Scyphozoa

14,000,000 Leading Edge Experts on the ideXlab platform

Scan Science and Technology

Contact Leading Edge Experts & Companies

Scan Science and Technology

Contact Leading Edge Experts & Companies

The Experts below are selected from a list of 306 Experts worldwide ranked by ideXlab platform

Gerhard Jarms - One of the best experts on this subject based on the ideXlab platform.

Michael N Dawson - One of the best experts on this subject based on the ideXlab platform.

  • Review of the diversity, traits, and ecology of zooxanthellate jellyfishes
    Marine Biology, 2019
    Co-Authors: Nicolas Djeghri, Philippe Pondaven, Herwig Stibor, Michael N Dawson
    Abstract:

    Many marine organisms form photosymbioses with zooxanthellae, but some, such as the medusozoans, are less well known. Here, we summarize the current knowledge on the diversity of zooxanthellate jellyfishes, to identify key traits of the holobionts, and to examine the impact of these traits on their ecology. Photosymbiosis with zooxanthellae originated at least seven times independently in Medusozoa; of these, five involve taxa with medusae. While most zooxanthellate jellyfishes are found in clades containing mainly non-zooxanthellate members, the sub-order Kolpophorae (Scyphozoa: Rhizostomeae) is comprised—bar a few intriguing exceptions—of only zooxanthellate jellyfishes. We estimate that 20–25% of Scyphozoa species are zooxanthellate (facultative symbiotic species included). Zooxanthellae play a key role in Scyphozoan life-cycle and nutrition although substantial variation is observed during ontogeny, or at the intra- and inter-specific levels. Nonetheless, three key traits of zooxanthellate jellyfishes can be identified: (1) zooxanthellate medusae, as holobionts, are generally mixotrophic, deriving their nutrition both from predation and photosynthesis; (2) zooxanthellate polyps, although capable of hosting zooxanthellae rarely depend on them; and (3) zooxanthellae play a key role in the life-cycle of the jellyfish by allowing or facilitating strobilation. We discuss how these traits might help to explain some aspects of the ecology of zooxanthellate jellyfishes—notably their generally low ability to outbreak, and their reaction to temperature stress or to eutrophication—and how they could in turn impact marine ecosystem functioning.

  • Species richness of jellyfishes (Scyphozoa : Discomedusae) in the Tropical Eastern Pacific: missed taxa, molecules, and morphology match in a biodiversity hotspot
    Invertebrate Systematics, 2017
    Co-Authors: Liza Gómez Daglio, Michael N Dawson
    Abstract:

    Species richness in the seas has been underestimated due to the combined challenges presented by the taxonomic impediment, delimitation of species, preponderance of cryptic species, and uneven sampling effort. The mismatch between actual and estimated diversity varies by region and by taxon, leaving open questions such as: are hotspots for well-known taxa also hotspots for poorly known taxa? We address these challenges and this question for shallow-water Scyphozoan jellyfishes in the Tropical Eastern Pacific (TEP). We increased sampling effort at 34 coastal locations along the TEP, and combined analyses of four molecular markers and up to 53 morphological characters. We applied phylogenetic analyses under Bayesian and maximum likelihood frameworks, barcoding, and statistical multivariate analyses of morphological data to estimate species richness. Where only five Discomedusae were reported previously, we found a total of 25 species. Of these, 22 species are new to science, two are non-indigenous, and one is a previous record; the other four prior records had been misidentified. The new discoveries evince the need to evaluate the evolutionary relationships with neighbouring regions to understand fully the origins of jellyfish diversity in the TEP and will lead to revision of the systematics and taxonomy of Scyphozoa.

  • SYMPOSIUM Evolutionary Relationships Among Scyphozoan Jellyfish Families Based on Complete Taxon Sampling and Phylogenetic Analyses of 18S and 28S Ribosomal DNA
    2016
    Co-Authors: Keith M. Bayha, Michael N Dawson, Allen G. Collins, Marcos S. Barbeitos, Steven H. D. Haddock
    Abstract:

