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This chapter focuses on a singular vermiform creature, the freshwater hydra or polyp. It provides a discussion of natural history writing to illustrate a kind of lyricism and aesthetic revaluation built into the articulation of empirical study. Discovered in 1740 by Abraham Trembley, the freshwater hydra or polyp displayed an astonishing variety of wormy behaviors, including the capacity to regenerate from cuttings as if it were a plant. Consequently, subsequent studies of Trembley’s investigations into the polyp emphasize this naturalist’s relative obscurity in the history of science against the polyp itself being a celebrated discovery. Trembley’s account of his investigations into the structure and behavior of this creature implores a way of aestheticizing the vermicular that fruitfully combines the more classic repulsion with an inventive appeal to mutability, indeterminacy, and the irrepressibility of the organic.

The toxicity of manufactured nanoparticles varies greatly depending on the test species in consideration and estimates of toxicity may also be confounded by test media in which the organisms are cultured. For a more comprehensive toxicity evaluation, species at different trophic levels or with life strategies, tested in different media should be included. In this study, we examined the toxicity of tyrosine-coated silver nanonparticles (tyr-AgNP) to three Australian freshwater invertebrates: Hydra vulgaris, Daphnia carinata and Paratya australiensis. Tyr-AgNPs were synthesized, characterized and their behaviour was examined in different media used for acute toxicity tests. Additionally, the sensitivity of tested organisms to tyr-AgNPs was compared to ionic silver (Ag+). Based on the LC50 values of both tyr-AgNPs and Ag+ ions at different time points, D. carinata was found to be the most sensitive species followed by P. australiensis and H. vulgaris. NP stability studies revealed that tyr-AgNPs were least stable in hydra medium followed by daphnid and shrimp media. This study demonstrates that significant differences in NP toxicity to aquatic organisms exist and the test media and the life strategy of the species play a key role in these differences. Therefore, it is recommended that a multispecies approach is used in predictive risk assessment of NPs and to ensure protection of native species from possible toxic effects from NPs released into aquatic systems. Also recommended is to carefully investigate the fate and behaviour of NPs in different media in assessing NP toxicity and emphasize the need to use native species in developing relevant regulatory frameworks.

Background Many metazoan genomes are characterized by highly conserved chromosomal homologies that predate the ancient origin of this clade. This conservation has been tested by expansions of selfish DNA elements, in particular transposable elements (TEs). While comparative genomics studies have highlighted their diversity across animal genomes, common principles underlying their evolution along deeply conserved chromosomes have been elusive. A detailed mechanistic understanding from phylogenetically key and early branching animal species has been lacking. Results We present a comprehensive stem-cell resolved genomic and transcriptomic study of the freshwater cnidarian Hydra , an animal characterized by its high regenerative capacity, the ability to propagate clonally, and an apparent lack of aging. Using single-haplotype telomere-to-telomere genome assemblies of two recently diverged strains and utilizing unique features of hydra biology allowed us to sequence and compare the individual genomes of hydra’s three stem cell lineages. We show that distinct TE families are active at both transcriptional and genomic levels via non-random insertions in each of these lineages. We show that the core set of these active TE families, primarily composed of DNA elements, is evolutionarily deeply conserved and contributes to consistent genomic expansions in metazoan lineages. These anciently active TEs differentially contribute to structural variants around loci associated with cell proliferation and long-range topological contacts. This is in strong contrast to the frequently observed and highly varied substantial genome expansions that often happen via retroelements. Conclusions Our study suggests an ancient and conserved role for these core TEs as self-renewing components of animal chromosomes.

A hydra effect occurs when the mean density of a species increases in response to greater mortality. We show that, in a stable multispecies system, a species exhibits a hydra effect only if maintaining that species at its equilibrium density destabilizes the system. The stability of the original system is due to the responses of the hydra‐effect species to changes in the other species’ densities. If that dynamical feedback is removed by fixing the density of the hydra‐effect species, large changes in the community make‐up (including the possibility of species extinction) can occur. This general result has several implications: (1) Hydra effects occur in a much wider variety of species and interaction webs than has previously been described, and may occur for multiple species, even in small webs; (2) conditions for hydra effects caused by predators (or diseases) often differ from those caused by other mortality factors; (3) introducing a specialist or a switching predator of a hydra‐effect species often causes large changes in the community, which frequently involve extinction of other species; (4) harvest policies that attempt to maintain a constant density of a hydra‐effect species may be difficult to implement, and, if successful, are likely to cause large changes in the densities of other species; and (5) trophic cascades and other indirect effects caused by predators of hydra‐effect species can exhibit amplification of effects or unexpected directions of change. Although we concentrate on systems that are originally stable and models with no stage‐structure or trait variation, the generality of our result suggests that similar responses to mortality will occur in many systems without these simplifying assumptions. In addition, while hydra effects are defined as responses to altered mortality, they also imply counterintuitive responses to changes in immigration and other parameters affecting population growth.

