Javier A. Calcagno

CHALLENGING THE COLD: CRABS RECONQUER THE ANTARCTIC

Recent records of lithodid crabs in deeper waters off the Antarctic continental slope raised the question of the return of crabs to Antarctic waters, following their extinction in the lower Miocene ;15 million years ago. Antarctic cooling may be responsible for the impoverishment of the marine high Antarctic decapod fauna, presently comprising only five benthic shrimp species. Effects of polar conditions on marine life, including lowered metabolic rates and short seasonal food availability, are discussed as main evolutionary driving forces shaping Antarctic diversity. In particular, planktotrophic larval stages should be vulnerable to the mismatch of prolonged development and short periods of food avail- ability, selecting against complex life cycles. We hypothesize that larval lecithotrophy and cold tolerance, as recently observed in Subantarctic lithodids, represent, together with other adaptations in the adults, key features among the life-history adaptations of lithodids, potentially enabling them to conquer polar ecosystems. The return of benthic top predators to high Antarctic waters under conditions of climate change would considerably alter the benthic communities.

Lipid, fatty acid and protein utilization during lecithotrophic larval development of Lithodes santolla (Molina) and Paralomis granulosa (Jacquinot)

During the larval development of the subantarctic king crab, Lithodes santolla, and stone crab, Paralomis granulosa, we compared changes in the carbon, fatty acid and protein contents of larvae reared under constant conditions from hatching to metamorphosis, either in presence or absence of food (Artemia spp. nauplii). In both species the feeding condition had no influence on any of the chemical parameters studied, indicating a fully lecithotrophic (i.e. non-feeding) mode of development from hatching of the first zoea to metamorphosis of the late megalopa. Dry mass and carbon contents at hatching were similar in the larvae of both species, but L. santolla contained initially higher total amounts of fatty acids and protein than P. granulosa. Both species utilized considerable portions of their total fatty acid pool which decreased logarithmically throughout the time of development. At metamorphosis, it was almost exhausted in P. granulosa, while L. santolla had consumed only about 60%. Protein utilization, in contrast, was higher in L. santolla (40%) than in P. granulosa (20%). Triacylglycerol was the principal storage lipid in both species, accounting initially for about 75% of the lipid fraction; it was strongly utilized during larval development. Phospholipid constituted the second largest lipid class; it also decreased in P. granulosa, but to a lesser extent in L. santolla. The major fatty acids of both species were 18:1(n9), 20:5(n3) and

Changes in biomass and chemical composition during lecithotrophic larval development of the southern king crab, Lithodes santolla (Molina)

Changes in biomass and elemental composition (dry mass, W; carbon, C; nitrogen, N; hydrogen, H) were studied in the laboratory during complete larval and early juvenile development of the southern king crab, Lithodes santolla (Molina), formerly known as Lithodes antarcticus (Jacquinot). At 6±0.5 °C, total larval development from hatching to metamorphosis lasted about 10 weeks, comprising three demersal zoeal stages and a benthic megalopa, with mean stage durations of 4, 7, 11 and 47 days, respectively. No differences in development duration or mortality were observed in larvae either fed with Artemia sp. nauplii or unfed, indicating that all larval stages of L. santolla are lecithotrophic. First feeding and growth were consistently observed immediately after metamorphosis to the first juvenile crab stage. Regardless of the presence or absence of food, W, C, N and H decreased throughout larval development. Also the C:N mass ratio decreased significantly, from 7.7 at hatching to 4.1 at metamorphosis, indicating that a large initial lipid store remaining from the egg yolk was gradually utilized as an internal energy source, while proteins played a minor role as a metabolic substrate. In total, 56–58% of the initial quantities of C and H present at hatching, and 20% of N were lost during nonfeeding larval development to metamorphosis. Nine to ten percent of the initially present C, N and H were lost with larval exuviae, half of these losses occurring in the three zoeal stages combined and another half in the megalopa stage alone. Metabolic biomass degradation accounted for losses of about 47–50% in C and H but for only 10% in N. Hence, most of the losses in C and H reflected metabolic energy consumption (primarily lipid degradation), while about half of the losses in N and two thirds of those in W were due to larval exuviation. Complete independence from food throughout larval development is based on an enhanced maternal energy investment per offspring and on energy-saving mechanisms such as low larval locomotory activity and low exuvial losses. These traits are interpreted as bioenergetic adaptations to food-limited conditions in Subantarctic regions, where a pronounced seasonality of day length limits the period of primary production, while low temperatures enforce a long duration of pelagic development.

