Sponges: Evolution and Diversity of Metazoans

1. 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 • Simplestbody form • Small and tube-shaped • Water enters a large cavity, the spongocoel • Lined with choanocytes • Choanocyte flagella pull water through • All Calcarea are asconoids • Leucosoleniaand 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 • Spongocoelis 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 archaeocytesfor 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. 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 Syconare marine shallow-water • Asconoid, syconoid and leuconoid body forms

33. 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 • Spiculesare siliceous but not six rayed • Absent or bound together by spongin • Leuconoidbody 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 • Highly varied in color and shape • Bath sponges • Lacks siliceous spicules • Have spongin skeletons

36. Phylum Porifera • Phylogeny and Adaptive Diversification • 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