Taxonomy of the European Hydra (Gnidaria: Hydrozoa): a re-examination of its history with emphasis on the species H. vulgaris Pallas, H. attenuata Pallas and H. circumcincta Schulze

The name Hydra attenuata Pallas is currently applied to the wrong animal. The common brown polyp, which is widely called H. attenuata, was described by Pallas (1766) as Hydra vulgaris. The name H. attenuata Pallas originally referred to an uncommon pale polyp, currently known as H. circumcincta Schulze. The history of this confusion is analysed here. The taxonomy of hydra was in disarray during the 18th and 19th centuries, and was clarified in 1917 with the monograph of Schulze. But Schulze misapplied the name for the common hydra, H. vulgaris, to an unusual form and thus was led to assign the name of a rare hydra, H. attenuata, to the common type. Schulze redescribed the rare, pale hydra that Pallas had named H. attenuata as H. circumcincta. The correct name of the common European brown, stalkless hydra is thus H. vulgaris Pallas, 1766. The name H. attenuata has priority for the uncommon pale hydra, but because of disuse of this application of the name, the pale hydra should be recognized by the current, generally accepted binominal H. circumcincta Schulze, 1914.     

中国广东水螅属一新种(水螅纲,无鞘螅目,水螅科)

报道采自中国广东省肇庆市的淡水水螅新种,即珠江水螅Hydra zhujiangensis sp.nov.,研究标本均来自1只水螅的单系繁殖群体,测量数据经生物统计学处理。新种名以水螅生境所属珠江水系命名。所有研究标本保存于深圳大学生命科学学院。     

Taxonomy of the European Hydra (Cnidaria: Hydrozoa): a re-examination of its history with emphasis on the species H. vulgaris Pallas,H. attenuata Pallas and H. circumcincta Schulze

The name Hydra attenuata Pallas is currently applied to the wrong animal. The common brown polyp, which is widely called H. attenuata, was described by Pallas (1766) as Hydra vulgaris. The name H. attenuata Pallas originally referred to an uncommon pale polyp, currently known as H. circumcincta Schulze. The history of this confusion is analysed here. The taxonomy of hydra was in disarray during the 18th and 19th centuries, and was clarified in 1917 with the monograph of Schulze. But Schulze misapplied the name for the common hydra, H. vulgaris, to an unusual form and thus was led to assign the name of a rare hydra, H. attenuata, to the common type. Schulze redescribed the rare, pale hydra that Pallas had named H. attenuata as H. circumcincta. The correct name of the common European brown, stalkless hydra is thus H. vulgaris Pallas, 1766. The name H. attenuata has priority for the uncommon pale hydra, but because of disuse of this application of the name, the pale hydra should be recognized by the current, generally accepted binominal H. circumcincta Schulze, 1914.     

Genome sizes and chromosomes in the basal metazoan Hydra

Hydras belong to one of the earliest eumetazoan animal groups, but to date very little is known about their genome sizes, gene numbers, and chromosomes. Here we provide genome size estimates and corresponding karyotypes for five Hydra species. Nuclear DNA contents were assessed by slide-based Feulgen microphotometry. Hydra oligactis possesses the largest genome of 1450 Mbp, followed by similar 1 C capacities in H. carnea (1350 Mbp), H. vulgaris (1250 Mpb) and H. circumcincta (1150 Mbp). The smallest genome of 380 Mbp was determined in H. viridissima. While the number of chromosomes is identical in all five Hydra species (2n = 30), the size of the chromosomes is strictly correlated to the size of the genome, with H. viridissima having conspicuously small chromosomes. The taxonomic and evolutionary significance of the C-value and chromosomal size variation in this ancient group of metazoans as well as its impact on genomic organization and forthcoming genome projects are discussed.     

Radiophryoides puytoraci sp. nov.: Astomatous Ciliate Parasite from a Fresh-Water Oligochaete

A new astomatous holotrichous ciliate, Radiophryoides puytoraci , parasitic in the gut of the Indian freshwater oligochaete is described. This is the second finding of the astomatous ciliate parasite from the microdrili from Asia. Key to all the known and valid species of Radiophryoides is given.     

