Porifera
Poriferans are more commonly known as sponges. A branching event early in the history of animals separated the sponges from the other metazoans. As one might expect based on their phylogenetic position, fossil sponges are among the oldest known animal fossils, dating from the Late Precambrian period. Since that time, sponges have been conspicuous members of many fossil communities; the number of described fossil genera exceeds 900. The approximately 5,000 living sponge species are classified in the phylum Porifera, which is composed of three distinct groups, the Hexactinellida (glass sponges), the Demospongia, and the Calcarea (calcareous sponges).
Sponges are characterized by the possession of a feeding system unique among animals. Poriferans don't have mouths, but instead, they have tiny pores in their outer walls through which water is drawn. Cells in the sponge walls filter food from the water as the water is pumped through the body and out other larger openings. The flow of water through the sponge is unidirectional, driven by the beating of flagella which line the surface of chambers connected by a series of canals. Sponge cells perform a variety of bodily functions and appear to be more independent of each other than are the cells of other animals.
Diversity, habitat, habit
~7000 described extant species in 682 valid extant genera (Hooper & van Soest, 2002)
Actual diversity estimated at 15,000+ species; Australia alone estimated to harbor 5000 species!
Mostly marine, few FW.
Almost all sponges are sessile, non are self-motile; thought to be plants until 1765
Growth form varies greatly, asymmetrical to radially symmetrical, often phenotypically plastic: changing form according to prevailing environmental conditions.
Sponges are often highly colorful, especially in shallow reefs; their colors can function in photoprotection or as aposomatic warning.
Few mm to 2m+
Primitive organization
The most “primitive” animals; do not conform to the body plan of other animals.
Very ancient (Precambrian-)
Probably evolved from a colonial choanoflagelate (see flash animation here)
No gut, nervous system, no organs for excretion, circulation, etc.
Obtain nourishment by filtering water through specialized, filtering collar cells (choanocytes).
Cellular grade of organization - although there are many cell types that are specialized to do different tasks, many of the various cell types can do multiple tasks. Little structural integration.
- individual cells do their own respiration, digestion (all intracellular), excretion, etc.
- small temporary aggregates of cells do reproduction and skeletogenesis
- permanent cell associations make up body wall and are involved in food capture
- considerable powers of trandifferentiation
Major cell types
pinacocytes (myocytes)
sclerocytes
amoebocytes (archeocytes)
collencytes
choanocytes
porocytes
Pinacoderm made of pinacocytes. Not a “proper” epithelium as lacks basal lamina, and cell junctions (desmosomes) are rare, or, usually, lacking. Some myocytes help control shape of openings, etc. Major exception: Homoscleromorpha, which do have basal lamina and cell junctions – thus their pinacoderm is an epithelium.
Sclerocytes secrete the mineralized spicules ("little spines") that form the skeletons of many sponges and in some species provide some defense against predators.
Archaeocytes (or amoebocytes) are amoeba-like cells that are totipotent, in other words each is capable of transformation into any other type of cell. They also have important roles in feeding and in clearing debris that block the ostia
- Lophocytes are amoeba-like cells that move slowly through the mesohyl and secrete collagen fibers.
- Collencytes are another type of collagen-producing cell.
- Rhabdiferous cells secrete polysaccharides that also form part of the mesohyl.
Choanoderm made of layer of choanocytes = collar cells; involved in food capture
Porocytes form the inlet pores (which are called ostia in asconoids & some syconoids)
Sponges built from 3 layers (see diagram): pinacoderm, mesohyl, choanoderm
In between: mesohyl - jello layer (proteins, polysaccharides, fibrous collagen fibers) w/ skeleton & loose cells in it. A variety
of cell types roam the mesohyl, including various ameoba-like amoebocytes that carry food, wastes, etc. (see below),
sclerocytes, collencytes, etc..
Link to a cool flash animation for how the different bodytypes feed
Three levels of structural organization: asconoid, syconoid, leuconoid.
All 3 in calcareans, demosponges almost exclusively leuconoid, a syconoid-type construction in hexactinellids.
Aquiferous system – canals to and from choanocyte chambers (as well as chambers themselves); canals lined by pinacoderm. Water flow: driven by choanocyte; also some passive flow induced by currents in sponges of appropriate shape and positioning. Water enters through:
- porocyte w/ ostium (diagram, in asconoids & some syconoids)
- or through ostia lined by several pinacoderm cells (other syconoids & leuconoids)
Then flows through:
- incurrent canals (lined by pinacocytes)
- prosopyle
- flagellated chambers
- apopyle
- excurrent canals (lined by pinacocytes) – may form large spongocoel at end
- osculum
Leuconoid sponges have 7,000-18,000 chambers / mm3, each filtering its volume ~1,200X a day (Syst.Por.).
