Thorp and Covich's Freshwater Invertebrates (Fourth Edition)
Chapter 17 - Phylum Tardigrada
Abstract
A sister group of the Arthropoda, the Tardigrada are micrometazoans that occupy a diversity of niches in freshwater, marine, and terrestrial habitats. Commonly called water bears because of their slow, lumbering gait, these molting lobopods have four pairs of legs, usually terminating in claws. Most are less than 1 mm in length, with a complete digestive tract, a dorsal gonad with one or two gonoducts, and a dorsal lobed brain with a ventral nerve cord and four bilobed ganglia, one per leg-bearing metamere. The body cavity (hemocoel) functions in respiration and circulation. Over 1200 species have been described based primarily on the morphology of the claws and buccal-pharyngeal apparatus. Individuals may be either gonochoric, unisexual, or hermaphroditic, with fertilized or unfertilized eggs deposited either freely or within the shed exuvium. Parthenogenesis occurs frequently in limnic and terrestrial tardigrades, allowing them to colonize new territories by passive dispersal of a single individual. Cryptobiosis (anhydrobiosis, anoxybiosis, cryobiosis, and osmobiosis) and diapause (encystment and resting eggs) occur during the life history. Active adults (surrounded by water) and cryptobiotic adults and eggs are primarily dispersed passively, but some active dispersal can also occur. Due to the characteristic patchy distributions of tardigrade populations, little is known about their population dynamics and trophic relationships. Improved methods for collection, microscopy, culturing, and molecular analyses have been have contributed much to our knowledge of tardigrades.
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Desiccation-tolerant organisms can survive dehydration in a state of anhydrobiosis. Tardigrades can recover from anhydrobiosis at any life stage and are considered among the toughest animals on Earth. However, the factors that influence recovery from anhydrobiosis are not well understood. The study aimed to evaluate the effect of sex, age, the presence of other individuals and the combination of the number and duration of anhydrobiosis episodes on the recovery of Paramacrobiotus experimentalis. The activity of 1200 individuals for up to 48 h after rehydration was evaluated using analysis of variance (ANOVA). Age was the main factor influencing return to activity, followed by the combination of number and duration of anhydrobiosis episodes, influence of the presence of other individuals, and sex. More individuals returned to activity after repeated short than repeated long anhydrobiosis episodes and older individuals were less likely to recover than younger individuals. In addition, when compared to single animals, the presence of other individuals resulted in higher number of active animals after dehydration and rehydration. The effect of sex was significant, but there was no general tendency for one sex to recover from anhydrobiosis better than the other one. The results contribute to a better understanding of the anhydrobiosis ability of Paramacrobiotus experimentalis and provide background for full explanation of molecular, cellular and environmental mechanisms of anhydrobiosis.
Morphological and molecular evidence for a new species of the genus Milnesium Doyère, 1840 (Tardigrada: Apochela) from south America
2024, Zoologischer AnzeigerTardigrades are aquatic microinvertebrates, that can be found from the poles to the tropical regions. However, knowledge about their diversity and distribution is still limited, especially regarding the Neotropical region. In addition, many species described in the past were considered cosmopolitan but new morphological, morphometric and especially molecular studies have demonstrated the existence of species complexes. The species Milnesium tardigradum Doyère, 1840 is an example that highlights this issue. Many M. tardigradum records made in South America date from the time when this species was considered cosmopolitan and exhibiting large intraspecific variability. However, current revisions of the genus using modern techniques indicate that the M. tardigradum sensu stricto is most likely restricted to the Palaearctic region. In this contribution, we describe a new species, Milnesium iniquum sp. nov. from south-eastern Brazil, with additional records from Argentina and French Guiana, utilising integrative taxonomy framework. The new species is characterised by having smooth cuticle, presence of dorsal pseudoplates, adult claw configuration [3-3]-[3-3] and [2-2]-[2-2] in hatchlings (early positive CC change), with internal and anterior spurs larger than external and posterior spurs and by presence of short cuticular bars on legs IV.