Absence of a prezygotic behavioural barrier to gene flow between the two sympatric morphs of the squat lobster Munida gregaria (Fabricius, 1793) (Decapoda: Anomura: Galatheidae)

Munida gregaria and M. subrugosa have been considered two different species for more than a century; however, after a recent molecular phylogenetic study, they are considered a single polymorphic species. Yet, the use of markers to diagnose species may be misleading when divergence between species is recent, since a speciation event may be obscured by the retention and stochastic sorting of ancestral polymorphisms. The morphs gregaria and subrugosa of Munida gregaria constitute an interesting case for the study of behavioural isolation since they are sympatric, breed at the same time of the year, and might have experienced a recent speciation. Mating behaviour observations and mate choice mating trials were conducted in order to investigate the potential existence of a behavioural prezygotic barrier to gene flow between these two morphs. Since factors involved in mate choice in galatheids are unknown, the four possible combinations of the two different morphs in trios were used to test for the existence of mate choice. Video recordings of all the possible trio combinations revealed that there was cross-attraction between males and females of different morphs. Females bearing partial broods participated in encounters as well as non-ovigerous females. The frequency and duration of homo- and heterotypic encounters were registered, and a reproductive isolation index was calculated for each variable for each trio. The isolation indexes calculated were not different from zero indicating random mating, and were not affected by the composition of the trio or the partial ovigerous condition of females. These results provided evidence of the absence of behavioural prezygotic barriers to gene flow between the morphs gregaria and subrugosa of M. gregaria.

Effects of physical disturbance on a sub-Antarctic middle intertidal bivalve assemblage

Abstract We present for the first time the vulnerability to disturbance of a bivalve assemblage situated in the middle intertidal zone of the Atlantic coast of the Southern South America (53°36′S, 67°58′W). This intertidal zone is characterized by a high level of sand movement preventing any establishment of sessile organisms on the vertical sides of rock outcrops. The coast is prone to potential spills from nearby hydrocarbon marine platforms, but this benthic assemblage has been poorly studied. In February 2001, we exposed the assemblage to two different experimental conditions: a complete removal of bivalves and a physical disturbance and tracked its recovery during the following 4 years. The disturbance lowered the diversity of associated fauna and made the sediment layer trapped among bivalves disappear, which was not restored. Mytilus chilensis was recruited into crevices or between byssi in the following settlement season, i.e. summer 2002. The recuperation of mussel coverage to predisturbance levels took 3 years after the total removal. In April 2004 the size frequency distributions, density and biomass/number ratio of M. chilensis were similar to those at the start of the experiment. Perumytilus purpuratus recovered more slowly than M. chilensis, probably due to its slower growth rate compared to M. chilensis and dependence on an adequate byssus matrix for settlement. The main apparent stressor was the irruption of sand, covering the bivalve assemblage for variable periods of time. This bivalve assemblage is characterized by low predatory pressure and therefore we hypothesize that it is predominantly controlled by competition for space.

Diel variation in catches of Euphausia superba Dana 1854 early larvae: vertical migration or avoidance reaction?

Abstract Samples were obtained with two pairs of black and white nets towed simultaneously at varying light conditions in the Weddell–Scotia confluence region. The statistical analysis (ANOVA, Regression and Kruskal–Wallis test) of the log transformed counts of Euphausia superba calyptopes larvae (CI, CII and CIII) shows that the significant difference between the white and black nets found in daytime are not present in darkness. A negative regression coefficient was detected between the difference in catches and suns height, pointing to a light dependent avoidance reaction. These results demonstrate that the day/night differences in observed densities of krill larvae are due to net avoidance rather than to vertical migration, a result confirmed by hydroacoustical observations; and that the habitat of these stages is restricted at all times to the upper layers of the ocean.

Relationship between energy allocation and gametogenesis in Aulacomya atra (Bivalvia: Mytilidae) in a sub-Antarctic environment

Resumen es: Se analizo la gametogenesis y la variacion temporal en la asignacion de energia a diferentes organos de Aulacomya atra del Canal Beagle (Tierra del Fuego...

Recruitment and zonation in a sub-Antarctic rocky intertidal community

Fil: Curelovich, Jessica Natalia. Universidad Maimonides. Area de Investigaciones Biomedicas y Biotecnologicas. Centro de Estudios Biomedicos, Biotecnologicos, Ambientales y de Diagnostico; Argentina. Consejo Nacional de Investigaciones Cientificas y Tecnicas; Argentina