Swimming behavior and energy metabolism of the calanoid copepod invader Sinodiaptomus sarsi

The appearance of invasive species threatens the integrity of aquatic ecosystems. Much is known about dispersal and introduction mechanisms while little is known on the biological properties of invasive species, such as behavior and energy efficiency, allowing them to successfully colonize new environments and compete with native species. This study examines the functional features of the Asian invasive copepod Sinodiaptomus sarsi (Rylov, 1923) that has invaded Europe since 2016. We focused on the energy metabolism and kinematic parameters of the main swimming types (i.e., gliding, hovering, small relocation jumps, and the escape reaction) of females and males of S. sarsi. Based on the above parameters, the mechanical energy for swimming and the respiration energy needed for movement were calculated. Females and males spend up to 95% of time hovering and slowly gliding at a speed of up to 0.5 cm s-1. During the remaining time, the average swimming speed was 8 cm s-1 by small jumps. In contrast, the average speed was 42 cm s-1 during escape swimming. Non-ovigerous females moved faster than ovigerous females during all relocation swimming types except for upward gliding. While performing small jumps with a frequency of 0.79 Hz, the respiration rate of active non-ovigerous females (0.32 ± 0.03 µg O2 ind-1 h-1) was 2.1 times higher than that of anesthetized individuals. The respiration energy associated with movement was 2.6 * 10-3 J h-1, while the total mechanical energy was only 4.2% of this value. The low energy cost of feeding along with the high speed of locomotion may explain the success of this Asian invader in European waters.

Ultrastructural characterization of the putative defensive glands (warts) in the sessile, colonial rotifer Sinantherina socialis (Gnesiotrocha; Flosculariidae)

Female Sinantherina socialis are freshwater, sessile, colonial rotifers that possess two pairs of distinctive glands (warts) located below the corona. Previous studies demonstrated that colonies are unpalatable to many invertebrate and vertebrate predators; those authors suggested that the warts were a possible source of a chemical deterrent to predation. Here we explore wart ultrastructure and cytochemisty to determine whether the warts function as exocrine glands and if their contents display any allomone-like chemistry, respectively. Externally, the warts appear as elevated bulges without pores. Internally, the warts are specialized regions of the integumental syncytium and therefore acellular. The lipid stain Nile Red labels all four warts. Two lipid membrane probes (sphingomyelin and phosphatidylinositol) also bind the warts and may be staining internal secretion vesicle membranes. In fact, wart ultrastructure is defined by hundreds of membrane-bound secretion vesicles packed tightly together. The vesicles are mostly electron-lucent and crowded into a well-defined cytoplasmic space. The cytoplasm also contains abundant ribosomes, rough endoplasmic reticulum, mitochondria, and Golgi, but nuclei are generally positioned peripheral to the packed vesicles. Absence of muscles around the warts or any signs of direct innervation suggests expulsion of gland contents is forced by general body contraction. A single specimen with 'empty' warts implies that secretions are released en masse from all glands simultaneously. The identity of the chemical secretion remains to be determined, but the lack of osmium and uranyl acetate staining suggests a low abundance or absence of phenols, unsaturated lipids, or NH2 and -COOH groups. This absence, combined with the positive Nile Red staining, is interpreted as evidence that vesicles contain saturated fatty acids such as lactones that are unpalatable to predators.

Axonal ion homeostasis and glial differentiation

The brain is the ultimate control unit of the body. It conducts accurate, fast and reproducible calculations to control motor actions affecting mating, foraging and flight or fight decisions. Therefore, during evolution, better and more efficient brains have emerged. However, even simple brains are complex organs. They are formed by glial cells and neurons that establish highly intricate networks to enable information collection, processing and eventually, a precise motor control. Here, we review and connect some well-established and some hidden pieces of information to set the focus on ion homeostasis as a driving force in glial differentiation promoting signalling speed and accuracy.

