Ch 12 Sponges and Placozoans

“First” multicellular animal

Sponge

Sponges finally recognized as animals

in 19th century

Animals sister to

Choanoflagellate algae

Choanocyte cells

Possessed by choanoflagellates and sponges; flagellum surrounded by collar of microvilli

Microvilli

Collect particles for filter feeding, usually bacteria

Evidence FOR the common ancestor of metazoans being colonial

“Prototype” multicellular organism, similar cell communication mechanisms exist between choanoflagellates and sponges

Evidence AGAINST the common ancestor of metazoans being colonial

Choanocytes only in adult sponges and not larval form; not seen in other phylogenetic groups, lost or suppressed

Phylum Porifera

Sponges; great filters with few parts, 2mm to 2m across

Porifera characteristics

Multicellular; pores that facilitate water movement; aquatic, radial symmetry; at least two cell types with nonliviing ECM; no organs or true tissue, digestion intracellular; absent or primitive nervous system; asexual reproduction with buds or gemmules, or sexual reproduction with eggs, sperm, and free swimming larvae

Prifera structure

Spicules imbedded in spongin for support, pinacoderm, dermal ostia, suspension feeders

Pinacoderm

Incurrent pores on cells

Dermal ostia

Incurrent pores for the sponge

Cellular and extracellular sponge components

Choanocytes, mesohyl, archaeocytes, pinacocytes

Mesohyl

Gelatinous extracellular matric (sometimes mesenchyme)

Archaeocytes

Ameboid cells that move in the mesohyl; sclerocytes, spongocytes, collencytes, lophocytes

Sclerocytes

Make spicules

Spongocytes

Make spongin

Collencytes

Make collagen

Lophocytes

Make collagen

Pinacocytes 

Analogous to epithelial cells, closest thing to true tissue a sponge has; Myocytes

Myocytes

Arranged in circular bands around pores, regulate water flow through contraction

Siliceous

Made of glass

Calcareous

Made of calcium

Leuconoid

Most common sponge form, more filters

Gemmules

Used for asexual sponge reproduction

Parenchymula

Free swimming larvae

Asexual advantages

No dilution of genes, favored in stable environments, all population is reproductive, more robust offspring

Asexual disadvantages

Offspring do not disperse long distances, identical offspring, not favored in changing environments, accumulate negative mutations over time

Sexual advantages

Genetic variation for selection to act upon, offspring disperse well

Sexual disadvantages

Not all of population is productive, fragile offspring, diluted gene pool, need to find a partner, most gametes/offspring do not reproduce

Class Calcarea

Asconoid, syconoid, or leuconoid; calcareous; small and vase shaped

Class Hexactinellida

Syconoid and leuconoid; siliceous; vary widely in size with vase or funnel shape; no mesohyl, rigid skeleton; deep sea; synctial cellular structure

Class Demospongiae

Leuconoid; siliceous; 95% of sponges, very diverse; mostly marine, but is the only group to contain freshwater sponges

Phylum Placozoa

One species — monotypic family and genus; scavengers; asexual