    Synopsis A stable phylogenetic hypothesis for families within jellyfish class Scyphozoa has been elusive. Reasons for the lack of resolution of Scyphozoan familial relationships include a dearth of morphological characters that reliably distin-guish taxa and incomplete taxonomic sampling in molecular studies. Here, we address the latter issue by using maximum likelihood and Bayesian methods to reconstruct the phylogenetic relationships among all 19 currently valid Scyphozoan families, using sequence data from two nuclear genes: 18S and 28S rDNA. Consistent with prior morphological hypoth-eses, we find strong evidence for monophyly of subclass Discomedusae, order Coronatae, rhizostome suborder Kolpophorae and superfamilies Actinomyariae, Kampylomyariae, Krikomyariae, and Scapulatae. Eleven of the 19 cur-rently recognized Scyphozoan families are robustly monophyletic, and we suggest recognition of two new families pending further analyses. In contrast to long-standing morphological hypotheses, the phylogeny shows coronate family Nausithoidae, semaeostome family Cyaneidae, and rhizostome suborder Daktyliophorae to be nonmonophyletic. Our analyses neither strongly support nor strongly refute monophyly of order Rhizostomeae, superfamily Inscapulatae, an

  • A review and synthesis on the systematics and evolution of jellyfish blooms: advantageous aggregations and adaptive assemblages
    Hydrobiologia, 2009
    Co-Authors: William M. Hamner, Michael N Dawson
    Abstract:

    Pelagic gelatinous invertebrates in many diverse phyla aggregate, bloom, or swarm. Although typically portrayed as annoying to humans, such accumulations probably are evolutionary adaptations to the environments of pelagic gelatinous zooplankton. We explore this proposition by systematic analysis completed in three steps. First, using the current morphological taxonomic framework for Scyphozoa, we summarize relevant information on species that aggregate, bloom, and swarm and on those species that do not. Second, we establish a molecular phylogenetic framework for assessing evolutionary relationships among classes and many orders of Medusozoa and among most families of Scyphozoa (particularly Discomedusae). Third, we interpret the phylogenetic distribution of taxa and of characteristics of jellyfish that aggregate, bloom, or swarm, in terms of species diversity—a proxy for evolutionary success. We found that: (1) Medusae that occur en masse are not randomly distributed within the Phylum Cnidaria but instead they are found primarily within the Scyphozoa which have a metagenic life history. (2) Midwater and deep-sea medusae rarely bloom or swarm. (3) Epibenthic medusae do not swarm. (4) Large carnivores that feed on large prey do not bloom strongly. (5) Large medusae that feed exclusively on small prey both bloom and swarm. (6) Pelagia , the only holoplanktonic, epipelagic scyphomedusan, both blooms and swarms, demonstrating that a metagenic life cycle is not required for blooming or swarming at sea. (7) Environmental change (overfishing, species introductions, and eutrophication) may induce or inhibit blooms. (8) Taxa that bloom or swarm are often more diverse than taxa that do not. (9) Speciation in Scyphozoans can occur rapidly. (10) Morphological stasis in holozooplankton masks genetic variability. (11) Selection for convergent evolution in the sea is strong because mass occurrence has evolved multiple times in independent evolutionary lineages under similar circumstances. Thus, attributes possessed by many taxa that occur en masse appear to be evolutionarily advantageous, i.e., adaptations.

  • Some implications of molecular phylogenetics for understanding biodiversity in jellyfishes, with emphasis on Scyphozoa
    Coelenterate Biology 2003, 1
    Co-Authors: Michael N Dawson
    Abstract:

    Statistical phylogenetic analyses of 111 5.8S and partial-28S ribosomal DNA sequences (total aligned length=434 nucleotides) including jellyfishes representing approximately 14% of known Scyphozoan morphospecies (21% genera, 62% families, and 100% orders) are presented. These analyses indicate stauromedusae constitute a fifth cnidarian class (Staurozoa) basal to a monophyletic Medusozoa (= Cubozoa, Hydrozoa, and Scyphozoa). Phylogenetic relationships among the medusozoans are generally poorly resolved, but support is found for reciprocal monophyly of the Cubozoa, Hydrozoa, Coronatae, and Discomedusae (= Semaeostomeae + Rhizostomeae). In addition, a survey of pairwise sequence differences in Internal Transcribed Spacer One within morphospecies indicates that Scyphozoan species diversity may be approximately twice recent estimates based on morphological analyses. These results highlight difficulties with traditional morphological treatments including terminology that obfuscates homologies. By integrating molecular phylogenetic analyses with old and new morphological, behavioural, developmental, physiological, and other data, a much richer understanding of the biodiversity and evolution of jellyfishes is achievable.

Ana Carolina Morandini - One of the best experts on this subject based on the ideXlab platform.

