Multicellular and

Chapter 9

Tissue Levels of

Organization

Origins of Multicellularity• Multicellular life arose quickly in the 100 million years prior to

Precambrian/ Cambrian boundary• The evolutionary events that leads

to multicellularity is a mystery• Scientist believe that multicellularity

could have arisen as dividing cells remained together.

called COLONIAL HYPOTHESIS (hypothesis)

• Second hypothesis – SYNCYTIAL HYPOTHESIS - a syncytium is a large,

multinucleated cell. The formation of plasma membranes in the cytoplasm of a syncytial protist, could have produced a small, multicellular organism

(Syncytial refers to protoplasm that contains numerous nuclei not separated from each other by plasma membrane).

Problems with Syncytial Theory • Radial symmetry occurs in "Radiates" (sponges, cnidarians, ctenophores, and placozoans) • No evidence of syncytial cells in basal metazoans.

Phylum Porifera - Sponges • Primarily marine animals that consist

of loosely organized cells; approx 9k spp, from < 1cm to > 1m

Characteristics of members of Phylum Porifera include:

1.  asymmetrical or radial symmetry

2.  3 types of cells - pinacocytes, mesenchyme cells (amoebocytes) and choanocytes

3.  Central cavity or several branching chambers, thru which water flows for filter

feeding

4.  no tissues or organs

Cell types, Body wall, and Skeletons 1.  sponge cells are specialized for particular

functions (division of labor) a. Pinacocytes = These cells are the "skin cells" of

sponges. They line the exterior of the sponge body wall. They are thin, leathery and tightly packed together.

= may be slightly contractile and help sponge change shape.

= Some pinacocytes specialized into porocytes, which regulate water circulation

• b.  jelly like layer under pinacocytes is termed mesohyl. Mesenchyme cells are amoeboid, and move about in the mesohyl. Specialized for reproduction, transporting and storing food, secreting skeletal elements (spicules)c.  beneath mesenchyme, lining inner chambers are choanocytes - collar cells. Flagellated cells with ring of microvilli surrounding flagella. Microfilaments connect microvilli, forming a net that helps filter edible particles

Sponges are supported by skeleton that may consist of spicules - needlelike spikes • spicules are formed by amoeboid cells • made of CaCO3 or silica • may take on a variety of shapes• alternatively, skeleton may be made of

spongin, a fibrous protein made of collagen - dried beaten and washed to produce commercial sponges

Spicules

Types of spicules

Water currents and body forms

• sponges lives depend on the water currents that choanocytes create 

1. water brings food and O2, removes wastes

2. methods of food filtration and circulation reflect body forms in the phylum

Three types of body forms:• a. ascon body form - simplest and

least common. Vaselike form»ostia are outer openings of

porocytes and lead directly to chamber called spongocoel

»choanocytes line spongocoel and their flagellar movements draw water into the spongocoel thru the ostia

»water exits sponge thru osculum, single large opening at the top of the sponge

b. sycon body form - sponge wall appears folded

• water enters thru dermal pores, which are openings of incurrent canals

• pores in body walls open to radial canals, and radial canals lead to spongocoel

• choanocytes line radial canals and beating of flagella moves water from ostia, thru incurrent and radial canals, to spongocoel and out the osculum.

c. leucon body forms have an extensively branched canal system. • Water enters the ostium and moves thru

branched incurrent canals,•   incurrent canals lead to choanocyte lined

chambers. Canals leading away from the chambers are called excurrent canals

•   proliferation of chambers and canals has resulted in absence of spongocoel. Often there are multiple exit points for water leaving sponge

Maintenance functions 1. sponges feed on particles that range in

size from .1 to 50 um. a. bacteria

b.  microscopic algaec.  protistsd.  other suspended particles

2. important in reducing coastal turbidity a. 1 leucon sponge, 1 cm in diameter and

10 cm high, filters 20 liters of water/day!

3. a few sponges are carnivorous - catch small crustaceans (deep water) with spicule-covered filaments.

4. feeding methods - choanocytes filter small suspended particles.

• a. Water passes thru collar near base and moves into spongocoel at open end of collar

• b. suspended food is trapped on collar and moved along microvilli to base of collar, where it is incorporated into a food vacuole

• c.  lysozymal enzymes and pH changes digest particle in vacuole

• partly digested food passed to amoeboid cells, that distribute it

• pinacocytes lining incurrent canals may phagocytize larger food particles. Sponges may also absorb nutrients in sea water thru active transport

Feeding – filter feeding

5.  Sponges get rid of waste thru diffusion, since all cells are in close contact with water

6. Sponges have no nerve cells for communication/coordination, but somehow choanocytes can cease activities more or less simultaneously, ceasing water circulation. chemical messages sent by ameboid cells is one possible method

Reproduction • most sponges are monoecious - both

sexes occur in same individual; do not usually self fertilize because eggs and sperm ready at different times.

• Asexual. buds or, in many freshwater species, gemmules

• Development usually internal. • Larvae free-swimming, and come in a

variety of forms.

• 1. certain choanocytes lose collars and flagella and undergo meiosis to form flagellated sperm2.  other choanocytes may undergo meiosis and form eggs. Eggs retained in mesohyl of parent

• 3.  sperm cells exit one sponge by osculum and enter another with incurrent water. they are trapped by choanocytes and put in vacuoles.

• 4.sperm lose collar and flagella, become ameboid and transfer sperm to eggs

• 5. early development occurs in mesohyl, then a flagellated larva forms. Larva breaks free, free-swims for up to 2 days before settling to substrate and develops into adult form (Fig. 9.8)

• 6.some sponges form resistant capsules called gemmules, which contain masses of ameboid cells. gemmules can survive freezing and drying (9.8c,d) When favorable conditions return, ameboid cells stream out of tiny opening, and organize into a sponge

• 7. Some sponges can regenerate from an individual cut or broken apart.

Classes of Porifera• Hexactinellida glass sponges, siliceous

lattice,  syconoid forms • Spicules – hexaxons • Lattice – siliceous w/ sieve plate over

osculum • Basal spicules w/ tufts for soft sediment • Structure and habitat •     Individualized cup, urns or vase shape •     Body wall- w/out pinacoderm, syncitium

externally and internally • Deep water (200 meters-abyss) /

Cosmopolitan w/ more in Antarctic

Euplectella

Class Calcarea (Calcispongeae)

• Spicules - Mon-, tri-, or tetraxon shapes

• Calcium carbonate, No spongin • Structure and habitat •     Small (<10 cm) • Occupy shallow water • Cosmopolitan • All 3 body types

Leucosolenia

Class Demospongiae• – most common form, leuconoid

forms • Spicules - Tri- or tetraxon • Along with spongin • Structure and habitat • Brightly colored (amebocytes w/

pigment) • Shape reflects habitat & resources

available • Encrusting on vertical surfaces or in

crevices

• Tubular (w/ branching) on limited substrates (conserves space)

• Shallow to deep water • Algal symbionts- non-motile

zooxanthella or cyanobacters in mesohyl or amebocytes

Grass sponge

Class Sclerospongiae• – Leuconoid forms, Found in grottos

or coral tunnels • Internal siliceous spicules & spngin • External calcareous portion

The End