula), Pushkin (1993, various zones) and Turpaeva (1998, 2000, Weddell Sea). These authors collate references and the historical background of previous work from this area, particularly Child’s papers. Other works (Arntz et al. 1990 and Galéron et al. 1992) provide only qualitative data about the occurrence of pycnogonids at 27 stations in the Weddell Sea, between 200 and 2000m depth. Gerdes et al. (1992) have provided SCIENTIA MARINA 71(4) December 2007, 699-704, Barcelona (Spain) ISSN: 0214-8358


INTRODUCTION
The most recent reports on pycnogonids from Antarctic and sub-Antarctic waters are those of Arntz et al. (2006), Bamber (1995, Falkland Islands and South Shetland Islands), Child (1994Child ( , 1995 diverse zones), Chimenz and Gravina (2001, Ross Sea), Munilla (1991, 2000, 2001a, 2002, Scotia Sea and Antarctic Peninsula), Munilla and Ramos (2005, Antarctic Peninsula), Pushkin (1993, various zones) and Turpaeva (1998, 2000, Weddell Sea).These authors collate references and the historical background of previous work from this area, particularly Child's papers.Other works (Arntz et al. 1990 andGaléron et al. 1992) provide only qualitative data about the occurrence of pycnogonids at 27 stations in the Weddell Sea, between 200 and 2000m depth.Gerdes et al. (1992) have provided quantitative data for this and other groups, sampled with a multibox corer, including biomass and abundance at 36 stations (170-2037) from the southeastern Weddell Sea.Some species from Livingston Island and surrounding waters (South Shetland Islands, Drake Passage, Bransfield Strait) have been documented previously, mainly by Gordon (1932Gordon ( , 1944)), Fry and Hedgpeth (1969), Pushkin (1993), and the Child and Bamber papers mentioned before.
The aim of the present study is to present quantitative data on the biomass and abundance of pycnogonids of the Bransfield Strait central basin in order to compare these data with other works of the same zone and with other volcanic zones.We also analysed the fauna from volcanic structures to deduce whether the volcanoes were active or inactive during 1996.

MATERIAL AND METHODS
The specimens were collected during the Gebrap-96 cruise (December 1996 to January 1997) aboard the Hespérides oceanographic vessel of the Spanish Navy.The benthonic sampling area was located on underwater volcanic structures located in the central basin of Bransfield Strait (CBB)(Fig.1).
A total of 9 stations were sampled, but pycnogonids were only present in 6 of them.The coordinates and characteristics of stations where pycnogonids were present are shown in Table 1.Samples were obtained between 647 and 1592 meters depth; the duration of dragging was between 41 (DR6) and 85 minutes (DR2), with a mean velocity of 2 knots.
A rocky dredge of 0.8 × 0.3m, with 10 mm mesh size and leather protection, was used in the hauls.Samples were sieved on board through sieves of decreasing mesh size, 10, 5 and 1 mm respectively.Specimens were fixed in 4% formalin solution and stored in 70% ethanol in the zoological collections of the Universidad Autónoma de Barcelona.
On the seafloor of the CBB, four large volcanic edifices (labelled A, D, E and F in Fig. 1, from west to east, following the labelling of Canals and Gracia, 1997) have been identified.They exhibit a range of circular (conical), semicircular, crescent and elongated or ridged forms, the latter arranged in an en échelon form.Edifice A showed linear volcanic ridges, and included two stations (DR5 and DR6); a series of spaced, subparallel ridges dominated edifice D, which included both DR4 and DR7; edifice E was dominated by conical seamounts, and included only one trawl, DR3; finally, edifice F, which was also associated with linear volcanic ridges, included DR2.

RESULTS
Only six of the nine stations sampled provided pycnogonids.
A total of 54 pycnogonids were collected, belonging to 22 species, 8 genera and six families.This represents 0.8% of the total abundance (a total of 6797 specimens were collected belonging to 35 different faunal groups).The most abundant pycnogonid families were Colossendeidae and Nymphonidae.Each family included 21 specimens.The two families together constituted 78% of the total pycnogonid abundance.Nymphon proximum (Calman, 1915) was the richest species, with 7 specimens, followed by Nymphon villosum (Hodgson, 1915), with 5 specimens.
Table 1 shows the species, abundance and biomass of the pycnogonids present in each station.
Table 2 shows the species collected with their respective life stages and their percentages of abundance and biomass in relation to the total number of pycnogonid specimens collected.
Total fresh weight for all pycnogonids was 34.73 g, which represented 0.23% of all faunal groups collected (15018 g).Note that three individuals presented teratological structures: i) right propodi with different spinulation and form to left ones in Austropallene brachyura; ii) functional chelae in adult of Ammothea carolinensis; iii) presence of a ventral bulge in the proboscis of Pycnogonum gaini.These features are normally the opposite to the normal ones.