The widest, terminal portions of excurrent canals are often strikingly apparent as star-like patterns converging toward oscula (astrorhizae). The largest openings on sponges are generally oscula; they are often elevated on raised, chimney-like projections, so as to expel wastewater far away and thus minimize the mixing of excurrent and incurrent water.
Current control
On several levels. Can stop beating of choanocytes and thus stop active flow (only in hexacts?). Can control flow by contracting ostia or oscula (in some sponges only – through muscular lining - myocytes) – oscular contraction water jets further. Current generated not only by active beating of flagellae, but also by passive induced flow - Bernoulli effect (high pressure, low velocity at base, low pressure, high velocity over raised object – sucks water out). Oscula placed on highest parts of sponge generate passive flow even in dead sponges; raising oscula also helps jet used water further – avoid mixing filtered water with intake.
Muscles
myocytes = pinacocytes that are also contractile, no “pure” muscle cells. Around in- and ex-current openings to help control water flow.
Food capture
- several levels of filtration: ostia = external filter to exclude big stuff, although porocytes/pinacocytes can phagocytize food that gets stuck outside. Pinacocytes lining channel walls can do same. If channel gets clogged by oversize particles, it can reorganize.
4 sets of filters in Callyspongia: Ostia: 50μm, incurrent canals: 25 μm, prosopyles very regular between choanocyte bodies: 1 μm, space between microvilli: 0.1 μm.
Pumping rate enormous: e.g. 10 cm high, 1 cm diam. Leuconia has 2,250,000 chambers & pumps 22.5 liters/day.
Great filtering efficiency, 0.1 um can be caught, most food is bacteria & phytoplankton & “invisible” small particles (80% of food = latter); capture 96% of such (Reiswig HM 1971 Biol. Bull. 141: 568-591).
Utilize size range available to few other metazoans (otherwise only ~ to mucus net feeders).
Thus if sponges common - great effect on microplankton & small particulates.
Other forms of nutrition
Predation - One group of sponges (Cladorhizidae) is predatory and has lost its choanoderm as well as aquiferous system! Instead uses long filaments extended from the body and armed with hooked spicules (Vacelet & Boury-Esnault, 1995), a fundamental abberation to the body plan of Porifera.
Trophic symbioses - In addition to filter feeding, many sponges, like many other sessile organisms on reefs, have microscopic algal symbionts, that may provide nourishment for the host. A diversity of cyanobacteria (= blue-green algae) and eukaryotic algae (zoochlorellae in spongillids, zooxanthellae in some clionaids) are involved in sponge photosymbiosis, and on many reefs photosymbiotic sponges dominate the sponge fauna, at least in biomass. One sponge is also known with chemoautotrophic symbiosis, with methanotrophic bacteria – lives in deep sea around mud volcanos, it is in the carnivorous sponge family (Cladorhizidae). Bacteria live intercellularly and are phagocytized periodically by amoebocytes.
“Circulation”
Food phagocytized by choanocyte/pinacocyte passed to amoebocyte for intracellular digestion, transport to other cells & storage.
Many sponges are slow growing and some (e.g. cave dwelling sclerosponges) can live for centuries (e.g. Ceratoporella – 0.2 mm/y, to 40 cm diameter – Benavides & Druffell, 1986). Some sponges grow very rapidly and have short lives though.
Osmoregulation / excretion
At cell level. FW spongillids have contractile vacuoles, like protists, for osmoregulating.
Skeleton:
Organic
– spongin (fibrous protein cf. collagen) - by collencytes – found only in (most, but not all) demosponges
– fibrous collagen – loose in mesohyl in all sponges
Inorganic
– spicules – CaCO3 (Calcarea) or SiO2 (Demosp., Hexact.)
– solid skeleton (sclerosponges - CaCO3 as aragonite or calcite)
– inorganic skeleton lacking in several major groups of demosponges – including bath sponges
Spicules: Diverse forms, often in 2 size classes: megascleres & microscleres. Very useful for identification – lots of terminology.
Spicule formation: by sclerocytes, which arise from exopinacocytes (latter can also make monaxon spicules in situ).
Calcarea: 3 cells come together - each divide to form thickeners and founders; form triradiate spicules.
Demosponiae: 1 cell divides 2X = 4, each makes ray of tetraxon (intracellularly?)
Nervous system: No nerve cells, little conduction in most sponges, if disturb animal, will slowly contract though. Electrical conduction - action potential - in hexactinellids - have continuous cell membrane because of syncytial morphology.
Reproduction & Development
Sexes separate or hermaphroditic
Spermatogenesis: from choanocytes, in Ephydatia divide & give off spermatogonia, which develop into spermatocysts surrounded by an epithelium. In others choanocytes of whole chambers may just drop their flagellae and become spermatogonia. Sperm always free spawned.
