Sponges are animals of the phylum Porifera
(pron.: /pɒˈrɪfərə/; meaning
"pore bearer"). They are multicellular organisms which 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 which 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.
Overview
Sponges are
like other animals in that they are multicellular, 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 silica.
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).
While most
of the approximately 5,000–10,000 known species feed on bacteria and other
food particles in the water, some host photosynthesizing micro-organisms as endosymbionts
and these alliances often produce more food and oxygen than they consume. A few
species of sponge that live in food-poor environments have become carnivores
that prey mainly on small crustaceans.
Most species
use sexual reproduction, releasing sperm cells into the water to fertilize ova
that in some species are released and in others are retained by the
"mother". The fertilized eggs form larvae which swim off in search of
places to settle. Sponges are known for regenerating from fragments that are
broken off, although this only works if the fragments include the right types
of cells. A few species reproduce by budding. When conditions deteriorate, for
example as temperatures drop, many freshwater species and a few marine ones
produce gemmules, "survival pods" of unspecialized cells that remain
dormant until conditions improve and then either form completely new sponges or
recolonize the skeletons of their parents.
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. Demosponges 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.
The sponge's
closest single-celled relatives are thought to be choanoflagellates, which
strongly resemble the cells sponges use to drive their water flow systems and
capture most of their food. Sponges are generally agreed, also, to not form a monophyletic
group, in other words do not include all and only the descendants of a
common ancestor, because Eumetazoa (more complex animals) are thought to be
descendants of a subgroup of sponges. However it is uncertain which group of
sponges is closest to Eumetazoa, as both calcareous sponges and a subgroup of
demosponges called Homoscleromorpha have been nominated by different
researchers. In addition, a study in 2008 suggested the earliest animals may
have been similar to modern comb jellies.
The few
species of demosponge that have entirely soft fibrous skeletons with no hard
elements have been used by humans over thousands of years for several purposes,
including as padding and as cleaning tools. By the 1950s, though, these had
been overfished so heavily that the industry almost collapsed, and most
sponge-like materials are now synthetic. Sponges and their microscopic
endosymbionts are now being researched as possible sources of medicines for
treating a wide range of diseases. Dolphins have been observed using sponges as
tools while foraging.
Distinguishing features
Sponges
constitute the phylum Porifera, and have been defined as sessile metazoans
(multi-celled animals) that have water intake and outlet openings connected by
chambers lined with choanocytes, cells with whip-like flagella. However, a few
carnivorous sponges have lost these water flow systems and the choanocytes. All known living sponges can remold their
bodies, as most types of their cells can move within their bodies and a few can
change from one type to another.
Like cnidarians
(jellyfish, etc.) and ctenophores (comb jellies), and unlike all other known
metazoans, sponges' bodies consist of a non-living jelly-like mass sandwiched
between two main layers of cells. Cnidarians and ctenophores have simple
nervous systems, and their cell layers are bound by internal connections and by
being mounted on a basement membrane (thin fibrous mat, also known as "basal
lamina"). Sponges have no nervous systems, their middle jelly-like layers
have large and varied populations of cells, and some types of cell in their
outer layers may move into the middle layer and change their functions.
Basic structure
Cell types
Mesohyl
Pinacocyte
Choanocyte
Lophocyte
Porocyte
Oocyte
Archeocyte
Sclerocyte
Spicule
Water flow
Main cell types of Porifera
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.
Other types
of cell live and move within the mesohyl:
- Lophocytes are amoeba-like cells that move slowly through the mesohyl and secrete collagen fibres.
- Collencytes are another type of collagen-producing cell.
- Rhabdiferous cells secrete polysaccharides that also form part of the mesohyl.
- Oocytes and spermatocytes are reproductive cells.
- Sclerocytes secrete the mineralized spicules ("little spines") that form the skeletons of many sponges and in some species provide some defense against predators.
