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Poriferans 1 The first metazoans Phylum Porifera (Sponges) • ~5,500 extant species • 98% are marine, 2% freshwater • Lack nerves and have no true musculature • No specialized organs • Adults are asymmetrical or superficially radially symmetrical • Cells are totipotent • Use choanocytes (unique flagellated cells) to drive water through canals and chambers which constitute the aquaferous system 2 The first metazoans Choanoflagellates (protozoan) – can operate in a colonial way. How are these guys any different? Sponges can differentiate self from non-self 3 Poriferan Body Plan 4 Poriferan Body Plan Choanoderm • inner layer of cells • Choanocytes (collar cells) – Cylindrical arrangement villi surrounding single flagellum Functions: 1. Circulation – generate currents to circulate water through sponge 2. Feeding - Capture of food particles 3. Reproduction- Capture incoming sperm 5 Poriferan Body Plan • Mesohyl – – – – Gel like matrix Acellular and non-living Contains archaeocytes Contains spicules Archaeocyts: • Amoeboid like cells • Digest food particles captured by choanocytes • Give rise to sperm and eggs • Role in self-recognition • Eliminate wastes • Differentiate to sclerocytes or spongocytes 6 Sclerocytes, spongocytes and spicules • Derived from archaeocytes • Produce spicules – calcium carbonate, silica, or collagenous protein Importance • Act as support elements for sponge structures and possibly to deter predation • Systematists use them for species identification 7 Poriferan Body Plan Pinacoderm (epidermis) – Continuous layer on the external surface of sponge – Lines the incurrent and excurrent canals * – Basement membrane is absent – Separates sponges from all other metazoa epithelia which have a basement membrane Pinocytes – thin flattened cells that make up the Pinacoderm 8 Poriferan Body Plan Three basic body types: 1. Asconoid 2. Synconoid 3. Leuconoid 9 Poriferan Body Plan Three basic body types: Asconoids • • • Choanoderm is one cell thick, simple and continuous Single osculum Small simple forms • Rarely exceed 10 cm in height 10 Leucosolenia sp. Poriferan Body Plan Three basic body types: Syconoids • • • • Folding of pinacoderm and choanoderm Simple and complex forms Choanocyte chambers • Open to atrium via apopyle Single osculum • Complex forms • • Incurrent canals Prosopyles – openings from inccurent chambers into choanocyte chambers Path of water flow in complex forms: Dermal pore incurrent canal prosopyle choanocyte chamber osculum atrium apopyle 11 Scypha sp. 12 Sycon sp. Poriferan Body Plan Three basic body types: Leuconoids • • • • • • Largest and most complex sponges up to 1 m in height Complex folding of choanoderm Thickening of mesohyl Discrete choanocyte chambers • Increase in number • Decrease in size Atrium is reduced to a series of excurrent canals which lead to oscula Dermal pore incurrent canal prosopyle choanocyte chamber osculum excurrent canal apopyle 13 osculum Excurrent canal Clathria sp 14 Poriferan feeding • most sponges are filterfeeders • rely on intracellular digestion – Phagocytosis – Pinocytosis • Coordinated efforts of choanocytes and Archaeocytes 15 Poriferan feeding • most sponges are filterfeeders • water is moved by current action and choanocyte flagellae • choanocyte collars capture food particles 16 The rate of water flow is important for efficient filter-feeding in sponges The increased surface area created by the compartmentalization in leuconoid sponges reduces the velocity of water flow to enhance particulate capture by choanocytes. 17 Poriferan feeding • Predatory Sponge!! – Only one species (Abestopluma) – Discovered 1995 – Mediterranean sea caves • Lack choanocyte lined aquaferous system • Trap prey on external spicules • Feeding cells migrate to envelope and digest prey 18 Reproduction Can have both asexual and sexual reproduction Asexual Reproduction 1. Budding • Small piece of sponge buds or breaks off • Carried by current • Anchors to new substrate and begins to grow a new individual 2. Reduction bodies • Marine sponges 19 Reproduction Can have both asexual and sexual reproduction Asexual Reproduction 1. Gemmule formation • Occurs mainly in freshwater sponges of family Spongillidae • Occurs in response to harsh conditions (temp or environmental) • Remarkable example of poriferan cell totipotency 20 Reproduction Gemmule formation • Archaeocytes aggregate in mesohyl and udergo rapid mitosis • Trophocytes (nurse cells) stream to archaeocyte mass and are engulfed by phagocytosis • Provide nutrition during dormancy • Mass of cells surrounded by three layers of spongin • Amphidisc spicules are transported to spongin envelope • Micropyle forms (absence of spicules in this region) • Gemmule begins “hibernation” while parent sponge dies and disintegrates 21 Reproduction Gemmule hatching • Favourable environmental conditions • Micropyle opens up • First archaeocytes released flow over gemmule and substratum • Form pinacoderm and choanoderm framework • Second wave of archaeocytes colonize this initial framework • Growth of a new sponge! 22 Reproduction Sexual Reproduction • Most sponges are hemaphroditic • Produce egg and sperm at different times • Cross fertilization is required • Sperm – arise from choanocytes • Oocyte – forms in meshyle surrounded by follicle cells and nurse cells • Sperm are released via the aquaferous system and broadcast by the current • Choanocytes capture sperm, phagocytise sperm (differentiate into amoeboid form) and deliver to the oocyte located in the mesohyl • Choanocytes digest the sperm of other sponge and invertebrate species but not their own (species recognition) 23
