Porifera (sponges)

SPONGES HAVE/ARE

SPONGES DON'T HAVE

Body with cells arranged around pores, canals and chambers for the passage of water. An organ system.
Skeletal structure of spicules and/or spongin. A nervous system.
All aquatic, mainly marine. Much co-ordination between cells.
Filter feeders. Tissue organization, they have some but it is very restricted.
Sessile adults; sessile, planktonic larvae. Symmetry, though some are radially symmetrical.
Sexual and asexual reproduction. A true body cavity or gut.
Gas exchange is by diffusion.  

Overview of the sponges

The sponges are an ancient group with a fossil record back as far as the Precambrian. There are about 10,000 known species of marine sponges, and about 150 freshwater sponges. They are relatively abundant in all marine waters at all depths. They vary in size from a few millimetres to over two metres across.

Their cells are structured around a system of pores, chambers and canals through which water is moved by the action of the flagellae of the choanocyte cells. The large pores called oscula (singular - osculum) are water outlets, and the small pores called ostia are water inlets.

Types of sponge cell

Pinaocytes. These are thin, flat epithelial cells covering the exterior surface and some interior surfaces. When they are located around pores they help to regulate water flow.

Choanocytes (see right) are responsible for maintaining the current of water through the sponge, and for processing food particles in the water current. The water current brings in oxygen and food, and removes waste. Eggs and sperm are also carried out of the sponge on this water current. The rate of water flow can be regulated by changing the size of the osculum. Choanocytes line the central cavity and small cavities in the canal systems (more below). In the drawings left and below the choanocyte layer is shown as a thickened black line. Each choanocyte has one flagellum ringed by a contractile collar (see right).

Archaeocytes digest food particles passed from the Choanocytes, secrete spicules, spongin and collagen.

Canal systems

There are three types of canal systems, in the following drawings the arrows show the direction of water flow:

  • Asconoids have the simplest, see the drawing on the right. These sponges are small and tube-shaped. The water enters through tiny ostia into one large internal cavity called a spongocoel, and is expelled through one large osculum. This type of canal system is found in the Calcarea Class (see below)
  • Synconoids, like asconiods, have a single, large osculum, but their body is thicker. The drawing below left shows two synconoid systems - a simple type on the left and a more complex type on the right. The water enters through numerous small ostia, and passes through incurrent canals before reaching the large central cavity. This system is found in Calcarea and Hexactinellida (see below)
  • Leuconoids have the most complex structure (see below right). They have many small ostia. The ostia lead to numerous incurrent canals, but there is no large central cavity.

 

choanocyte sponge cellasconoid canal system
two synconoid canal systems

 

leucaniod canal system

Sponge skeleton

The skeleton of spicules and spongin provides the support to keep the pores open. The spicules can be either siliceous or calcareous of a variety of shapes (see right), and can be used in identifying species. The spongin is a form of collagen.

Regeneration. Sponges have the amazing ability to regenerate an entire individual from just a few cells.

Porifera spicules
Euplectella sp. glass sponge


Above Euplectella sp.

Regadrella okinoseana, lacy basket sponge

Class Hexactinellida

These are commonly known as the glass sponges and include the Venus' flower basket (Euplectella spp.) which can be seen on the right, and Regardrella okinoseana, a lacy basket sponge below right. They are often radially symmetrical and vase- or funnel-shaped and can be up to 1 m across and 1 m high, and are very beautiful. Their distinguishing feature is the network formed by the six-rayed siliceous spicules. There are about 500 species. They occur mainly in deep, cold waters between 200 - 2000 m, although some can be found below 6000.

Euplectella sp. is rather atypical as it is found in tropical waters at around 150 m, and is rarely taller than 30 cm. Some species of Euplectella have a commensal relationship with a shrimp, Spongicola venusta. A young male and female shrimp enter the central cavity and live there. As they grow they become too large to escape through the sieve-like covering of the osulum, and so spend their entire life inside the sponge. Synconoid and leuconoid canal systems occur in this class.

The Glass rope sponge, Hyalonema sp. below is found worldwide in deep waters. It ranges in length from 18 - 50 cm - most of this being the stem. The body averages 5 - 8 cm long with a diameter of 3 - 6 cm. The long stalk of the sponge anchors in soft sediments on the sea floor.

Glass rope sponge, Hyalonema sp.

Class Calcarea

As the name suggests sponges in this class have spicules of calcium carbonate. The spicules are either free or fused. They tend to be relatively small, mostly less than 10 cm, and tubular or vase-shaped. All three types of canal system occur in this class. There are about 100 species, mainly marine in water no deeper than 1000m.

Class Sclerospongiae

This is a small group of sponges that resemble corals. They are usually found in dark tunnels in coral reefs. The skeleton consists of siliceous spicules and spongin on a thick basal layer of calcium carbonate. All in this class have the leuconoid canal system.

Class Demospongiae