Graciela B. Esnal

Benthic faunal associations on soft substrates at Potter Cove, King George Island, Antarctica

Abstract Associations of benthic invertebrates from Potter Cove (Antarctica) were defined from photo-transects. Density, percentage cover, species richness S′, diversity index H′, evenness index J′ and mean-size estimations were studied in relation to water depth down to 30 m. A clear bathymetric pattern was evident, with two different communities at 15 and 30 m, and a transition area between 20 and 25 m. At 15 m we observed a small number of taxa (nine in total), a high percentage of bare substratum (95%), and the dominant species were pennatulids and the bivalve Laternula elliptica. From 20 to 30 m the dominant species was the ascidian Molgula pedunculata and there was a slight increase in S′, H′ and J′, as well as in the mean size of individuals, especially in M. pedunculata, while the proportion of bare substratum showed a constant decrease with depth. Using multivariate analyses, three faunal assemblages related to depth were defined and a strong association of some species, mainly predators and opportunistic necrophages, with M. pedunculata was revealed. Ice impact (icebergs and anchor ice) seems to be the major regulating factor of benthic assemblages in shallow waters.

A mechanism for swarming in the tunicate Salpa thompsoni (Foxton, 1961)

Two populations of Salpa thompsoni, collected from the Weddell–Scotia confluence area at the end of two summers (1994 and 1995) with contrasting densities, are compared. The present study was aimed at corroborating whether fluctuations in abundance could be related to some life history parameters considered as indicators of growth rate in salp populations, such as the body size of solitary reproductive individuals, number of buds per block or chain in the stolon, and the offspring to parent ratio. No differences were observed in the sizes reached by aggregate individuals, or in the size at which oocyte maturity was attained (moment at which the development of the embryo can be detected). There were no differences in the size reached by the embryos during their residence inside the mothers blastozooid. In 1995, the percentage of non-fecundated blastozooids (which included even the largest individuals) was higher than in 1994. The mean size of the solitary individuals was significantly higher in 1994 than in 1995. The number of buds per block was also significantly higher in most of the stages, confirming that this is the mechanism used by these organisms to produce swarms. This species eventually attains high population densities under favorable environmental conditions such as poor sea-ice cover in winter. Under optimal conditions, the number of potential descendants produced by a single solitary individual would exceed 800.

Food availability and gut contents in the ascidian Cnemidocarpa verrucosa at Potter Cove, Antarctica

Abstract. A high seasonality characterizes Antarctic environments, and generates marked differences in availability and composition of food for benthic filter feeders. During a year-round period at Potter Cove, Antarctica, algal pigment concentration (chlorophyll a) and organic matter were measured in water column and sediment samples. Chemical composition of gut contents of the ascidian Cnemidocarpa verrucosa was also analyzed. Despite the low standing stock, capture and absorption of organic matter were detected year-round, suggesting intake of other particles besides microalgae. The mechanism that provides food to the ascidians and epibenthic communities may be related to the supply of allochtonous particles, bottom resuspension events, and microbial community dynamics. Sea-ice cycles may affect food availability in terms of water-column mixing and benthic resuspension. The scarce primary production and the high amount of sedimented material are not limiting conditions in Potter Cove, which presents a rich ascidian community.

Diet components in the food of Antarctic ascidians living at low levels of primary production

Coupling between pelagic and benthic systems has been described in numerous shallow water communities. In Potter Cove, where pelagic primary production is low (not only during the Antarctic winter but also during the summer), the rich benthic community present there must depend on other food sources than phytoplankton. Over a year-round period we determined the abundance of the different seston particles which constituted the stomach contents of the Antarctic ascidian Cnemidocarpa verrucosa (Lesson, 1830) at Potter Cove. Stomach repletion was highest in November and lowest in June. Ascidians took in a wide range of particles from large detritus (macroalgal debris and faecal pellets) to minute particles < 5 μm. Large detritus and minute particles together represent the main percentage of contents throughout the year (mean 91%). Diatoms were a low percentage (mean 4.5%). Unidentified flagellates, dinoflagellates and coccolithophorids were scarce, with mean values lower than 4%. Among diatoms benthic species were more abundant in summer and pelagic ones prevailed from March to November. Resuspension of benthic material due to wind mixing and the input of allochthonous particles by currents are important mechanisms that ensure food for ascidians and the community of suspension feeders in Potter Cove.

ASCIDIANS (TUNICATA, ASCIDIACEA) OF POTTER COVE, SOUTH SHETLAND ISLANDS, ANTARCTICA

Certain physical factors, substrate type and ice action appear to be important determinants for ascidian distribution. Three different substrate types were sampled at depths between 0–30 m by SCUBA diving: soft bottoms, hard bottoms and moraine deposits. The species found were Aplidium radiatum , Synoicum adareanum , Distaplia cylindrica , Sycozoa gaimardi , Sycozoa sigillinoides , Tylobranchion speciosum , Corella eumyota , Ascidia challengeri , Cnemidocarpa verrucosa , Styela wandeli , Dicarpa insinuosa , Pyura bouvetensis , Pyura discoveryi , Pyura obesa , Pyura setosa , Molgula enodis and Molgula pedunculata. Highest diversity and patchy distribution was found in less stringent environments, where epibiosis is a commmon phenomenon on stolidobranch ascidians. Differences in stalk development were found in the most abundant species Molgula pedunculata and Cnemidocarpa verrucosa. Ice action may be the main factor that determines the absence of ascidians above 15 m in all the stations sampled.