  • jellyfish chrysaora lactea cnidaria semaeostomeae aggregations in southern brazil and consequences for human stings
    Submission article platform - Latin American Journal of Aquatic Research, 2017
    Co-Authors: Antonio C Marques, Vidal Haddad, Lenora Rodrigo, Emanuel Marquesdasilva, Ana Carolina Morandini
    Abstract:

    The frequency of blooming is generating a world-wide discussion about medusae population explosions, mainly those associated with accidents. We report over 20,000 accidents caused by Chrysaora lactea (Scyphozoa) in the State of Parana (Brazil) during the austral summer of 2011-2012. Accidents were considered mild, but almost 600 cases were treated in emergency services, with either toxic and allergic reactions, some with systemic manifestations. We proposed non-exclusive hypotheses to explain this large number of cases.

  • Succession of generations is still the general paradigm for Scyphozoan life cycles
    Bulletin of Marine Science, 2016
    Co-Authors: Ana Carolina Morandini, Agustín Schiariti, Sérgio N. Stampar, Maximiliano M. Maronna, Ilka Straehler-pohl, Antonio C Marques
    Abstract:

    A recent study proposed an unorthodox view of the long-known metagenetic life cycle of Scyphozoan jellyfish. We argue that misinterpretations and imprecise information generated a misleading view of such life cycle patterns. In favor of our reasoning, we present the historical understanding of metagenesis, and contend that it can still be used as a shared general life cycle pattern for Scyphozoa, as well as for other medusozoans.

  • jellyfish chrysaora lactea cnidaria semaeostomeae aggregations in southern brazil and consequences of stings in humans
    Latin American Journal of Aquatic Research, 2014
    Co-Authors: Antonio C Marques, Vidal Haddad, Lenora Rodrigo, Emanuel Marquesdasilva, Ana Carolina Morandini
    Abstract:

    The frequency of jellyfish blooms is generating a world-wide discussion about medusae population explosions, mainly those associated with stings. We report over 20,000 envenomations caused by Chrysaora lactea (Scyphozoa) in the State of Parana (southern Brazil) during the austral summer of 2011-2012. Envenomations were considered mild, but almost 600 cases were treated in emergency services, with either toxic and allergic reactions, some with systemic manifestations. We proposed non-exclusive hypotheses to explain this large number of cases.

  • New records of scyphomedusae from Pakistan coast: Catostylus perezi and Pelagia cf. noctiluca (Cnidaria: Scyphozoa)
    Marine Biodiversity Records, 2013
    Co-Authors: Shahnawaz Gul, Ana Carolina Morandini
    Abstract:

    Jellyfish are generally unfavourably known by their negative impact on human activities related to the sea such as fishing, aquaculture and tourism (Purcell et al., 2007). Among the cnidarian jellyfish groups (known as medusozoans), the class Scyphozoa Goette, 1887 comprises about 200 described species occurring in all oceans (Mianzan & Cornelius, 1999; Daly et al., 2007). Scyphozoan jellyfish are important macrozooplankton predators, unique due to their large size and beautiful colour patterns (Cornelius, 1997). They play a significant role in marine food webs, not only predating on several groups (Purcell, 1997) but also serving as food for many marine animals (Arai, 2005). In different countries, some scyphomedusae species are also consumed by humans as a delicacy food (Hsieh et al., 2001; Omori & Nakano, 2001). The group presents venomous species that sometimes even cause severe medical problems to humans (Burnett, 2001). The occurrence of large populations or blooms (Graham et al., 2001; Miranda et al., 2012) in coastal waters can cause disturbance in fishing activities (Purcell et al., 2007; Nagata et al., 2009). For the coast of Pakistan (North Arabian Sea) there are only two reports mentioning Scyphozoan jellyfish species, both rhizostomes (Tahera & Kazmi, 2006; Muhammed & Sultana, 2008): Rhizostoma pulmo (Macri, 1778) and Catostylus mosaicus (Quoy & Gaimard, 1824). Here we report the occurrence of another scyphomedusae for Pakistan, and a discussion concerning previous identification of Catostylus mosaicus.