- In addition to or instead of sclerocytes, demosponges have spongocytes that secrete a form of collagen that polymerizes into spongin, a thick fibrous material that stiffens the mesohyl.
- Myocytes ("muscle cells") conduct signals and cause parts of the animal to contract.
- "Grey cells" act as sponges' equivalent of an immune system.
- Archaeocytes (or amoebocytes) are amoeba-like cells that are totipotent, in other words each is capable of transformation into any other type of cell. They also have important roles in feeding and in clearing debris that block the ostia.
Glass sponges' syncytia
Water flow
Main syncitium
Spicules
Choanosyncitium
and collar bodies
showing interior
and collar bodies
showing interior
The glass sponge Euplectella
Glass
sponges present a distinctive variation on this basic plan. Their spicules,
which are made of silica, form a scaffolding-like framework between whose rods
the living tissue is suspended like a cobweb that contains most of the cell
types. This tissue is a syncytium that in some ways
behaves like many cells that share a single external membrane, and in others
like a single cell with multiple nuclei. The mesohyl is absent or minimal. The
syncytium's cytoplasm, the soupy fluid that fills the interiors of cells, is organized
into "rivers" that transport nuclei, organelles ("organs"
within cells) and other substances. Instead of choanocytes they have further
syncytia, known as choanosyncytia, which form bell-shaped chambers which water
enters via perforations. The insides of these chambers are lined with
"collar bodies", each consisting of a collar and flagellum but
without a nucleus of its own. The motion of the flagella sucks water through
passages in the "cobweb" and expels it via the open ends of the
bell-shaped chambers.
Some types
of cells have a single nucleus and membrane each, but are connected to other
single-nucleus cells and to the main syncytium by "bridges" made of cytoplasm.
The sclerocytes that build spicules have multiple nuclei, and in glass sponge
larvae they are connected to other tissues by cytoplasm bridges; such
connections between sclerocytes have not so far been found in adults, but this
may simply reflect the difficulty of investigating such small-scale features.
The bridges are controlled by "plugged junctions" that apparently
permit some substances to pass while blocking others.
Water flow and body structures
Asconoid
Syconoid
Leuconoid
Pinacocytes
Choanocytes
Mesohyl
Water flow
Porifera body structures
Most sponges
work rather like chimneys: they take in water at the bottom and eject it from
the osculum ("little mouth") at the top. Since ambient currents are
faster at the top, the suction effect that they produce does some of the work
for free. Sponges can control the water flow by various combinations of wholly
or partially closing the osculum and ostia (the intake pores) and varying the
beat of the flagella, and may shut it down if there is a lot of sand or silt in
the water.
Although the
layers of pinacocytes and choanocytes resemble the epithelia of more complex
animals, they are not bound tightly by cell-to-cell connections or a basal
lamina (thin fibrous sheet underneath). The flexibility of these layers and
re-modeling of the mesohyl by lophocytes allow the animals to adjust their
shapes throughout their lives to take maximum advantage of local water
currents.
The simplest
body structure in sponges is a tube or vase shape known as
"asconoid", but this severely limits the size of the animal. The body
structure is characterized by a stalk-like spongocoel surrounded by a single
layer of choanocytes. If it is simply scaled up, the ratio of its volume to
surface area increases, because surface increases as the square of length or
width while volume increases proportionally to the cube. The amount of tissue
that needs food and oxygen is determined by the volume, but the pumping
capacity that supplies food and oxygen depends on the area covered by
choanocytes. Asconoid sponges seldom exceed 1 mm (0.039 in) in
diameter.
Some sponges
overcome this limitation by adopting the "syconoid" structure, in
which the body wall is pleated. The inner pockets of the pleats are lined with
choanocytes, which connect to the outer pockets of the pleats by ostia. This
increase in the number of choanocytes and hence in pumping capacity enables
syconoid sponges to grow up to a few centimeters in diameter.
