Which Of These Traits Is Unique To Poriferans Among The Animals?

Invertebrates

142 Phylum Porifera

Learning Objectives

By the finish of this section, you will exist able to do the following:

• Draw the organizational features of the simplest multicellular organisms
• Explain the diverse body forms and bodily functions of sponges

As we accept seen, the vast majority of invertebrate animals exercise non possess a defined bony vertebral endoskeleton, or a bony cranium. However, one of the near ancestral groups of deuterostome invertebrates, the Echinodermata, practise produce tiny skeletal "bones" called ossicles that make upward a true endoskeleton, or internal skeleton, covered by an epidermis.

We volition commencement our investigation with the simplest of all the invertebrates—animals sometimes classified within the clade Parazoa ("abreast the animals"). This clade currently includes just the phylum Placozoa (containing a single species, Trichoplax adhaerens), and the phylum Porifera, containing the more than familiar sponges ((Effigy)). The dissever between the Parazoa and the Eumetazoa (all animal clades above Parazoa) probable took identify over a billion years agone.

We should reiterate here that the Porifera do not possess "true" tissues that are embryologically homologous to those of all other derived animal groups such every bit the insects and mammals. This is considering they do not create a true gastrula during embryogenesis, and equally a result do not produce a truthful endoderm or ectoderm. But even though they are non considered to accept true tissues, they practise take specialized cells that perform specific functions like tissues (for instance, the external "pinacoderm" of a sponge acts like our epidermis). Thus, functionally, the poriferans can be said to have tissues; however, these tissues are likely not embryologically homologous to our own.

Sponge larvae (e.g, parenchymula and amphiblastula) are flagellated and able to swim; all the same, adults are non-motile and spend their life attached to a substratum. Since h2o is vital to sponges for feeding, excretion, and gas exchange, their body construction facilitates the motility of h2o through the sponge. Various canals, chambers, and cavities enable water to move through the sponge to permit the exchange of food and waste matter as well as the exchange of gases to nearly all body cells.

Sponges. Sponges are members of the phylum Porifera, which contains the simplest invertebrates. (credit: Andrew Turner)

Morphology of Sponges

There are at least 5,000 named species of sponges, probable with thousands more nonetheless to be classified. The morphology of the simplest sponges takes the shape of an irregular cylinder with a large fundamental cavity, the spongocoel, occupying the within of the cylinder ((Figure)). Water enters into the spongocoel through numerous pores, or ostia, that create openings in the body wall. H2o entering the spongocoel is expelled via a large mutual opening chosen the osculum. However, we should note that sponges exhibit a range of variety in body forms, including variations in the size and shape of the spongocoel, likewise as the number and organization of feeding chambers within the torso wall. In some sponges, multiple feeding chambers open off of a central spongocoel and in others, several feeding chambers connecting to one another may lie between the entry pores and the spongocoel.

While sponges exercise non exhibit truthful tissue-layer organization, they do accept a number of functional "tissues" composed of unlike prison cell types specialized for distinct functions. For instance, epithelial-similar cells called pinacocytes form the outermost body, called a pinacoderm, that serves a protective function similar that of our epidermis. Scattered among the pinacoderm are the ostia that allow entry of water into the body of the sponge. These pores have given the sponges their phylum name Porifera—pore-bearers. In some sponges, ostia are formed past porocytes, single tube-shaped cells that act as valves to regulate the flow of h2o into the spongocoel. In other sponges, ostia are formed by folds in the trunk wall of the sponge. Between the outer layer and the feeding chambers of the sponge is a jelly-like substance called the mesohyl, which contains collagenous fibers. Various cell types reside inside the mesohyl, including amoebocytes, the "stem cells" of sponges, and sclerocytes, which produce skeletal materials. The gel-like consistency of mesohyl acts like an endoskeleton and maintains the tubular morphology of sponges.