Ensheathment and Myelination of Axons: Evolution of Glial Functions

Myelination of axons provides the structural basis for rapid saltatory impulse propagation along vertebrate fiber tracts, a well-established neurophysiological concept. However, myelinating oligodendrocytes and Schwann cells serve additional functions in neuronal energy metabolism that are remarkably similar to those of axon-ensheathing glial cells in unmyelinated invertebrates. Here we discuss myelin evolution and physiological glial functions, beginning with the role of ensheathing glia in preventing ephaptic coupling, axoglial metabolic support, and eliminating oxidative radicals. In both vertebrates and invertebrates, axoglial interactions are bidirectional, serving to regulate cell fate, nerve conduction, and behavioral performance. One key step in the evolution of compact myelin in the vertebrate lineage was the emergence of the open reading frame for myelin basic protein within another gene. Several other proteins were neofunctionalized as myelin constituents and help maintain a healthy nervous system. Myelination in vertebrates became a major prerequisite of inhabiting new ecological niches.

Neurotoxicity in Marine Invertebrates: An Update

Invertebrates represent about 95% of existing species, and most of them belong to aquatic ecosystems. Marine invertebrates are found at intermediate levels of the food chain and, therefore, they play a central role in the biodiversity of ecosystems. Furthermore, these organisms have a short life cycle, easy laboratory manipulation, and high sensitivity to marine pollution and, therefore, they are considered to be optimal bioindicators for assessing detrimental chemical agents that are related to the marine environment and with potential toxicity to human health, including neurotoxicity. In general, albeit simple, the nervous system of marine invertebrates is composed of neuronal and glial cells, and it exhibits biochemical and functional similarities with the vertebrate nervous system, including humans. In recent decades, new genetic and transcriptomic technologies have made the identification of many neural genes and transcription factors homologous to those in humans possible. Neuroinflammation, oxidative stress, and altered levels of neurotransmitters are some of the aspects of neurotoxic effects that can also occur in marine invertebrate organisms. The purpose of this review is to provide an overview of major marine pollutants, such as heavy metals, pesticides, and micro and nano-plastics, with a focus on their neurotoxic effects in marine invertebrate organisms. This review could be a stimulus to bio-research towards the use of invertebrate model systems other than traditional, ethically questionable, time-consuming, and highly expensive mammalian models.

Wrapping glia regulates neuronal signaling speed and precision in the peripheral nervous system of Drosophila

The functionality of the nervous system requires transmission of information along axons with high speed and precision. Conductance velocity depends on axonal diameter whereas signaling precision requires a block of electrical crosstalk between axons, known as ephaptic coupling. Here, we use the peripheral nervous system of Drosophila larvae to determine how glia regulates axonal properties. We show that wrapping glial differentiation depends on gap junctions and FGF-signaling. Abnormal glial differentiation affects axonal diameter and conductance velocity and causes mild behavioral phenotypes that can be rescued by a sphingosine-rich diet. Ablation of wrapping glia does not further impair axonal diameter and conductance velocity but causes a prominent locomotion phenotype that cannot be rescued by sphingosine. Moreover, optogenetically evoked locomotor patterns do not depend on conductance speed but require the presence of wrapping glial processes. In conclusion, our data indicate that wrapping glia modulates both speed and precision of neuronal signaling.

Copepod feeding strategy determines response to seawater viscosity: videography study of two calanoid copepod species

Calanoid copepods, depending on feeding strategy, have different behavioral and biological controls on their movements, thereby responding differently to environmental conditions such as changes in seawater viscosity. To understand how copepod responses to environmental conditions are mediated through physical, physiological and/or behavioral pathways, we used high-speed microvideography to compare two copepod species, Acartia hudsonica and Parvocalanus crassirostris, under different temperature, viscosity and dietary conditions. Acartia hudsonica exhibited 'sink and wait' feeding behavior and typically responded to changes in seawater viscosity; increased seawater viscosity reduced particle-capture behavior and decreased the size of the feeding current. In contrast, P. crassirostris continuously swam and did not show any behavioral or physical responses to changes in viscosity. Both species showed a physiological response to temperature, with reduced appendage beating frequency at cold temperatures, but this did not generally translate into effects on swimming speed, feeding flux or active time. Both copepod species swam slower when feeding on diatom rather than dinoflagellate prey, showing that prey type mediates copepod behavior. These results differentiate species-specific behaviors and responses to environmental conditions, which may lead to better understanding of niche separation and latitudinal patterns in copepod feeding and movement strategies.

