There is neither an excretory system nor organs, and nitrogenous wastes simply diffuse from the cells into the water outside the animal or into the gastrovascular cavity. The phylum Cnidaria contains about 10, described species divided into two monophyletic clades: the Anthozoa and the Medusozoa. The Anthozoa include the corals, sea fans, sea whips, and the sea anemones.
The Medusozoa include several classes of Cnidaria in two clades: The Hydrozoa include sessile forms, some medusoid forms, and swimming colonial forms like the Portuguese man-of-war. The other clade contains various types of jellies including both Scyphozoa and Cubozoa. The Anthozoa contain only sessile polyp forms, while the Medusozoa include species with both polyp and medusa forms in their life cycle.
Sea anemones are usually brightly colored and can attain a size of 1. Individual animals are cylindrical in shape and are attached directly to a substrate. The mouth of a sea anemone is surrounded by tentacles that bear cnidocytes. The slit-like mouth opening and flattened pharynx are lined with ectoderm.
This structure of the pharynx makes anemones bilaterally symmetrical. A ciliated groove called a siphonoglyph is found on two opposite sides of the pharynx and directs water into it. The pharynx is the muscular part of the digestive system that serves to ingest as well as egest food, and may extend for up to two-thirds the length of the body before opening into the gastrovascular cavity.
This cavity is divided into several chambers by longitudinal septa called mesenteries. Each mesentery consists of a fold of gastrodermal tissue with a layer of mesoglea between the sheets of gastrodermis. Mesenteries do not divide the gastrovascular cavity completely, and the smaller cavities coalesce at the pharyngeal opening. The adaptive benefit of the mesenteries appears to be an increase in surface area for absorption of nutrients and gas exchange, as well as additional mechanical support for the body of the anemone.
Sea anemones feed on small fish and shrimp, usually by immobilizing their prey with nematocysts. Some sea anemones establish a mutualistic relationship with hermit crabs when the crab seizes and attaches them to their shell. In this relationship, the anemone gets food particles from prey caught by the crab, and the crab is protected from the predators by the stinging cells of the anemone.
Some species of anemone fish, or clownfish, are also able to live with sea anemones because they build up an acquired immunity to the toxins contained within the nematocysts and also secrete a protective mucus that prevents them from being stung.
The structure of coral polyps is similar to that of anemones, although the individual polyps are usually smaller and part of a colony, some of which are massive and the size of small buildings. Coral polyps feed on smaller planktonic organisms, including algae, bacteria, and invertebrate larvae. Some anthozoans have symbiotic associations with dinoflagellate algae called zooxanthellae. The mutually beneficial relationship between zooxanthellae and modern corals—which provides the algae with shelter—gives coral reefs their colors and supplies both organisms with nutrients.
This complex mutualistic association began more than million years ago, according to a new study by an international team of scientists. That this symbiotic relationship arose during a time of massive worldwide coral-reef expansion suggests that the interconnection of algae and coral is crucial for the health of coral reefs, which provide habitat for roughly one-fourth of all marine life.
Reefs are threatened by a trend in ocean warming that has caused corals to expel their zooxanthellae algae and turn white, a process called coral bleaching. Male or female gametes produced by a polyp fuse to give rise to a free-swimming planula larva. The larva settles on a suitable substratum and develops into a sessile polyp. The medusa is the prominent stage in the life cycle, although there is a polyp stage in the life cycle of most species.
Most jellies range from 2 to 40 cm in length but the largest scyphozoan species, Cyanea capillata , can reach a size of two meters in diameter.
Scyphozoans display a characteristic bell-like morphology Figure. In the sea jelly, a mouth opening is present on the underside of the animal, surrounded by hollow tentacles bearing nematocysts.
Scyphozoans live most of their life cycle as free-swimming, solitary carnivores. The mouth leads to the gastrovascular cavity, which may be sectioned into four interconnected sacs, called diverticuli.
In some species, the digestive system may branch further into radial canals. Like the septa in anthozoans, the branched gastrovascular cells serve two functions: to increase the surface area for nutrient absorption and diffusion, and to support the body of the animal.
In scyphozoans, nerve cells are organized in a nerve net that extends over the entire body, with a nerve ring around the edge of the bell. Clusters of sensory organs called rhopalia may be present in pockets in the edge of the bell.