The
"leuconoid" pattern boosts pumping capacity further by filling the
interior almost completely with mesohyl that contains a network of chambers
lined with choanocytes and connected to each other and to the water intakes and
outlet by tubes. Leuconid sponges grow to over 1 m (3.3 ft) in
diameter, and the fact that growth in any direction increases the number of
choanocyte chambers enables them to take a wider range of forms, for example
"encrusting" sponges whose shapes follow those of the surfaces to
which they attach. All freshwater and most shallow-water marine sponges have
leuconid bodies. The networks of water passages in glass sponges are similar to
the leuconid structure. In all three types of structure the cross-section area
of the choanocyte-lined regions is much greater than that of the intake and
outlet channels. This makes the flow slower near the choanocytes and thus makes
it easier for them to trap food particles. For example in Leuconia, a
small leuconoid sponge about 10 centimetres (3.9 in) tall and 1 centimetre
(0.39 in) in diameter, water enters each of more than 80,000 intake canals
at 6 cm per minute. However, because Leuconia has more than
2 million flagellated chambers whose combined diameter is much greater
than that of the canals, water flow through chambers slows to 3.6 cm per hour,
making it easy for choanocytes to capture food. All the water is expelled
through a single osculum at about 8.5 cm per second, fast enough to
carry waste products some distance away.
Pinacocyte
Choanocyte
Archeocytes and other cells in
mesohyl
mesohyl
Mesohyl
Spicules
Calcium
carbonate
Seabed /
rock
Water flow
Sponge with calcium carbonate skeleton
Skeleton
In zoology a
skeleton is any fairly rigid structure of an animal, irrespective of whether it
has joints and irrespective of whether it is biomineralized. 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. Spicules may be made
of silica or calcium carbonate, and vary in shape from simple rods to
three-dimensional "stars" with up to six rays. Spicules are produced
by sclerocyte cells, and may be separate, connected by joints, or fused.
Some sponges
also secrete exoskeletons that lie completely outside their organic components.
For example sclerosponges ("hard sponges") have massive calcium
carbonate exoskeletons over which the organic matter forms a thin layer with choanocyte
chambers in pits in the mineral. These exoskeletons are secreted by the pinacocytes
that form the animals' skins.
Classes
Sponges were
traditionally distributed in three classes: calcareous sponges (Calcarea),
glass sponges (Hexactinellida) and demosponges (Demospongiae). However, studies
have shown that the Homoscleromorpha, a group thought to belong to the Demospongiae,
is actually phylogenetically well separated. Therefore, they have recently been
recognized as the fourth class of sponges.
Sponges are
divided into classes mainly according to the composition of their skeletons:
Type of cells
|
Spicules
|
Spongin fibers
|
Massive exoskeleton
|
Body form
|
|
Calcarea
|
Single
nucleus, single external membrane
|
Calcite
May be individual or large masses |
Never
|
Common.
Made of calcite if present. |
Asconoid,
syconoid, leuconoid or solenoid
|
Glass sponges
|
Mostly syncytia
in all species
|
Silica
May be individual or fused |
Never
|
Never
|
Leuconoid
|
Demosponges
|
Single
nucleus, single external membrane
|
Silica
|
In many
species
|
In some
species.
Made of aragonite if present. |
Leuconoid
|
Homoscleromorpha
|
Single
nucleus, single external membrane
|
Silica
|
In many
species
|
Never
|
Sylleibid
or leuconoid
|
Vital functions
Spongia
officinalis, "the
kitchen sponge", is dark grey when alive
Movement
Although
adult sponges are fundamentally sessile animals, some marine and freshwater
species can move across the sea bed at speeds of 1–4 mm (0.039–0.16 in) per
day, as a result of amoeba-like movements of pinacocytes and other cells. A few
species can contract their whole bodies, and many can close their oscula and ostia.
Juveniles drift or swim freely, while adults are stationary.
