Cnidaria (Coelenterates)

Abstract

The study of cnidarians has contributed to concepts of biodiversity, evolution and development. Aquatic, mostly marine metazoans, cnidarians are distinguished from members of other phyla by the presence of cnidocytes typically on tentacles. A cnidocyte contains a cnidocyst, a subcellular capsule containing an extrusible tubule capable of everting as an arrow‐like cnida functioning primarily in predation and defence. Cnidarians are predominantly tubular polyps or disk‐ to bell‐shaped medusae. Sexual reproduction generally results in solid planula larvae that develop into polyps, while asexual reproduction performed by polyps results in both polyps and medusae. Polyps bud individuals and members of colonies (e.g. corals). Cnidaria is broken into three subphyla: (1) Anthozoa, composed exclusively of polyps, comprises Octocorallia (soft corals) and Hexacorallia (hard corals); (2) Medusozoa contains both polyps and medusae. This subphylum encompasses Staurozoa, Cubozoa, Scyphozoa and Hydrozoa; (3) Endocnidozoa contains highly reduced endoparasites (myxozoans) plus some larger outliers. Key Concepts Cnidarians are aquatic, mostly marine metazoans, once lumped with ctenophores in the phylum Coelenterata. By sorting out many competing phylogenetic claims, taxonomists have brought cnidarian classification to its present level of clarity. Cnidarians are typically in the form of cylindrical polyps or bell‐shaped medusae. The subphylum Anthozoa consists exclusively of polyps; the subphylum Medusozoa has both polyps and medusae and the subphylum Endocnidozoa is epitomised by myxozoa parasites of annelid or bryozoan and fish, Polypodium hydriforme, an oligocellular parasite of sturgeon and paddlefish eggs, and Buddenbrockia plumatellae, a worm‐like parasite of bryozoans. Uniquely, members of the phylum Cnidaria possess cnidocytes (Gr. knī́dē nettle, sea nettle + kyto hollow vessels) aka nematocytes (Gr. nema‐ thread + hollow vessels), an alternative name for cnidocysts (with the exception of spirocytes and ptychocytes). These cells develop from cnidoblasts and produce cnidocysts (aka nematocysts), encapsulated inside‐out tubules that evert to become adhesive, ensnaring, entangling, penetrating, constructing or poisoning darts (cnidae). Cnidocysts are concentrated in the epidermis of tentacles protruding from the oral end of polyps and from the rim of medusae. Cnidocysts also occur elsewhere on the body both externally and internally. Both asexual and sexual reproduction occur among polyps, but sexual reproduction in the Medusozoa is predominantly the function of medusae. Fertilised eggs generally give rise to solid, ciliated planula larvae that grow into polyps. These polyps may subsequently produce medusae asexually, typically by fragmentation (strobilation). The studies of evolution, cladistics and development gained momentum with the advent of molecular biology in the twentieth century, isoelectric focusing and immunological methods, followed by multiple deoxyribonucleic acid ( DNA )‐based approaches; DNA hybridisation; restriction enzyme digestion; random polymerase chain reaction ( PCR ) amplification of ribosomal genes, mitochondrial (mt) small subunit (S) 12S and 16S and large subunit 18S ribosomal ribonucleic acid ( rRNA ) and mitochondrial deoxyribonucleic acid ( mtDNA ); amplification, use of species‐specific PCR primers; DNA sequencing and so on. The application of these methods now dominates cnidarian classification, perspectives on mechanism of development, and cnidarian evolution.