Fabiana L. Capitanio

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.

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.

Otolith morphology and feeding ecology of an Antarctic nototheniid, Lepidonotothen larseni

Abstract This study is based on the analysis of the stomach content and the morphology and morphometry of the three pairs of otoliths (sagitta, asteriscus and lapillus) of Lepidonotothen larseni (Lönnberg) collected at the South Shetland Islands and Antarctic Peninsula during summer, in order to find possible relationships between ontogenetic change of sagittal otolith shape and feeding ecology. Length-weight relationship resulted in a positive allometric growth, with juveniles and adults in good nutritional condition (Le Cren condition index > 1), and with a decreasing trend from noon to late evening of the stomach repletion index. The stomach content consisted of several prey, with copepods and amphipods more frequent and abundant in juveniles, whereas euphausiids were in adults. The morphometric analysis of otoliths enabled us to relate different measurements with fish size, and those contributing mostly to separate juveniles from adults were the otolith and rostrum length and their percentage (R index). Juveniles proportionally showed a shorter and wider sagitta than adults reflected in a major E index because of a rounded shape and a minor R index because of a less developed rostrum. This pattern can be tentatively linked to the different habitat of juveniles and adults of this species, being respectively pelagic and epibenthic, as also evidenced by the ontogenetic change of feeding habits.

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.

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.

The classification of Antarctic appendicularians: the Oikopleura gaussica group

The validity of the Antarctic species of the “Oikopleura gaussica group” (O. gaussica, O. valdiviae, O. drygalskii, and O. weddelli) is discussed. The characteristics of new material from South Georgia and the Bellingshausen Sea are compared with the original descriptions. We examined 495 specimens at different stages of maturity and concluded that the characters used to differentiate these species of Oikopleura have not enough weight to justify their separation. The morphological variations found were ascribed either to intraspecific variability and/or to the different degree of maturity of the specimens examined of a single species, Oikopleura gaussica. The main taxonomic characters for the identification of this species are: the particular way in which the flat triangular gonads surround the intestinal ball like a sheath, the equal size of the two lobes of the stomach, the oesophagus entering the postero-dorsal edge of the rounded left lobe of the stomach, the presence of oval and some times lobate oral glands, and the variable number of subchordal cells (4 to 14) arranged in a single row on the right (ventral) side of the chorda.