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Comparative neuroanatomy of the central nervous system in web-building and cursorial hunting spiders
(2023)
Spiders (Araneae) include cursorial species that stalk their prey and more stationary species that use webs for prey capture. While many cursorial hunting spiders rely on visual cues, web-building spiders use vibratory cues (mechanosensation) for prey capture. We predicted that the differences in primary sensory input between the species are mirrored by differences in the morphology/architecture of the central nervous system (CNS). Here, we investigated the CNS anatomy of four spider species, two cursorial hunters Pardosa amentata (Lycosidae) and Marpissa muscosa (Salticidae), and two web-building hunters Argiope bruennichi (Araneidae) and Parasteatoda tepidariorum (Theridiidae). Their CNS was analyzed using Bodian silver impregnations, immunohistochemistry, and microCT analysis. We found that there are major differences between species in the secondary eye pathway of the brain that pertain to first-order, second-order, and higher order brain centers (mushroom bodies [MB]). While P. amentata and M. muscosa have prominent visual neuropils and MB, these are much reduced in the two web-building species. Argiope bruennichi lacks second-order visual neuropils but has specialized photoreceptors that project into two distinct visual neuropils, and P. tepidariorum lacks MB, suggesting that motion vision might be absent in this species. Interestingly, the differences in the ventral nerve cord are much less pronounced, but the web-building spiders have proportionally larger leg neuropils than the cursorial spiders. Our findings suggest that the importance of visual information is much reduced in web-building spiders, compared to cursorial spiders, while processing of mechanosensory information requires the same major circuits in both web-building and cursorial hunting spiders.
Foraging behavior, neuroanatomy and neuroplasticity in cursorial and stationary hunting spiders
(2023)
The central nervous system (CNS) is the integration center for the coordination and regulation of
all body activities of animals and the source of behavioral patterns, behavioral plasticity and
personality. Understanding the anatomy and the potential for plastic changes of the CNS not only
widens the knowledge on the biology of the respective species, but also enables a more
fundamental understanding of behavioral and ecological patterns. The CNS of species with
different sensory ecologies for example, will show specific differences in the wiring of their CNS,
related to their lifestyle. Spiders are a group of mesopredators that include stationary hunting
species that build webs for prey capture, and cursorial hunting species that do not build capture
webs. These distinct lifestyles are associated with major differences in their sensory equipment,
such as size of the different eyes.
In this thesis, I aimed to answer if a cursorial mesopredator would change its behavior due to
different levels of perceived predation risk, and if this behavior would be influenced by individual
differences (chapter 1); how the visual pathways in the brain of the cursorial hunting jumping
spider Marpissa muscosa differs from that of the nocturnal cursorial hunting wandering spider
Cupiennius salei (chapter 2); to what degree the visual systems of stationary and cursorial hunting
spiders differ and whether CNS areas that process vibratory information show similar differences
(chapter 3); and finally if the CNS in stationary and cursorial hunting spiders shows different
patterns of neuroplasticity in response to sensory input and deprivation during development
(chapter 4).
In chapter 1, I found that jumping spiders adjust their foraging behavior to the perceived level of
risk. By favoring a dark over a light substrate, they displayed a background-matching strategy.
Short pulses of acute risk, produced by simulated bird overflights, had only small effects on the
behavior. Instead, a large degree of variation in behavior was due to among-individual differences
in foraging intensity. These covaried with consistent among-individual differences in activity,
forming a behavioral syndrome. Our findings highlight the importance of consistent amongindividual
differences in the behavior of animals that forage under risk. Future studies should
address the mechanisms underlying these stable differences, as well as potential fitness
consequences that may influence food-web dynamics.
In chapter 2, I found that the visual pathways in the brain of the jumping spider M. muscosa differ
from that in the wandering spider C. salei. While the pathway of the principal eyes, which are
responsible for object discrimination, is the same in both species, considerable differences occur
in the pathway of the secondary eyes, which detect movement. Notably, M. muscosa possesses
an additional second-order visual neuropil, which is integrating information from two different
secondary eyes, and may enable faster movement decisions. I also showed that the tiny posterior
median eye is connected to a first-order visual neuropil which in turn connects to the arcuate body
(a higher-order neuropil), and is thus not vestigial as suggested before. Subsequent studies should
focus on exploring the function of the posterior median eyes in different jumping spider species,
Foraging behavior, neuroanatomy, and neuroplasticity in cursorial and stationary hunting spiders
as they show considerable inter-specific size differences that may be correlated with a differing
connectivity in the brain.
