This phylum contains about 5000 species of sponges.
The members of this phylum are commonly called as sponges. They are also regarded by another name i.e. pore bearers as their body walls possess numerous tiny pores.
All members of this phylum are aquatic. Most of them belong to the marine ecosystem but a few of them live in fresh water also. For Example Spongilla is a fresh water sponge. It is found in lakes and slow streams. This phylum constitutes sessile organisms that remain attached to an underwater object called substratum. They cannot move about here and there.
Now a question arises that how these organisms obtain food. The answer to this question lies in their porous body structure which enables them to obtain food and other substances from all sides.
|Sponges have cellular level of organization i.e. the cells in their body do
not form tissues, organs or organ system. All the cells present in their body
act independently and show very little coordination. However some division of
labour is present among the cells.
They are multi-cellular organisms that have bodies full of pores and channels allowing water to circulate through them, consisting of jelly-like mesohyl sandwiched between two thin layers of cells. Sponges have unspecialized cells that can transform into other types and that often migrate between the main cell layers and the mesohyl in the process. Sponges do not have nervous, digestive or circulatory systems. Instead, most rely on maintaining a constant water flow through their bodies to obtain food and oxygen and to remove wastes.
They are also known as the primitive multi-cellular animals. Most of them are asymmetric as any plane passing through their body center cannot divide them into equal halves.
Fig.- A STOVE PIPE SPONGE
They have canal system because they have a porous body. It is the distinguishing feature of all the sponges. These numerous pores are meant for the entry and exit of water current. Minute pores present in the body wall through which water enters into the body are called Ostia. The entering water provides food to the sponges and the water leaving their body carries their waste material that is it helps in the removal of waste matter.
Sponges are similar to other animals in that they are multi-cellular, heterotrophic, lack cell walls and produce sperm cells. Unlike other animals, they lack true tissues and organs, and have no body symmetry. The shapes of their bodies are adapted for maximal efficiency of water flow through the central cavity, where it deposits nutrients, and leaves through a hole called the osculum. Many sponges have internal skeletons of spongin and/or spicules of calcium carbonate or silicon dioxide. All sponges are sessile aquatic animals. Although there are freshwater species, the great majority are marine (salt water) species, ranging from tidal zones to depths exceeding 8,800 m (5.5 mi).
Fig. - OSCULUM IN A SPONGE
The body wall of a common sponge consists of the following layers:
I. Pinacoderm (Dermal Layer): This is the outermost layer which consists of two types of cell
1. Flattened Pinacocytes
2. Oval Porocytes
II. Choanoderm (Gastral Layer): It is inner cellular layer which consists of highly specialized cells called choanocytes (collar cells). They are the characteristic cells of this phylum responsible for ingestion of food, secretion of mesohyal and differentiation of sex cells.
III. Mesohyl layer (Mesenchyme): Basically it is a non-cellular layer found between the pinacoderm and the choanoderm. It has fine dispersed sponging fibers and numerous spicules. The mesohyl functions as an endoskeleton in most sponges, and is the only skeleton in soft sponges that encrust hard surfaces such as rocks. More commonly, the mesohyl is stiffened by mineral spicules, by spongin fibers or both. Demosponges use spongin, and in many species, silica spicules and in some species, calcium carbonate exoskeletons. Demo sponges constitute about 90% of all known sponge species, including all freshwater ones, and have the widest range of habitats. Calcareous sponges, which have calcium carbonate spicules and, in some species, calcium carbonate exoskeletons, are restricted to relatively shallow marine waters where production of calcium carbonate is easiest. The fragile glass sponges, with "scaffolding" of silica spicules, are restricted to polar regions and the ocean depths where predators are rare. Fossils of all of these types have been found in rocks dated from 580 million years ago. In addition Archaeocyathids, whose fossils are common in rocks from 530 to 490 million years ago, are now regarded as a type of sponge.
A sponge's body is hollow and is held in shape by the mesohyl, a jelly-like substance made mainly of collagen and reinforced by a dense network of fibers also made of collagen. The inner surface is covered with choanocytes, cells with cylindrical or conical collars surrounding one flagellum per choanocyte. The wave-like motion of the whip-like flagella drives water through the sponge's body. All sponges have ostia, channels leading to the interior through the mesohyl, and in most sponges these are controlled by tube-like porocytes that form closable inlet valves. Pinacocytes, plate-like cells, form a single-layered external skin over all other parts of the mesohyl that are not covered by choanocytes, and the pinacocytes also digest food particles that are too large to enter the ostia, while those at the base of the animal are responsible for anchoring it.
The body of spoges is organized into such a manner as to form a complex system of pores and canals. This system is called canal system. Three types of canal system are found in sponges.
I. ASCON TYPE: It is the simplest type of canal system which is found in leucosolenia and few other sponges.
II. SYCON TYPE: It is more complex than the asycon type. It is found in sycon and some other sponges.
Ingressing water-through dermal ostia-incurrent canals-through prosopyles to-Radial canals-through apopyles to-spongocoel-through osculum-to outside.
III. LEUCON TYPE- It is most complex canal system which is found in Spongilla and some other sponges.
Ingressing water-through dermal ostia to-incurrent canals-through prosopyles to-flagellated chambers- through apopyles to- excurrent canals-through osulum- to outside.