Respiration, feeding and excretion
Sponges do
not have distinct circulatory, respiratory, digestive, and excretory systems –
instead the water flow system supports all these functions. They filter food
particles out of the water flowing through them also called intracellular
digestion. Particles larger than 50 micrometers cannot enter the ostia and
pinacocytes consume them by phagocytosis (engulfing and internal digestion). Particles
from 0.5 μm to 50 μm are trapped in the ostia, which taper from the
outer to inner ends. These particles are consumed by pinacocytes or by archaeocytes
which partially extrude themselves through the walls of the ostia.
Bacteria-sized particles, below 0.5 micrometers, pass through the ostia
and are caught and consumed by choanocytes. Since the smallest particles are by far the
most common, choanocytes typically capture 80% of a sponge's food supply. Archaeocytes transport food packaged in vesicles
from cells that directly digest food to those that do not. At least one species
of sponge has internal fibers that function as tracks for use by
nutrient-carrying archaeocytes, and these tracks also move inert objects.
It used to
be claimed that glass sponges could live on nutrients dissolved in sea water
and were very averse to silt. However a study in 2007 found no evidence of this
and concluded that they extract bacteria and other micro-organisms from water
very efficiently (about 79%) and process suspended sediment grains to extract
such prey. Collar bodies digest food and distribute it wrapped in vesicles that
are transported by dynein "motor" molecules along bundles of microtubules
that run throughout the syncytium.
Sponges'
cells absorb oxygen by diffusion from water into cells as water flows through
body, into which carbon dioxide and other soluble waste products such as ammonia
also diffuse. Archeocytes remove mineral particles that threaten to block the
ostia, transport them through the mesohyl and generally dump them into the
outgoing water current, although some species incorporate them into their
skeletons.
Carnivorous sponges
A few
species that live in waters where the supply of food particles is very poor
prey on crustaceans and other small animals. Most belong to the family Cladorhizidae,
but a few members of the Guitarridae and Esperiopsidae are 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.
Endosymbionts
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 cyanobacteria but in some
cases dinoflagellates. Symbiotic cyanobacteria 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.
"Immune" system
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.
Reproduction
Asexual
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 the
mesohyl 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.
Sexual
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.
Life cycle
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.
Coordination of activities
Adult
sponges lack neurons or any other kind of nervous tissue. However most species
have the ability to perform movements that are coordinated all over their
bodies, mainly contractions of the pinacocytes, squeezing the water channels
and thus expelling excess sediment and other substances that may cause
blockages. Some species can contract the osculum independently of the rest of
the body. Sponges may also contract in order to reduce the area that is
vulnerable to attack by predators. In cases where two sponges are fused, for
example if there is a large but still unseparated bud, these contraction waves
slowly become coordinated in both of the "Siamese twins". The
coordinating mechanism is unknown, but may involve chemicals similar to neurotransmitters.
However glass sponges rapidly transmit electrical impulses through all parts of
the syncytium, and use this to halt the motion of their flagella if the
incoming water contains toxins or excessive sediment. Myocytes are thought to be responsible for
closing the osculum and for transmitting signals between different parts of the
body.
Sponges
contain genes very similar to those that contain the "recipe" for the
post-synaptic density, an important signal-receiving structure in the neurons
of all other animals. However in sponges these genes are only activated in
"flask cells" that appear only in larvae and may provide some sensory
capability while the larvae are swimming. This raises questions about whether
flask cells represent the predecessors of true neurons or are evidence that
sponges' ancestors had true neurons but lost them as they adapted to a sessile
lifestyle.
Ecology
Euplectella
aspergillum is a deep
ocean glass sponge; seen here at a depth of 2,572 metres (8,438 ft) off
the coast of California.
Habitats
Sponges are
worldwide in their distribution, from the polar regions to the tropics. Most live in quiet, clear waters, because
sediment stirred up by waves or currents would block their pores, making it
difficult for them to feed and breathe. The greatest numbers of sponges are usually
found on firm surfaces such as rocks, but some sponges can attach themselves to
soft sediment by means of a root-like base.
