Seasonal variation in the relative strength of bottom-up <em>vs</em> top-down effects in pelagic cladoceran populations identified through contribution analysis of birth rate

Pelagic freshwater communities are characterized by the presence of strong trophic interactions, with the dominance of either food limitation (bottom-up effect) or predation pressure (top-down effect) alternating in time. Though this temporal variation should have a major impact on zooplankton dynamics, few studies have examined it. We consider here an approach that involves identifying signatures of food availability and size-selective fish predation based on the demographic characteristics of cladoceran populations. The relative strength of top-down vs bottom-up effects was assessed on short sampling intervals as contributions of the proportion of adults and fecundity, respectively, to the resulting change in birth rate. The dominant effect on each sampling interval was determined based on the absolute value of the ratio of those contributions (R). From previous experiments, we expected that under the dominant effect of food, R should be less than unity, while under fish predation it should be 1 < R < 3.4. We analyzed two datasets – one collected for a pelagic cladoceran community of three species, and another retrieved from the published data for Daphnia catawba in 1986. In the former case, the temporal variation in the dominant effect was assumed from the pattern of cladoceran populationdynamics as well as limited data on zooplankton consumption by planktivorous fish; in the latter case, the seasonal change in predation pressure by fish on the daphnids was known from the original data. Our results show that the probability density functions for R values from the two datasets indicate an increased probability of the ratio values associated with the abovementioned ranges, suggesting that both bottom-up and top-down effects should have been driving cladoceran population dynamics during the study periods. Based on the results of the Generalized Additive Models (GAMs), the fitted R values for the most abundant species from the first dataset - Bosmina longirostris - changed from the values indicative of strong food effect at the beginning of the study period to those indicative of strong top-down effect when fish with substantial numbers of bosminids in the gut were caught. In the second dataset, for the two time intervals associated with increased predation pressure by fish, the fitted R values were predominantly located between 1 and 3.4, as expected. For both datasets, our results indicate that contribution analysis of birth rate can be used as an informative, albeit preliminary, tool to identify trophic interactions driving zooplankton seasonal population fluctuations in freshwater communities

Updated functional segregation of retinal ganglion cell projections in the tectum of a cyprinid fishfurther elaboration based on microelectrode recordings

Single-unit responses of retinal ganglion cells (GCs) were recorded extracellularly from their axonal terminals in the tectum opticum (TO) of the intact fish (goldfish, carp). The depths of retinal units consecutively recorded along the track of the microelectrode were measured. At the depth of around 50m, the responses of six types of direction-selective (DS) GCs were regularly recorded. Responses of two types of orientation-selective (OS) GCs and detectors of white and black spots occurred approximately 50m deeper. Responses of GCs with dark- and light-sustained activity were recorded deeper than all others, at about 200m. The receptive fields of consecutively recorded units overlap, so they analyze the same fragment of the visual scene, focused by eye optic on the photoreceptor raster. The responses of pairs of DS GCs (ON and OFF units that preferred same direction of stimulus movement) and OS GCs (detectors of vertical and horizontal lines) were often simultaneously recorded at one position of the microelectrode. (The paired recordings of certain units amounted about fourth part of all recordings.) This suggests that their axonal arborizations are located close to each other in the tectal retinorecipient layer. Electrophysiological method, thus, allows to indirectly clarify and make precise the morphology of the retino-tectal connections and to establish a morpho-physiological correspondence

SPARK: A US Cohort of 50,000 Families to Accelerate Autism Research

The Simons Foundation Autism Research Initiative (SFARI) has launched SPARKForAutism. org, a dynamic platform that is engaging thousands of individuals with autism spectrum disorder (ASD) and connecting them to researchers. By making all data accessible, SPARK seeks to increase our understanding of ASD and accelerate new supports and treatments for ASD