中国水螅属一新种(水螅纲,水螅科)

报道采自广东省肇庆市的淡水水螅1新种,多形水螅Hydra polymorphus sp.nov.,研究标本均来自1只水螅的单系繁殖群体,测量数据经生物统计学处理。新种的钩刺丝囊外形及内部刺丝盘旋均有多种形态,故新种名以此命名。所有研究标本保存于深圳大学生命科学学院。     

Value of the Hydra model system for studying symbiosis

Green Hydra is used as a classical example for explaining symbiosis in schools as well as an excellent research model. Indeed the cosmopolitan green Hydra (Hydra viridissima) provides a potent experimental framework to investigate the symbiotic relationships between a complex eumetazoan organism and a unicellular photoautotrophic green algae named Chlorella. Chlorella populates a single somatic cell type, the gastrodermal myoepithelial cells (also named digestive cells) and the oocyte at the time of sexual reproduction. This symbiotic relationship is stable, well-determined and provides biological advantages to the algal symbionts, but also to green Hydra over the related non-symbiotic Hydra i.e. brown hydra. These advantages likely result from the bidirectional flow of metabolites between the host and the symbiont. Moreover genetic flow through horizontal gene transfer might also participate in the establishment of these selective advantages. However, these relationships between the host and the symbionts may be more complex. Thus, Jolley and Smith showed that the reproductive rate of the algae increases dramatically outside of Hydra cells, although this endosymbiont isolation is debated. Recently it became possible to keep different species of endosymbionts isolated from green Hydra in stable and permanent cultures and compare them to free-living Chlorella species. Future studies testing metabolic relationships and genetic flow should help elucidate the mechanisms that support the maintenance of symbiosis in a eumetazoan species.     

Prototypes of Hyalomma scupense Schulze, 1918 and H. detritum Schulze, 1919 (Acari: Ixodidae) in connection with microevolution within the genus

Type series of two species of the genus Hyalomma Koch, 1844 deposited in the Museum of Zoology in Berlin have been examined. Examination of the H. detritum Schulze, 1919 holotype has shown that this name is actually a junior synonym (syn. nov.) of H. marginatum turanicum Pomerantsev, 1946. At the same time it was also found out that the paratypes of H. detritum are conspecific to the syntypes of H. scupense Schulze, 1818, among which the lectotype has been designated. Taxonomic errors of Schulze and other authors, which had led to a worldwide use in acorological literature the name H. detritum instead of the valid name H. scupense, are discussed. It is suggested that the reasons of microevolution within the polymorphic species H. scupense could be explained by unequal climatic conditions. Microevolutionary process in this species is most well expressed in a tendency to reduce the number of contacts with a host during the life cycle. It has resulted in the reformation of the two-host cycle into the one-host cycle.     

Hydra, a model system for environmental studies

Hydra have been extensively used for studying the teratogenic and toxic potential of numerous toxins throughout the years and are more recently growing in popularity to assess the impacts of environmental pollutants. Hydra are an appropriate bioindicator species for use in environmental assessment owing to their easily measurable physical (morphology), biochemical (xenobiotic biotransformation; oxidative stress), behavioural (feeding) and reproductive (sexual and asexual) endpoints. Hydra also possess an unparalleled ability to regenerate, allowing the assessment of teratogenic compounds and the impact of contaminants on stem cells. Importantly, Hydra are ubiquitous throughout freshwater environments and relatively easy to culture making them appropriate for use in small scale bioassay systems. Hydra have been used to assess the environmental impacts of numerous environmental pollutants including metals, organic toxicants (including pharmaceuticals and endocrine disrupting compounds), nanomaterials and industrial and municipal effluents. They have been found to be among the most sensitive animals tested for metals and certain effluents, comparing favourably with more standardised toxicity tests. Despite their lack of use in formalised monitoring programmes, Hydra have been extensively used and are regarded as a model organism in aquatic toxicology.     