Sponges are
more abundant but less diverse in temperate waters than in tropical waters,
possibly because organisms that prey on sponges are more abundant in tropical
waters. Glass sponges are the most common in polar
waters and in the depths of temperate and tropical seas, as their very porous
construction enables them to extract food from these resource-poor waters with
the minimum of effort. Demosponges and calcareous sponges are abundant and
diverse in shallower non-polar waters.
The
different classes of sponge live in different ranges of habitat:
Water type
|
Depth
|
Type of
surface
|
|
Calcarea
|
Marine
|
less than
100 m (330 ft)
|
Hard
|
Glass
sponges
|
Marine
|
Deep
|
Soft or
firm sediment
|
Demosponges
|
Marine,
brackish; and about 150 freshwater species
|
Inter-tidal
to abyssal; a carnivorous demosponge has been found at 8,840 m
(5.49 mi)
|
Any
|
As primary producers
Sponges with
photosynthesizing endosymbionts produce up to three times more oxygen than they
consume, as well as more organic matter than they consume. Such contributions
to their habitats' resources are significant along Australia's Great Barrier
Reef but relatively minor in the Caribbean.
Defenses
Holes made
by clionaid sponge (producing the trace Entobia) after the death of a
modern bivalve shell of species Mercenaria mercenaria, from North
Carolina
Close-up of
the sponge boring Entobia in a modern oyster valve. Note the chambers
which are connected by short tunnels.
Many sponges
shed spicules, forming a dense carpet several meters deep that keeps away echinoderms
which would otherwise prey on the sponges. They also produce toxins that prevent
other sessile organisms such as bryozoans or sea squirts from growing on or
near them, making sponges very effective competitors for living space. One of
many examples includes ageliferin.
A few
species, such as the Caribbean fire sponge Tedania ignis, cause a severe
rash in humans who handle them. Turtles and
some fish feed mainly on sponges. It is often said that sponges produce
chemical defenses against such predators. However an experiment showed that there is no
relationship between the toxicity of chemicals produced by sponges and how they
taste to fish, which would diminish the usefulness of chemical defenses as
deterrents. Predation by fish may even help to spread sponges by detaching
fragments.
Glass
sponges produce no toxic chemicals, and live in very deep water where predators
are rare.
Predation
Sponge
flies, also known as spongilla-flies (Neuroptera, Sisyridae), are specialist
predators of freshwater sponges. The female lays her eggs on vegetation
overhanging water. The larvae hatch and drop into the water where they seek out
sponges to feed on. They use their elongated mouthparts to pierce the sponge
and suck the fluids within. The larvae of some species cling to the surface of
the sponge while others take refuge in the sponge's internal cavities. The
fully grown larvae leave the water and spin a cocoon in which to pupate.
Bioerosion
The
Caribbean chicken-liver sponge Chondrilla nucula secretes toxins that
kill coral polyps, allowing the sponges to grow over the coral skeletons. Others, especially in the family Clionaidae,
use corrosive substances secreted by their archeocytes to tunnel into rocks,
corals and the shells of dead mollusks. Sponges may remove up to 1 m
(3.3 ft) per year from reefs, creating visible notches just below low-tide
level.
Diseases
Caribbean
sponges of the genus Aplysina suffer from Aplysina red band syndrome.
This causes Aplysina to develop one or more rust-colored bands,
sometimes with adjacent bands of necrotic tissue. These lesions may completely
encircle branches of the sponge. The disease appears to be contagious. The
rust-colored bands are caused by a cyanobacterium, but it is unknown whether
this organism actually causes the disease.
Collaboration with other organisms
In addition
to hosting photosynthesizing endosymbionts, sponges are noted for their wide
range of collaborations with other organisms. The relatively large encrusting
sponge Lissodendoryx colombiensis is most common on rocky surfaces, but
has extended its range into seagrass meadows by letting itself be surrounded or
overgrown by seagrass sponges, which are distasteful to the local starfish and
therefore protect Lissodendoryx against them; in return the seagrass
sponges get higher positions away from the sea-floor sediment.