In chapter 3, I described all neuropils and major tracts in the CNS of two stationary (Argiope
bruennichi and Parasteatoda tepidariorum) and two cursorial hunting spiders (Pardosa amentata
and M. muscosa). I found major differences in the visual systems of the secondary eyes between
cursorial and stationary hunting spiders, but also within the groups. A. bruennichi has specialized
retinula cells in two of the secondary eyes, which connect to different higher-order neuropils. P.
tepidariorum has only a single visual neuropil connected to all secondary eyes, and lacks
recognizable mushroom bodies. The neuroanatomy of CNS areas that process mechanosensory
information on the other hand, is remarkably similar between cursorial and stationary hunting
species. This suggests that the same major circuits are used for the processing of mechanosensory
information in both cursorial and stationary hunting spiders. Future studies on functional aspects
of sensory processing in spiders can build on the findings of our study.
In chapter 4, I found that developmental neuroplasticity in response to sensory input differs
between a cursorial (M. muscosa) and a stationary hunting spider (P. tepidariorum). While
deprivation of sensory input leads to a volume increase in several visual and mechanosensory
neuropils M. muscosa, neither sensory deprivation nor sensory enrichment had an effect on the
volume of neuropils in P. tepidariorum. However, exposure to mechanical cues during
development had an effect on the allometric scaling slope of the leg neuropils in both M. muscosa
and P. tepidariorum. Future studies should focus on the genetic and cellular basis of
developmental neuroplasticity in response to sensory input in order to explain the observed
patterns.
Introduction: At the cellular level, acute temperature changes alter ionic conductances, ion channel kinetics, and the activity of entire neuronal circuits. This can result in severe consequences for neural function, animal behavior and survival. In poikilothermic animals, and particularly in aquatic species whose core temperature equals the surrounding water temperature, neurons experience rather rapid and wide-ranging temperature fluctuations. Recent work on pattern generating neural circuits in the crustacean stomatogastric nervous system have demonstrated that neuronal circuits can exhibit an intrinsic robustness to temperature fluctuations. However, considering the increased warming of the oceans and recurring heatwaves due to climate change, the question arises whether this intrinsic robustness can acclimate to changing environmental conditions, and whether it differs between species and ocean habitats.
Methods: We address these questions using the pyloric pattern generating circuits in the stomatogastric nervous system of two crab species, Hemigrapsus sanguineus and Carcinus maenas that have seen a worldwide expansion in recent decades.
Results and discussion: Consistent with their history as invasive species, we find that pyloric activity showed a broad temperature robustness (>30°C). Moreover, the temperature-robust range was dependent on habitat temperature in both species. Warm-acclimating animals shifted the critical temperature at which circuit activity breaks down to higher temperatures. This came at the cost of robustness against cold stimuli in H. sanguineus, but not in C. maenas. Comparing the temperature responses of C. maenas from a cold latitude (the North Sea) to those from a warm latitude (Spain) demonstrated that similar shifts in robustness occurred in natural environments. Our results thus demonstrate that neuronal temperature robustness correlates with, and responds to, environmental temperature conditions, potentially preparing animals for changing ecological conditions and shifting habitats.
Animals often respond to climate change with changes in morphology, e.g., shrinking body size with increasing temperatures, as expected by Bergmann’s rule. Because small body size can have fitness costs for individuals, this trend could threaten populations. Recent studies, however, show that morphological responses to climate change and the resulting fitness consequences cannot be generalized even among related species. In this long-term study, we investigate the interaction between ambient temperature, body size and survival probability in a large number of individually marked wild adult female Natterer’s bats (Myotis nattereri). We compare populations from two geographical regions in Germany with a different climate. In a sliding window analysis, we found larger body sizes in adult females that were raised in warmer summers only in the northern population, but not in the southern population that experienced an overall warmer climate. With a capture-mark-recapture approach, we showed that larger individuals had higher survival rates, demonstrating that weather conditions in early life could have long-lasting fitness effects. The different responses in body size to warmer temperatures in the two regions highlight that fitness-relevant morphological responses to climate change have to be viewed on a regional scale and may affect local populations differently.
Background
The ‘wallflower’ hypothesis proposes females mate indiscriminately to avoid reproductive delays. Post-copulatory mechanisms may then allow ‘trading up’, favouring paternity of future mates. We tested links between pre- and post-copulatory choice in Latrodectus geometricus female spiders paired sequentially with two males. These females copulate as adults or as subadults and store sperm in paired spermathecae. Choosy adults have a higher risk of delays to reproduction than subadults.
