Porifera (Sponges) The phylum Porifera (the sponges) includes about 5000 species almost all of which are marine (there are about 150 freshwater species, members of the family Spongillidae). Sponges occur worldwide at all latitudes from the intertidal zone to the deep sea. Range in size from a few millimeters to 2 meters across. Porifera means “pore-bearing” and refers to the numerous pores and channels that permeate a sponge’s body.

Yellow tube sponge Barrel sponge

Porifera Sponges are the simplest multi-cellular organisms, but they lack the germ layers of more complex. Have a cellular level of organization lacking true tissues and organs. Body is a mass of cells imbedded in a gelatinous matrix (mesohyl) which is supported by a framework of spicules, as well as collagen and spongin fibers.

Glass sponge spicules

6.8 Spicules form the stiffening skeletal structure of the sponge and may be made of calcium carbonate, silica or spongin [collagen].

Porifera feeding Sponges are sessile (they don’t move) when adults and depend on water movement to bring in food and oxygen and remove wastes. Sponges generate their own flow of water having a unique water current system.

Porifera feeding Water enters through many small pores called ostia and exits through fewer, larger oscula.

Oscula

Porifera feeding Openings are connected by a series of canals, which are lined by choanocytes (the flagellated collar cells) that maintain the current and filter out food particles. Choanocytes

Porifera feeding The choanocyte’s collar consists of microvilli joined together by delicate microfibrils,which filter out tiny food particles. The beating of the flagellum draws water through the collar and out the top. Particles too big to pass through the collar get trapped in mucus and slide down the collar to the base where they are phagocytized.

Porifera feeding Sponges can filter enormous volumes of water as much as 20,000 times the volume of the sponge in 24 hours. Sponges mostly consume bacteria and may filter as much as 90% of those passing through.

Carnivorous sponges A few sponges, however, capture small prey, such as crustaceans. Members of the family Cladorhizidae capture their prey using spicules that act like velcro to hold the prey. Cells then surround and digest the prey extracellularly.

Types of cells in Porifera Sponge cells occur scattered through a gelatinous matrix called mesohyl. Spicules are distributed through the sponge as are several different specialized cells types.

Sponge cell types Archaeocytes: move around within the mesohyl. They are ameboid in appearance and carry out several tasks. Phagocytize particles and receive particles for digestion from choanocytes. Can differentiate into other specialized cell types. Secrete structural components. Specialized archaeocytes called sclerocytes, spongocytes and collenocytes secrete respectively spicules, spongin and collagen.

Sponge cell types Collar Cells engage in filter feeding. One end is imbedded in mesohyl and the other end protrudes. The protruding end of the choanocyte has a flagellum that moves water through a mesh-like collar where small particles are trapped.

Sponge cell types Porocytes: These are tubular cells that in the simplest type of sponge form tubes through the wall of the sponge and allow water to flow into the central chamber.

Canal systems Most sponges have a canal system through with water enters from one place and exits through another:

Fig 6.3 a

Sponge reproduction Sponges reproduce both sexually and asexually. Most sexually reproducing species are hermaphrodites (individuals produce both male and female gametes at different times). Sperm are shed into the water and taken up by other sponges. Individuals with eggs use special cells called archaeocytes to transport sperm to the eggs.

Sponge reproduction Zygotes develop into ciliated larvae that are released into the water and eventually settle and develop into a sponge. Asexual reproduction is either by budding or more commonly the production of gemmules which are clusters of cells unspecific cells called Amoebocytes surrounded by a protective coat.

Gemmule

Totipotency Sponge possess several different types of cells. All sponge cells are totipotent and can give rise to any of the other types of cell. A single cell can give rise to a new sponge or can self-assemble with other cells into a sponge. (a sponge separated into its constituent cells will spontaneously reassemble).

Clathrina canariensis (Calcarea)

Calcarea Hexactinellida

Marine Demospongiae on a Caribbean coral reef

Coral Clips NG http://video.nationalgeographic.com/video/coral-reefs Spawning http://video.nationalgeographic.com/video/coralreef_spawning

Phylum Placozoa Placozoans are tiny (<3mm), odd animals. Consist of a few thousand cells that form a flat transparent layer, but with ventral and dorsal sides that differ in structure. They feed by flowing over surfaces and digesting small items they encounter. Only 4 cell types occur in Placozoans (cf. 16-20 in sponges and >200 in mammals).

http://www.ucmp.berkeley.edu/phyla/placozoa/placozoa.html

Placozoans Nothing is known about Placozoan behavior in the wild. They were discovered first in a European aquarium in the 1880’s and have only been studied in the lab.

Placozoans Because they are so simple, when these animals were first discovered it was thought they were the most primitive metazoans, even simpler than sponges. However, the dorsal surface of Placozoans is a tissue layer of ciliated cylinder cells and non-ciliated gland cells which suggests they diverged after the sponges branched off from other animals. There is some evidence that Placozoans may have diverged from other animals after the cnidarians (jellyfish) split off and secondarily became simplified, but the phylogenetic picture is still unresolved.