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Sponges and Placozoans

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Sponges and Placozoans CHAPTER 12 Sponges and Placozoans Powerpoint revised by Franklyn Tan Te 12-1 Copyright © 2013 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. A Caribbean demosponge, Aplysina fistularis. 12-2 Origins of Multicellularity A. Cells are the elementary units of life 1. Nature’s experiments with larger organisms without cellular differentiation are limited such that large, single celled marine algae are rare 2. Sponges are the simplest multicellular animals but their cell assemblages are distinct from other metazoans. 3. Sponges have cells embedded in an extracellular matrix supported by a skeleton with needle-like spicules and 12-3 protein. Origins of Multicellularity ◼ Increasing the size of a cell causes problems of exchanging molecules with the environment. ◼ Multicellularity prevents surface-to-mass problems as smaller units greatly increase surface area for metabolic activities ◼ Highly adaptive towards larger body size ◼ Sponges neither look like or behave as animals but molecular evidence demonstrates that they are phylogenetically grouped with animals 12-4 Origin of Metazoa ◼ Evolution of the Metazoa (animals) ◼ Eukaryotic cells evolved and diversified into many lineages that led to modern day descendants ◼ Includes all unicellular protozoans, colonial and multicellular plants, animals, and fungi ◼ Multicellular organisms were collectively called metazoans and are now also termed “animals” ◼ Metazoans are placed in the Opisthokont clade which include fungi, choanoflagellates, and other groups 12-5 Origin of Metazoa ◼ Choanoflagellates ◼ Solitary or colonial aquatic eukaryotes ◼ Each cell has a flagellum surrounded by a collar of microvilli ◼ Flagellum beats and draws water into collar where the microvilli collect particles like bacteria ◼ Most are sessile but one species attaches to floating diatom colonies and feed midwater ◼ Strongly resemble sponge feeding cells called choanocytes, which have been argued to be ancestral to choanoflagellates 12-6 Origin of Metazoa ◼ Evidences of common ancestry between Choanoflagellates and Metazoans ◼ By comparing the genomes or proteomes of sponges with more complex taxa, scientists can discover what cell transmitters or morphogens the first metazoans possessed. ◼ Shared characteristics would have been inherited from the most recent common ancestor of animals. ◼ Molecular phylogeny indicates that colonial bodies evolved early in the lineage. 12-7 Origin of Metazoa ◼ Recent research indicates proteins used by colonial choanoflagellates for cell communication and adhesion are homologous to those that metazoans use in cell-to-cell signaling. ◼ Sponge genome contains elements that code for regulatory pathways of more complex metazoans ◼ Sponges have proteins that code for spatial patterning that specify anterior and posterior pole of larvae. 12-8 Origin of Metazoa ◼ Sponges today are less complex than their ancestors ◼ Sponges have simple bodies composed of aggregates of several cell types held together by extracellular matrix ◼ Sponge bodies are not symmetrical and have no mouth or digestive tract ◼ Placozoans share features with other animal groups. ◼ Have small nuclear genome and the largest mitochondrial genome in the animal kingdom ◼ Placozoan bodies are as puzzling as sponges: they also have no heads or tails 12-9 Phylum Porifera ◼ General Features of Sponges ◼ Mostly sessile ◼ Body designed for efficient aquatic filter feeding ◼ Porifera means “pore-bearing”; sac-like bodies are perforated by many pores ◼ Use flagellated “collar cells”, or choanocytes, to move water to bring food and oxygen while removing wastes ◼ Most of the 8600 sponges are marine, found in all seas and all depths, while few live in brackish water and 150 live in fresh water 12-10 Figure 12.1 Some growth habits and forms of sponges. 12-11 Figure 12.2 Sponge choanocytes have a collar of microvilli 12-12 surrounding a flagellum. Phylum Porifera ◼ Sponges vary in size from a few millimeters to over 2 meters in diameter ◼ Many species are brightly colored because of pigments in dermal cells ◼ Embryos are free-swimming while adult sponges always attached ◼ Some appear radially symmetrical but many are irregular in shape ◼ Some stand erect, some are branched, and others are encrusting ◼ Growth patterns depend on shape of substratum, direction of water, speed of flow 12-13 and availability of space Phylum Porifera ◼ Many animals like crabs, nudibranchs, fish, and other species do live as commensals or parasites in or on sponges ◼ Sponges can also grow on a variety of other living organisms with some crabs using sponges for camouflage and protection ◼ Sponges and microorganisms living on them often have a noxious odor and produce a variety of bioactive compounds ◼ Certain sponge extracts have manifested medical and pharmaceutical effectiveness. 