Staurozoan classification is highly subjective, based on phylogeny-free inferences, and suborders, families, and genera are commonly defined by homoplasies. Additionally, many characters used in the taxonomy of the group have ontogenetic and intraspecific variation, and demand new and consistent assessments to establish their correct homologies. Consequently, Staurozoa is in need of a thorough systematic revision. The aim of this study is to propose a comprehensive phylogenetic hypothesis for Staurozoa, providing the first phylogenetic classification for the group. According to our working hypothesis based on a combined set of molecular data (mitochondrial markers COI and 16S, and nuclear markers ITS, 18S, and 28S), the traditional suborders Cleistocarpida (animals with claustrum) and Eleutherocarpida (animals without claustrum) are not monophyletic. Instead, our results show that staurozoans are divided into two groups, herein named Amyostaurida and Myostaurida, which can be distinguished by the absence/presence of interradial longitudinal muscles in the peduncle, respectively. We propose a taxonomic revision at the family and genus levels that preserves the monophyly of taxa. We provide a key for staurozoan genera and discuss the evolution of the main characters used in staurozoan taxonomy.

Stauromedusae have relatively few macromorphological characters, making both their taxonomy and identification difficult. For this reason, histological characters are also employed in the taxonomy of the group. This study presents a detailed description of the histomorphology of Haliclystus antarcticus Pfeffer, 1889 (Cnidaria, Staurozoa). We make new observations for the species and for the class, and address functional, taxonomical, and evolutionary aspects of staurozoan histo-anatomy. A complete reconstruction of H. antarcticus body plan is used to guide a more detailed observation, based on light microscopy, of structures rarely cited in the literature, such as the intertentacular lobules, the ostia between adjacent perradial pockets, and the male and female gonadal vesicles. Two possible regions of nematocyst formation are hypothesized and discussed. We also provide a review of the current use of histological characters in the taxonomy of the group. Understanding the body plan of stauromedusae is a challenge, because each single individual presents characters found in medusae and in polyps of other medusozoans. Comprehensive histological descriptions are important to establish relations of homology within Staurozoa and Cnidaria, providing crucial data on their evolution.

BackgroundLife cycles of medusozoan cnidarians vary widely, and have been difficult to document, especially in the most recently proposed class Staurozoa. However, molecular data can be a useful tool to elucidate medusozoan life cycles by tying together different life history stages.Methodology/Principal FindingsGenetic data from fast-evolving molecular markers (mitochondrial 16S, nuclear ITS1, and nuclear ITS2) show that animals that were presumed to be a hydrozoan, Microhydrula limopsicola (Limnomedusae, Microhydrulidae), are actually an early stage of the life cycle of the staurozoan Haliclystus antarcticus (Stauromedusae, Lucernariidae).Conclusions/SignificanceSimilarity between the haplotypes of three markers of Microhydrula limopsicola and Haliclystus antarcticus settles the identity of these taxa, expanding our understanding of the staurozoan life cycle, which was thought to be more straightforward and simple. A synthetic discussion of prior observations makes sense of the morphological, histological and behavioral similarities/congruence between Microhydrula and Haliclystus. The consequences are likely to be replicated in other medusozoan groups. For instance we hypothesize that other species of Microhydrulidae are likely to represent life stages of other species of Staurozoa.

We have compiled available records in the literature for medusozoan cnidarians and ctenophores of South America. New records of species are also included. Each entry (i.e., identified species or still as yet not determined species referred to as "sp." in the literature) includes a synonymy list for South America, taxonomical remarks, notes on habit, and information on geographical occurrence. We have listed 800 unique determined species, in 958 morphotype entries: 5 cubozoans, 905 hydrozoans, 25 scyphozoans, 3 staurozoans, and 20 ctenophores. Concerning nomenclatural and taxonomical decisions, two authors of this census (Miranda, T.P. & Marques, A.C.) propose Podocoryna quitus as a nomen novum for the junior homonym Hydractinia reticulata (Fraser, 1938a); Euphysa monotentaculata Zamponi, 1983b as a new junior synonym of Euphysa aurata Forbes, 1848; and Plumularia spiralis Milstein, 1976 as a new junior synonym of Plumularia setacea (Linnaeus, 1758). Finally, we also reassign Plumularia oligopyxis Kirchenpauer, 1876 as Kirchenpaueria oligopyxis (Kirchenpauer, 1876) and Sertularella margaritacea Allman, 1885 as Symplectoscyphus margaritaceus (Allman, 1885).

