Biology 320 Invertebrate Zoology Fall 2005

Biology 320Invertebrate ZoologyFall 2005 Chapter 5 – Phylum Porifera

Phylum Porifera • The sponges • Name Porifera means “pore bearers” • First metazoan phylum we will discuss • Posses epithelioid and rudimentary connective tissue • Lack true muscle and nervous tissues • Not considered to be eumetazoans

Approximately 8000 spp. of sponges • Mostly marine, with approx. 150 described freshwater spp. • Primitive, sessile filter feeders • Most are asymmetrical, but some display radial symmetry • Can be erect, branching, or encrusting on substratum

Body Design • Three main forms of sponges, classified according to complexity • Asconoid • Syconoid • Leuconoid • Asconoid sponges • Simplest body plan • Resemble a hollow tube, with the base attached to the substrate • Possesses one large spongocoel lined with a single layer of choanoderm • Choanocytes beat and draw water through ostia in the epidermis

Asconoid sponges cont… • Water exits spongocoel through a single large osculum • Smallest of all sponges (1mm in diameter) • Thinnest body walls • Leucosolenia • Growth is limited by spongocoel diameter • If asconoid sponges had larger diameters, body volume would exceed pumping capacity of choanoderm • Therefore, a only a change in this simple body plan would permit the evolution of larger body sizes in sponges

Syconoid Sponges • Body wall contains pockets, known as choanocyte chambers, that are lined with choanoderm • Increases choanoderm surface area • Decreases spongocoel volume • Larger than asconoid sponges (one cm – a few cm) • Body walls are thicker than in asconoid sponges • Grantia and Sycon (formerly Scypha)

Leuconoid sponges • Contains thousands of choanocyte chambers, thereby further increasing the choanoderm SA • Spongocoel is further reduced • Therefore, these are the largest sponges (few cm to one meter, or more) • Thickest body walls • May possess more than one osculum

Body Wall Composition • The body walls of sponges are classified as either being cellular or syncytial • Cellular sponges have two primitive primary tissues • Epithelioid – resembles epithelium • Mesohyl – connective tissue middle layer • There are two specific types of epithelioid tissue • Pinacoderm – lines inside (except where choanoderm is present) and outside • Choanoderm – choanocytes posses a flagellum and collar of microvilli

Mesohyl is a fibrous (proteinaceous) ECM containing cells and skeletal elements • All mesohyl cells are totipotent and amoeboid • Archeocytes – can differentiate into any type of sponge cell. Aide in digestion (via phagocytosis) and internal transport • Lophocytes – secrete and maintain collagen fibers • Spongocytes – responsible for producing thick skeletal fibers known as spongin

Mesohyl cells cont… • Sclerocytes – secrete spicules, interesting skeletal elements made of silica or calcium • Myocytes – essentially muscle cells; usually concentrated around osculum. Constrict or dilate to control water flow • Germ cells – reproductive cells

Syncytial sponges DO posses cells, but not at the same level of organization as cellular sponges • Cytoplasm is continuous and lacks membranes that compartmentalize separate cells • Lack pinacoderm • Lack choanoderm. Posses collar bodies instead; these are located in collar body chambers, individually rather than in epitheliod sheets • Mesohyl is present and contains archeocytes, sclerocytes, and germ cells

Syncytial sponges cont… • Body wall resembles a 3D cobweb-like pattern and is called a trabecular syncytium • Each strand of the trabecular syncytium encloses an axis of mesohyl • Collagen and spicules are present

Taxonomy • Two subphyla • Subphylum Symplasma (Hexactinellida) • Subphylum Cellularia • Subphylum Symplasma consists of glass sponges that posses syncytial tissues • Most sponges belong to Subphylum Cellularia, which is divided into two main classes, both possessing cellular tissues • Class Demospongiae – have siliceous spicules and spongin • Class Calcarea – have calcareous spicules

Skeleton • Any cell, organism, etc. needs some form of support • Mesohyl acts as an endoskeleton • Diversity of mesohylar structure • Fine collagen fibers only • May be supplemented with spicules, spongin, or both • Incredible diversity of spicules; and some can project through mesohyl to protect the outside of the sponge

Water Pumping • Most pump a volume of water equal to their body volume, every 5 seconds • Can slow / stop water flow to avoid taking in silt • Can control rate of water flow via • Contracting or relaxing myocytes, which controls osculum diameter • Closing ostia • Adjusting flagellar beat of choanoderm / collar bodies

Locomotion • Some have limited capacity for locomotion • Can move 1 to 4 mm per day • Result of collected amoeboid movement of cells • Osculum contraction • Whole body contraction

Nutrition • Filter feeders • Typically phagocytize particles of 50µm or less • Particle size determines which cells phagocytize • All cells can phagocytize • Choanocytes transfer particles to vacuoles for digestion • Archeocytes remove wastes / inorganics from system

Feed on dinoflagellates, bacteria, viruses, debris, etc. • Some are carnivorous (don’t filter); trap small animals such as crustaceans

May posses photosynthetic endosymbionts • Cyanobacteria • Dinoflagellates • Chlorophytes • Symbionts may cause sponge to be brightly colored • Sponges must live in shallow water to photosynthesize • Some obtain up to 80% of nutrients from photosynthate

Internal Transport • Gas / waste transport is carried out via simple diffusion • Sponges are “leaky,” so water penetrates almost the entire animal • Sheets of cells are only one cell layer thick • Mobile amoeboid cells • Ammonia is the main metabolic waste • Almost always the case for animals in aqueous environments • Uric acid or urea in terrestrial habitats • Archeocytes transfer wastes / nutrients • Some individual cells posses contractile vacuoles

Nervous Tissue • Lack nerve cells • Some have localized impulses responsible for myocyte contraction • Glass sponges can generate action potentials that travel all across their syncytium at a rapid rate • Used to arrest flagellar beating

Ecology • Many produce toxins to prevent predation • Some animals are spongivores • Nudibranchs • Fish • Turtles – Hawksbill turtle feces can be up to 95% siliceous spicules • Some release chemicals that kill competing sessile organisms, such as corals

Some animals such as shrimps and brittle stars live in sponges • Decorator crabs may place sponges on their carapaces • Cliona breaks down calcareous shells. Bores into shells for protection

Reproduction • Sponges are excellent regenerators • Reproduce clonally through several methods • Fragmentation – usually due to wave damage or grazing • Budding • Gemmules (winter bodies) – spore-like structures that are essentially a mass of nutrient-laden archeocytes that are surrounded by a shell. Undergo diapause

Sponges frequently reproduce sexually • Sponges are hermaphrodites (monoecious) • Germ cells occur throughout mesohyl • Choanocytes can also release sperm, and form eggs

Sperm are broadcasted into water column • Choanocytes phagocytize incoming sperm, but don’t digest • They differentiate into an amoeboid cell and deliver sperm head to egg • Most eggs are fertilized through phagocytosis (therefore, most sponge sperm lack an acrosome)

Some sponges are oviparous, and release zygotes into water column • Most are viviparous, and retain zygotes in their body, later releasing larvae • Sponge larvae are very diverse

Larvae are short lived • Settle within a few days • Creep across substrate until a suitable spot is found • Metamorphose into a juvenile • Varied lifespans • May live one to a few years in temperate zones • Tropical or deep sea spp. May live 200 years or more • Some only grow 0.2 mm/yr, and could be 5000 years old if that growth rate is constant

Biology 320 Invertebrate Zoology Fall 2005