Jellies have a ring of muscles lining the dome of the body, which provides the contractile force required to swim through water, as well as to draw in food from the water as they swim.
Neurons may even be present in clusters called rhopalia. These animals possess a ring of muscles lining the dome of the body, which provides the contractile force required to swim through water. Scyphozoans are dioecious animals, having separate sexes. The gonads are formed from the gastrodermis with gametes expelled through the mouth. Planula larvae are formed by external fertilization; they settle on a substratum in a polypoid form known as scyphistoma.
These forms may produce additional polyps by budding or may transform into the medusoid form. The life cycle of these animals can be described as polymorphic because they exhibit both a medusal and polypoid body plan at some point. Lifecycle of a jellyfish : The lifecycle of a jellyfish includes two stages: the medusa stage and the polyp stage.
The polyp reproduces asexually by budding,while the medusa reproduces sexually. Cubozoans live as box-shaped medusae while Hydrozoans are true polymorphs and can be found as colonial or solitary organisms. Cubozoans display overall morphological and anatomical characteristics that are similar to those of the scyphozoans. A prominent difference between the two classes is the arrangement of tentacles.
This is the most venomous group of all the cnidarians. Cubozoans : The a tiny cubazoan jelly Malo kingi is thimble shaped and, like all cubozoan jellies, b has four muscular pedalia to which the tentacles attach. Two people in Australia, where Irukandji jellies are most-commonly found, are believed to have died from Irukandji stings.
The cubozoans contain muscular pads called pedalia at the corners of the square bell canopy, with one or more tentacles attached to each pedalium. These animals are further classified into orders based on the presence of single or multiple tentacles per pedalium. In some cases, the digestive system may extend into the pedalia.
Nematocysts may be arranged in a spiral configuration along the tentacles; this arrangement helps to effectively subdue and capture prey. Cubozoans exist in a polypoid form that develops from a planula larva. These polyps show limited mobility along the substratum. As with scyphozoans, they may bud to form more polyps to colonize a habitat.
Polyp forms then transform into the medusoid forms. Hydrozoa includes nearly 3, species; most are marine, although some freshwater species are known. Animals in this class are polymorphs: most exhibit both polypoid and medusoid forms in their lifecycle, although this is variable. The polyp form in these animals often shows a cylindrical morphology with a central gastrovascular cavity lined by the gastrodermis.
The gastrodermis and epidermis have a simple layer of mesoglea sandwiched between them. A mouth opening, surrounded by tentacles, is present at the oral end of the animal. Many hydrozoans form colonies that are composed of a branched colony of specialized polyps that share a gastrovascular cavity, such as in the colonial hydroid Obelia.
Other species are solitary polyps Hydra or solitary medusae Gonionemus. The true characteristic shared by all these diverse species is that their gonads for sexual reproduction are derived from epidermal tissue, whereas in all other cnidarians they are derived from gastrodermal tissue.
Privacy Policy. Skip to main content. Search for:. Phylum Cnidaria. Phylum Cnidaria Cnidarians are diploblastic, have organized tissue, undergo extracellular digestion, and use cnidocytes for protection and to capture prey. A merger of these three images is also shown.
To characterize maturation stage-specific expression profiles of cnidocytes, we employed two different comparisons: i positives vs negatives and ii super-positives vs negatives. The principal component analysis revealed distinct gene expression profiles of the biological conditions positives or super-positives vs their respective negatives and uniform expression of biological replicates within the respective condition Fig.
Differential expression analyses by two different methods in concert DESeq2 and edgeR enabled the stringent identification of a large number of differentially expressed genes in positive and super-positive cell populations, in comparison to the negative cells Additional file 3 : Figure S3.
Furthermore, certain genes that are known to lack expression in cnidocytes [ 20 , 21 , 22 ], such as the neuronal markers FMRFamide and ELAV —the latter only identified by DESeq2—were found to be downregulated in both the positive and super-positive cells, in comparison to the negative cells Additional file 4 : Figure S4; Additional file 5 : Figure S5.
These results demonstrate the robustness of our experimental approach in isolating cnidocytes and accurately characterizing their transcriptional profiles.
However, failing to identify cnidocyte markers as differentially expressed in the super-positive cells, despite their upregulation in positive cells, was surprising Additional file 4 : Figure S4; Additional file 5 : Figure S5.