Life history theory predicts that reproduction and somatic maintenance are negatively related, but the strength of this relationship is expected to depend on food availability. In this study, we investigated asexual reproduction (budding rate) and oxidative stress tolerance as two opposing facets of life history trade-offs in 17 strains of five freshwater hydra species under experimentally simulated low, medium, and high food availability. Stress tolerance was quantified by exposing animals to exogenous H₂O₂, which mimics reactive oxygen species arising in vivo. The five species differed in life history traits (low budding rate and high stress tolerance in Hydra vulgaris and H. circumcincta and the opposite in H. oligactis and H. viridissima; low budding rate combined with relatively low stress tolerance in H. oxycnida). Stress tolerance and asexual reproduction increased with food, but there were clear interspecific differences in this relationship. Across all strains, stress tolerance and budding rate were significantly negatively related on the low and medium, but not the high food level. These results suggest that resource allocation trade-offs are involved in determining life history traits in hydra; populations/species can be broadly positioned on a fast–slow life history continuum, and response to variation in food varies along this continuum.

It has been proposed that holobionts (host-symbiont units) could swap endosymbionts, rapidly alter the hologenome (host plus symbiont genome), and increase their stress tolerance. However, experimental tests of individual and combined contributions of hosts and endosymbionts to holobiont stress tolerance are needed to test this hypothesis. Here, we used six green hydra (Hydra viridissima) strains to tease apart host (hydra) and symbiont (algae) contributions to thermal tolerance. Heat shock experiments with (1) hydra with their original symbionts, (2) aposymbiotic hydra (algae removed), (3) novel associations (a single hydra strain hosting different algae individually), and (4) control hydra (aposymbiotic hydra re-associated with their original algae) showed high variation in thermal tolerance in each group. Relative tolerances of strains were the same within original, aposymbiotic, and control treatments, but reversed in the novel associations group. Aposymbiotic hydra had similar or higher thermal tolerance than hydra with algal symbionts. Selection on the holobiont appears to be stronger than on either partner alone, suggesting endosymbiosis could become an evolutionary trap under climate change. Our results suggest that green hydra thermal tolerance is strongly determined by the host, with a smaller, non-positive role for the algal symbiont. Once temperatures exceed host tolerance limits, swapping symbionts is unlikely to allow these holobionts to persist. Rather, increases in host tolerance through in situ adaptation or migration of pre-adapted host strains appear more likely to increase local thermal tolerance. Overall, our results indicate green hydra is a valuable system for studying aquatic endosymbiosis under changing environmental conditions, and demonstrate how the host and the endosymbiont contribute to holobiont stress tolerance.

The regenerative ability of Hydra vulgaris was tested as potential biomarker for the development of a new eco-toxicological index. The test is based on the regeneration rate and the aberration frequency of the columna (body and adhesive foot) after separation from head and tentacles by a bistoury. Particularly, 45 columnae were submerged in the rearing solution (that is Hydra medium) to have control, and 285 in potential contaminated waters to have treatments, collected from 19 sites along 10 rivers in central Italy. ANCOVA and chi-square tests were used to compare values from each site to a laboratory control. Subsequently the values on regeneration rate and aberration frequency were inserted in a double entry matrix, where the match of the two entries in the matrix provides the score of the proposed Teratogenic Risk Index (TRI). Each score corresponded to one of the 5 teratogenic risk classes, to which a risk level was associated: from 1 (no risk) to 5 (very high risk). On the whole, 32% of the studied sites were classified as no teratogenic risk while the remaining showed a variable risk level from low to very high. This study proposed for the first time an early warning system to detect the presence of teratogens in running waters, providing a rapid and cost-effective evaluation method. Therefore, TRI may contribute to initiate adequate measures to manage riverine habitats, and to monitor the running water teratogenic status . Specifically, this index may provide the opportunity to identify the disturbance sources and then to drive the decisions, together with competent authorities, on the catchment and landscape management and on the possible use of waters for urban, agricultural, and industrial activities, since they may show significant effects on the human health.