Oogenesis: from choanocytes (e.g. Scypha - Franzen 1988) or from archeocytes (e.g. Ephydatia - Saller 1988), nutrition gained in part by phagocytizing other cells - trophocytes. Eggs retained in most sponges until fertilization, although some sponges (e.g. Cliona, Xestospongia) free spawn eggs. If eggs retained, sperm is captured by choanocyte, engulfed into vesicle – choanocyte looses collar & flagella and becomes carrier cell – carries to oocyte – fertilization.
Some oviparous (zygote spawned) many viviparous (larvae released).
Irregular, variable cleavage. 3 larval types: amphiblastula (hollow interior, partly flagellated; typical of Calcarea & Oscarella among demosponges), parenchymella (=parenchymula, solid, ciliated; demosponges), coeloblastula.
Amphiblastula from stomoblastula by inversion, flagellae get to outside – then at metamorphosis, turns inside out again - see diagram - now internalized flagellas become choanocytes.
Parenchymella has outer flagellated cells that loose flagellation at metamorphosis (Franzen 1988 p. 355). Inside filled with various degrees of complexity depending on species: amoebocytes, also spicules (in some/all?); most complex in FW sponges, where they have fully formed choanocyte chambers and some canals.
Asexual reproduction
- can produce gemmules in FW sponges for overwintering, also gemmule like bodies in some marine species (e.g., some Haliclona spp.). Gemmules filled w/ aggregated amoebocytes.
- many forms of budding
- also common for sponges to just break apart and grow separately – latter is used in propagation of commercial sponges.
- great powers of regeneration: cut up - heals easily. Certain spp. if strain through filter thus separate cells - will reaggregate to form new sponge. Such reaggregation can be spp. specific - mix red and yellow and get them back; or unspecific - some even form aggregates w/ anemone cells.
Ecology
Predation – Like many sessile reef organisms, sponges are reknown for their chemical weaponry, and perhaps because of their old ancestry and abundant bacterial symbionts have a more diverse array of protective, nasty chemicals than any other animal group. In addition to the secondary metabolites, they have loads of spicules – which can inflict wounds, make digestion difficult, and generally decrease the density of digestible materials.
Only a few animal groups are capable of eating sponges without suffering ill consequences; these animals tend to be specialized feeders on sponges, often feeding on but one sponge species. Major sponge predators include some polychaete worms, some opisthobranchs (e.g. some dorid nudibranchs, cephalaspids), some prosobranch snails (e.g. pleurotomariids, some fissurellids, some cowries), several sea stars (e.g. Pteraster, Henricia, some oreasterids and ophidiasterids), a few reef fish such as pomacanthids, moorish idol (Zanclus cornutus) and hawksbill turtles.
Sponge predators often sequester nasty sponge chemicals to defend themselves against predation. The bright colors of many nudibranchs appear to be aposomatic to advertise this. Eating hawksbill meat has caused human fatalities. In other cases predators mimic the color of their host sponge and are difficult to find – e.g. Ophlitaspongia - Rostanga, former source of color for latter, or Notodoris serenae – Leucetta avocado.
Sponges are hotels for symbionts – their spongy bodies are often full of a variety of organisms. These include a diversity of prokaryotes, some of which may be responsible for the great chemical arsenal of sponges. Many inverts and fish can also live in sponges, and some species are literally crawling with worms, shrimp, crabs, brittle stars and the like. These animals gain protection from the sponges arsenal and may also benefit from (and “parasitize”) the flow of water into the sponge, or clean off accumulated large particles on, and thus benefit, the sponge. Many of the associations are obligate and species specific, and some are trophic – e.g. some tiny cephalaspids live on sponges and are micropredatory on them. Chemistry can be too nasy though, and there is some evidence that invert symbionts are more common in chemically less nasty sponges.
Competition
Sponges have to compete with other sessile biota. Their chemical arsenal helps in this as well. Some can be very successful in dominating space – e.g. Terpios hoshinota.
Bioerosion
Sponges are one of the major forces of bioerosion on coral reefs, and on limestone substrata in general –Spheciospongia, Cliona, etc. Cliona (see Seilacher notes): etch pieces chemically by amoebocytres, chips thrown out through excurrent, such chips abundant in reef sediments.
Systematic relationships
Collar cells = choanocytes appear to establish relation to choanoflagellates, a phylum of protists that also feed in a similar way to sponges, filtering w/ microvillar ring & phagocytizing food. Choanoflagellates may be colonial, e.g. in jelly mess – Proterospongia – these really similar to sponges. Choanoflagellates have flat mt. cristae like few other protists, but like metazoans and fungi. Genetic data supports relationship (fungi (choanos sponges)). Some have even suggested that choanoflagellates – a quite divergent group of protists overall – may be degenerate sponges. Collar type cells are also found throughout the Eumetazoa –e.g. many monociliated cells in deuterostomes and some protostomes bear rudimentary microvillar collars.