PHYLUM PORIFERA

Read Also

The sponges are the first metazoans (multicellular animals) that we will study.   The principal features of phylum Porifera are listed below.

1.   While some sponges are radially symmetrical, the majority of sponges are asymmetrical in body form.   Sponges are considered to be at a cellular grade of construction; that is, they have cellular differentiation (tissues) without cellular coordination.

2.   The outermost tissue layer of sponges is composed of cells called pinacocytes.   In some sponges this outer tissue layer is syncytial while in others the pinacocytes are all distinctly separated from one another by cell membranes.   The innermost tissue layer is composed of cells called choanocytes or collar cells (see S&S, p.45) which have flagella that beat to produce water currents through the sponge body.  Between these two tissue layers is a gelatinous layer called the mesoglea (mesohyl).   The mesoglea is not considered to be a tissue since it contains a number of different kinds of independently functioning cells.   Each cell type in the mesoglea has a specific name, but the general term for all of these wandering cells is amoebocyte.

3.   Some of the amoebocytes in the mesoglea are specialized for secreting a skeleton.   The sponge skeleton may be composed of mineral spicules, spongin fibers or a combination of these two, depending on the kind of sponge.   Spicules may be calcareous (composed of Ca CO₃) or siliceous (composed of H₂Si₂O₇).   Spongin fibers are composed of a sulfur-containing schleroprotein.

4.   Water enters the body of a sponge by way of a number of minute incurrent pores or ostia.   Water leaves the body by way of one or more large excurrent pores or oscula.   Within the body of the sponge, the water may pass through a large cavity (the spongocoel) through a system of canals and chambers, or through a combination of these two.

5.   Movement of water through the sponge body is accomplished by the beating of the choanocyte flagella.   The choanocyte cells line either a spongocoel or a number of small chambers, depending on the sponge.   A choanocyte cell consists of a nucleus, one or more vacuoles, a long flagellum and a delicate, collarlike structure which surrounds the base of the flagellum.   Electron microscope studies show the collar of a choanocyte to be composed of a circular arrangement of microvilli-like structures extending outward from the cell body.   The rotary motion of the flagellum forces solid food particles in the incoming water to adhere to the outside surface of the collar.   The streaming protoplasm of the collar transfers the food to the collar base where ingestion can occur.

6.   Sponges may be divided into three basic grades or types based upon the arrangement of their water canal systems.   Note that these grades or types are not taxonomic groupings.   The three types of sponges are described below and are shown diagrammatically.

·         Asconoid Type -  Water entering the sponge passes through ostia which are actually openings within doughnut-shaped cells called porocytes, which are found only in asconoid sponges.   The water enters the large central cavity called the spongocoel, which is lined with choanocytes.   Water exits from the spongocoel through a single large osculum.

·         Syconoid Type -  Water enters the sponge through ostia which are openings between cells, rather than within cells as in asconoid sponges.   Water then passes into radially arranged incurrent canals which lead to flagellated chambers lined with choanocytes.   Water leaves the flagellated chambers by way of excurrent canals that lead to the spongocoel, which is lined a simple flat epithelium.   Water exits from the spongocoel by way of a single large osculum.   Note that the body wall of syconoid sponges is thicker than that of asconoid sponges and that the syconoid spongocoel is not lined by choanocytes as is the asconoid spongocoel.

·         Leuconoid Type -  The ostia of a leuconoid sponge are like those of a syconoid sponge.   These ostia lead into a complex system of canals and flagellated chamgers that penetrate the very thick, dense mesoglea.  There is no spongocoel in a leuconoid sponge.   Rather, water reaches the oscula by way of large excurrent canals.   The complex canal system of leuconoid allows for greater surface area over which water may pass and consequently creates an increased area for food and oxygen uptake and for waste removal.   It is not surprising, therefore, that leuconoid sponges are the largest in size of all the types and that the vast majority of sponges are leuconoid.

7.  Sponge taxonomy is based on skeletal composition.   The four classes in phylun Porifera are listed below along with distinguishing characteristics for each class.   The grades of sponges found in each class are given in parenthesis, although this is not distinguishing since there is overlap between the classes.

            a) Class Calcarea - contains sponges having calcareous spicules with 1 to 4 rays.  (asconoid, syconoid, leuconoid)

            b) Class Hexactinellida - contains sponges having siliceous spicules with 6 rays.   These spicules are often fused to form a beautiful lattice-like cylinder, as in the so-called Venus' flower basket. (syconoid)

           

            c) Class Demospongiae - contains sponges having siliceous spicules (not 6-rayed) and/or spongin fibers.  (leuconoid)

            d) Class Sclerospongiae - contains sponges having an internal skeleton of siliceous spicules and spongin fibers plus an outer encasement of calcium carbonate.  Only six species from the Caribbean have been described to date.  (leuconoid)

8.   Sponges are capable of both sexual and asexual reproduction and they also have great powers of tissue regeneration and reassociation. Sexual reproduction is accomplished by production of eggs and sperm which unite to form a zygote.   The zygote divides repeatedly to produce a free-swimming larval form.   Depending on the sponge, this larva may be either a uniformly ciliated parenchymula larva or an amphiblastula larva, which has flagella only at one pole (Fig.  2.7, p.55 of S&S).   The larvae eventually settle and metamorphose into the adult form.

Sponges may reproduce asexually by budding.   In addition, all freshwater sponges and some marine forms produce resistant overwintering bodies called gemmules.   These gemmules consist of aggregations of food laden amoebocytes surrounded by a resistant covering.   They are produced during periods of cold or drought and can survive to produce a new sponge body when conditions improve (Fig. 2.8, p.55 of S&S).

            In today's laboratory you will examine the structure of the sponge species available and then perform experiments on the reassociation of porifera cells.

ASCONOID SPONGES

            Asconoid sponges are the simplest and most primitive sponge architectural type and are all relatively small due to their inefficient filtering system.   Asconoid structure is demonstrated in Leucosolenia.   Obtain a small colony of Leucosolenia sponges and place it in a dish filled with seawater.   Examine it under a dissecting microscope.   Does it respond to a stimulus?   Do you detect movement?   To observe the filtering mechanism of Leucosolenia, prepare a dilute suspension of carmine powder and seawater and then gently place a drop of the suspension near the colony.   Describe the water movement through Leucosolenia.   Where do the carmine particles enter?   Where do they exit?   Do particles enter the sponge at the same velocity that they exit?   Explain.   Do you see budding on your Leucosolenia colony?   If so, where are the buds positioned?   Examine the colonial structure of Leucosolenia.  Describe how you think the ultimate colonial form develops from a single sponge tube.

From your Leucosolenia colony remove a single sponge tube for closer examination and then rinse the colony in fresh seawater and return it to the holding tank.   Cut the sponge tube longitudinally into two halves (from osculum to base) and place the two halves on a slide so that half the tube shows the inner surface and the other half shows the outer surface.   Add a drop of saltwater and cover with a coverslip.   Examine both surfaces under the compound microscope.   Try to identify porocytes, pinacocytes, and choanocytes, or evidence of their presence (refer to S&S, pp.  45‑47 for diagrams).   Next, tease apart the sections of sponge with a dissecting needle and examine for spicules and amoebocytes.   Describe the shape and arrangement of the spicules.   How are spicules formed?   Do you see any evidence of this in your preparation?

Examine the prepared slides of asconoid sponges.  The staining of these slides will make the cellular structures easier to identify.