Seasonal variability of metazooplankton in coastal sub-Antarctic waters (Beagle Channel)

Abstract The aim of this study was to analyse the variability in the species composition and abundance of the metazooplankton community in different coastal areas of the Beagle Channel (southern tip of South America) during a seasonal cycle. Sampling was conducted during November (spring) 2005, March (summer), June (autumn) and September (winter) 2006 at 12 coastal stations. Copepods were the most abundant group throughout the study, and their assemblages were composed of a mixture of species typical of the south-western Atlantic, the south-eastern Pacific and the Southern Ocean. Among them, Oithona similis, Ctenocalanus citer and Drepanopus forcipatus were the dominant species. The copepod Acartia tonsa was the only taxon that displayed a spatial pattern of abundance, showing higher densities in areas with lower salinities. The community structure showed a strong temporal pattern. The metazooplankton community in March and June was mainly composed of copepods, while in November and September the community showed a greater diversity. In these two months high densities of meroplanktonic larvae were found, in coincidence with higher chlorophyll-a concentration. This temporal pattern seems to be more dependent on primary production than on physical factors such as temperature or salinity. The absence of a clear spatial pattern may suggest that the studied area of the Beagle Channel behaves as a semi-enclosed water body.

Gonadal development, allometric growth and ecological impact of Appendicularia sicula (Appendicularia: Fritillariidae) from the south-western Atlantic Ocean

The gonadal development and morphometry of Appendicularia sicula (Appendicularia: Fritillariidae) were studied by means of histological sections and from the measurement of several characters. Based on morphometric measurements, the ingestion and clearance rates and the percentages of consumed chlorophyll-a were also estimated. The gonadal development is similar to that of Fritillaria pellucida .A small dorsal ovotestis, which originates ovary and testicle, shows large somatic nuclei inside. In the ovary these somatic nuclei are surrounded by a single layer of germ cells nuclei, whereas in the testicle they are found between the small nuclei of the germ cells. As the development proceeds, the large somatic nuclei are resorbed in both gonads, and the testicle ¢nally envelopes the ovary posteriorly. After spermatozoids emission, the rectum is shown occupying most of the body volume. The length of the rectum shows a positive allometric growth in respect to the length of the trunk, which can be ascribed to its continuous accumulation of matter due to the lack of an anus. The relationship obtained between trunk length (TL, mm) and dry weight (DW, mg) was DW¼2.24� 10 711 TL 4.098 . Applying this equation, a two-fold increase in length would correspond to a 42-fold weight increase. For specimens with an advanced sexual maturity (400^450mm TL), we estimated a mean autotrophic ingestion rate of 0.17mg C ind 71 d 71 and a mean clearance rate of 5.01ml ind 71 d 71 . The percentage of total chlorophyll-a consumed by the A. sicula population was 5.13%. This value is similar to those reported for other appendicularian species in diierent temperate seas.

Differences in embryological development in two closely related species: Ihlea racovitzai and Ihlea magalhanica (Tunicata, Thaliacea)

Differences in the early stages of embryological development confirm the validity of the closely related salp species Ihlea racovitzai and Ihlea magalhanica. In the latter 8–12 oocytes and thus 8–12 follicles and uterine sacs are observed in the ovary; however most of them undergo a progressive degradation and only two embryos complete their development. In Ihlea racovitzai two embryos also develop but from the very beginning only the two successful follicles are found.

Diel vertical distribution of the larvacean Oikopleura dioica in a North Patagonian tidal frontal system (42°–45°S) of the SW Atlantic Ocean

Abstract In frontal systems, larvaceans generally comprise a significant fraction of the mesozooplankton. Given the low mobility of these organisms, their vertical distribution is directly influenced by the physical conditions of the water column. The main goal of this study was to investigate the day and night vertical distribution of the larvaceans Oikopleura dioica in the different sectors of the Península Valdés tidal front (42°–45°S; SW Atlantic Ocean), focusing on possible causes such as oceanographic conditions (well-mixed, frontal and stratified), feeding activities, reproductive behaviour and predation risks. Day and night samples were collected with a MultiNet of 67 µm and 300 µm mesh size, from two depth layers. Conductivity-temperature-depth (CTD) profiles were also recorded. In the well-mixed waters, the size structure of O. dioica was similar in the upper and lower layers both in the day and night samples, probably due to the continuous vertical movement caused by the turbulent mixing of waters. In the frontal and stratified areas, mature animals were found in the upper layers during the night, possibly due to their aggregation in surface waters for synchronized spawning. In these areas, a high percentage of well-fed larvaceans was also found. Principal component analysis showed that, at night, the abundance of potential predators such as chaetognaths, ctenophores and anchovy larvae was correlated with the parameter of stability of the water column (Simpson parameter), and that these were more abundant in the upper layers of the stratified area containing larvaceans. Our results suggest that the degree of stratification of the water column was the main factor affecting the vertical distribution of O. dioica.

Stolon development and some aspects of musculature in the characterization of Ihlea racovitzai (van Beneden, 1913) and Ihlea magalhanica (Apstein, 1894) (Tunicata, Thaliacea)

SummaryA new contribution to the characterization of the salps Ihlea racovitzai and I. magalhanica, particularly their reproductive features, is presented. In both species two oocytes develop in the aggregate form, in the solitary form stolon growth is continuous. Buds grow continuously from the zone of deployment to the distal region without forming blocks or intermediate pieces. In Ihlea racovitzai the exit of the stolon and the resulting release of buds occurs later than in I. magalhanica. In the latter it takes place at the beginning of the strobilation of the stolon, which means an earlier and perhaps more continuous release of buds. The opening for the release of the buds is ventral and anterior to the nucleus in I. magalhanica and ventrolateral and posterior to the nucleus in I. racovitzai. Differences in the pattern of body muscle VII and the atrial retractors in the solitary forms are underlined.