Results
We predicted low pre-copulatory, but high post-copulatory choice at first matings for adults and the opposite for subadults. At second matings, we expected all females would prefer males superior to their first. We found all females mated indiscriminately at their first pairing, but in contrast to subadults, adults usually allowed only a single insertion (leaving one of their paired spermatheca empty); a mechanism of post-copulatory choosiness. Adult-mated females were more likely to remate than subadult-mated females when they became adults, showing a preference for larger males, while subadult-mated females tended to prefer males of greater size-corrected mass.
Conclusions
Our results show that the ‘wallflower’ effect and ‘trading up’ tactics can be utilized at different life stages, allowing females to employ choice even if rejecting males is costly.
Podocytes are highly specialized kidney cells that are attached to the outer aspect of the glomerular capillaries and are damaged in more than 75% of patients with an impaired renal function. This specific cell type is characterized by a complex 3D morphology which is essential for proper filtration of the blood. Any changes of this unique morphology are directly associated with a deterioration of the size-selectivity of the filtration barrier. Since podocytes are postmitotic, there is no regenerative potential and the loss of these cells is permanent. Therefore, identification of small molecules that are able to protect podocytes is highly important. The aim of this work was to establish an in vivo high-content drug screening in zebrafish larvae. At first, we looked for a reliable podocyte injury model which is fast, reproducible and easy to induce. Since adriamycin is commonly used in rodents to damage podocytes, we administered it to the larvae and analyzed the phenotype by in vivo microscopy, (immuno-) histology and RT-(q)PCR. However, adriamycin did not result in a podocyte-specific injury in zebrafish larvae. Subsequently, we decided to use a genetic ablation model which specifically damages podocytes in zebrafish larvae. Treatment of transgenic zebrafish larvae with 80 µM metronidazole for 48 hours generated an injury resembling focal and segmental glomerulosclerosis which is characterized by podocyte foot process effacement, cell depletion and proteinuria. Following this, we established an in vivo high-content screening system by the use of a specific screening zebrafish strain. This screening strain expresses a circulating 78 kDa eGFP-labeled Vitamin D-binding fusion protein, which passes the filtration barrier only after glomerular injury. Therefore, we had an excellent readout to follow podocyte injury in vivo. We generated a custom image analysis software that measures the fluorescence intensity of podocytes and the vasculature automatically on a large scale. Furthermore, we screened a specific drug library consisting of 138 compounds for protective effects on larval podocytes using this in vivo high-content system. The analysis identified several initial hits and the subsequent validation experiments identified belinostat as a reliable and significant protective agent for podocytes. These results led to a patent request and belinostat is a promising candidate for a clinical use and will be tested in mammalian podocyte injury models.
Hibernation is a widespread adaptation in animals to seasonally changing environmental conditions. In the face of global anthropogenic change, information about plastic adjustments to environmental conditions and associated mortality costs are urgently needed to assess population persistence of hibernating species. Here, we used a five-year data set of 1047 RFID-tagged individuals from two bat species, Myotis nattereri and Myotis daubentonii that were automatically recorded each time they entered or left a hibernaculum. Because the two species differ in foraging strategy and activity pattern during winter, we expected species–specific responses in the timing of hibernation relative to environmental conditions, as well as different mortality costs of early departure from the hibernaculum in spring. Applying mixed-effects modelling, we disentangled population-level and individual-level plasticity in the timing of departure. To estimate mortality costs of early departure, we used both a capture mark recapture analysis and a novel approach that takes into account individual exposure times to mortality outside the hibernaculum. We found that the timing of departure varied between species as well as among and within individuals, and was plastically adjusted to large-scale weather conditions as measured by the NAO (North Atlantic Oscillation) index. Individuals of M. nattereri, which can exploit milder temperatures for foraging during winter, tuned departure more closely to the NAO index than individuals of M. daubentonii, which do not hunt during winter. Both analytical approaches used to estimate mortality costs showed that early departing individuals were less likely to survive until the subsequent hibernation period than individuals that departed later. Overall, our study demonstrates that individuals of long-lived hibernating bat species have the potential to plastically adjust to changing climatic conditions, although the potential for adjustment differs between species.