Components, structure, biogenesis and function of the Hydra extracellular matrix in regeneration, pattern formation and cell differentiation

The body wall of Hydra is organized as an epithelial bilayer (ectoderm and endoderm) with an intervening extracellular matrix (ECM), termed mesoglea by early biologists. Morphological studies have determined that Hydra ECM is composed of two basal lamina layers positioned at the base of each epithelial layer with an intervening interstitial matrix. Molecular and biochemical analyses of Hydra ECM have established that it contains components similar to those seen in more complicated vertebrate species. These components include such macromolecules as laminin, type IV collagen, and various fibrillar collagens. These components are synthesized in a complicated manner involving cross-talk between the epithelial bilayer. Any perturbation to ECM biogenesis leads to a blockage in Hydra morphogenesis. Blockage in ECM/cell interactions in the adult polyp also leads to problems in epithelial transdifferentiation processes. In terms of biophysical parameters, Hydra ECM is highly flexible; a property that facilitates continuous movements along the organism's longitudinal and radial axis. This is in contrast to the more rigid matrices often found in vertebrates. The flexible nature of Hydra ECM can in part now be explained by the unique structure of the organism's type IV collagen and fibrillar collagens. This review will focus on Hydra ECM in regard to: 1) its general structure, 2) its molecular composition, 3) the biophysical basis for the flexible nature of Hydra's ECM, 4) the relationship of the biogenesis of Hydra ECM to regeneration of body form, and 5) the functional role of Hydra ECM during pattern formation and cell differentiation.     

The ichnogenous<Emphasis Type="Italic">Palaxius</Emphasis> (crustacean coprolite) and description of<Emphasis Type="Italic">P. hydranensis</Emphasis> n. sp. from the Upper Triassic (Norian part of “Pantokrator” limestone) of Hydra (Greece)

The fossil coprolite ichnogenusPalaxius is known with 22 species from the Permian up to the Tertiary. The diagnostic characteristics and the stratigraphie range of all species is given in this paper. The ichnogenusAgantaxia Kristan-Tollmann is a younger synonym of the ichnogenusPalaxius. The new crustacean coprolitePalaxius hydranensis n. sp. is described from the reef limestones of Norian part of “Pantokrator” limestone of the Island Hydra (Greece). This is the first report of coprolites from Hydra. The organism association of coprolite bearing limestone is mentioned.      

GFP expression in Hydra: lessons from the particle gun

The cnidarian Hydra is an important model organism to study pattern formation and tem cell differentiation. In the past, however, it has been difficult to study gene function in Hydra because the animals have hot been accessible to gene transfection studies, we have now developed a method to transiently express GFP-tagged proteins in Hydra using a green fluorescent protein (GFP) expression plasmid under the control of the Hydra actin promoter and a particle gun to introduce it into Hydra cell nuclei. We achieve strong transient GFP expression in a small but reproducible number of epithelial and interstitial cells. Implications for the use of this method to carry out single cell assays with GFP-tagged Hydra proteins are discussed.     

Germline stem cells and sex determination in Hydra

The sex of germline stem cells (GSCs) in Hydra is determined in a cell-autonomous manner. In gonochoristic species like Hydra magnipapillata or H. oligactis, where the sexes are separate, male polyps have sperm-restricted stem cells (SpSCs), while females have egg-restricted stem cells (EgSCs). These GSCs self-renew in a polyp, and are usually transmitted to a new bud from a parental polyp during asexual reproduction. But if these GSCs are lost during subsequent budding or regeneration events, new ones are generated from multipotent stem cells (MPSCs). MPSCs are the somatic stem cells in Hydra that ordinarily differentiate into nerve cells, nematocytes (stinging cells in cnidarians), and gland cells. By means of such a backup system, sexual reproduction is guaranteed for every polyp. Interestingly, Hydra polyps occasionally undergo sex-reversal. This implies that each polyp can produce either type of GSCs, i.e. Hydra are genetically hermaphroditic. Nevertheless a polyp possesses only one type of GSCs at a time. We propose a plausible model for sex-reversal in Hydra. We also discuss so-called germline specific genes, which are expressed in both GSCs and MPSCs, and some future plans to investigate Hydra GSCs.     

