Among shallow water sea urchin genera, Arbacia is the only genus that contains species found in both high and low latitudes. In order to determine the geographical origin of the genus and its history of speciation events, we constructed phylogenies based on cytochrome oxidase I and sperm bindin from all its species. Both the mitochondrial and the nuclear gene genealogies show that Arbacia originated in the temperate zone of the Southern Hemisphere and gave rise to three species in the eastern Pacific, which were then isolated from the Atlantic by the Isthmus of Panama. The mid-Atlantic barrier separated two additional species. The bindin data suggest that selection against hybridization is not important in the evolution of this molecule in this genus. Metz et al. in a previous publication found no evidence of selection on bindin of Arbacia and suggested that this might be due to allopatry between species, which obviated the need for species recognition. This suggestion formed the basis of the conclusion, widely spread in the literature, that the source of selection on sea urchin bindin (where it does occur) was reinforcement. However, the range of Arbacia spatuligera overlaps with that of two other species of Arbacia, and our data show that it is hybridizing with one of them. We found that even in the species that overlap geographically, there are no deviations from selective neutrality in the evolution of bindin.

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A population of Galaxias maculatus, a native species in southern South America, living under cold-temperate conditions (0°C water temperature with an ice layer covering the stream during the coldest days) and 7 h of light in winter undergoes high energetic demands in the population studied. We analyzed the energy density of gonads, liver, fat and muscle, through calorimetry for the Wrst time in this species. Energy density of fat and liver were extremely high (34.77-56.52 and 29.54-40.77 kJ/g respectively). While perivisceral fat reserves were likely used for overwintering, liver and muscle reserves were used for reproduction. High energy densities were also found in gonads (27.76 kJ/g in ovaries and 25.84 kJ/g in testes). High investment in gonads of males suggests the presence of sperm competition. The temporal variation of the energy content of gonads, liver, fat and muscle indicates that the allocation of energy occurs by the internal transference of energy between organs and tissues.

This study establishes the reproductive cycle, batch fecundity and oocyte diameter of a diadromic population of the puyen, Galaxias maculatus, in the Rı´o Ovando, Tierra del Fuego, (54°S), at the southernmost limit of its distribution. Given the short Ôfavourable seasonÕ in relation to other populations of the species (in terms of temperature, photoperiod and food availability), the study also explores the trade-offs between feeding and oocyte production and between phases of growth and reproduction. The reproductive cycle was analyzed by the monthly evolution of the proportion of gonadal maturity stages (determined from microscopic examination of the gonads). Oocytes were measured using a micrometric ocular scale and classified by diameter and morphological characteristics; absolute fecundity was established as the total number of hydrated oocytes per ovary. The studied population exhibits a repetitive spawning strategy, given the coexistence of postovullatory follicles and hydrated oocytes in histological sections and the presence in ripe ovaries of an intermediate cohort of yolked oocytes ready for hydration. Individual fecundity is lower (1422 ± 422 oocytes ⁄ ovary) than in other puyen populations, but the egg production increases through individual repetitive spawnings during the protracted spawning period (from October to February). Females attain larger sizes than those of other South American populations (the largest female reaching 115 mm total length), maximizing its potential fecundity. The present paper contributes to the knowledge of the variability of reproductive traits of G. maculatus in relation to diadromic populations, given that the bulk of information in South America refers to landlocked populations.

SUMMARYIn the present study, the influence of short-term osmotic variations on some electrophysiological properties related to NaCl absorption across posterior gills of Chasmagnathus granulatus was investigated. The transepithelial potential difference (Vte) of isolated and perfused gills increased significantly when hyposmotic saline (699 mosmol l-1) was used instead of isosmotic solution (1045 mosmol l-1). A reduction of the concentration of Na+ or Cl- at constant osmolarity did not produce any change in Vte. Transepithelial short-circuit current(Isc) and conductance (Gte), measured with split gill lamellae mounted in a modified Ussing chamber, also increased after changing to hyposmotic salines (Isc: from-89.0±40.8 μA cm-2 to -179.3±37.0 μA cm-2; Gte: from 40.5±16.9 mS cm-2 to 47.3±15.8 mS cm-2). The observed effects of reduced osmolarity were fast, reversible and gradually dependent on the magnitude of the osmotic variation. The activity of the Na+/K+-ATPase increased significantly after perfusion with hyposmotic saline, from 18.73±6.35 μmol Pih-1 mg-1 to 41.84±14.54 μmol Pih-1 mg-1. Theophylline maintained part of the elevated Vte induced by hyposmotic saline, suggesting that an increased cellular cyclic AMP level is involved in the response to reduced osmolarity. In summary, the results indicate that the hemolymph osmolarity regulates active transbranchial NaCl absorption by modulating the activity of the basolateral Na+/K+-ATPase and by changing a conductive pathway, probably at the apical membrane.

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