Phylum Porifera: The Sponges

 

Iintroduction 

 

Sponges consist of any of several thousand species constituting a phylum of simple invertebrate animals.

 

Sponges come in an incredible variety of colors and an amazing array of shapes.

 

Sponges are either radially symmetrical or asymmetrical.

 

Sponges are mainly marine, with a few freshwater species.

 

Sponges are ubiquitous sea creatures, found at all latitudes beneath the world's oceans, and from the intertidal zone to the deep-sea.

 

Generally, they are sessile, though it has been shown that some are able to move slowly (up to 4 mm per day) within aquaria.

 

It is unknown whether this movement is important for sponge ecology under natural conditions.

 

They are abundant throughout the world and especially in tropical waters, where they and other invertebrates such as corals are important in the formation of calcareous deposits.

 

All major sponge groups in existence today had representatives living in the Cambrian period. 

 

Fossil sponges are among the oldest known animal fossils, dating from the Late Precambrian.

 

Since then, sponges have been conspicuous members of many fossil communities; the number of described fossil genera exceeds 900.

 

There are approximately 5,000 living sponge species.

 

Considerable debate exists as to the relation of sponges to other invertebrate groups.

 

That they are a multicellular line derived from single‑celled protozoans is generally accepted, but their relationship to the coelenterates (jellyfish and corals) is less clear.

 

Many species contain toxic substances, probably to discourage predators.

 

Certain other marine animals take advantage of this characteristic of sponges by placing adult sponges on their bodies, where the sponges attach and grow.

The chemicals also probably play a role in competition among sponges and other organisms, as they are released by sponges to insure themselves space in the marine ecosystem.

 

Some of these chemicals have been found to have beneficial pharmaceutical effects for humans, including compounds with respiratory, cardiovascular, gastrointestinal, anti-inflammatory, antitumor, and antibiotic activities.

 

Characteristics

 

The primitive cellular construction of sponges consists of an outer layer of covering cells called epidermis and an inner layer of flagellated cells called collar cells or choanocytes that move water through the animals.

 

In the mesoglea between the two layers are numerous wandering, amoeboid cells and supporting skeletal structures, often in the form of hard, crystal‑like spines called spicules.

 

Sponges are characterized by the possession of a feeding system unique among animals.

 

Sponges feed on fine particulate material by moving water into lateral incoming pores called ostia (incurrent openings) and then out a large topmost excurrent opening called the osculum.

 

Each flagellated cell, or choanocyte, has a thin collar around a single flagellum.

 

Choanocytes line either large chambers or small ones called flagellated chambers.

 

If the flagellated chambers are present, they are connected by canals, and the entire sponge can build up pressure and shoot water long distances from the osculumBapparently a mechanism to avoid reuse of oxygen‑ and food‑depleted water.

 

Water flowing through sponges provides oxygen, as well as a means for waste removal.

 

Choanocytes in the sponge walls filter food particles from the water as the water is pumped through the body and out other larger openings.

 

The flow of water through the sponge is unidirectional, driven by the beating of flagella which line the surface of chambers connected by a series of canals.

 

Sponges are capable of regulating the amount of flow through their bodies by the constriction of various openings.

 

The volume of water passing through a sponge can be enormous, up to 20,000 times its volume in a single 24 hour period.

 

In general, sponges feed by filtering bacteria from the water that passes through them.

 

Some sponges trap roughly 90 percent of all bacteria in the water they filter.

 

Other sponges, in particular glass sponges, appear to be less efficient at capturing bacteria and may specialize in feeding on smaller bits of organic matter.

 

Food items are taken into individual cells by phagocytosis, and digestion occurs within individual cells.

Amoebocytes deliver food to all cells since they share in the division of labor.

 

Still other sponges harbor symbionts such as green algae, dinoflagellates, or cyanobacteria, from which they also derive nutrients.

 

The water movement through some sponges is aided by water currents passing over raised excurrent openings. This moving water creates an area of low pressure above the excurrent openings that assists in drawing water out of the sponge.

 

Sponges, then, fall into three main groups according to how their bodies are organized.

 

The simplest sponges are the asconoid sponges.

 

These are shaped like a simple tube perforated by pores.

 

The open internal part of the tube is called the spongocoel; it contains the collar cells.

 

There is a single opening to the outside, the osculum.

 

The next-most complicated group is the syconoids.

 

These tend to be larger than asconoids.

