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Phylum Cnidaria

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Phylum Cnidaria An Introduction to the Invertebrates, Part One Phyla Placozoa, Porifera, Cnidaria, Ctenophora Reference: Chapter 33.1, 33.2 Overview: Life Without a Backbone v Invertebrates are animals that lack a backbone § 95% of known animal species § Morphologically very diverse How are invertebrates related to protists? Colonial Choanoflagellate Protists – Is this our Great Grandpappy? Metazoa vs. Eumetazoa v Kingdom Animalia = Metazoa (animals) § Eukaryotic, multicellular, heterotrophs § No cell walls § Motile at some point in life history v Eumetazoa (“true” animals) § In addition to metazoan characteristics, this group has: § True tissues organized into germ layers § Neurons § An embryo that goes through a gastrula stage The Basal Metazoans – Phyla Placozoa and Porifera v The basal animals lack true tissues and organs § Phylum Placozoa – one species (relation to other basal groups not clear) § Phylum Porifera (sponges) – 5500 species v Resemble choanoflagellate protists Phylum Placozoa v Only one formally recognized species, Trichoplax adhaerens § Most simply organized metazoan known v Three-layered “sandwich” organization § Upper (protective) and lower (nutritive) monociliated epithelia § Middle layer a meshwork of connected fiber cells v Global distribution- crawls along rocks, corals, etc. v Avoids predation by using chemical defenses Phylum Porifera (“sponges”) v Sessile animals with simple body plan, but diverse shapes and colors v Live in marine waters (most species) or fresh water Sponges consist of three layers 1. Outer epidermis 2. Mesohyl is the middle, gelatinous noncellular layer § Mobile amoebocytes play roles in digestion and structure § Spicules are skeletal fiber structures of calcium carbonate or silica 3. Choanocytes are flagellated collar cells making up the inner lining 1. Generate a water current through the sponge and ingest suspended food Sponges are suspension feeders v Capture food particles suspended in the water that passes through their body § Water is drawn through pores into a cavity called the spongocoel and out through an opening called the osculum § Choanocytes trap food in “collar” and ingest it by phagocytosis § Food particles may be transferred to amoebocytes (named for their use of pseudopodia) Most Sponges are hermaphrodites v Each individual sponge functions as both male and female § Most are sequential hermaphrodites- function first as one sex, then the other § Gametes arise from either choanocytes or amoebocytes § Eggs stay within the mesohyl but sperm disperse on the water current § Zygotes disperse as flagellated larvae before settling v Sponges are harvested for their bodies, composed of spicules Sponges create current to aid feeding & reproduction Sponge-dye experiment (~5-min video): https://www.youtube.com/watch?v=8ueKlShb9Ls The Eumetazoan Diploblasts: Phyla Cnidaria & Ctenophora v Eumetazoan = “true” tissues v Diploblasts § Two primary germ layers § Endoderm and ectoderm § Radial symmetry Phylum Cnidaria v Cnidarians have diversified into a wide range of both sessile and motile forms including jellies, corals, and hydras v The basic body plan of a cnidarian is a sac with a central digestive compartment, the gastrovascular cavity v A single opening functions as mouth and anus Cnidarian Body Plan v Depending on species, there are two variations on the body plan: the sessile polyp and motile medusa § Polyp: adheres to the substrate by the aboral end of its body (the end opposite the mouth) § Medusa: bell-shaped body with the mouth/anus on the underside v Example of polymorphism – the presence of more than one morphological type in a taxon Cnidarian are carnivores v Use tentacles to capture prey v Tentacles are armed with cnidocytes § Specialized cells for defense and capture of prey § Contain Nematocysts § Specialized organelles within cnidocytes that eject a stinging thread Nematocysts in action Excerpt from video, https://www.youtube.com/watch?v=7WJCnC5ebf4 ~1:08-3:50 Cnidarian Tissues v No brain or nervous system – but do have a nerve net v No true muscles (diploblastic!) § Instead, epitheliomuscular cells control locomotion Phylum Cnidaria is divided into four major classes 1. Hydrozoa- alternate between polyp and medusa 2. Scyphozoa- “true” jellies 3. Cubozoa- box jellies 4. Anthozoa- anemones and corals Phylum Cnidaria: Class Hydrozoa v Most alternate between polyp and medusa forms v Hydra, a freshwater cnidarian, exists only in polyp form and reproduces asexually by budding Life cycle of Hydrozoan Obelia - MEMORIZE IT! Reproductive Feeding polyp polyp Medusa MEIOSIS bud Gonad Medusa Egg Sperm SEXUAL ASEXUAL REPRODUCTION Portion REPRODUCTION of a (BUDDING) colony FERTILIZATION of polyps Zygote Developing Planula polyp (larva) Mature polyp Key Haploid (n) 1 mm Diploid (2n) Phylum Cnidaria: Unusual Hydrozoan v The Portuguese Man of War § Often mistaken for true jellyfish (scyphozoa) § Has no polyp form § Severe sting can cause major harm § Preyed upon by Glaucus, a nudibranch (mollusc) § Incorporates the nematocysts into its own body for defense Phylum Cnidaria: Class Scyphozoa v In the class Scyphozoa, jellies (medusae) are the dominant form of the life cycle Phylum Cnidaria: Class Cubozoa v In the class Cubozoa, which includes box jellies and sea wasps, the medusa is box-shaped and has complex eyes v Cubozoans often have highly toxic cnidocytes- painful and possibly lethal v Most common in tropical Indo-Pacific Phylum Cnidaria: Class Anthozoa v “Flower animals” v Include the corals and sea anemones v Occur only as polyps v “Hard” corals secrete a calcareous exoskeleton; “soft” corals do not Phylum Cnidaria: Class Anthozoa v Many corals form obligate symbioses with algae § Polyp tissue (green) around mouth and base of the tentacles § Symbiotic zooxanthellae (red fluorescence from chlorophyll) in tissue between polyps James Nicholson, Olympus BioScapes Phylum Ctenophora v “Comb jellies” § 8 comb-like rows of cilia used for locomotion § One group has tentacles; another does not § Swim “mouth first,” unlike cnidarian medusa § No polyp morph v Predators § No cnidocytes; instead have colloblasts arrayed along tentacles that produce sticky substance to trap prey v Many capable of bioluminescence Ctenophore gallery .
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