1. Sponges and Placozoans
CHAPTER 12
Sponges and Placozoans

3. Origin of Metazoa Evolution of the Metazoa
Evolution of eukaryotic cell followed by diversification
Modern descendants
Protozoa, plus multicellular animals
Multicellular animals
Referred to collectively as metazoans

4. Origin of Metazoa Choanoflagellates
Solitary or colonial aquatic eukaryotes
Each cell (choanocyte) has a flagellum surrounded by collar of microvilli
Beating the flagellum draws water into collar
Microvilli collect mostly bacteria
Most are sessile
One species attaches to floating diatom colonies
Strongly resemble sponge feeding cells
Much debate whether sponge choancytes are ancestral to choanoflagellates

5. Phylum Porifera General Features Sessile sponges are filter feeders
Porifera means “pore‑bearing”
Sac-like bodies perforated by many pores
Use flagellated “collar cells”, or choanocytes, to move water
Body is efficient aquatic filter
Approximately 15,000 species of sponges
Most are marine
Few live in brackish water, 150 in fresh water

8. Phylum Porifera
Marine sponges found in all seas at all depths and vary greatly in size
Many species are brightly colored because of pigments in dermal cells
Embryos are free-swimming, adult sponges always attached
Some appear radially symmetrical but many are irregular in shape
Some stand erect, some are branched, and some are encrusting

9. Phylum Porifera Many live as commensals or parasites in or on sponges
Also grow on a variety of other living organisms
Few predators
Sponges may have an elaborate skeletal structure and often have a noxious odor

10. Phylum Porifera
Skeletal structure of a sponge can be fibrous and/or rigid
If present, rigid skeleton consists of calcareous or siliceous spicules
Fibrous portion
Collagen fibrils in intercellular matrix
Several types of one form of collagen, spongin, exists
Composition and shape the spicules
Forms the basis of sponge classification

13. Phylum Porifera Form and Function
Body openings consist of small incurrent pores or dermal ostia
Inside the body
Water is directed past the choanocytes where food particles are collected
Choanocytes (flagellated collar cells) line some of the canals
Keep the current flowing by beating of flagella

14. Phylum Porifera Types of Canal Systems
Asconoids: Flagellated Spongocoels
Simplest body form
Small and tube-shaped
Water enters a large cavity, the spongocoel
Lined with choanocytes
Choanocyte flagella pull water through
All Calcarea are asconoids
Leucosolenia and Clathrina are examples.

16. Phylum Porifera Syconoids: Flagellated Canals
Resemble asconoids but larger with a thicker body wall
Wall contains choanocyte-lined radial canals that empty into spongocoel
Spongocoel is lined with epithelial cells rather than choanocytes
Food is digested by choanocytes

19. Phylum Porifera Leuconoids: Flagellated Chambers
Most complex and are larger with many oscula
Clusters of flagellated chambers are filled from incurrent canals, and discharge to excurrent canals
Most sponges are leuconoid
The leuconoid system
Evolved independently many times in sponges
System increases flagellated surfaces compared to volume
More collar cells can meet food demands
Large sponges filter 1500 liters of water per day

22. Phylum Porifera Types of Cells
Sponge cells are arranged in a gelatinous matrix, mesohyl
Connective “tissue” of sponges
Only visible activities of sponges are
Slight alterations in shape, local contraction, and closing and opening of incurrent and excurrent pores
Movements occur very slowly
Apparently excitation spreads from cell to cell by an unknown mechanism

24. Phylum Porifera Choanocytes
Oval cells with one end embedded in mesohyl
Exposed end has one flagellum surrounded by a collar
Collar consists of adjacent microvilli
Forms a fine filtering device to strain food
Particles too large to enter collar are trapped in mucous
Food engulfed by choanocytes is passed to archaeocytes for digestion

25. Phylum Porifera Archaeocytes Move about in the mesohyl
Phagocytize particles
Can differentiate into any other type of cell

26. Phylum Porifera Pinacocytes Form pinacoderm Flat epithelial-like cells
Somewhat contractile
Help regulate flow of water

27. Phylum Porifera Cell Independence: Regeneration
Sponges have a great ability to regenerate lost parts and repair injuries
Process of reorganization differs in sponges of differing complexity
Regeneration following fragmentation is one means of asexual reproduction

28. Phylum Porifera Asexual reproduction can occur by bud formation
External buds
Small individuals that break off after attaining a certain size
Internal buds or gemmules
Formed by archaeocytes that collect in mesohyl
Coated with tough spongin and spicules
Survive harsh environmental conditions

29. Phylum Porifera Sexual Reproduction Most are monoecious
Sponges provide nourishment to zygote until it is released as a ciliated larva
In some, one sponge releases sperm which enter the pores of another sponge
Some sponges release both sperm and oocytes into water
The free-swimming larva of sponges is a solid parenchymula

30. Phylum Porifera Classification Class Calcarea Class Hexactinellida
Class Demospongiae

31. Class Calcarea (Calcispongiae)
Phylum Porifera
Class Calcarea (Calcispongiae)
Calcareous sponges with spicules of calcium carbonate
Spicules are straight or have three or four rays
Most are small with tubular or vase shapes
Many are drab in color, but some are bright yellow, green, red, or lavender
Leucosolenia and Sycon are marine shallow-water
Asconoid, syconoid and leuconoid body forms

33. Class Hexactinellida (Hyalospongiae)
Phylum Porifera
Class Hexactinellida (Hyalospongiae)
Glass sponges with six-rayed spicules of silica
Nearly all are deep-sea forms
Most are radially symmetrical
Stalks of root spicules attach them to substrate
Chambers appear to correspond to both syconoid and leuconoid types
Adapted to a deep-water habitat with a large and easy flow of water

34. Phylum Porifera Class Demospongiae
Contains 95% of living sponge species
Spicules are siliceous but not six rayed
Absent or bound together by spongin
Leuconoid body form
All marine except for Spongillidae, the freshwater sponges
Freshwater sponges
Widely distributed in well-oxygenated ponds and springs
Reproduce sexually, but existing genotypes may also reappear annually from gemmules

35. Phylum Porifera Marine demosponges Bath sponges
Highly varied in color and shape
Bath sponges
Lacks siliceous spicules
Have spongin skeletons

36. Phylum Porifera Phylogeny and Adaptive Diversification One theory
Sponges appeared before the Cambrian
Glass sponges expanded in the Devonian
One theory
Sponges arose from choanoflagellates
Molecular rRNA evidence suggests
Common ancestor for choanoflagellates and metazoans
Classes of sponges
Distinguished on basis of spicule form and chemical composition
Phylogenetic studies indicate
Sponges with calcareous spicules in class Calcarea belong in a separate clade than those with spicules made of silica