Polyps, peptides and patterning

Peptides serve as important signalling molecules in development and differentiation in the simple metazoan Hydra. A systematic approach (The Hydra Peptide Project) has revealed that Hydra contains several hundreds of peptide signalling molecules, some of which are neuropeptides and others emanate from epithelial cells. These peptides control biological processes as diverse as muscle contraction, neuron differentiation, and the positional value gradient. Signal peptides cause changes in cell behaviour by controlling target genes such as matrix metalloproteases. The abundance of peptides in Hydra raises the question of whether, in early metazoan evolution, cell-cell communication was based mainly on these small molecules rather than on the growth-factor-like cytokines that control differentiation and development in higher animals.     

A new case of neuropeptide coexpression (RGamide and LWamides) in Hydra,found by whole-mount,two-color double-labeling in situ hybridization

The freshwater polyp Hydra has a primitive nervous system that expresses at least six different neuropeptide genes: (1) three genes, coding for the preprohormones-A, -B, and -C that each gives rise to a variety of peptides with the C-terminal sequence Arg-Phe-NH(2) (the Hydra-RFamides); (2) one gene, coding for a preprohormone that gives rise to five peptides with the C-terminal sequence Leu-Trp-NH(2) (the Hydra-LWamides); (3) one gene, coding for a preprohormone that produces a peptide with the C-terminal sequence Lys-Val-NH(2) (Hydra-KVamide, also called Hym-176); and (4) one gene, coding for a preprohormone that gives rise to a peptide with the C-terminal sequence Arg-Gly NH(2) (Hydra-RGamide, also called Hym-355). In a previous paper, we described that a population of neurons in the peduncle (a region just above the foot) of Hydra coexpresses the preprohormone-A and KVamide genes, whereas neurons in the other regions only express either the preprohormone-A, -B, -C, LWamide, or the KVamide genes. Here, we investigated the RGamide gene expression, using whole-mount, two-color double-labeling in situ hybridization, and found that neurons in the basal disk (foot), gastric region, hypostome (a region around the mouth), and tentacles coexpress this gene together with the LWamide gene. A small population of neurons in the hypostome and upper gastric region expresses only the LWamide gene. No coexpression of the RGamide gene with any of the other neuropeptide genes was observed. This is the second example of coexpression of two neuropeptide genes in cnidarians. It demonstrates that many neurons in the primitive nervous systems of cnidarians use combinations ("cocktails") of neuropeptides for their signaling. It also shows that Hydra has at least seven neurochemically different populations of neurons.     

Taxonomic revision of the Mediterranean Plakina Schulze (Porifera, Demospongiae, Homoscleromorpha)

Only three species of the sponge genus Plakina Schulze have been described from the Mediterranean since 1880, in spite of a large amount of allegedly intraspecific variability in morphological characters. However, recent genetic studies based on electrophoretic techniques have revealed extensive cryptic speciation in north-western Mediterranean Plakina , demonstrating that most of this variation was interspecific rather than intraspecific. We describe in detail the morphology and anatomy of four new species of Plakina from the Mediterranean– P. crypta, P. weinbergi, P. endoumensis and P. jani –of which the latter two were discovered through allozyme electrophoresis. Plakina monolopha Schulze and P. trilopha Schulze are redescribed, and their morphological and geographical limits are discussed along with those of P. dilopha Schulze. Accurate analysis of the internal anatomy and of the shape and ramification pattern of lophose spicules in scanning electron microscopy provides new, powerful morphological criteria for species discrimination in Plakina. More widespread use of such new taxonomic characters should provide evidence against the alleged cosmopolitanism of some Plakina species, thus generating an increase in estimates of the biodiversity of plakinids.