In this review, we present the current state of biodiversity knowledge for the class Staurozoa (Cnidaria), including richness estimates, geographical and bathymetric distributions, substrate use, feeding, behavior, life cycle, and conservation. Based on non-parametric, statistical incidence estimators, the global inventory of 50 known and accepted species of stalked jellyfishes might be regarded as close to complete, but we discuss possible bias related to the lower research effort applied in the Southern Hemisphere. Most of the species occur at mid-latitudes, presenting a distributional pattern that disagrees with the classic pattern of diversity (higher richness near the Equator). Specimens are frequently found on algae, but they have also been reported attached to rocks, seagrasses, shells, mud, sand, coral/gorgonian, sea cucumber, and serpulid tube. Most of the species are found in the intertidal and shallow subtidal regions, but species of Lucernaria have been reported at more than 3000 m deep. Amphipods and copepods are the prey items most frequently reported, and stauromedusae have been observed being actively preyed upon by nudibranch mollusks and pycnogonids. Apparently, stalked jellyfishes have a high sensitivity to anthropic impacts in the environment, and promotion of the class, one of the least studied among Cnidaria, is perhaps the best possible conservation strategy.

The benthic polyp phase of Medusozoa (Staurozoa, Cubozoa, Scyphozoa, and Hydrozoa) has endoskeletal or exoskeletal support systems, but their composition, development, and evolution is poorly known. In this contribution the variation in synthesis, structure, and function of the medusozoan exoskeleton was examined. In addition, an evolutionary hypothesis for its origin and diversification is proposed for both extinct and extant medusozoans. We also critically reviewed the literature and included data from our own histological and microstructural analyses of some groups. Chitin is a characteristic component of exoskeleton in Medusozoa, functioning as support, protection, and a reserve for various ions and inorganic and organic molecules, which may persuade biomineralization, resulting in rigid biomineralized exoskeletons. Skeletogenesis in Medusozoa dates back to the Ediacaran, when potentially synergetic biotic, abiotic, and physiological processes resulted in development of rigid structures that became the exoskeleton. Of the many types of exoskeletons that evolved, the corneous (chitin-protein) exoskeleton predominates today in polyps of medusozoans, with its greatest variation and complexity in the polyps of Hydroidolina. A new type of bilayered exoskeleton in which there is an exosarc complementing the perisarc construction is here described.

Comparative efforts to understand the body plan evolution of stalked jellyfishes are scarce. Most characters, and particularly internal anatomy, have neither been explored for the class Staurozoa, nor broadly applied in its taxonomy and classification. Recently, a molecular phylogenetic hypothesis was derived for Staurozoa, allowing for the first broad histological comparative study of staurozoan taxa. This study uses comparative histology to describe the body plans of nine staurozoan species, inferring functional and evolutionary aspects of internal morphology based on the current phylogeny of Staurozoa. We document rarely-studied structures, such as ostia between radial pockets, intertentacular lobules, gametoducts, pad-like adhesive structures, and white spots of nematocysts (the last four newly proposed putative synapomorphies for Staurozoa). Two different regions of nematogenesis are documented. This work falsifies the view that the peduncle region of stauromedusae only retains polypoid characters; metamorphosis from stauropolyp to stauromedusa occurs both at the apical region (calyx) and basal region (peduncle). Intertentacular lobules, observed previously in only a small number of species, are shown to be widespread. Similarly, gametoducts were documented in all analyzed genera, both in males and females, thereby elucidating gamete release. Finally, ostia connecting adjacent gastric radial pockets appear to be universal for Staurozoa. Detailed histological studies of medusozoan polyps and medusae are necessary to further understand the relationships between staurozoan features and those of other medusozoan cnidarians.

The collapse of the Fundão tailings dam at Mariana (State of Minas Gerais, Brazil) started a huge human tragedy and likely the most serious environmental disaster in recent Brazilian history. The dam had contained waste from processing iron ore from mines owned by Samarco, a joint venture company of the Brazilian Vale S.A. and the Anglo-Australian BHP Billiton Ltd. Following ineffective attempts to contain the disaster, after 16 days the mud flood reached the sea, where its impact is expected to affect thousands of marine fauna and flora species. Here, we provide an example of one of these species, the cnidarian Kishinouyea corbini Larson 1980 (Staurozoa), emblematic because it is extremely rare, poorly studied, and its known distribution overlaps the threatened area on the Brazilian coast. Based on this case, we discuss the need for efforts to monitor and minimize the possible impacts of this socio-environmental crime, as well as to identify and punish all responsible players in this tragedy, including negligent licensing and supervisory state agencies, in order to prevent future similar tragedies.