Shrimps of
the genus Synalpheus form colonies in sponges, and each shrimp species
inhabits a different sponge species, making Synalpheus one of the most
diverse crustacean genera. Specifically, Synalpheus regalis utilizes the sponge
not only as a food source, but also as a defense against other shrimp and
predators. As many as 16,000 individuals inhabit a single
loggerhead sponge, feeding off the larger particles that collect on the sponge
as it filters the ocean to feed itself.
Evolutionary history
Fossil record
Fossil
sponge Raphidonema faringdonense from Cretaceous rocks in England
1: Gap 2: Central cavity
3 Internal wall 4: Pore (all walls have
pores) 5 Septum 6 Outer wall 7 Holdfast
Archaeocyathid structure
24-isopropylcholestane
is a stable derivative of 24-isopropylcholesterol, which is thought to be
produced by demosponges but not by eumetazoans ("true animals", i.e. cnidarians
and bilaterians). Since choanoflagellates are thought to be animals' closest
single-celled relatives, a team of scientists examined the biochemistry and genes
of one choanoflagellate species. They concluded that this species could not
produce 24-isopropylcholesterol but that investigation of a wider range of
choanoflagellates would be necessary in order to prove that the fossil
24-isopropylcholestane could only have been produced by demosponges. Although a previous publication reported
traces of the chemical 24-isopropylcholestane in ancient rocks dating to 1,800 million
years ago, recent
research using a much more accurately dated rock series has revealed that these
biomarkers only appear before the end of the Marinoan glaciation approximately 635
million years ago, and that "Biomarker analysis has yet to reveal any
convincing evidence for ancient sponges pre-dating the first globally extensive
Neoproterozoic glacial episode (the Sturtian, ~713 million years ago in
Oman)".
Although molecular
clocks and biomarkers suggest sponges existed well before the Cambrian
explosion of life, Silica spicules like those of demosponges are absent from
the fossil record until the Cambrian, although one unsubstantiated report
exists of spicules in rocks dated around 750 million years ago, although this
appears unlikely based on the above reference. Well-preserved fossil sponges
from about 580 million years ago in the Ediacaran period have been found in the
Doushantuo Formation. These fossils, which include spicules, pinacocytes, porocytes,
archeocytes, sclerocytes and the internal cavity, have been classified as
demosponges. Fossils of glass sponges have been found from around 540 million
years ago in rocks in Australia, China and Mongolia. Early Cambrian sponges from Mexico belonging
to the genus Kiwetinokia show evidence of fusion of several smaller
spicules to form a single large spicule. Calcium carbonate spicules of calcareous
sponges have been found in Early Cambrian rocks from about 530 to 523
million years ago in Australia. Other probable demosponges have been found in
the Early Cambrian Chengjiang fauna, from 525 to 520 million years
ago. Freshwater sponges appear to be much younger,
as the earliest known fossils date from the Mid-Eocene period about 48 to 40
million years ago. Although about 90% of modern sponges are demosponges,
fossilized remains of this type are less common than those of other types
because their skeletons are composed of relatively soft spongin that does not
fossilize well.
Archaeocyathids,
which some classify as a type of coralline sponge, are common in the Cambrian
period from about 530 million years ago, but apparently died out by the end of
the Cambrian 490 million years ago.
Family tree
A choanoflagellate
In the 1990s
sponges were widely regarded as a monophyletic group, in other words all of
them descended from a common ancestor that was itself a sponge, and as the
"sister-group" to all other metazoans (multi-celled animals), which
themselves form a monophyletic group. On the other hand some 1990s analyses
also revived the idea that animals' nearest evolutionary relatives are choanoflagellates,
single-celled organisms very similar to sponges' choanocytes – which would imply
that most Metazoa evolved from very sponge-like ancestors and therefore that
sponges may not be monophyletic, as the same sponge-like ancestors may have
given rise both to modern sponges and to non-sponge members of Metazoa.
