6) Specialized cell types -- the "lego blocks" -- of sponges include:
pinacocytes- form the pinacoderm= inner & outer body covering; not a true epithelium because cells primitively lack one or more of: a) a basal lamina, b) intercellular junctions (can pull apart!), 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 layer, 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 capture method still unclear
pinacocytes & amoebocytes may also phagocytize food particles
some sponges are predatory! hooked spicules act like velcro
8) Reproduction: both asexual (budding & fragmentation) and sexual
most sponges are hermaphroditic (sperm from choanocytes, eggs from choanocyte or amoebocyte 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; can be big (>1m); silica spicules (6-rays); odd 'modified leucon' canal system; no pinacoderm or spongin; syncytial sheets form body, even choanocytes lack nuclei! many weird features, even for a sponge
11) Phylogenetic relations unclear:, but still sister group to other Metazoa