12-14 Phylum Porifera ◼ Skeletal structure of a sponge can be fibrous and/or rigid consisting of calcareous or siliceous spicules ◼ Fibrous portion comes from collagen protein fibrils in intercellular matrix ◼ There are several types of collagen, which vary in chemical composition; sponges contain spongin ◼ Composition and shape the spicules form the basis of sponge classification ◼ Modern materials science view spicules for possible nanoparticle products ◼ The simplistic exterior of sponges often mask their chemical and functional sophistication 12-15 Figure 12.3 Diverse forms of spicules that support a sponge body. 12-16 Phylum Porifera ◼ Sponges date back to the early Cambrian and maybe even Precambrian period ◼ Traditionally grouped in three classes based on spicules and chemical composition ◼ Calcarea: calcium carbonate spicules with one, three, or four rays ◼ Hexactinellids: glass sponges with six-rayed siliceous spicules ◼ Demospongiae: siliceous spicules around an axial filament, spongin fibers, or both ◼ Homoscleromorpha, was formed to contain sponges without a skeleton or with siliceous spicules without an axial filament 12-17 Figure 12.4 Cladogram depicting evolutionary relationships among the four classes of sponges. 12-18 Phylum Porifera ◼ Form and Function ◼ Body openings consist of small incurrent pores or dermal ostia in the outer layer of cells called pinacoderm ◼ Sponges feed by collecting suspended particles from the water through internal canal systems ◼ Water is directed past the choanocytes, which are flagellated collar cells that keep the current flowing via beating of flagella ◼ Microvilli in the collar trap and phagocytize food particles that pass by. 12-19 Phylum Porifera ◼ Sponges non-selectively consume food particles (detritus, plankton, and bacteria) ◼ The smallest particles (80%) are taken into choanocytes by phagocytosis ◼ Protein molecules may be taken in by pinocytosis ◼ Two other cell types, pinacocytes and archaeocytes, facilitate feeding ◼ Dissolved nutrients can also be absorbed by sponges ◼ Efficiency of food capture is dependent on water movement through the sponge body 12-20 Phylum Porifera ◼ Three types of sponge body designs ◼ Asconoids ◼ Simplest body organization ◼ Small and tube-shaped to allow water to flow directly across cells so no “dead space” ◼ Choanocytes are in a large internal chamber, the spongocoel ◼ Choanocyte flagella pull water through the pores and extract food particles ◼ Used water is expelled through a large single osculum ◼ All Calcarea are asconoids ◼ Leucosolenia sp. and Clathrina sp, for example 12-21 Figure 12.5 Three types of sponge structure. 12-22 Figure 12.6 Clathrina canariensis (class Calcarea) is a common Asconoid on Caribbean reefs. 12-23 Phylum Porifera ◼ Syconoids ◼ Resemble asconoids but larger and with a thicker more complex body wall ◼ Body wall is folded outwards with choanocyte- lined radial canals that empty into spongocoel ◼ Water enters through dermal ostia and move into tiny openings called prosopyles into the radial canals ◼ Food is ingested by choanocytes and used water is pumped through internal pores called apopyles then outwards via osculum ◼ Spongocoel is lined with epithelial cells rather than choanocytes as in asconoids 12-24 Phylum Porifera ◼ Developmental evidence of being derived from asconoid ancestors ◼ Syconoids pass through an asconoid stage in development but do not form highly branched colonies ◼ Flagellated canals form by evagination of the body wall ◼ Syconoid body plan is not homologous among all sponges that have it ◼ Classes Calcarea and Hexactinellida have syconoid species (ex: Sycon sp.) 12-25 Figure 12.7 Cross section through wall of sponge Sycon sp., showing choanocytes in canals within the wall but do not line spongocoel. 12-26 Phylum Porifera ◼ Leuconoids ◼ Most complex and larger, for more food- collecting regions ◼ These regions have choanocytes lining in small chambers that effectively filter all water present ◼ Clusters of flagellated chambers are filled from incurrent canals and discharge to excurrent canals which lead to osculum ◼ After food is removed, used water is pooled to form an exit stream that leaves through an exit pore at very high velocity ◼ This high rate of exit flow prevents the sponge from re-filtering used water and wastes 12-27 ◼ Most sponges are leuconoid type Phylum Porifera ◼ The leuconoid system has high adaptive value to efficiently meet high food demands of larger body size ◼ Has the highest proportion of flagellated surface per volume of cell tissue ◼ More collar cells can filter more particles ◼ Water flow slows down inside due to greater surface
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