Introduction to Cnidaria Jellyfish, 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!