Moreover, the transcripts encoding memOrange2 and NvNcol-3 were not identified as significantly upregulated in both positive and super-positive cnidocytes. Differentially expressed genes within cnidocytes. Panels a and b show the clustering of biological replicates memOrange2 negative: Neg1—3 and Neg5—7, positive: Pos1—3, and super-positive cell populations: Pos5—8 in a principal component analysis, where the axes represent the first two principal components, labeled with the percentages of variance associated with each axis.
Panels c and d show MA-plots, which represent the log ratio of differential expression as a function of the mean intensity for each feature. The total number of upregulated and downregulated genes, highlighted in the plot as blue and red circles, respectively, is also indicated. Panel e depicts a Venn diagram, showing a comparison of differentially expressed genes identified in the positive and super-positive populations.
The total number of genes uniquely identified as either upregulated or downregulated in positive and super-positive cells can be attributed to early-stage and mature cnidocytes, respectively.
Enriched and depleted annotation features of each of these cell populations are also indicated. A total of and differentially expressed genes were identified in common by DESeq2 and edgeR in positive and super-positive cell populations, respectively Fig.
Overall, and genes were commonly identified as upregulated or downregulated, respectively, in the positive and super-positive cell populations, in comparison to the negative cells Fig.
Interestingly, and genes were identified as significantly upregulated and downregulated, respectively, in the positive cell population alone. These differentially expressed genes, identified only in the positive cells, represent genes that are either upregulated or downregulated during the earlier stages of cnidocyte maturation but not in the mature cnidocyte-rich super-positive population.
Similarly, we identified and genes as uniquely upregulated and downregulated, respectively, in the mature cnidocytes of the super-positive population but not in the earlier stages of maturation Fig. The absence of significant enrichment of NvNcol-3 and memOrange2 transcripts in both the positive and super-positive cell populations was surprising.
We suspected that this might be connected to the maturation stage of the isolated cnidocyte and that, perhaps, the temporal differences in expression levels of memOrange2 mRNA and protein may result in an inability to capture very early-stage cnidocytes.
To test this hypothesis, we performed an ISH experiment, followed by immunostaining. These combined assays enabled us to distinguish between NvNcol-3 mRNA and protein expression in the 3-day-old wild-type planulae Fig.
Variability between the transcriptional and proteomic expression levels of cnidocyte-expressed genes. This figure shows that the expression patterns of NvNcol-3 and memOrange2 transcripts stained in FastRed panels a and d differ from the expression pattern of their protein products stained with Alexa fluor panels b and e. Examples for cells containing only RNA, only protein or both RNA and protein are indicated by magenta, gray and orange arrowheads, respectively panels c and f.
Quantifications of the cells positive for only RNA, only protein, or both RNA and protein each from seven animals are shown as column graphs panels g and h. This experiment revealed spatiotemporal differences in the mRNA and protein expression patterns of both these genes within cnidocytes. In both the wild-type and NvNcol-3 transgenic planulae, a significant proportion of cnidocytes exclusively stained either at the mRNA level or at the protein level Fig.
In seven wild-type planulae, we observed cells that were positive only to NvNcol-3 RNA, 44 cells that were positive only for the antibody, and cells that were positive for both Fig. In seven transgenic planulae, we detected 62 cells that were positive only to memOrange2 RNA, 58 cells that were positive only for the antibody, and cells that were positive for both Fig.
The cells that only expressed mRNA lacked capsules or contained undeveloped capsules and multiple vesicles and are probably cnidoblasts or early-stage cnidocytes Fig. Only a subset of the cnidocytes, most of which were characterized by relatively small and immature capsules—typical of developing cnidocytes—exhibited expression at both mRNA and protein levels Fig. These findings are in agreement with the recently reported co-localization experiments of NvNcol-3 [ 23 ].
One of the major objectives of this study was to identify novel genes with cnidocyte-specific expression, as this would advance our knowledge regarding the cnidocyte biology, their genetic basis of development, and evolutionary origin. First, to test the accuracy of our experimental and bioinformatic approaches for identifying differentially expressed genes in cnidocytes, we chose ten genes that were found in our current analyses to be significantly upregulated in positive cells log 2 fold change in the range of 2.
