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Introduction to CnidariaJellyfish, corals, and other stingers. . .
Cnidarians are incredibly diverse in form • Yet, these diverse animals are all armed with stinging structures called nematocysts (cnidocytes) • The name Cnidaria comes from the Greek word "cnidos“ • Many thousands of cnidarian species
Fossil Records • A few mineralized coral-like fossils have turned up in the Cambrian Period • Identifiable corals began an evolutionary radiation in the Early Ordovician • Scleractinian corals appeared in the Middle Triassic, about 15 million years after the Permian extinction.
Scleractinian became the dominant hermatypic (reef-building) organisms in shallow tropical marine habitats. • Corals are sensitive to changes in light, temperature, water quality, and salinity
Cnidarian TAXONOMY • There are four major groups of cnidarians: • Anthozoa, which includes true corals, anemones, and sea pens; • Cubozoa, the amazing box jellies with complex eyes and potent toxins; • Hydrozoa, the most diverse group with siphonophores, hydroids, fire corals, and many medusae; and • Scyphozoa, the true jellyfish.
CNIDARIAN Morphology ***Versus polyp
Features: • Two tissue layers with nerve and muscle tissues • Nematocysts: structures contained in special cells called cnidocytes or cnidoblasts that can act in both offense and defense • Two main life forms: free-swimming medusa (e.g., jellyfish) or stationary polyp (e.g., anemone)
Introduction to the Anthozoa • Anthozoans are probably the most famous cnidarians: they include the corals that build great reefs in tropical waters, as well as sea anemones, sea fans, and sea pens. • True corals living today did not appear until the middle Triassic, at about the same time that the first dinosaurs were evolving
Introduction to Cubozoa:The Box Jellies • They look like your basic jellyfish, but they can swim pretty fast, maneuver around things, and see fairly well despite not having a brain. • Cubozoans have a square shape when viewed from above. • They also have four evenly spaced out tentacles or bunches of tentacles and well-developed eyes • Chironex fleckeri
Introduction to the Hydrozoa • The best-known hydrozoan is Hydra. Hydra never goes through a medusoid stage, and spends its entire life as a polyp. • Most hydrozoans alternate between a polyp and a medusa stage -- they spend part of their lives as "jellyfish" which are hard to distinguish from scyphozoan jellyfish. • The "Portuguese man-o'war" and "by-the-wind-sailors" that often wash up on beaches are examples of these unusual colonial hydrozoans.
Introduction to the Scyphozoa “the true jellyfish” • Scyphozoans include most of the jellyfish other similar organisms • Their stings may cause skin rashes, muscle cramps, or even death. • Jellyfish range in size from a mere twelve millimeters to more than two meters across. • The largest is Cyanea arctica, which may have tentacles over 40m long!