Marco Uttieri

Seasonal adaptations of Daphnia pulicaria swimming behaviour: the effect of water temperature

Daphnia swimming behaviour is controlled by a variety of external factors, including light, presence of food and predators. Temperature represents a key driver in the dynamics of Daphnia populations, as well as on their motion. In this study, we have investigated the behavioural adaptations of adult Daphnia pulicaria to two different temperatures, representative of the mean winter (3°C) and summer (22°C) temperatures to which these organisms are exposed to in the real environment. Video observations were conducted both in the presence and in the absence of light to investigate possible day/night modifications in the motion strategy. Analyses of mean speed, velocity power spectral density and trajectory fractal dimension point out specific adaptations that allow D. pulicaria to successfully adjust to the changing conditions of the environment. Independently of the light conditions, in cold waters D. pulicaria swim almost vertically with defined motional frequencies, likely to increase the encounter with food items diluted in the fluid. A similar behaviour is displayed by the animals at summertime temperatures in the presence of light; however, in this case the vertical swimming is coupled with the absence of peaks in the power spectra and might be exploited to avoid predators. In contrast, at 22°C in dark conditions D. pulicaria move horizontally with lateral motions to take advantage of possible patches of phytoplankton. This information sheds new light into the complex and dynamic adaptations of D. pulicaria in response to external stimuli.

Validation of HF Radar-Derived Currents in the Gulf of Naples With Lagrangian Data

A massive drifter deployment in the Gulf of Naples (Southern Tyrrhenian Sea, Western Mediterranean Sea) over a ten-day multidisciplinary in situ experiment in Summer 2012 provided sea-truth data for validating the performance of a high-frequency (HF) radar network. The buoys were frequently retrieved and relaunched to ensure an optimal coverage of the domain. The total velocity of the drifters, together with the associated zonal and meridional components, was compared with the HF radar surface current estimates. Divergence between virtual and real drifter trajectories, and also the simulated movement of clouds of particles give useful insights for scenarios of search and rescue. All comparisons were performed, considering both ideal and antenna radiation pattern-corrected fields. The results of the investigation testify the high precision of HF radars and confirm the necessity of periodically verifying the antenna pattern to ensure the optimal functionality of these systems.

Morphology of antennular sensors in Clausocalanus furcatus (Copepoda: Calanoida)

The success of planktonic copepods in aquatic environments is dependent on efficient sensing of their three-dimensional surroundings. The detection of external cues is of importance for the localization of other organisms (prey, predators and mates), and is mediated by an array of mechano- and chemoreceptors located on the paired antennules (A1). We investigated the morphology and distribution pattern of A1 sensory structures in the adult female of Clausocalanus furcatus (Copepoda: Calanoida) using different techniques (camera lucida, scanning and transmission electron microscopy and laser scanning confocal microscopy) each focusing on a specific aspect of the structures analysed. Integration of the information collected shows that C. furcatus possesses an array of mechanical, chemical and dual-function sensors over its A1, by which the copepod can detect different stimuli from the environment. Results are discussed in the light of the unique swimming behaviour displayed by this widespread epipelagic copepod.

ITS2 in calanoid copepods: reconstructing phylogenetic relationships and identifying a newly introduced species in the Mediterranean

Abstract Phylogenetic inference and molecular taxonomy are becoming increasingly important approaches to classical morphological systematics and marine ecology. The number of molecular markers suitable for such goals is quite high, but general use restricts the list to a few of them, mainly mitochondrial (namely cytochrome c oxidase subunit I, COI and Cytochrome b), especially in copepods. The ribosomal cistronic regions have been widely used for broad phylogenetic analyses in different taxa. Among them, the internal transcribed spacers (ITS rDNA) are powerful tools for phylogenetic reconstructions at the different taxonomic levels, although not yet extensively used for copepods. In the present work, we tested the suitability of ITS2 rDNA marker to reconstruct the phylogenetic relationships of calanoid copepods using sequences retrieved from GenBank, complementing the phylogenetic positions of the species studied with their morphological and ecological traits. Through ITS2 rDNA we provided the first molecular evidence for the invasive calanoid Pseudodiaptomus marinus from the Mediterranean Sea (Lake Faro, Sicily, Italy), and compared it with the GenBank ITS2 sequences for P. marinus from Korea and other calanoid species. The divergence of the sequences of our P. marinus from those of Korean specimens was quite prominent (4.4%) and allowed us to hypothesise either a new forma living in the Mediterranean or a cryptic species. This study highlights the appropriateness of ITS2 for phylogenetic reconstructions and species identification, as well as for barcoding, meta-barcoding and phylogeographic approaches, and evidences the need for a more thorough knowledge of ribosomal regions in copepods from different sites.