Analyses
since 2001 have concluded that Eumetazoa (more complex than sponges) are more
closely related to particular groups of sponges than to the rest of the
sponges. Such conclusions imply that sponges are not monophyletic, because the
last common ancestor of all sponges would also be a direct ancestor of the
Eumetazoa, which are not sponges. A study in 2001 based on comparisons of ribosome
DNA concluded that the most fundamental division within sponges was between glass
sponges and the rest, and that Eumetazoa are more closely related to Calcareous
sponges, those with calcium carbonate spicules, than to other types of sponge. In 2007 one analysis based on comparisons of RNA
and another based mainly on comparison of spicules concluded that demosponges and
glass sponges are more closely related to each other than either is to
calcareous sponges, which in turn are more closely related to Eumetazoa.
Other
anatomical and biochemical evidence links the Eumetazoa with Homoscleromorpha,
a sub-group of demosponges. A comparison in 2007 of nuclear DNA, excluding
glass sponges and comb jellies, concluded that: Homoscleromorpha are most
closely related to Eumetazoa; calcareous sponges are the next closest; the
other demosponges are evolutionary "aunts" of these groups; and the chancelloriids,
bag-like animals whose fossils are found in Cambrian rocks, may be sponges. The sperm of Homoscleromorpha share with those
of Eumetazoa features that those of other sponges lack. In both
Homoscleromorpha and Eumetazoa layers of cells are bound together by attachment
to a carpet-like basal membrane composed mainly of "type IV" collagen,
a form of collagen not found in other sponges – although the spongin fibers
that reinforce the mesohyl of all demosponges is similar to "type IV"
collagen.
A comb jelly
The analyses
described above concluded that sponges are closest to the ancestors of all
Metazoa, in other words of all multi-celled animals including both sponges and
more complex groups. However, another comparison in 2008 of 150 genes in each
of 21 genera, ranging from fungi to humans but including only two species of
sponge, suggested that comb jellies (ctenophora) are the most basal lineage of
the Metazoa included in the sample. If this is correct, either modern comb
jellies developed their complex structures independently of other Metazoa, or
sponges' ancestors were more complex and all known sponges are drastically
simplified forms. The study recommended further analyses using a wider range of
sponges and other simple Metazoa such as Placozoa. The results of such an analysis, published in
2009, suggest that a return to the previous view may be warranted. 'Family
trees' constructed using a combination of all available data – morphological,
developmental and molecular – concluded that the sponges are in fact a
monophyletic group, and with the cnidarians form the sister group to the
bilaterians.
Archaeocyathids
are very common fossils in rocks from the Early Cambrian about 530 to 520
million years ago but are not found after the Late Cambrian. It has been
suggested that they were produced by: sponges; cnidarians; algae; foraminiferans;
a completely separate phylum of animals, Archaeocyatha; or even a completely
separate kingdom of life, labeled Archaeata or Inferibionta. Since the 1990s
archaeocyathids have been regarded as a distinctive group of sponges.
= skin
= aragonite
= flesh
Halkieriid sclerite structure
It is
difficult to fit chancelloriids into classifications of sponges or more complex
animals. An analysis in 1996 concluded that they were closely related to
sponges on the grounds that the detailed structure of chancellorid sclerites
("armor plates") is similar to that of fibers of spongin, a collagen protein,
in modern keratose (horny) demosponges such as Darwinella. However
another analysis in 2002 concluded that chancelloriids are not sponges and may
be intermediate between sponges and more complex animals, among other reasons
because their skins were thicker and more tightly-connected than those of
sponges. In 2008 a detailed analysis of chancelloriids'
sclerites concluded that they were very similar to those of halkieriids, mobile
bilaterian animals that looked like slugs in chain mail and whose fossils are
found in rocks from the very Early Cambrian to the Mid Cambrian. If this is
correct, it would create a dilemma, as it is extremely unlikely that totally
unrelated organisms could have developed such similar sclerites independently,
but the huge difference in the structures of their bodies makes it hard to see
how they could be closely related.
