Porifera – the sponges

porus (pore) + fera (bearing) Abundant marine grp w some fresh water (f.w.) species. Can range in size fr. small crusts or growths or simple vases few mm high to 1-2 meters.

Definition Porifera are asymmetrical or radially

symmetrical Metazoans of the cellular grade of construction, without organs, mouth, digestive system or nervous tissue, with a body permeated with pores, canals and chambers thru which a water current flows, and with one to many internal cavities lined with choanocytes.

 Symmetry

Arrangement of parts into geometrical designs (but these are idealized forms never found in nature).

Asymmetry. No slice through the body will divide it into two like halves. Most sponges. Asymmetrical animals are plant-like and display irregular growth.

Symmetry Radial symmetry. The body has the

general form of a short or tall cylinder with one main longitudinal axis around and along which parts are arranged.

Any plane thru this longitudinal axis divides the body into 2 like halves.

Examples: Some of the sponges which are vase-shaped, the Cnidaria and the echinoderms.

Symmetry

 Bilateral symmetry. Only one plane of symmetry which divides an animal into symmetrical left and right halves which are comparatively the same.

Sagittal plane divides body into dorsal and ventral halves which are unalike.

Most animals.

Metazoa Multicellular animals. Eukaryotic, heterotrophic organisms

lacking cell walls and having more than one differentiated cell type.

Separate from plants because not photosynthetic.

Fungi have cell walls. Bacteria are not eukaryotic. Protista are not multicellular with more

than one differentiated cell type.

Cellular Construction

Loose aggregation of cells hardly formed into tissues (group of morphologically and functionally similar cells).

Sponges consist entirely of epithelia and mesohyl = mesenchyme.

More Defining Characters

No mouth organs, systems, digestive tract, anus, nerve cells.

Surface perforated by numerous pores. Incurrent openings are small and

numerous Excurrent - few and large

More Defining Characters

Internal cavities. Interior is hollow or permeated by

numerous channels. Some or all interior spaces are lined

by specialized cells called choanocytes.

Cell types Pinacocytes. = thin,

flat epithelial-type cells that cover the exterior surface and some interior surfaces- nearest thing to true tissue is the arrangement of these cells of the epidermis.

Somewhat contractile. Help regulate surface area of the sponge and can control pore openings to regulate rate of water flow.

Cell Types Amoebocytes.

(archaeocytes, sclerocytes, etc.) wandering cells which move in the mesohyl and perform a variety of functions.

carry food aid in digestion carry pigments form reproductive cells

(archaeocytes) secrete spongin fibers of

skeleton (spongocytes) secrete spicules and arrange

them (sclerocytes) some appear to contract

(muscular fn), and some appear to have nerve cell fn.

Cell Types choanocytes =

collar cells. Line canals and

chambers and are responsible for water flow through the sponge and food capture.

Choanocytes one end anchored in mesohyl and

other exposed with flagellum surrounded by the collar.

b. Beat of flagella pulls water thru sieve-like collar and forces it out thru the open top of collar.

Particles too large to enter collar become trapped in mucus and slide down collar to base where phagocytized by cell body.

Larger particles don't get past pores - strained out there. Food engulfed by the cells either digested in food vacuoles of choanocyte or passed on to amoebocytes where digested.

Types of Canal Systems Asconoids - flagellated

spongocoels. Simplest - small and tube

shaped. Very few sponges of this type.

Water enters through microscopic pores into large spongocoel lined with choanocytes.

Choanocyte flagella pull water thru pores and out single large osculum (excurrent opening).

Types of Canal Systems

Syconoids - flagellated canals Somewhat larger and derived from asconoids

During development - pass thru asconoid stage. Few sponges

Tubular body and single osculum, but body wall thicker and more complex. Contains choanocyte-lined radial canals that empty into spongocoel.

c. Spongocoel lined with epithelial-type cells, not choanocytes.

d. Water in thru large numbers of dermal pores --> incurrent canals --> filters into choan. - lined radial canals --> forced into spongocoel by choanocyte flagella --> out thru osculum.

Types of Canal Systems Leuconoids - flagellated

chambers. Most complex - leads to increase in

size. Large colonial masses, each individual its with own osculum, but individuals poorly defined and often impossible to distinguish.

Clusters of flagellated chambers filled from incurrent canals - discharge water into excurrent canals that eventually lead to osculum

  Types of Canal Systems

 Evolutionary trends within phylum Increased flagellated surfaces in

proportion to volume --> increased no. of choanocytes to meet feeding demands.