Interestingly, nine of these ten genes were not previously described in the cnidocytes of Nematostella. The localization of expression using ISH highlighted the cnidocyte-specific expression of these ten genes Fig. Unexpectedly, the expression pattern of individual genes exhibited a distinct spatiotemporal variability Fig. For example, the expression of the NOWA-like gene was limited to very few cnidocytes in the oral region of the planula larva and the tentacles of the primary polyp, while the expression of Cnido-Jun was localized to both the oral and the central part of the planula, followed by an increased expression in the tentacles of the primary polyp Fig.
The three protein disulfide isomerases also showed noticeably distinct expression patterns, strongly indicating functional specialization of these enzymes Fig. To validate the cnidocyte-specific expression of these novel genes, we further conducted a double ISH experiment using NvNcol-3 as a cnidocyte marker.
An overlapping expression was observed, demonstrating that these novel genes, indeed, exhibit cnidocyte-specific expression profiles Additional file 7 : Figure S6. Novel genes with cnidocyte-specific expression. In situ hybridization expression patterns of novel cnidocyte-specific genes identified in this study are shown in late planula and primary polyp.
The elongated cells in the ectoderm, seen in the closeup image, can clearly be identified as cnidocytes according to the large unstained space, which is the cnidocyst capsule.
Heatmaps of expression levels of each gene in positive P and super-positive SP cell populations, each across three technical replicates positives and super-positive indicated as Pos1—3 and negatives as Neg1—3 are also provided.
A color code for expression values, ranging from a gradient of red downregulated to blue upregulated , is depicted at the bottom. Further, gene enrichment analyses enabled the identification of functional categories that are significantly enriched or depleted in positive and super-positive cell populations, providing novel insights into the biochemical pathways of these enigmatic cells Fig. The enrichment of the terms related to the extracellular matrix Fig. By retrieving homologs from the genomes and transcriptomes of a diversity of cnidarian species, we reconstructed the phylogenetic histories of two transcription factors that were found to be specifically upregulated in cnidocytes NVE and NVE ; Fig.
We named these transcription factors Cnido-Jun 4 and 1. These transcription factors are known to dimerize into an activation protein-1 AP-1 complex, which is involved in various stress responses in Bilateria and Cnidaria [ 28 , 29 , 30 ]. We show that Cnido-Jun and Cnido-Fos1 protein-coding genes originated via gene duplication in the common ancestor of Hexacorallia—sea anemones and stony corals Fig.
However, only in sea anemones, the c-Fos gene underwent an additional round of duplication and led to the origination of Cnido-Fos2 NVE but was not identified as differentially expressed in a significant manner.
Domain scanning of these transcription factors revealed cnidarian-specific insertions between the Jun and bZIP domains of the c-Jun proteins—the latter is required for DNA binding Fig. In a complete contrast, we found insertions in bilaterian c-Fos proteins that were missing in their cnidarian counterparts. Interestingly, Cnido-Fos1 was the only protein that had different residues than those implicated in the heterodimer formation Fig.
The deep origin of novel cnidocyte-specific transcription factors in Cnidaria. This figure depicts the phylogenetic histories of a the c-Jun and b c-Fos family of proteins in Cnidaria. Duplication events leading to the origin of Cnido-Jun and Cnido-Fos1—the two cnidocyte-specific transcription factors identified in this study labeled in red font —and c-Jun and c-Fos indicated in green font in Hexacorallia are shown in red circles.
Domain organization of these proteins are also depicted, where the Jun-like domain, the bZIP domains, and the c-Fos-related domains are indicated in blue, green, and purple, respectively.
The DNA binding domain is marked by black arrows, while the dimerization domains are depicted in red bars. To understand whether Cnido-Jun, which exhibits cnidocyte-specific expression, plays a significant role in nematogenesis, we manipulated its expression in Nematostella embryos. To verify the specificity of the effect, we repeated these experiments with a second non-overlapping MO against Cnido-Jun. We counted two different criteria that represent normal cnidogenesis in Nematostella : i presence of high NvNCol-3 expression in a ring-shaped domain around the oral pole of the planula.
In this count, only one of 58 animals in the Cnido-Jun MO-injected group exhibited such proportion, whereas 47 of 74 animals exhibited this proportion in the control MO-injected group.