Taxonomy
Levels in the Linnean taxonomy.
Linnaeus,
who classified most kinds of sessile animals as belonging to the order Zoophyta
in the class Vermes, mistakenly identified the genus Spongia as plants
in the order Algae. For a long time thereafter sponges were
assigned to a separate subkingdom, Parazoa ("beside the animals"),
separate from the Eumetazoa which formed the rest of the kingdom Animalia. They
are now classified as a paraphyletic phylum, from which the higher animals have
evolved.
The phylum
Porifera is further divided into classes mainly according to the composition of
their skeletons:
- Hexactinellida (glass sponges) have silicate spicules, the largest of which have six rays and may be individual or fused. The main components of their bodies are syncytia in which large numbers of cell share a single external membrane.
- Calcarea have skeletons made of calcite, a form of calcium carbonate, which may form separate spicules or large masses. All the cells have a single nucleus and membrane.
- Most Demospongiae have silicate spicules or spongin fibers or both within their soft tissues. However a few also have massive external skeletons made of aragonite, another form of calcium carbonate. All the cells have a single nucleus and membrane.
- Archeocyatha are known only as fossils from the Cambrian period.
In the 1970s
sponges with massive calcium carbonate skeletons were assigned to a separate
class, Sclerospongiae, otherwise known as "coralline sponges". However in the 1980s it was found that these
were all members of either the Calcarea or the Demospongiae.
So far
scientific publications have identified about 9,000 poriferan species, of
which: about 400 are glass sponges; about 500 are calcareous species; and the
rest are demosponges. However some types of habitat, such as vertical rock and
cave walls and galleries in rock and coral boulders, have been investigated
very little, even in shallow seas.
Use
By dolphins
A report in
1997 described use of sponges as a tool by bottlenose dolphins in Shark Bay in
Western Australia. A dolphin will attach a marine sponge to its rostrum, which
is presumably then used to protect it when searching for food in the sandy sea
bottom. The behavior, known as sponging, has
only been observed in this bay, and is almost exclusively shown by females. A
study in 2005 concluded that mothers teach the behavior to their daughters, and
that all the sponge-users are closely related, suggesting that it is a fairly
recent innovation.
Skeleton
The calcium
carbonate or silica spicules of most sponge genera make them too rough for most
uses, but two genera, Hippospongia and Spongia, have soft,
entirely fibrous skeletons. Early Europeans used soft sponges for many
purposes, including padding for helmets, portable drinking utensils and
municipal water filters. Until the invention of synthetic sponges, they were
used as cleaning tools, applicators for paints and ceramic glazes and discreet contraceptives.
However by the mid-20th century, over-fishing brought both the animals and the
industry close to extinction. See also sponge
diving.
Many objects
with sponge-like textures are now made of substances not derived from
poriferans. Synthetic sponges include personal and household cleaning tools, breast
implants, and contraceptive sponges. Typical materials used are cellulose foam, polyurethane
foam, and less frequently, silicone foam.
The luffa
"sponge", also spelled loofah, which is commonly sold for use
in the kitchen or the shower, is not derived from an animal but from the
fibrous "skeleton" of a gourd (Cucurbitaceae).
Antibiotic compounds
Sponges have
medicinal potential due to the presence in sponges themselves or their
microbial symbionts of chemicals that may be used to control viruses, bacteria,
tumors and fungi
Other biologically active compounds
Lacking any
protective shell or means of escape, sponges have evolved to synthesize a
variety of unusual compounds. One such class is the oxidized fatty acid
derivatives called oxylipins. Members of this family have been found to have
anti-cancer, anti-bacterial and anti-fungal properties. One example isolated
from the Okinawan plakortis sponges, plakoridine A, has shown potential
as a cytotoxin to murine lymphoma cells.
Source
:
http://en.wikipedia.org/wiki/Sponge
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