The feeding chambers inside the sponge are lined past choanocytes ("collar cells"). The structure of a choanocyte is critical to its role, which is to generate a directed water current through the sponge and to trap and ingest microscopic food particles past phagocytosis. These feeding cells are similar in appearance to unicellular choanoflagellates (Protista). This similarity suggests that sponges and choanoflagellates are closely related and likely share common ancestry. The trunk of the choanocyte is embedded in mesohyl and contains all the organelles required for normal cell function. Protruding into the "open infinite" inside the feeding chamber is a mesh-like collar composed of microvilli with a single flagellum in the heart of the column. The beating of the flagella from all choanocytes draws water into the sponge through the numerous ostia, into the spaces lined past choanocytes, and eventually out through the osculum (or osculi, if the sponge consists of a colony of attached sponges). Food particles, including waterborne bacteria and unicellular organisms such as algae and diverse creature-similar protists, are trapped by the sieve-similar collar of the choanocytes, slide down toward the body of the prison cell, and are ingested past phagocytosis. Choanocytes also serve another surprising part: They can differentiate into sperm for sexual reproduction, at which time they become dislodged from the mesohyl and get out the sponge with expelled water through the osculum.

Link to Learning

Watch this video to see the movement of h2o through the sponge body.

The amoebocytes (derived from stalk-cell-like archaeocytes), are so named because they move throughout the mesohyl in an amoeba-similar fashion. They have a variety of functions: In addition to delivering nutrients from choanocytes to other cells within the sponge, they too give rise to eggs for sexual reproduction. (The eggs remain in the mesohyl, whereas the sperm cells are released into the water.) The amoebocytes can differentiate into other cell types of the sponge, such as collenocytes and lophocytes, which produce the collagen-like protein that support the mesohyl. Amoebocytes tin can too requite rise to sclerocytes, which produce spicules (skeletal spikes of silica or calcium carbonate) in some sponges, and spongocytes, which produce the protein spongin in the majority of sponges. These unlike cell types in sponges are shown in (Figure).

Visual Connexion

Elementary sponge body plan and jail cell types. The sponge'due south (a) basic body plan and (b) some of the specialized cell types found in sponges are shown.

Which of the following statements is false?

1. Choanocytes take flagella that propel h2o through the trunk.
2. Pinacocytes can transform into any cell type.
3. Lophocytes secrete collagen.
4. Porocytes command the flow of water through pores in the sponge body.

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Link to Learning

Have an up-close tour through the sponge and its cells.

Equally we've seen, nigh sponges are supported by small bone-like spicules (commonly tiny pointed structures made of calcium carbonate or silica) in the mesohyl. Spicules provide support for the body of the sponge, and may as well deter predation. The presence and composition of spicules course the basis for differentiating three of the four classes of sponges ((Effigy)). Sponges in class Calcarea produce calcium carbonate spicules and no spongin; those in class Hexactinellida produce six-rayed siliceous (glassy) spicules and no spongin; and those in class Demospongia contain spongin and may or may not take spicules; if nowadays, those spicules are siliceous. Sponges in this last class have been used as bathroom sponges. Spicules are most conspicuously present in the glass sponges, class Hexactinellida. Some of the spicules may achieve gigantic proportions. For case, relative to typical glass sponge spicules, whose size generally ranges from 3 to 10 mm, some of the basal spicules of the hexactinellid Monorhaphis chuni are enormous and grow up to 3 meters long! The glass sponges are also unusual in that about of their body cells are fused together to form a multinucleate syncytium. Considering their cells are interconnected in this way, the hexactinellid sponges accept no mesohyl. A quaternary class of sponges, the Sclerospongiae, was described from species discovered in underwater tunnels. These are likewise called coralline sponges after their multilayered calcium carbonate skeletons. Dating based on the rate of deposition of the skeletal layers suggests that some of these sponges are hundreds of years former.

Several classes of sponges. (a) Clathrina clathrus belongs to form Calcarea, (b) Staurocalyptus spp. (common name: yellow Picasso sponge) belongs to class Hexactinellida, and (c) Acarnus erithacus belongs to class Demospongia. (credit a: modification of piece of work past Parent Géry; credit b: modification of work past Monterey Bay Aquarium Enquiry Establish, NOAA; credit c: modification of piece of work past Sanctuary Integrated Monitoring Network, Monterey Bay National Marine Sanctuary, NOAA)

Link to Learning

Use the Interactive Sponge Guide to identify species of sponges based on their external form, mineral skeleton, fiber, and skeletal compages.

