Chapter 12: Phylum Porifera and Phylum Placozoa

Opisthokont Clade

* fungi, choanoflagellates, multicellular animals
* metazoa = multicellular

Choanoflagellates + Metazoa

* Synapomorphy - cell-to-cell signaling proteins

Origin of Multicellularity

* Single cell organisms limited in size (surface area to volume)
* Multicellularity increases efficiency; likely evolved from colonial organisms
* __**Choanoflagellates**__ - solitary or colonial
* cell-to-cell signaling proteins homologous to those of metazoans
* similar to choanocytes in sponges

Phylum Porifera Synapomorphies

* cellular level of organization (no tissues)
* possess __**choanocytes**__
* internal system of pores and canals

Other features of Phylum Porifera

* multicellular with incipient tissues (no basal lamina)
* no body symmetry (sometimes radial)
* sessile (don’t move)
* almost all marine; some brackish, few FW

Sponge Skeletal Framework & Body Form

* Skeleton: __**spicules**__ in a fibrous network of __**spongin**__ (collagen)
* __**Ostium (pl. ostia)**__ = incurrent opening (small pores)
* __**Osculum**__ = excurrent opening (larger)

3 types of canal systems in sponges

* __**Asconoid**__ : simplest; choanocytes line spongocoel
* __**Syconoid**__ : body wall folded; choanocytes line radial canals
* __**Leuconoid**__ : most complex; choanocytes line internal chambers

Mesohyl

* extracellular gelatinous matrix

Pinacocytes

* outer layer (called __**Pinacoderm**__)
* can ingest food particles (phagocytosis)
* some contracile (myocytes); especially around ostia or oscula
* not a true tissue (no basal lamina)

Archaeocytes

* ameboid cells; phagocytize particles esp. particles for digestion
* differentiate into other types of cells
* sclerocytes - secrete spicules
* spongocytes - secrete spongin
* collencytes - secrete collagen

Choanocytes

* flagellated; create current flow

Sponge feeding

* sponges are suspension feeders
* __**choanocyte**__ - flagellated “collar cell”
* flagella beats; pulls water in through ostia
* “collar” of microfibrils traps particles
* choanocytes phagocytize particles & pass them to archaeocytes for digestion
* digestive system is intracellular

Sponge Asexual reproduction

* __**fragmentation**__ (somatic embryogenesis)
* __**budding**__ (external buds)
* __**gemmules**__ (internal buds; mostly FW sponges)
* archaeocytes become surrounded by spongin coat & spicules

Sponge Sexual reproduction

* most monoecious (have both male & female sex cells)
* gametes arise from choanocytes (archaeocytes in some)
* most sponges: sperm released; phagocytized by archaeocytes carried to oocytes; parent released ciliated larva (__**parenchymula**__)
* in others: egg & sperm released, form parenchymula
* some sponges have inversion (similar to Volvox); blastula turns inside-out

Class Homoscleromorpha

* no spicules or silaceous spicules in some
* pinacoderm with true basal lamina, but cell-cell connections not same as in true tissues

Class Calcispongiae

* calcium carbonate spicules

Class Demospongiae

* silaceous spicules __**not**__ 6-rayed; may be absent
* 95% of all sponges in this class

Class Hexactinellida

* 6-rayed silaceous spicules

some sponges are carnivorous!

* deep sea; \~120 species
* ensnare prey, phagocytize and digest intracellularly

Phylum Placozoa

* one species; marine; tiny (2-3 mm)
* platelike body, no symmetry, no organs, no basal lamina or ECM (extra cellular matrix)
* glide over food, secrete digestive enzymes; absorb products
* interesting because genome contains genes to encode ECM