A) ASCON TYPE B) SYCON TYPE C) LEUCON TYPE
A few species that live in waters where the supply of food particles is very poor prey on crustaceans and other small animals. So far only 137 species have been discovered. Most belong to the family Cladorhizidae, but a few members of the Guitarridae and Esperiopsidaeare also carnivores. In most cases little is known about how they actually capture prey, although some species are thought to use either sticky threads or hooked spicules.
Most carnivorous sponges live in deep waters, up to 8,840 m (5.49 mi), and the development of deep-ocean exploration techniques is expected to lead to the discovery of several more. However one species has been found in Mediterranean caves at depths of 17–23 m (56–75 ft), alongside the more usual filter feeding sponges. The cave-dwelling predators capture crustaceans under 1 mm (0.039 in) long by entangling them with fine threads, digest them by enveloping them with further threads over the course of a few days, and then return to their normal shape; there is no evidence that they use venom.
Most known carnivorous sponges have completely lost the water flow system and choanocytes. However the genus Chondrocladia uses a highly modified water flow system to inflate balloon-like structures that are used for capturing prey.
Freshwater sponges often host green algae as endosymbionts within archaeocytes and other cells, and benefit from nutrients produced by the algae. Many marine species host other photosynthesizing organisms, most commonly cyan bacteria but in some cases dinoflagellates. Symbiotic cyan bacteria may form a third of the total mass of living tissue in some sponges, and some sponges gain 48% to 80% of their energy supply from these micro-organisms. In 2008 a University of Stuttgart team reported that spicules made of silica conduct light into the mesohyl, where the photosynthesizing endosymbionts live. Sponges that host photosynthesizing organisms are most common in waters with relatively poor supplies of food particles, and often have leafy shapes that maximize the amount of sunlight they collect.
A recently discovered carnivorous sponge that lives near hydrothermal vents hosts methane-eating bacteria, and digests some of them.
Sponges do not have the complex immune systems of most other animals. However they reject grafts from other species but accept them from other members of their own species. In a few marine species, gray cells play the leading role in rejection of foreign material. When invaded, they produce a chemical that stops movement of other cells in the affected area, thus preventing the intruder from using the sponge's internal transport systems. If the intrusion persists, the grey cells concentrate in the area and release toxins that kill all cells in the area. The "immune" system can stay in this activated state for up to three weeks.
The freshwater sponge Spongilla lacustris
Sponges have three asexual methods of reproduction: after fragmentation; by budding; and by producing gemmules. Fragments of sponges may be detached by currents or waves. They use the mobility of their pinacocytes and choanocytes and reshaping of themesohyl to re-attach themselves to a suitable surface and then rebuild themselves as small but functional sponges over the course of several days. The same capabilities enable sponges that have been squeezed through a fine cloth to regenerate. A sponge fragment can only regenerate if it contains both collencytes to produce mesohyl and archeocytes to produce all the other cell types. A very few species reproduce by budding.
Gemmules are "survival pods" which a few marine sponges and many freshwater species produce by the thousands when dying and which some, mainly freshwater species, regularly produce in autumn. Spongocytes make gemmules by wrapping shells of spongin, often reinforced with spicules, round clusters of archeocytes that are full of nutrients. Freshwater gemmules may also include phytosynthesizing symbionts. The gemmules then become dormant, and in this state can survive cold, drying out, lack of oxygen and extreme variations in salinity. Freshwater gemmules often do not revive until the temperature drops, stays cold for a few months and then reaches a near-"normal" level. When a gemmule germinates, the archeocytes round the outside of the cluster transform into pinacocytes, a membrane over a pore in the shell bursts, the cluster of cells slowly emerges, and most of the remaining archeocytes transform into other cell types needed to make a functioning sponge. Gemmules from the same species but different individuals can join forces to form one sponge. Some gemmules are retained within the parent sponge, and in spring it can be difficult to tell whether an old sponge has revived or been "recolonized" by its own gemmules.
Most sponges are hermaphrodites (function as both sexes simultaneously), although sponges have no gonads (reproductive organs). Sperm are produced by choanocytes or entire choanocyte chambers that sink into the mesohyl and form spermatic cysts while eggs are formed by transformation of archeocytes, or of choanocytes in some species. Each egg generally acquires a yolk by consuming "nurse cells". During spawning, sperm burst out of their cysts and are expelled via the osculum. If they contact another sponge of the same species, the water flow carries them to choanocytes that engulf them but, instead of digesting them, metamorphose to an ameboid form and carry the sperm through the mesohyl to eggs, which in most cases engulf the carrier and its cargo.
|A few species release fertilized eggs into the water, but most retain the
eggs until they hatch. There are four types of larvae, but all are balls of
cells with an outer layer of cells whose flagellae or cilia enable the larvae to
move. After swimming for a few days the larvae sink and crawl until they find a
place to settle. Most of the cells transform into archeocytes and then into the
types appropriate for their locations in a miniature adult sponge.
Glass sponge embryos start by dividing into separate cells, but once 32 cells have formed they rapidly transform into larvae that externally are ovoid with a band of cilia round the middle that they use for movement, but internally have the typical glass sponge structure of spicules with a cobweb-like main syncitium draped around and between them and choanosyncytia with multiple collar bodies in the center. The larvae then leave their parents' bodies.
Sponges in temperate regions live for at most a few years, but some tropical species and perhaps some deep-ocean ones may live for 200 years or more. Some calcified demosponges grow by only 0.2 mm (0.0079 in) per year and, if that rate is constant, specimens 1 m (3.3 ft) wide must be about 5,000 years old. Some sponges start sexual reproduction when only a few weeks old, while others wait until they are several years old.