Physiological Processes in Sponges

Sponges, despite being simple organisms, regulate their different physiological processes through a diversity of mechanisms. These processes regulate their metabolism, reproduction, and locomotion.

Digestion

Sponges lack complex digestive, respiratory, circulatory, and nervous systems. Their food is trapped as water passes through the ostia and out through the osculum. Bacteria smaller than 0.5 microns in size are trapped past choanocytes, which are the principal cells engaged in feeding, and are ingested by phagocytosis. Withal, particles that are larger than the ostia may exist phagocytized at the sponge's surface by pinacocytes. In some sponges, amoebocytes ship food from cells that have ingested food particles to those that practise not. In sponges, in spite of what looks like a big digestive cavity, all digestion is intracellular. The limit of this type of digestion is that food particles must exist smaller than individual sponge cells.

All other major trunk functions in the sponge (gas commutation, circulation, excretion) are performed by diffusion betwixt the cells that line the openings within the sponge and the water that is passing through those openings. All cell types within the sponge obtain oxygen from h2o through improvidence. Likewise, carbon dioxide is released into seawater past improvidence. In addition, nitrogenous waste produced equally a byproduct of protein metabolism is excreted via improvidence by private cells into the water every bit it passes through the sponge.

Some sponges host green algae or blue-green alga as endosymbionts inside archeocytes and other cells. It may exist a surprise to learn that there are near 150 species of carnivorous sponges, which feed primarily on tiny crustaceans, snaring them through sticky threads or hooked spicules!

Although at that place is no specialized nervous organisation in sponges, there is intercellular communication that tin regulate events like wrinkle of the sponge's trunk or the activity of the choanocytes.

Reproduction

Sponges reproduce by sexual besides as asexual methods. The typical means of asexual reproduction is either fragmentation (during this procedure, a piece of the sponge breaks off, settles on a new substrate, and develops into a new individual), or budding (a genetically identical outgrowth grows from the parent and somewhen detaches or remains attached to form a colony). An atypical type of asexual reproduction is establish only in freshwater sponges and occurs through the germination of gemmules. Gemmules are environmentally resistant structures produced past adult sponges (due east.g., in the freshwater sponge Spongilla). In gemmules, an inner layer of archeocytes (amoebocytes) is surrounded by a pneumatic cellular layer that may be reinforced with spicules. In freshwater sponges, gemmules may survive hostile environmental atmospheric condition like changes in temperature, and then serve to recolonize the habitat in one case environmental atmospheric condition amend and stabilize. Gemmules are capable of attaching to a substratum and generating a new sponge. Since gemmules can withstand harsh environments, are resistant to desiccation, and remain dormant for long periods, they are an excellent means of colonization for a sessile organism.

Sexual reproduction in sponges occurs when gametes are generated. Oocytes ascend by the differentiation of amoebocytes and are retained within the spongocoel, whereas spermatozoa result from the differentiation of choanocytes and are ejected via the osculum. Sponges are monoecious (hermaphroditic), which means that one individual can produce both gametes (eggs and sperm) simultaneously. In some sponges, production of gametes may occur throughout the year, whereas other sponges may show sexual cycles depending upon water temperature. Sponges may also become sequentially hermaphroditic, producing oocytes first and spermatozoa subsequently. This temporal separation of gametes produced past the same sponge helps to encourage cantankerous-fertilization and genetic multifariousness. Spermatozoa carried along by water currents can fertilize the oocytes borne in the mesohyl of other sponges. Early larval evolution occurs within the sponge, and gratuitous-swimming larvae (such every bit flagellated parenchymula) are then released via the osculum.