First get outpushing of spongocoel of asconoid --> radial canals of syconoid. Then further folding of body wall --> complex channels and chambers of leuconoids.

Reproduction - Asexual Buds - detach --> new sponges or remain

attached --> colonies Gemmules - f.w. and some marine -

totipotent amoebocytes (archaeocytes) collect inside shell.

when parent dies gemmules released. gemmules dormant thru freezing or severe

drought. good conds. cells escape --> new sponges. In f.w. = adaptation to changing conds. and way

of colonizing new habitats - carried by streams or animals.

Reproduction - sexual Most are monoecious (having male and female

sex organs in the same individual). Usually no self fertilization because sperm and eggs are not produced at the same time. Why is this a good idea?

In some sponges certain choanocytes loose collars and flagella and undergo meiosis to form flagellated sperm.

These exit one sponge through its osculum and enter another with the incurrent water. Sperm are trapped, incorporated into a vacuole and transported to an egg by amoebocytes.

Eggs come from other choanocytes or amoebocytes.

They are retained in mesohyl of parent. Early development in mesohyl in most sponges.

Embryology

Weird and different from any other animal. Larval stage is usually a flagellated

parenchymella. Interior has all cells of sponges but no choanocytes.

Settles. Creeps around on flagella and eats extracellularly.

Embryology External flagellated cells loose their

flagella and move inside where they regrow flagella and become choanocytes.

Then goes through development into asconoid, syconoid, leuconoid.

Cells differentiate before organism attains final form, unlike most animals where gross form precedes specialization of cells.

Embryology In some sponges larva is hollow

amphiblastula. Micromeres with flagella Macromeres nonflagellated

Reorganizes by cells growing inwards (invagination) by cells growing over others (epiboly) or both.

Embryology Macromeres overgrow micromeres.

Unique! Macromeres produce pinacoderm

and micromeres become choanocytes.

Both layers produce amoebocytes of mesohyl.

No way to homologize with germ layers of other metazoa. Nothing like endoderm is found in sponges!

Regeneration

Tremendous ability to repair and restore lost parts.

Can also reconstitute selves if totally disintegrated.

Sponge tissue has some similarity to human connective tissue. Could lead to aid in tissue transplantation.

Phylogenetic Considerations Furnish us with some idea of the

construction of the early Metazoa because of primitive (like the ancestor) traits.

Slight differentiation and independence of cells

Limited tissue formation Surface layer remains undeveloped. In

other animals generally differentiates at an early stage. Differentiation in sponges comes from the mesohyl.

Phylogenetic Considerations

Epidermis and mesohyl from same embryonic cells. Mesohyl theoretically ectoderm?

No cephalization or digestive sac and protozoan mode of food intake and digestion has been retained by all of the cells.

Mode of skeleton formation is protozoan, concerning only single cells or groups of cells.

Phylogenetic Considerations Choanocytes lining internal cavities and

embryology are unique. Some cells similar to choanocytes are found

in other animals. Certainly find flagellated cells in all animal groups (sperm).

rRNA evidence supports idea that choanocytes are remnants of a choanoflagellate ancestry for sponges. (see paper)

Collar cells were especially retained in sponges.

Phylogenetic Considerations Best guess today is that sponges

evolved from single-celled flagellated protists (choanoflagellates).

Animal line is monophyletic and includes choanoflagellates.

Sponges diverged and went their own way.

Cnidarians and Placozoa diverged along a different pathway.

Phylogenetic Considerations

Phylogenetic Considerations

Phylum Placozoa: Trichoplax adhaerans

Phylum Placozoa: Trichoplax adhaerans

Animal with the simplest construction.

Only 4 cell types. transparent, flat, round (up to 3

millimeters across), and have two distinct sides.

Ventral tissue surface has two types of cells, column-shaped cylinder cells with cilia and gland cells

The upper dorsal surface consists of a layer of cover cells, which are ciliated and flattened toward the outside of the animal.

Middle layer contains star-shaped fiber cells connected to each other.

Phylum Placozoa: Trichoplax adhaerans Feed by moving over food (algae

usually) and digesting extracellularly. Easy to see how they could give rise

to Cnidarians, but they are not radially symmetrical and this is problem.

Could be degenerate from bilateral metazoa.