 

They also have a tubular body with a single osculum, but their body wall is thicker and the pores that penetrate it are longer, forming a system of simple canals.

 

These canals are lined by collar cells, the flagellae of which move water from the outside, into the spongocoel and out the osculum.

 

The third category of body organization is leuconoid.

 

These are the largest and most complex sponges.

 

These sponges are made up of masses of tissue penetrated by numerous canals.

 

Canals lead to numerous small chambers lined with flagellated cells.

 

Water moves through the canals, into these chambers, and out via a central canal and osculum.

 

Some sponges bore into the shells of bivalves, gastropods, and the colonial skeletons of corals by slowly etching away chips of calcareous material.

 

The corals or molluscs are not eaten; rather, the sponge is probably seeking protection for itself by sinking into the hard structures it erodes.

 

Even this process has some beneficial effects, however, in that it is an important part of the process by which calcium is recycled.

 

Another interesting thing can happen when a sponge settles on a snail shell that is being used by a hermit crab; an unusual association  can be formed.

 

This sponge/crab association results in a sponge that gets around.

 

Imagine your surprise if you were to see a sponge crawl away when you reached out for it!

 

Sponges also provide a home for a number of small marine plants, which live in and around their pore systems.

 

The carnivorous exception -

 

A recently discovered sponge from the Mediterranean Sea lacks the choanocytes and canals necessary to filter feed.

 

Instead this carnivorous sponge passively traps, envelops, and digests crustaceans on its surface with a series of tiny hook‑shaped filaments. 

 

Surprisingly, they typically feed by capturing and digesting whole animals.

 

They capture small crustaceans with their spicules which act like Velcro when they come in contact with the crustacean exoskeletons.

 

Cells then migrate around the helpless prey and digestion takes place extracellularly.

 

 

Reproduction occurs either sexually or asexually.

 

Sexual reproduction -

 

Some sponges are monoecious (separate sexes).

 

The sponges are usually hermaphroditic but cross‑fertilize one another.

 

They produce eggs and sperm at different times.

 

Sperm are frequently "broadcast" into the water column.

 

That is, sperm are created, concentrated and sent out the excurrent openings, sometimes in masses so dense that the sponges appear to be smoking.

 

These sperm are subsequently captured by female sponges of the same species.

 

Inside the female, the sperm are "captured" by collar cells, which then lose their collars and transform into specialized, amoeba-like cells that carry the spermatozoa to the eggs.

 

Fertilization occurs in the mesoglea and the zygotes develop into ciliated larvae.

 

Some sponges release their larvae, where others retain them for some time.

 

Once the larvae are in the water column they settle and develop into juvenile sponges.

 

Asexual reproduction -

 

Asexual reproduction may be by means of external buds which form and eventually break off to become an independent sponge.

 

Reproduction can also occur by small, internal asexual buds called gemules.

 

Gemmules are formed during unfavorable conditions. 

 

A gemmule is composed of amoebocytes and spicules encrusted in organic matter. 

 

Each one able to give rise to a new sponge when favorable conditions return.

Fresh water sponges of the Spongillidae often produce gemmules prior to winter. These then develop into adult sponges beginning the following spring.

 

Regeneration - sponges have also been of great interest to developmental biologists because sponges are able to reconstitute themselves if their cells are separated into a suspension.

 

Commercial Sponges

 

Six species of sponge are considered marketable, with numerous varieties.

 

The skeleton of these sponges is composed only of spongin tissue and contains no hard spicules.

 

The Mediterranean sponges are the softest and best; those of the Red Sea are next in quality, and the West Indian species are coarser and less durable.

 

The sponges are gathered by divers, and the living tissue is allowed to decompose.

 

The remaining undecomposed skeleton of spongin fibers is then washed, bleached, sometimes dyed, and cut into the familiar blocks seen on store counters.

 

Scientific classification:

 

Sponges make up the phylum Porifera.

 

Three major classes exist.

 

All the Calcarea are marine, with skeletal spicules composed of calcium carbonate.

 

The Hexactinellida (glass sponges) are found in the deep sea; because their skeleton is made of silica in beautiful six‑pointed arrangements, they are called glass sponges.

 

The Demospongiae (95 percent of all living species) include the few freshwater forms.

 

Their skeletal network is made of spongin, a rather flexible protein material (that of a bath sponge made from a real sponge). 

 

In some species silica spicules are also present.

 

The Demospongiae include the carnivorous Mediterranean sponge.