Geometric regularity of spider webs has been intensively studied in orb‐weaving spiders, although it is not exclusive of orb weavers. Here, we document the geometrically regular, repetitive elements in the webs of the non‐orb‐weaving groups Leptonetidae and Telemidae for the first time. Similar to orb weavers, we found areas with regularly spaced parallel lines in the webs of Calileptoneta helferi, Sulcia sp., and cf. Pinelema sp. Furthermore, we provide a detailed account of the regular webs of Ochyrocera (Ochyroceratidae). The sections of the web with regularly disposed parallel lines are built as U‐shaped modules reminiscent of orb webs. It has been suggested that the regularly spaced parallel lines in the webs of Ochyroceratidae and Psilodercidae may be produced in a single sweep of their posterior lateral spinnerets, which have regularly spaced aciniform gland spigots, perhaps involving expansion of the spinnerets. To test this hypothesis, we compared the spacing between parallel lines with the spacing between spigots, searched for expansible membranes in the spinnerets, and examined the junctions of regularly spaced lines. The distance between parallel lines was 10–20 times the distance between spigots, and we found no expansible membranes, and the intersection of parallel lines are cemented, which opposes the single sweep hypothesis. Furthermore, we found cues of viscid silk in the parallel lines of the psilodercid Althepus and broadened piriform gland spigots that may be responsible of its production. Finally, we evaluated the presence or absence of geometrically regular web elements across the spider tree of life. We found reports of regular webs in 31 spider families, including 20 families that are not orb weavers and hypothesize that the two basic aspects of regularity (parallel lines spaced at regular intervals, and radial lines spaced at regular angles) probably appeared many times in the evolution of spiders.
Copulatory mechanics of ghost spiders reveals a new self-bracing mechanism in entelegyne spiders
(2023)
Spiders evolved a distinctive sperm transfer system, with the male copulatory organs located on the tarsus of the pedipalps. In entelegyne spiders, these organs are usually very complex and consist of various sclerites that not only allow the transfer of the sperm themselves but also provide a mechanical interlock between the male and female genitalia. This interlocking can also involve elements that are not part of the copulatory organ such as the retrolateral tibial apophysis (RTA)—a characteristic of the most diverse group of spiders (RTA clade). The RTA is frequently used for primary locking i.e., the first mechanical engagement between male and female genitalia. Despite its functional importance, some diverse spider lineages have lost the RTA, but evolved an apophysis on the femur instead. It can be hypothesized that this femoral apophysis is a functional surrogate of the RTA during primary locking or possibly serves another function, such as self-bracing, which involves mechanical interaction between male genital structures themselves to stabilize the inserted pedipalp. We tested these hypotheses using ghost spiders of the genus Josa (Anyphaenidae). Our micro-computed tomography data of cryofixed mating pairs show that the primary locking occurs through elements of the copulatory organ itself and that the femoral apophysis does not contact the female genitalia, but hooks to a projection of the copulatory bulb, representing a newly documented self-bracing mechanism for entelegyne spiders. Additionally, we show that the femoral self-bracing apophysis is rather uniform within the genus Josa. This is in contrast to the male genital structures that interact with the female, indicating that the male genital structures of Josa are subject to different selective regimes.
Extra-organismal DNA (eoDNA) from material left behind by organisms (noninvasive DNA, e.g., feces, hair) or from environmental samples (eDNA, e.g., water, soil) is a valuable source of genetic information. However, the relatively low quality and quantity of eoDNA, which can be further degraded by environmental factors, results in reduced amplification and sequencing success. This is often compensated for through cost- and time-intensive replications of genotyping/sequencing procedures. Therefore, system- and site-specific quantifications of environmental degradation are needed to maximize sampling efficiency (e.g., fewer replicates, shorter sampling durations), and to improve species detection and abundance estimates. Using 10 environmentally diverse bat roosts as a case study, we developed a robust modeling pipeline to quantify the environmental factors degrading eoDNA, predict eoDNA quality, and estimate sampling-site-specific ideal exposure duration. Maximum humidity was the strongest eoDNA-degrading factor, followed by exposure duration and then maximum temperature. We also found a positive effect when hottest days occurred later. The strength of this effect fell between the strength of the effects of exposure duration and maximum temperature. With those predictors and information on sampling period (before or after offspring were born), we reliably predicted mean eoDNA quality per sampling visit at new sites with a mean squared error of 0.0349. Site-specific simulations revealed that reducing exposure duration to 2–8 days could substantially improve eoDNA quality for future sampling. Our pipeline identified high humidity and temperature as strong drivers of eoDNA degradation even in the absence of rain and direct sunlight. Furthermore, we outline the pipeline's utility for other systems and study goals, such as estimating sample age, improving eDNA-based species detection, and increasing the accuracy of abundance estimates.