Locomotion

Sponges are generally sessile as adults and spend their lives attached to a stock-still substratum. They exercise not show motion over big distances similar other free-swimming marine invertebrates. Even so, sponge cells are capable of creeping along substrata via organizational plasticity, i.e., rearranging their cells. Nether experimental weather condition, researchers have shown that sponge cells spread on a physical support demonstrate a leading edge for directed motion. It has been speculated that this localized creeping movement may help sponges adjust to microenvironments well-nigh the point of attachment. It must be noted, however, that this pattern of movement has been documented in laboratories, it remains to be observed in natural sponge habitats.

Link to Learning

Watch this BBC video showing the array of sponges seen along the Cayman Wall during a submersible dive.

Section Summary

Animals included in phylum Porifera are parazoans because they do non show the formation of true embryonically derived tissues, although they take a number of specific cell types and "functional" tissues such as pinacoderm. These organisms show very uncomplicated organization, with a rudimentary endoskeleton of spicules and spongin fibers. Drinking glass sponge cells are connected together in a multinucleated syncytium. Although sponges are very uncomplicated in organization, they perform nigh of the physiological functions typical of more than circuitous animals.

Visual Connectedness Questions

(Figure) Which of the following statements is simulated?

1. Choanocytes have flagella that propel water through the trunk.
2. Pinacocytes can transform into any cell type.
3. Lophocytes secrete collagen.
4. Porocytes control the flow of h2o through pores in the sponge body.

(Figure) B

Review Questions

Mesohyl contains:

1. a polysaccharide gel and dead cells.
2. a collagen-like gel and suspended cells for various functions.
3. spicules composed of silica or calcium carbonate.
4. multiple pores.

B

The large key opening in the parazoan body is called the:

1. gemmule.
2. spicule.
3. ostia.
4. osculum.

D

Nigh sponge trunk plans are slight variations on a uncomplicated tube-within-a-tube design. Which of the following is a primal limitation of sponge body plans?

1. Sponges lack the specialized cell types needed to produce more than complex body plans.
2. The reliance on osmosis/diffusion requires a design that maximizes the surface area to volume ratio of the sponge.
3. Choanocytes must be protected from the hostile exterior environment.
4. Spongin cannot support heavy bodies.

B

Disquisitional Thinking Questions

Describe the different cell types and their functions in sponges.

Pinacocytes are epithelial-similar cells, form the outermost layer of sponges, and enclose a jelly-like substance called mesohyl. In some sponges, porocytes form ostia, single tube-shaped cells that act equally valves to regulate the menses of water into the spongocoel. Choanocytes ("neckband cells") are nowadays at various locations, depending on the blazon of sponge, only they e'er line some space through which water flows and are used in feeding.

Describe the feeding mechanism of sponges and identify how information technology is different from other animals.

The sponges describe water carrying food particles into the spongocoel using the beating of flagella on the choanocytes. The food particles are caught by the neckband of the choanocyte and are brought into the cell past phagocytosis. Digestion of the food particle takes identify inside the cell. The difference between this and the mechanisms of other animals is that digestion takes place within cells rather than exterior of cells. It means that the organism can feed only on particles smaller than the cells themselves.

Glossary

amoebocyte

sponge jail cell with multiple functions, including nutrient delivery, egg germination, sperm delivery, and prison cell differentiation

choanocyte

(as well, collar cell) sponge prison cell that functions to generate a water current and to trap and ingest food particles via phagocytosis

gemmule

construction produced by asexual reproduction in freshwater sponges where the morphology is inverted

invertebrata

(too, invertebrates) category of animals that practise not possess a attic or vertebral cavalcade

mesohyl

collagen-similar gel containing suspended cells that perform various functions in the sponge

osculum

large opening in the sponge's body through which water leaves

ostium

pore present on the sponge's body through which water enters

pinacocyte

epithelial-like cell that forms the outermost layer of sponges and encloses a jelly-like substance called mesohyl

Porifera

phylum of animals with no truthful tissues, but a porous body with rudimentary endoskeleton

sclerocyte

cell that secretes silica spicules into the mesohyl

spicule

structure made of silica or calcium carbonate that provides structural support for sponges

spongocoel

central cavity inside the body of some sponges

Source: https://opentextbc.ca/biology2eopenstax/chapter/phylum-porifera/

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