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Phylum Porifera: Sponges

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Sponges are the simplest multicelled animals. Advantages of multicellularity: increase in surface area available for metabolic activities. However, sponges are phylogenetically distinct from other metazoans. Phylogenetically, sponges appeared to split off from the metazoa before the origin of the radiates. Parazoa and Mesozoa are multicellular, but cellular layers are not homologous to germ layers of eumetozoa; do not share developmental patterns with other metazoa. Parazoa = "beside animals"

The sponge body is an assemblage of cells embedded in a gelatinous matrix, supported by a skeleton of minute needle-like spicules (a minute, pointy, skeletal element found inside sponges, radiolarians, corals and sea cucumbers) and protein. They neither look nor behave like other animals.

They are metazoans, or many-celled animals, unlike the protozoans (single celled). They have the simplest type of metazoan organization- a cellular level of organization. What would the next level of organization be?

Sponges lack true tissues and organs; division of labor is confined to a few types of specialized cells.

I. Sponges belong to the phylum Porifera. They are the ultimate couch potatoes of the universe.

A. Characteristics:

1. sessile

2. bodies - myriads of pores and canals- huge filter feeding system, depend on water currents

carried through their unique canal system to bring food and water and remove wastes

3. inactive life style

4. mass of cell embedded in a gelatinous matrix

5. spicules of calcium or silica, or a collagen substance called spongin.

6. no organs or true tissues

7. no nervous system or sense organs, simple contractile elements

B. Body forms

1. most colonial

2. can be a few millimeters to 2+ m.

3. brightly colored - red, yellow, orange, green, purple; fade when removed from water.

4. some are radially symmetrical but many are irregular in shape

5. Some are erect, some are crusty, some are branched and lobed, some low

6. some bore holes into shells or rock

C. Phylogeny

1. sponge fossils from Cambrian period; some claim Precambrian

2. 3 classes, and a possible 4th book will retain

a. Calcarea - calcareous spicules

b. Hexactinellida - six rayed siliceous spicules

c. Demospongiae - skeleton of siliceous spicules or spongin or both

d. Sclerospongiae - massive calcareous skeleton and siliceous spicules

D. Ecological Relationships

1. Most of the 5000 sponge species are marine, some are freshwater, some brackish

2. present in all seas at all depths

3. larvae are free-swimming, adults all attached, usually to rocks, shells, coral or submerged

objects; some benthic forms - sand and mud

4. crabs, nudibranchs, mites, bryozoans live as commensals or parasites in or on sponges

5. sponges live on other animals - molluscs, barnacles, brachiopods, corals or hydroids

6. some crabs attach pieces of sponge to carapace for camouflage or protection; most

predators find sponge distasteful

7. some produce toxic or noxious substances

8. some reef fish may graze on shallow water sponges

9. one sponge preys on shrimp

E. Body functions

1. many tiny pores for incoming water called ostia

2. one or a few large pores for outgoing water called oscula (osculum sing.)

3. openings connected by a series of canals, some lined with choanocytes - flagella maintain a current of water through canals. Water enters canals through dermal ostia (tiny incurrent pores)

4. cells lining passageways are very loosely organized

5. skeleton prevents collapse of canals;

6. types of canals

a. asconoids- flagellated spongocoels (central cavity of a sponge)

1. simplest organization of sponges; small, tube shaped

2. water enters dermal pores into spongocoel, lined with choanocytes (flagellated collar cells); flagella pull water through pores and expel it through osculum.

3. Leucosolenia - asconoid that grows as individuals in groups attached to an object in shallow water by a common stem; Clathrina is an asconoid with bright yellow intertwined tubes (fig 17.4)

4. Asconoids are found only in Calcarea.

b. syconoids - flagellated canals

1. look like larger asconoids, from which they were derived.

2. Tubular body and singular osculum

3. thicker body and more complex than asconoids

4. receive water through incurrent canals that deliver it to choanocyte lined radial canals, which empty into spongocoel lined with epithelial cells, not choanocytes, as in asconoids (Fig. 17.3 and 17.5)

5. syconoids are found only in Calcarea

c. leuconoids - flagellated chambers

1. most complex organization of the sponges

2. best adapted fro increase in sponge size

3. form large colonial masses, each member has own osculum, but individualspoorly defined

4. clusters of flagellated chambers are filled from incurrent canals

5. water goes to excurrent canals that eventually lead to osculum (17.3)

6. most sponges are of this type, in all 4 classes

7. These 3 types of canals demonstrate increasing complexity and efficiency in water pumping system, but this is not an evolutionary sequence

a. leuconoid plan - clear adaptive value

1. increases proportion of flagellated surfaces compared to volume

2. much larger body size possible

8. Types of cells, which are loosely arranged in gelatinous matrix called mesohyl (also called mesoglea or mesenchyme) Mesohyl is the connective tissue of sponges (Fig. 17.6)

a. Pinacocytes- nearest thing to a true tissue in sponges. Pinacocytes on the external epithelium- thin, flat cells that cover the exterior and some interior surfaces. Some are Tshaped. They are somewhat contractile and help regulate surface area of sponge. Some are modified into contractile myocytes- arranged in contractile bands around oscula or pores to regulate water flow; myocytes have microfilaments similar to those found in muscle cells of animals

b. Porocytes - tubular cells that pierce the wall of asconoids; water flows through

c. choanocytes - line flagellated canals and channels; one end embedded in mesohyl and other end exposed; exposed end has flagellum surrounded by collar; (Fig 17.6 and 17.7)collar is fine filtering device for straining food from water. (Fig 17.7 B and C).