Behaviour-dependent predation risk in swimming zooplankters

BackgroundThe survival of zooplanktonic organisms is determined by their capability of moving in a fluid environment, trading off between the necessities of finding prey and avoiding predators. In previous numerical experiments, we concentrated on the relationship between natatorial modality and encounter success of a virtual copepod swimming in the presence of prey distributed either in patches or uniformly in the environment.ResultsIn this contribution, we extend this simulation framework to the encounter with chaetognaths, the primary copepod predators, considering different motion rules as a proxy of different swimming strategies and looking at the influence of the concentration of predators and the size of their detection radius in posing a risk on copepod survival. The outcomes of our simulations indicate that more convoluted trajectories are more vulnerable to predator encounter while straighter motions reduce predation risk.ConclusionsOur results are then complemented with those obtained in our previous studies to perform a general cost-benefit analysis of zooplankton motion.

Behavioural interseasonal adaptations in Daphnia pulicaria (Crustacea: Cladocera) as induced by predation infochemicals

A wide range of chemical signals have been demonstrated to affect the swimming behaviour of Daphnia, including molecules associated with predation threat. In this contribution, we investigate how the concomitant presence of kairomones from the predaceous fish Lepomis macrochirus and alarm pheromones from crushed conspecifics affect the small-scale swimming behaviour of Daphnia pulicaria. In particular, we studied both the light/dark and summer/winter variations in the swimming motion of the cladoceran in the presence and absence of these infochemicals, and related them to the limnology of the environment. At summertime temperature, in the presence of light and infochemicals D. pulicaria confined its motion to the first few centimetres of the observation vessel, expressing positive phototaxis. In all other conditions, instead, no significant difference in the time spent in the upper and lower parts of the chamber was recorded. The upward displacement of D. pulicaria recorded in our experiments deviates from the most traditionally accepted downward migration used to move into darker layers to avoid visual predation. Our results highlight additional behavioural mechanisms that might be efficiently exploited by D. pulicaria to contrast predation by L. macrochirus.

Distinctive diffusive properties of swimming planktonic copepods in different environmental conditions

Abstract.Suspensions of small planktonic copepods represent a special category in the realm of active matter, as their size falls within the range of colloids, while their motion is so complex that it cannot be rationalized according to basic models of self-propelled particles. Indeed, the wide range of individual variability and swimming patterns resemble the behaviour of much larger animals. By analysing hundreds of three-dimensional trajectories of the planktonic copepod Clausocalanus furcatus, we investigate the possibility of detecting how the motion of this species is affected by different external conditions, such as the presence of food and the effect of gravity. While this goal is hardly achievable by direct inspection of single organism trajectories, we show that this is possible by focussing on simple average metrics commonly used to characterize colloidal suspensions, such as the mean square displacement and the dynamic correlation functions. We find that the presence of food leads to the onset of a clear localization that separates a short-time ballistic from a long-time diffusive regime. Such a benchmark reflects the tendency of C. furcatus to remain temporally feeding in a limited space and disappears when food is absent. Localization is clearly evident in the horizontal plane, but is negligible in the vertical direction, due to the effect of gravity. Our results suggest that simple average descriptors may provide concise and useful information on the swimming properties of planktonic copepods, even though single organism behaviour is strongly heterogeneous.Graphical abstract