For the second non-overlapping MO, the observed morphology and percentage of affected animals counted by the same indices were highly similar, providing strong support for the specificity of the assay Fig.
Knockdown of Cnido-Jun results in decreased NvNcol-3 expression. The AUG translation start site is underlined. Panels d and e present the effects of a second non-overlapping MO against Cnido-Jun and the control MO that was injected in parallel.
The numbers of embryos exhibiting a normal NvNcol-3 phenotype out of each injected group based on the two indexes described in the text appear below each merged picture. Among the large number of upregulated genes detected within the positive cell population Fig. Though there was consistency with regard to the downregulation of FMRFamide and ELAV , we did not identify the upregulation of the aforementioned cnidocyte marker genes in the super-positive cell population Additional file 5 : Figure S5 , which was enriched with mature cnidocytes.
This can be explained by the fact that in mature cnidocytes, the capsule, which is a very tight polymer of various peptides and proteoglycans, is already formed and the secretion of structural proteins is no longer required and is most probably wasteful.
As a result, the expression of such genes diminishes with the maturation of cnidocytes. This is clearly evident when examining the expression of memOrange2 across various cell types. In our reporter line, memOrange2 was integrated into the genomic locus of NvNcol-3 —the gene coding a vital structural protein of the capsule Fig. This clearly demonstrates that the expression of structural proteins drops significantly with the maturation of capsules in the fully developed cnidocytes.
Since the development of the mature capsule impedes the migration of secreted toxins, a similar pattern of expression can be expected for toxin-coding genes at this stage of development. As explained in the following section, these results can also be attributed to the inherent dynamics of gene expression across developing cnidocytes.
It should be noted that because the secreted memOrange2 protein requires 4. As a result, despite the increased expression in positive cells in comparison to the negative cells, memOrange2 was not identified as a differentially expressed transcript. However, the robustness of our experimental approach in isolating cnidocytes for generating cell type-specific transcriptomic profiles is strongly supported by multiple lines of evidence: i the significant upregulation of cnidocyte-specific markers ranging between 1.
Moreover, the microscopic examination of the positive cell populations revealed many cnidocytes in a very early stage of maturation Additional file 1 : Figure S1. These cells can clearly be seen to contain undeveloped capsules and multiple secretory vesicles carrying premature cnidocyst components, which indicate early stages of nematogenesis when the capsule starts to form by massive Golgi secretions [ 1 ].
Many were even completely missing capsules but contained only memOrange2-filled vesicles Additional file 1 : Figure S1 , suggesting that our approach can capture cnidocytes in early stages of maturation. However, the transgenic approach might have some limitations in detecting genes expressed very early in cnidocyte development since the reporter fluorescence is by nature temporally lagging behind the expression of these genes i.
We speculate that this is the reason why some genes previously reported to be involved in early nematogenesis such as PaxA and Mef2 were not recovered in our analysis [ 23 , 35 ]. The dramatic difference we discovered in the expression profiles of certain genes in the positive and super-positive cell populations can be further attributed to the inherent dynamics of transcription and translation across various stages of cnidocyte maturation. This was revealed by a combination of ISH and immunostaining experiments, where we co-localized the transcripts and proteins encoded by memOrange2 or NvNcol-3 genes and is in agreement with previous results [ 23 ].
With this approach, we detected three different populations of cnidocytes: i cnidocytes in the very early stages of maturation that only contained abundant transcripts but no proteins, suggesting that they were yet to develop a capsule; ii cnidocytes with high levels of both transcripts and proteins, suggesting that these were developing cells but contained an immature capsule; and iii mature cnidocytes with fully developed capsules that only contained proteins and completely lacked transcripts for memOrange2 and NvNcol-3 genes Fig.
This supports our hypothesis that transcription of toxin and structural protein-coding genes drastically falls in mature cnidocytes with the complete formation of the tightly polymerized capsule.
Thus, we reveal spatiotemporal dynamics of transcription and translation of certain genes within cnidocytes Fig. Genetic Modification 4: Ecology 1. Energy Flow 3. Carbon Cycling 4. Climate Change 5: Evolution 1. Evolution Evidence 2. Natural Selection 3. Classification 4. Cladistics 6: Human Physiology 1. Digestion 2. The Blood System 3. Disease Defences 4. Gas Exchange 5.
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