Beat of flagellum pulls water through sievelike collar, forced out through open top of collar. Particles too large to enter collar are trapped in mucus and slide down collar to base where they are phagocytized by cell body. Larger particles already screened by small size of dermal pores and prosopyles. The food engulfed by cells is passed onto neighboring archaeocyte for digestion.

d. Archaeocytes - amoeboid cells that move around in mesohyl and carry out a number of functions (Fig 17.6)

1. phagocytize particles received from choanocytes or at external epithelium

2. differentiate into any other types of more specialized sponge cells

a. sclerocytes secrete spicules

b. spongocytes secrete spongin

c. collencytes secrete collagen

9. Types of skeletons - give support to sponge and prevent collapse of canals and chambers. (Fig. 17.8) structural variations of spicules very important in taxonomy

a. Major structural protein of animal kingdom is collagen; fibrils of collagen found throughout the intercellular matrix of all sponges.

b. Demosponges secrete a form a collagen called spongin.

c. Demosponges also secrete siliceous spicules, as do sclerosponges.

d. Calcareous sponges secrete spicules of crystalline calcium carbonate that have 1, 3 or 4 rays (Fig. 17.8)

e. glass sponges (Hexactinellida) have siliceous spicules with 6 rays

G. Sponge physiology -

1. feed on particles suspended in water pumped through canal system

2. detritus, plankton and bacteria all consumed

3. size of particulates 50 microns (size of ostia - incoming pores) to .1 micron (size of openings in collar cells)

4. larger particles consumed by archaeocytes in canals. Smallest particles (about 80% of food are phagocytized by choanocytes.

5. digestion is all intracellular, performed by archaeocytes

6. no respiratory or excretory organs- occurs through diffusion.

7. sponges are amazing pumps. some large ones can filter 1500 l/day

H. Reproduction and development - sexual and asexual

1. most sponges are monoecious - male and female sex cells in same organism

2. sperm arise from transformation of choanocytes

3. in Calcarea and some Demospongiae, oocytes develop from choanocytes

4. in other sponges, oocytes develop from archaeocytes

5. sperm released into water of one individual and taken in by another. The choanocytes phagocytize them, carry them to the oocytes through mesohyl; zygotes develop into flagellated larvae, break away, and carried away by water currents

6. Fig. 17.9 other sponges are oviparous - sperm and egg expelled into water, on their own; fertilization produces a larva, which settles down in a likely spot. flagellated cells migrate to the interior and become choanocytes in flagellated chambers.

7. asexually, all sponges can form external buds; if they remain, they are colonial, or they may detach.

8. gemmules - dormant masses of encapsulated archaeocytes produced in unfavorable conditions. Survive drought and freezing. When conditions favorable, new sponges can develop

I. Survey of Sponges

1. Calcarea - tend to be small (10 cm), tubular or vase shaped. spicules- 1,3 or 4 rays,  composed of calcium carbonate; may be asconoid, leuconoid or syconoid in structure; drab or bright in color

2. Hexactinellida - glass sponges- most are deep sea forms. Most are radially symmetrical, range from 7 cm to > 1m in size; skeleton - 6 rays in siliceous spicules bound in glass latticework (Fig 17.8A); little mesohyl, no myocytes; living tissue composed of fused pseudopods of archaeocytes. Adapted for slow constant current of sea bottom; channels and pores relatively large

3. Demospongia - 80% of all sponges- includes larger sponges. skeletons are siliceous spikes, spongin fibers or both; all are leuconoid, most are marine with one freshwater family. Freshwater sponges form gemmules in autumn, die and disintegrate so gemmules survive the winter. Marine Demosponges are varied in both form and color: tall or encrusting, shaped like fans, vases, cushions or balls (Fig 17.11) Some bore into coral skeletons or mollusk shells; loggerheads grow several m. bath sponges are called horny sponges, which have only spongin skeletons.

4. Sclerospongiae - small group w/ massive skeleton, called coralline sponges. Living tissue extend up to 3 mm into skeleton but only 1mm above it. Leuconoid organization and siliceous spicule and spongin most of the time. they live in caves, crevices on coral reefs and deep water. relict from ancient groups from Cambrian period

J. Phylogeny and Adaptive Radiation

1. phylogeny - originated before Cambrian period; ribosomal RNA evidence supports common ancestor of both choanoflagellates and metazoans. suggests that sponges and eumetazoa are sister groups

2. adaptive radiation - successful- branched into several thousand species, variety of freshwater and marine habitats. leuconoid sponges most efficient, allows for more size and adaptive radiation in this group. Asconoids and syconoids too small to move much water.

 

 

1. simplest organization of all metazoa, but more integration of form and function than protozoans. These animals have a cellular level of organization.

2. Sponges have several types of cells differentiated for various functions, some of which are organized into incipient (just beginning to exist) tissues.

3. Developmental patterns of sponges are different from other phyla; embryonic layers are not homologous to germ layers of eumetazoa.

4. sponges have developed a unique system of water currents. They depend on these for food and O2.

1. Multicellular; body a loose aggregation of cells of mesenchymal origin

2. Body with pores (ostia), canals, and chambers that serve for passage of water

3. All aquatic; mostly marine

4. Symmetry radial or none

5. Epidermis of flat pinacocytes (help regulate surface area- can be myocytes); most interior cells lined with choanocytes (flagellated collar cells); gelatinous protein matrix called mesohyl contains amebocytes, collencytes and skeletal elements

6. Skeletal structure of fibrillar collagen (a protein) and calcareous or siliceous crystalline spicules, often combined with variously modified collagen (spongin) fibrils

7. No organs or true tissues; digestion intracellular, excretion and respiration by diffusion

8. Reactions to stimuli local and independent; nervous system probably absent

9. All adults sessile and attached to substrate

10. Asexual reproduction by buds or gemmules, sexual repro by eggs and sperm; free swimming ciliated larvae