Capecitabine (CAP, prodrug) and 5-fluorouracil (5-FU, its active metabolite) are two of the most prominent cytostatics, for which no clear picture can be drawn regarding potential concentrations of effect for freshwater biota, with CAP being grouped in the least studied cytostatic, whereas 5-FU has been classified as of no and of high environmental risk. Accordingly, the present work aimed to assess the ecotoxicity of CAP and 5-FU in three freshwater species, which included a 72-h assay with the producer Raphidocelis subcapitata ; a 96-h assay with the invertebrate secondary consumer Hydra viridissima ; and a 96-h assay with embryos of the vertebrate secondary consumer Danio rerio . The following endpoints were monitored: yield and population growth rate for the algae; mortality, morphological alterations, and post-exposure feeding rates for the cnidarian; and mortality, hatching, and malformations for the fish. Overall, organisms’ sensitivity to CAP decreased in the following order: R. subcapitata  >  H. viridissima  >  D. rerio , whereas for 5-FU, it decreased in the following order: H. viridissima  >  D. rerio  >  R. subcapitata . For CAP, no median lethal effective concentrations (LC/EC 50 ) were possible to compute for D. rerio , with no significant mortality or malformations registered in embryos exposed at concentrations up to 800 mg L −1 . For R. subcapitata , the EC 50 s were 0.077 and 0.63 mg L −1 for yield and growth rate, respectively, and for H. viridissima , the EC 50,30 min for feeding was 22.0 mg L −1 . For 5-FU, no EC 50s could be computed for R. subcapitata , whilst the EC 50s for H. viridissima mortality and feeding were 55.4 and 67.9 mg L −1 , respectively, and for D. rerio , the LC 50,96 h and EC 50,96 h (hatching and abnormalities) were 4546, 4100, and 2459 mg L −1 , respectively. Assuming similar modes of action for both compounds and their co-occurrence, the combined risk quotient of the two chemicals was determined to be 7.97, which represents a risk for freshwater biota. Anticipating the increased consumption of these compounds and cancer development trends worldwide, these impacts may be further aggravated.

Sodium metasilicate (SM) is a synthetic hazardous water-soluble salt used in industry as an active ingredient in household cleaning products. The impact of SM on the aquatic environment has been discussed worldwide, but its toxicity has not been well documented and researched. Studies have only been performed on a handful of aquatic organisms: algae, plants, blackworms, water fleas, dipteran larvae, and two fish species. Hydra is a simple freshwater cnidarian with diploblastic organisation, where all the cells are in permanent contact with the surrounding aqueous media, and represents a sensitive model organism for environmental toxicity assessments. This research aimed to determine and compare the effect of SM on the morphology, excitability, and behaviour of green and brown hydra and endosymbiotic microalgae as a microbiome of green hydra. The hydras were treated with four sublethal SM concentrations (0.050, 0.365, 0.380, and 0.390 g/L) for 72 h. Standard preparations were made for the cyto-histological analysis of green hydra, and damaged cellular layers and mesoglea and a changed distribution of microalgae were recorded. The SM caused muted responses to mechanical stimuli and damage to the tentacles in both hydra species. The changes were more pronounced in brown hydra, while green hydra showed better adaptability to unfavourable environmental conditions.

In this study, we investigate the main ecological interactions between fouling aquatic organisms (both invasive and native) present in the reservoir of the Governador José Richa hydroelectric plant, located in southern Brazil, and to identify the most suitable period for the interruption of machinery operation for cleaning and maintenance of the hydraulic systems of this plant. A total of 32 experimental plates were fixed to a metallic structure positioned close to the plant's water intake. Three species of invasive fouling were identified in our samples (Limnoperna fortunei [Mollusca], Cordylophora sp., and Hydra sp. [Cnidaria]) and six native taxa belonging to the phyla Protozoa, Ciliophora, Amoebozoa, and Arthropoda. Spring and summer were the seasons with the highest fouling rates, as well as densities of fouling organisms. The highest levels of diversity were recorded during the colder seasons. Several interactions between the organisms were identified, such as mutualism, commensalism, competition, epibiosis, cannibalism, and predation. The data obtained suggest that, from the biological point of view, the most suitable period for machine shutdown destined for the removal of biological fouling in the hydraulic systems of the studied plant is between the end of spring and the beginning of summer.