Sponges and Placozoans CHAPTER 12 Sponges and Placozoans

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

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

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

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

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

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

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

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.

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

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

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

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

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

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

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

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

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

Phylum Porifera Classification Class Calcarea Class Hexactinellida Class Demospongiae

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

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

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

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

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