  • Neritic Jellyfishes (Cnidaria: Cubozoa and Scyphozoa) from the coast of Rio Grande do Norte state, northeast of Brazil
    Check List, 2009
    Co-Authors: Marcelo De Oliveira Soares, Ana Carolina Morandini, Helena Matthews-cascon
    Abstract:

    For the entire Brazilian coast, there are 22 published records of Scyphozoans. On the other hand, only 35 species of cubozoans were described worldwide, four of them reported for the Brazilian coast. However, little is known about the species of cubozoans and Scyphozoans in the Northeastern states of Brazil. The aim of this study was to perform a survey of the jellyfish (Cnidaria: Cubozoa and Scyphozoa) on the coast of Rio Grande do Norte state, Northeast of Brazil. Specimens were collected using trawl net on beaches in the counties of Natal (in 2003) and Tibau (in 2004). For the Rio Grande do Norte coast there were few records of large jellyfish, and new records of the following cubozoan and Scyphozoan species were verified: Chiropsalmus quadrumanus; Chrysaora lactea; Lychnorhiza lucerna and Stomolophus meleagris. The studied species had their distributions expanded in the coast to the State of Rio Grande do Norte.

Henry Tiemann - One of the best experts on this subject based on the ideXlab platform.

  • Organ-like gonads, complex oocyte formation, and long-term spawning in Periphylla periphylla (Cnidaria, Scyphozoa, Coronatae)
    Marine Biology, 2009
    Co-Authors: Henry Tiemann, Gerhard Jarms
    Abstract:

    An account is given about the development of the gametes of the holopelagic coronate scyphomedusa Periphylla periphylla (Peron and Lesueur 1809). The gonads of the species are complex and differ from those thus far described in Scyphozoa in having this characteristic composition of trophocytes, follicle cells, gamete-releasing pores, mucus cells, and resorption cells. Our results differ from those of previous coronate studies with respect to the contact of the oocytes with the gonad tissue throughout the duration of development. Among the Medusozoa, follicle cells have thus far been considered as an apomorphy for the Staurozoa, but their presence in the Scyphozoa casts some doubts on this assumption. From morphological structure and examination of the gonads and gametes, it can be deduced that P. periphylla has true organs and that gamete release is continuous. These results supplement our knowledge of the reproductive biology of this mesopelagic species.

  • Calcium sulfate hemihydrate (bassanite) statoliths in the cubozoan Carybdea sp.
    Zoologischer Anzeiger - A Journal of Comparative Zoology, 2006
    Co-Authors: Henry Tiemann, Gerhard Jarms, Ilka Sötje, Alexander Becker, Matthias Epple
    Abstract:

    Abstract The chemistry and physical structure of statoliths of young cubozoan medusae (Carybdea sp.) were examined by X-ray spectroscopy (EDX) and X-ray powder diffractometry (XRD). These concretions, associated with sensory receptors, were found to consist of bassanite (calcium sulfate hemihydrate), a dense but hygroscopic biomineral. Bassanite occurs in a cluster of radially oriented crystals in a druse, which contains perfect hexagonal crystals. This discovery provides evidence that the Rhopaliophora (Scyphozoa and Cubozoa) originated from an ancestor having statoliths of bassanite.

  • calcium sulfate hemihydrate is the inorganic mineral in statoliths of Scyphozoan medusae cnidaria
    Dalton Transactions, 2005
    Co-Authors: A Becker, Henry Tiemann, Ilka Sötje, C Paulmann, Felix Beckmann, Tilman Donath, Roland Boese, Oleg Prymak, Matthias Epple
    Abstract:

    Scyphomedusae use inorganic crystals (statoliths) for gravity sensing. The organs which contain the statoliths are called rhopalia. Rhopalia of five different species of the three different orders of the class Scyphozoa were studied with high-end solid-state chemical methods to elucidate the crystallographic nature of the biomineral: synchrotron powder diffraction, synchrotron single-crystal diffraction, synchrotron microtomography, scanning electron microscopy, and energy dispersive X-ray spectroscopy. Each rhopalium contains a large number of statoliths in an ordered way. The statoliths of all species consist of calcium sulfate hemihydrate, a water-deficient phase. This is remarkable for sea-living organisms consisting mostly of water. The phylogenetic relationships within the class Scyphozoa are discussed.

  • Calcium sulfate hemihydrate is the inorganic mineral in statoliths of Scyphozoan medusae (Cnidaria).
    Dalton transactions (Cambridge England : 2003), 2005
    Co-Authors: Alexander Becker, Henry Tiemann, Ilka Sötje, C Paulmann, Felix Beckmann, Tilman Donath, Roland Boese, Oleg Prymak, Matthias Epple
    Abstract:

    Scyphomedusae use inorganic crystals (statoliths) for gravity sensing. The organs which contain the statoliths are called rhopalia. Rhopalia of five different species of the three different orders of the class Scyphozoa were studied with high-end solid-state chemical methods to elucidate the crystallographic nature of the biomineral: synchrotron powder diffraction, synchrotron single-crystal diffraction, synchrotron microtomography, scanning electron microscopy, and energy dispersive X-ray spectroscopy. Each rhopalium contains a large number of statoliths in an ordered way. The statoliths of all species consist of calcium sulfate hemihydrate, a water-deficient phase. This is remarkable for sea-living organisms consisting mostly of water. The phylogenetic relationships within the class Scyphozoa are discussed.