Biological richness vs. abiotic factors
Although Antarctic and Subantarctic pycnogonids are well studied, it is not unusual for a cruise to find new species or new geographical or bathymetric data, and few collections have been made in the basin of Bransfield Strait (Munilla, 2000(Munilla, , 2001a)).
The bottom bathymetry map of the central Bransfield Basin, west Antarctica, is dominated by a series of isolated volcanoes and associated ridges (formation of new oceanic crust) with distinct morphologies which illustrate three tectonovolcanic evolutionary stages: (I) conical seamounts, indicative of continuous, focused, point-source volcanism, represented by the small scattered conical volcanoes and edifice E; (II) the flat top volcanic cones are split by extensional faults in two crescent halves by a longitudinal and central volcanic ridge (this situation is well illustrated by edifices A and F); and (III) series of spaced, subparallel ridges (edifice D), resulting from the progressive disfigurement of the volcanic builds, which means that they have become volcanically inactive (Canals and Gracia, 1997).
The zone with the highest biological richness in pycnogonids and other zoological groups (Ramil and Ramos, 1997) in the Bransfield Strait, both in abundance and biomass, was DR6 station, located on edifice A, which has a bottom of mud with some gravel.The reasons for this biological richness are difficult to explain, taking into account that the dragging of DR6 station was the shortest (Table 1), but two hypotheses are presented.
The abundance of pycnogonids and other benthic groups of the preliminary study (Ramil and Ramos, 1997) decrease with depth.Depth is an approximate measure of distance from the planktonic source (Smith, 1955); that is to say, food is more abundant at the upper stations because distance to the planktonic source is shorter.Food can be acquired more easily in shallow stations (DR6), and so pycnogonids taxocoenosis may increase in abundance.
The slightly inclined orography of DR6 (Canals and Gracia, 1997) may favour the arrival of resuspended organic material in slight currents, which would be filtered by bryozoa, hydrozoa and porifera, or other suspension-feeders; these organisms increase their biomass (Ramil and Ramos, 1997) and may serve as food for pycnogonids.
San Vicente et al. (1997) mention that depth is the only important abiotic factor of six factors tested in waters around Livingston Island.Munilla (2001b) found that two-thirds of the Antarctic and Subantarctic pycnogonid species have only been found on the continental shelf and upper slope (also the most sampled zones) and that the number of species decreases dramatically down to 1000 m.

Are there adaptations to volcanic structures?
There are only six reports (Child, 1982(Child, , 1987(Child, , 1989(Child, , 1996;;Turpaeva, 1988;Brescia and Tunnicliffe, 1998) on Pycnogonida taken from five different deep sea hydrothermal vents.All recorded species (Table 3) had a normal aspect without special morphological adaptations for the vents, except that deep-sea species did not have eyes (Turpaeva, 2002).It is interesting to note that five of the six known species of Sericosura have been recorded exclusively on hydrothermal vent exposures, sometimes in elevated densities (Brescia and Tunnicliffe, 1998), and no Sericosura species were found by the Gebrap-96 cruise despite sampling similar volcanic areas.
None of the pycnogonids or other taxa (Ramil and Ramos, 1997) sampled by this cruise found specimens with specific volcanic adaptations (e.g.external mucus-filamentous bacteria, dark sulphide crust-vent Pycnogonida), leading us to believe that these sampled volcanic zones were inactive during 1996.The same occurred on the hydroids sampled in this cruise (Peña Cantero and Ramil, 2006).

Faunistic and taxonomic traits
In this study, the genus Colossendeis was found to be as abundant as Nymphon.This differs somewhat from other Antarctic collections (Munilla, 2000(Munilla, , 2001a)), in which the most abundant genus was Nymphon (the most speciose genus), and the second most abundant was Ammothea.
Colossendeis and Nymphon are two genera with very large legs, adapted to moving on muddy bottoms (DR6 station), and in deep currents (Turpaeva, 2002).
Although the Colossendeidae and Nymphonidae are equal in abundance, biomass is much greater for the Colossendeidae (Fig. 2) due to their generally larger size compared to the smaller Nymphonidae.Munilla (1991) found a similar situation, and ranked the Colossendeids (heavier to lighter) like this: Decolopoda australis, Colossendeis australis and C. robusta.Since the Colossendeids are much larger than an average pycnogonid, this may help to account for their greater longevity (Munilla, 1991), and indicates that they are probably employing the K-strategy (environmental stability, slow growth, big size and low metabolism) instead of the r-strategy, which is typical of smaller pycnogonids such as nymphonids.
We consider Ammothea gibbosa of Bouvier (1913, Fig. 81), (not Colossendeis gibbosa of Möbius, 1902, pl.XXX, Fig. 1-5, which is a juvenile of Ammothea confused with a juvenile of Colossendeis), to be a valid species and not a synonym of A. carolinensis (Leach, 1814), as suggested by Fry and Hedgpeth (1969, p. 75).Ammothea gibbosa has a rounded ocular tubercle, a basal tubercle on the inclined abdomen, a curved fourth article of the palp and present dorsal tubercles on the trunk segments.In comparison, A. carolinensis has a conical ocular tubercle, does not have a basal tubercle on the inclined abdomen, the fourth palpal article is not curved and the dorsal tubercles of the trunk are pointed and oriented backwards.
Finally, it is necessary to carry out more volcanic deep water expeditions in order to expand the knowledge of the specific biodiversity, bathymetrical and zoogeographical distribution of pycnogonids, and other benthic groups.
FIG. 1. -Sampling area and location of dredged stations.

TABLE . 2
. -Life stages, fresh weight (g), percentage of abundance and biomass of the pycnogonid species recorded in the Gebrap-96 cruise.

TABLE . 4
. -Geographical distribution and depth range in meters.E, east Antarctic zone; C, circumpolar; P, Antarctic Peninsula; R, Ross Sea; S, Scotia Sea; W, Weddell Sea; * indicates a new zone or depth of distribution.