Rotational Maneuvers of Copepod Nauplii at Low Reynolds Number

Copepods are agile microcrustaceans that are capable of maneuvering freely in water. However, the physical mechanisms driving their rotational motion are not entirely clear in small larvae (nauplii). Here we report high-speed video observations of copepod nauplii performing acrobatic feats with three pairs of appendages. Our results show rotations about three principal axes of the body: yaw, roll, and pitch. The yaw rotation turns the body to one side and results in a circular swimming path. The roll rotation consists of the body spiraling around a nearly linear path, similar to an aileron roll of an airplane. We interpret the yaw and roll rotations to be facilitated by appendage pronation or supination. The pitch rotation consists of flipping on the spot in a maneuver that resembles a backflip somersault. The pitch rotation involved tail bending and was not observed in the earliest stages of nauplii. The maneuvering strategies adopted by plankton may inspire the design of microscopic robots, equipped with suitable controls for reorienting autonomously in three dimensions.

Evolution of glial wrapping: A new hypothesis

Animals are able to move and react in numerous ways to external stimuli. Thus, environmental stimuli need to be detected, information must be processed and finally an output decision must be transmitted to the musculature to get the animal moving. All these processes depend on the nervous system which comprises an intricate neuronal network and many glial cells. In the last decades, a neurono-centric view on nervous system function channeled most of the scientific interest toward the analysis of neurons and neuronal functions. Neurons appeared early in animal evolution and the main principles of neuronal function from synaptic transmission to propagation of action potentials are conserved during evolution. In contrast, not much is known on the evolution of glial cells that were initially considered merely as static support cells. Although it is now accepted that glial cells have an equally important contribution as their neuronal counterpart to nervous system function, their evolutionary origin is unknown. Did glial cells appear several times during evolution? What were the first roles glial cells had to fulfil in the nervous system? What triggered the formation of the amazing diversity of glial morphologies and functions? Is there a possible mechanism that might explain the appearance of complex structures such as myelin in vertebrates? Here, we postulate a common evolutionary origin of glia and depict a number of selective forces that might have paved the way from a simple supporting cell to a wrapping and myelin forming glial cell.

Predatory posture and performance in a precocious larval fish targeting evasive copepods

Predatory fishes avoid detection by prey through a stealthy approach, followed by a rapid and precise fast-start strike. Although many first-feeding fish larvae strike at non-evasive prey using an S-start, the clownfish Amphiprion ocellaris feeds on highly evasive calanoid copepods from a J-shaped position, beginning 1 day post-hatch (dph). We quantified this unique strike posture by observing successful predatory interactions between larval clownfish (1 to 14 dph) and three developmental stages of the calanoid copepod Bestiolina similis The J-shaped posture of clownfish became less tightly curled (more L-shaped) during larval development. Larvae were also less tightly curled when targeting adult copepods, which are more evasive than younger copepod stages. Strike performance measured as time to capture and as peak speed improved only slightly with larval age. Therefore, the J-posture may allow first-feeding larvae to minimize disturbance during their approach of sensitive prey, and may represent an alternative predatory strategy to the prototypical S-start.

Going with the flow: hydrodynamic cues trigger directed escapes from a stalking predator

In the coevolution of predator and prey, different and less well-understood rules for threat assessment apply to freely suspended organisms than to substrate-dwelling ones. Particularly vulnerable are small prey carried with the bulk movement of a surrounding fluid and thus deprived of sensory information within the bow waves of approaching predators. Some planktonic prey have solved this apparent problem, however. We quantified cues generated by the slow approach of larval clownfish ( Amphiprion ocellaris) that triggered a calanoid copepod ( Bestiolina similis) to escape before the fish could strike. To estimate water deformation around the copepod immediately preceding its jump, we represented the body of the fish as a rigid sphere in a hydrodynamic model that we parametrized with measurements of fish size, approach speed and distance to the copepod. Copepods of various developmental stages (CII-CVI) were sensitive to the water flow caused by the live predator, at deformation rates as low as 0.04 s-1. This rate is far lower than that predicted from experiments that used artificial predator-mimics. Additionally, copepods localized the source, with 87% of escapes directed away (greater than or equal to 90°) from the predator. Thus, copepods' survival in life-threatening situations relied on their detection of small nonlinear signals within an environment of locally linear deformation.