  • the holopelagic life cycle of the deep sea medusa periphylla periphylla Scyphozoa coronatae
    Sarsia, 1999
    Co-Authors: Gerhard Jarms, Henry Tiemann, Ulf Bamstedt, Monica B Martinussen, Jan Helge Fossa, Tore Hoisœter
    Abstract:

    Abstract The life cycle of Periphylla periphylla (Peron & Lesueur, 1809) is described. it is the only known direct development of a coronate Scyphozoa, showing a holopelagic life cycle without any sessile stage. It also lacks an ephyra stage that normally precedes the medusa stage and it is the first Scyphozoan life cycle with sexual propagation where the planula stage is missing. The morphology and histology in the early development is described and a description of eight well recognised developmental stages, covering the period from egg to young medusa, is presented. A brief comparison of the population structure in 1992 and 1997 in Lurefjorden, Norway shows that there has been a dramatic change. The population in 1992 showed a normal size distribution with a median individual wet weight of 540 g, eggs and stages did not occur in the plankton and there was no significant recruitment. Eggs and larvae were first observed in 1993 and since then they have been present in all seasons. The population therefor...

Allen Gilbert Collins - One of the best experts on this subject based on the ideXlab platform.

  • box stalked and upside down draft genomes from diverse jellyfish cnidaria acraspeda lineages alatina alata cubozoa calvadosia cruxmelitensis staurozoa and cassiopea xamachana Scyphozoa
    GigaScience, 2019
    Co-Authors: Aki Ohdera, Cheryl Lewis Ames, Rebecca B Dikow, Ehsan Kayal, Marta Chiodin, Ben Busby, Stacy Pirro, Allen Gilbert Collins
    Abstract:

    Background Anthozoa, Endocnidozoa, and Medusozoa are the 3 major clades of Cnidaria. Medusozoa is further divided into 4 clades, Hydrozoa, Staurozoa, Cubozoa, and Scyphozoa-the latter 3 lineages make up the clade Acraspeda. Acraspeda encompasses extraordinary diversity in terms of life history, numerous nuisance species, taxa with complex eyes rivaling other animals, and some of the most venomous organisms on the planet. Genomes have recently become available within Scyphozoa and Cubozoa, but there are currently no published genomes within Staurozoa and Cubozoa. Findings Here we present 3 new draft genomes of Calvadosia cruxmelitensis (Staurozoa), Alatina alata (Cubozoa), and Cassiopea xamachana (Scyphozoa) for which we provide a preliminary orthology analysis that includes an inventory of their respective venom-related genes. Additionally, we identify synteny between POU and Hox genes that had previously been reported in a hydrozoan, suggesting this linkage is highly conserved, possibly dating back to at least the last common ancestor of Medusozoa, yet likely independent of vertebrate POU-Hox linkages. Conclusions These draft genomes provide a valuable resource for studying the evolutionary history and biology of these extraordinary animals, and for identifying genomic features underlying venom, vision, and life history traits in Acraspeda.

  • Cnidarian phylogenetic relationships as revealed by mitogenomics
    BMC evolutionary biology, 2013
    Co-Authors: Ehsan Kayal, Allen Gilbert Collins, Béatrice Roure, Hervé Philippe, Dennis V. Lavrov
    Abstract:

    Cnidaria (corals, sea anemones, hydroids, jellyfish) is a phylum of relatively simple aquatic animals characterized by the presence of the cnidocyst: a cell containing a giant capsular organelle with an eversible tubule (cnida). Species within Cnidaria have life cycles that involve one or both of the two distinct body forms, a typically benthic polyp, which may or may not be colonial, and a typically pelagic mostly solitary medusa. The currently accepted taxonomic scheme subdivides Cnidaria into two main assemblages: Anthozoa (Hexacorallia + Octocorallia) – cnidarians with a reproductive polyp and the absence of a medusa stage – and Medusozoa (Cubozoa, Hydrozoa, Scyphozoa, Staurozoa) – cnidarians that usually possess a reproductive medusa stage. Hypothesized relationships among these taxa greatly impact interpretations of cnidarian character evolution. We expanded the sampling of cnidarian mitochondrial genomes, particularly from Medusozoa, to reevaluate phylogenetic relationships within Cnidaria. Our phylogenetic analyses based on a mitochogenomic dataset support many prior hypotheses, including monophyly of Hexacorallia, Octocorallia, Medusozoa, Cubozoa, Staurozoa, Hydrozoa, Carybdeida, Chirodropida, and Hydroidolina, but reject the monophyly of Anthozoa, indicating that the Octocorallia + Medusozoa relationship is not the result of sampling bias, as proposed earlier. Further, our analyses contradict Scyphozoa [Discomedusae + Coronatae], Acraspeda [Cubozoa + Scyphozoa], as well as the hypothesis that Staurozoa is the sister group to all the other medusozoans. Cnidarian mitochondrial genomic data contain phylogenetic signal informative for understanding the evolutionary history of this phylum. Mitogenome-based phylogenies, which reject the monophyly of Anthozoa, provide further evidence for the polyp-first hypothesis. By rejecting the traditional Acraspeda and Scyphozoa hypotheses, these analyses suggest that the shared morphological characters in these groups are plesiomorphies, originated in the branch leading to Medusozoa. The expansion of mitogenomic data along with improvements in phylogenetic inference methods and use of additional nuclear markers will further enhance our understanding of the phylogenetic relationships and character evolution within Cnidaria.

  • Cladistic analysis of Medusozoa and cnidarian evolution
    Invertebrate Biology, 2005
    Co-Authors: Antonio C Marques, Allen Gilbert Collins
    Abstract:

    Abstract. A cladistic analysis of 87 morphological and life history characters of medusozoan cnidarians, rooted with Anthozoa, results in the phylogenetic hypothesis (Anthozoa (Hydrozoa (Scyphozoa (Staurozoa, Cubozoa)))). Staurozoa is a new class of Cnidaria consisting of Stauromedusae and the fossil group Conulatae. Scyphozoa is redefined as including those medusozoans characterized by strobilation and ephyrae (Coronatae, Semaeostomeae, and Rhizostomeae). Within Hydrozoa, Limnomedusae is identified as either the earliest diverging hydrozoan lineage or as the basal group of either Trachylina (Actinulida (Trachymedusae (Narcomedusae, Laingiomedusae))) or Hydroidolina (Leptothecata (Siphonophorae, Anthoathecata)). Cladistic results are highly congruent with recently published phylogenetic analyses based on 18S molecular characters. We propose a phylogenetic classification of Medusozoa that is consistent with phylogenetic hypotheses based on our cladistic results, as well as those derived from 18S analyses. Optimization of the characters presented in this analysis are used to discuss evolutionary scenarios. The ancestral cnidarian probably had a sessile biradial polyp as an adult form. The medusa is inferred to be a synapomorphy of Medusozoa. However, the ancestral process (metamorphosis of the apical region of the polyp or lateral budding involving an entocodon) could not be inferred unequivocally. Similarly, character states for sense organs and nervous systems could not be inferred for the ancestral medusoid of Medusozoa.

  • Phylogeny of Medusozoa and the evolution of cnidarian life cycles
    Journal of Evolutionary Biology, 2002
    Co-Authors: Allen Gilbert Collins
    Abstract:

    Cubozoa; development; Hydrozoa; Scyphozoa; Stauromedusae. Abstract To investigate the evolution of cnidarian life cycles, data from the small subunit of the ribosome are used to derive a phylogenetic hypothesis for Medusozoa. These data indicate that Cnidaria is monophyletic and composed of Anthozoa and Medusozoa. While Cubozoa and Hydrozoa are well supported clades, Scyphozoa appears to be paraphyletic. Stauromedusae is possibly the sister group of either Cubozoa or all other medusozoans. The phylogenetic results suggest that: the polyp probably preceded the medusa in the evolution of Cnidaria; within Hydrozoa, medusa development involving the entocodon is ancestral; within Trachylina, the polyp was lost and subsequently regained in the parasitic narcomedusans; within Siphonophorae, the float originated prior to swimming bells; stauromedusans are not likely to be descended from ancestors that produced medusae by strobilation; and cubozoan polyps are simplified from those of their ancestors, which possessed polyps with gastric septa and four mesogleal muscle bands and peristomial pits.