E. D. Spivak

Seasonal variation of peracarid assemblages in natural and artificial marine environments of the Southwestern Atlantic Ocean

The diversity and seasonal variations of two assemblages of marine benthic peracarids were studied between a natural rocky shore and an artificial harbour area over a 12-month period. Samples were obtained monthly in La Estafeta, a rocky intertidal zone with low human impact, and Mar del Plata Harbour, a polluted environment, between March 2011 and March 2012. The two sites differed markedly in the composition and abundance of species across all seasons: the tanaid Tanais dulongii was most abundant in La Estafeta rocky shore, followed by the amphipods Monocorophium acherusicum, Hyale grandicornis, Ampithoe valida, the isopod Idotea balthica, the tanaid Leptochelia sp. and the isopod Sphaeroma serratum. In contrast, M. acherusicum was most abundant in the harbour area, followed by T. dulongii, S. serratum, Ericthonius punctatus, I. balthica, Caprella equilibra and C. dilatata. Total density of peracarids varied between months in La Estafeta rocky shore and Mar del Plata Harbour. In La Estafeta rocky shore mean density increased from March to May 2011 (autumn in the southern hemisphere; ca. 45,000 ind/m2), decreased sharply until August and then increased in January 2012. In Mar del Plata Harbour the mean density was lower from March to October (ca. 500,000 ind/m3), then increased and reached a maximum in January 2012 (more than 1,500,000 ind/m3), and decreased until the following March. This study suggests that the differences in peracarid assemblages, diversity and seasonality could be related to an effect of temperature, but we should not rule out a synergistic effect of other factors, such as pollution, food availability and hydrodynamic factors.

Effects of Removal from the Mother and Salinity on Embryonic Development of Palaemonetes Argentinus (Decapoda: Caridea: Palaemonidae)

Parental removal experiments are a simple method for understanding the adaptive value of egg brooding in decapod crustaceans. Embryos of several decapods have been successfully cultured in vitro; however, simultaneous comparison between sibling embryos attached and isolated from their mother remains largely unexplored. In this study, we kept sibling embryos of Palaemonetes argentinus Nobili, 1901 isolated and attached to the female throughout embryogenesis in two salinity treatments (2 and 15‰). Four times during embryonic development, we individually photographed embryos and measured their volume, yolk consumption, and developmental rates. We also measured hatching time, and determined the survival time of newly-hatched larvae in absence of food. The volume and yolk consumption increased throughout embryonic development, but isolated embryos always reached a larger size and had less yolk than the attached ones. The enhancement in volume and yolk consumption for both attached and isolated embryos was higher at the low salinity treatment. Moreover, isolated embryos always hatched earlier than those attached to their mothers, and the shortest time to hatch was registered at the low salinity treatment. The first zoeal stage of P. argentinus can successfully develop from hatching through the moult to the following stage without food (facultative lecithotrophy). This result was affected by embryo culture conditions and salinity indicating that embryonic experience carries over into the larval phase. The embryos/larvae without maternal care seem to have better performance than the ones that enjoyed maternal brooding during embryo development and these results were consistent across two different salinities. It is likely that brooding of embryos persist in this species because motherless embryos in the natural environment most likely would have no survival at all. In such a sense, our results suggest a trade-off where maternal care has certain costs for embryos (longer development, lower larval survival), but where the benefits (high survival of protected embryos in natural environments) most likely outweigh these costs.

Effects of Salinity on the Reproductive Cycle of Female Freshwater Shrimp, Palaemonetes argentinus

Abstract The South American shrimp Palaemonetes argentinus inhabits inland freshwater habitats such as lakes, rivers and streams, but also coastal brackish marshes and lagoons. Hence, this so-called “freshwater” shrimp is expected to be able to reproduce in a wide range of salinities. Here we examined effects of three salinities (1, 15, 25 ppt) on survival and reproductive cycle of females originating from two separate populations in Argentina: 1) Lake Chascomús (LC), a shallow inland lake with low and stable ionic concentrations; and 2) Vivoratá Creek (VC), a lotic stream draining into the adjacent brackish lagoon Mar Chiquita, where salinity is tide-dependent. After an observation period of four months, survival was generally high, without showing significant differences between salinities or populations. Ovarian maturation was measured on a scale ranging from “empty” ovaries ( =  females with newly laid eggs) to fully developed ovaries ( =  females ready to spawn). Most LC shrimps (> 80%) developed their ovaries again after an initial egg extrusion (beginning of the experiment) and produced new eggs at all tested salinities. In VC shrimps, ovarian maturation and egg deposition were successful in both the lowest and the highest salinity (1, 25 ppt), but developed ovaries were resorbed in most individuals at the intermediate salinity (15 ppt). Ovarian resorption was never observed at the highest salinity treatment, thus this phenomenon may have been provoked by unknown factors other than ionic concentration. At 15 and 25 ppt, the beginning of vitellogenesis started later and the total time to ovary maturation was longer than at 1 ppt, lengthening in both populations the time between consecutive spawning of eggs by nearly 10 days. Irrespective of the origin of a population, female P. argentinus survived and completed all reproductive events under a wide range of salinities, which supports the hypothesis that this species is in an early stage of evolutionary invasion of freshwater habitats.