Glia: from 'just glue' to essential players in complex nervous systems: a comparative view from flies to mammals

In the last years, glial cells have emerged as central players in the development and function of complex nervous systems. Therefore, the concept of glial cells has evolved from simple supporting cells to essential actors. The molecular mechanisms that govern glial functions are evolutionarily conserved from Drosophila to mammals, highlighting genetic similarities between these groups, as well as the great potential of Drosophila research for the understanding of human CNS. These similarities would imply a common phylogenetic origin of glia, even though there is a controversy at this point. This review addresses the existing literature on the evolutionary origin of glia and discusses whether or not insect and mammalian glia are homologous or analogous. Besides, this manuscript summarizes the main glial functions in the CNS and underscores the evolutionarily conserved molecular mechanisms between Drosophila and mammals. Finally, I also consider the current nomenclature and classification of glial cells to highlight the need for a consensus agreement and I propose an alternative nomenclature based on function that unifies Drosophila and mammalian glial types.

Zooplankton can actively adjust their motility to turbulent flow

Calanoid copepods are among the most abundant metazoans in the ocean and constitute a vital trophic link within marine food webs. They possess relatively narrow swimming capabilities, yet are capable of significant self-locomotion under strong hydrodynamic conditions. Here we provide evidence for an active adaptation that allows these small organisms to adjust their motility in response to background flow. We track simultaneously and in three dimensions the motion of flow tracers and planktonic copepods swimming freely at several intensities of quasi-homogeneous, isotropic turbulence. We show that copepods synchronize the frequency of their relocation jumps with the frequency of small-scale turbulence by performing frequent relocation jumps of low amplitude that seem unrelated to localized hydrodynamic signals. We develop a model of plankton motion in turbulence that shows excellent quantitative agreement with our measurements when turbulence is significant. We find that, compared with passive tracers, active motion enhances the diffusion of organisms at low turbulence intensity whereas it dampens diffusion at higher turbulence levels. The existence of frequent jumps in a motion that is otherwise dominated by turbulent transport allows for the possibility of active locomotion and hence to transition from being passively advected to being capable of controlling diffusion. This behavioral response provides zooplankton with the capability to retain the benefits of self-locomotion despite turbulence advection and may help these organisms to actively control their distribution in dynamic environments. Our study reveals an active adaptation that carries strong fitness advantages and provides a realistic model of plankton motion in turbulence.

Directional Hydrodynamic Sensing by Free-Swimming Organisms

Many aquatic organisms detect the presence of moving objects in their environment, such as predators, by sensing the hydrodynamic disturbances the movements produce. The resultant water flow is readily detectable by stationary organisms, but free-swimming organisms are carried with the surrounding water and may not detect the bulk surrounding flow, which limits the available information about the source. We have developed a theory that clarifies what information is contained in disturbances generated by an attacking predator that is available to a free-swimming organism and might be extracted from local flow deformations alone. The theory shows that, depending on how well the deformations can be measured in space and time, an organism can reduce the range of possible locations, speeds, sizes, and arrival times of the predator. We apply the theory to planktonic copepods that have mechanosensory hairs along a pair of antennules. The study reveals the presence of "blind spots," potential ambiguities in resolving from which of two sides a predator attacks, and whether it generates a bow wave or suction. Our findings lead to specific testable hypotheses concerning optimal escape strategies, which are helpful for interpreting the behavior of evasive prey and designing free-swimming robots with sensory capabilities.