Presence of zooplankton, eggs, and resting stages in ballast water samples from the Laurentian Great Lakes
2024, Journal of Great Lakes ResearchTo minimize the introduction and spread of aquatic nuisance species, the United States Coast Guard (USCG) and the International Maritime Organization (IMO) regulate the allowed microbe, protist, and zooplankton densities present in ballast water discharged from commercial ships. However, the density of eggs and resting stages in ballast discharge are not routinely assessed when determining the effectiveness of ballast water treatment systems. While the density of eggs and resting stages found in residual sediment has been examined by various researchers, the density in ballast water uptake and discharge samples was previously unexamined. Untreated uptake and discharge ballast water samples from 2017, 2022, and 2023, collected on ships within the Laurentian Great Lakes from August through January, were analyzed for eggs and resting stages. The samples contained a total density of 325–140,859 eggs and resting stages per cubic meter. These results demonstrate that uncounted eggs and resting stages are present in ballast water, contributing to the risk of spreading aquatic nuisance species within the Great Lakes.
Negative impact of freeze–thaw cycles on the survival of tardigrades
2023, Ecological IndicatorsGlobal warming effects in temperate and polar regions include higher average temperatures and a decrease in snow cover, which together lead to an increase in the number of freeze–thaw cycles (FTC). These changes could affect the fitness of both terrestrial and aquatic species. In this study, we tested how tardigrades, ubiquitous microscopic invertebrates, face FTC. Tardigrades are amongst the most resistant animals to unfavorable conditions, including long and deep freezing periods, and are an emerging model group for invertebrate ecology and evolution. We used 12 populations of tardigrades, representing different families within order Parachela, inhabiting different ecosystems (glaciers, snow, terrestrial, aquatic), found in various substrates (mosses, sediments in lakes, cryoconite on glaciers, and snow), and originating from different latitudes and altitudes. We estimated the number of cycles required to kill 50% of individuals and tested for its association with ecological characteristics of the natural habitat (e.g., number of months with predicted FTC), while accounting for phylogeny. The most resistant tardigrades to FTC were the ones from mountain areas and glaciers. The estimated number of cycles required to kill 50% of individuals was the highest for mountainous species inhabiting rock pools and cryoconite holes on glaciers (30 and 14 FTC, respectively). Tardigrades from lowlands were the most sensitive to changes, with 50% of individuals dying after three FTC, while lacustrine and subtropical tardigrades required only one FTC to reach 50% mortality. Our study shows that the response to recurrent freezing stress is taxon dependent and related to the local environmental conditions. The predicted increase of FTC cycles will negatively impact tardigrade populations. Considering the abundance and various trophic roles of tardigrades, reduction in population sizes or the disappearance of some fragile species could affect the functioning of both aquatic and terrestrial ecosystems. Tardigrades are candidate indicators of how freeze–thaw cycles impact ubiquitous microscopic metazoans with similar physiological capabilities.
The effects of sample storage duration on tardigrade density and community composition in moss samples
2023, PedobiologiaIn studies on micrometazoans, sample storage and processing methods are mostly decided based on sample quality (e.g., substrate type and moisture level), and the choice of methods may affect the reliability of the data. However, these methods are poorly studied and rarely reported in detail. Our aim was to determine the methodological compromise between efficiency and reliability required for large-scale quantitative meiofaunal ecological studies. Specifically, we tested whether storage duration (necessary for large number of samples) affects the density or community composition of tardigrades in moss samples. We focus on a largely unexplored limnoterrestrial ecosystem – boreal peatlands, where moss moisture levels are naturally variable across different microhabitats and moss species. We collected seven moss samples from a peatland in Central Finland, kept them in a refrigerator and extracted tardigrades using the Baermann wet funnel at 1, 24, 48 and 96 h post sampling. We found a significant decrease in tardigrade density (32 % on average), but no changes in community composition, after the first 24 h of storage. Based on these results, we recommend that samples collected from wet limnoterrestrial habitats should be processed within 24 h to ensure accuracy and comparability of large-scale quantitative data on tardigrade ecology.
Benthic Animals
2023, Wetzel's Limnology: Lake and River Ecosystems, Fourth EditionAnimals (and protozoans) are highly diverse organisms in the benthos of freshwater ecosystems. Their distribution, abundance, and productivity are determined by several factors, from historical events to local abiotic and biotic conditions. Representatives of nearly every animal phylum occur in freshwaters, with aquatic insects being the most abundant and diverse benthic group. They have a large variety of morphological, physiological, reproductive, dispersal, feeding, and life history traits that have made them very successful in these ecosystems. In lakes benthic animals are found in littoral and profundal habitats, whereas in rivers and streams they are segregated by flow and velocity. Benthic metacommunities are explained by a mixture of local environmental (species sorting) and spatial (dispersal) factors, with the former being more important in freshwater ecosystems than the latter. Their responses to the environment have made them ideal organisms to assess past (e.g., paleolimnology), current (e.g., biomonitoring), and future global change effects.