Z250 - Lec7 Outline- Porifera II: Cell types, feeding, development (2013)

Zoology 250 (2013)

PORIFERA II: Cell types,
feeding, development, and diversity

(Porifera study images;
click on tree to see full cladogram)

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6) Specialized cell types -- the "lego blocks" -- of sponges include:
- pinacocytes- form the pinacoderm= inner & outer covering; not true epithelium because cells lack one or more of: a) intercellular junctions (can pull apart!), b) a basal lamina, c) apical-basal polarity, or d) ability (as a layer) to regulate ion levels
- archaeocytes (=amoebocytes)- amoeboid cells that can transform into all other cell types (totipotent); special amoebocytes make spicules and spongin fibers (collagen); some may transport food and make eggs
- myocytes (=contractile cells)- encircle oscula and flagellated chambers, can open and close oscula in response to environmental stimuli
- choanocytes- in choanocyte chambers, function in pumping and feeding; a ring of microvilli surrounds the flagellum; flagellum has a vane like the choanoflagellate flagellum
- porocyte- a tubular cell; forms ostia in some groups
7) Choanocytes feed like choanoflagellates:
- they phagocytize very tiny particles: bacteria, spores, fine organic debris
- particles are trapped on the 'collar' of microvilli (remember: sponges have no mouth and no gut); actual method still unclear
- pinacocytes & amoebocytes may also phagocytize food particles
- some sponges are predatory! hooked spicules act like velcro
8) Reproduction: budding and fragmentation are common
- most sponges are hermaphroditic (sperm from choanocytes, eggs from choanocytes or mesenchyme cells; usually only one sex at a time)
- fertilization is internal! (collar cell captures sperm, carries to egg)
- early cleavage is radial; in most a ciliated blastula develops inside the adult (brooding); the larva swims only briefly before settlement
- overwintering cysts (gemmules): all freshwater spp., some marine spp.
9) Two general body forms: encrusting and upright
10) Three classes of Porifera to note (a fourth class is not covered):
- CALCARAE: calcareous spicules (3-rays), no spongin, small: less than 15cm
- DEMOSPONGIAE: silica spicules (many shapes; never 6-rays), some have spongin; most successful group (80-90% of sp.); can be >2m!
- HEXACTINELLIDA: "glass sponges"; mostly deep sea taxon; can be large (>1m); silica spicules (6-rays); lack: canal system, pinacoderm, spongin; body formed of syncytial sheets, even choanocytes lack nuclei! many bizarre features, even for a sponge
11) Phylogenetic relations unclear:, but clearly the sister group to the Metazoa

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