|Porifera - the sponges|
|This phylum's 'place'
relative to other phyla:
|Selected taxa within this taxon:|
But first, a discussion on the origin of metazoa
Metazoans (= animals) - Criteria based on:
Theories origin of metazoans
Syncytial theory - Metazoa evolved from multinucleated ciliate based on:
(Syncytial refers to protoplasm that contains numerous nuclei not separated from each other by plasma membrane)
- Many protozoans are multinucleated, including ciliates
- Bilaterality in ciliates as in most metazoa
- Such an ancestor might be similar to ciliated 'planula' larva found in cnidarians (and some porifera and one ctenophore) and to acoel flatworms
Planula larva from a cnidarian
Problems with Syncytial Theory
- Radial symmetry occurs in "Radiates" (sponges, cnidarians, ctenophores, and placozoans)
- Little evidence of syncytial cells in most basal metazoans.
Colonial Theory -Metazoa evolved from a colonial flagellate
A colonial sphere of cells, termed a blastaea, invaginated to form cnidarian-like gut resulting in differentation of cell types.Support for Colonial Theory
- Similar to the embryology of many metazoans
- Similarity of Cambrian fossil embryos to blastaea
- Similarity between Choanoflagellates and Choanocytes of sponges (not only basic morphology, but with respect to their internal organelles)
- Molecular evidence indicating a close relationship between chaonoflagellates and metazoans
Theory has been modified to explain planula larva and 'primitive' bilateral animals by ingression:
Numerous other theories have been proposed. Recent molecular data supports a monophyletic origin of metazoans. Does the evidence of monophylogeny suggest that two or more of these theories might be correct concerning the origin of metazoans?
Phylum Porifera: The
Diverse: ~5,000 living species
typically been considered an early branching event in the
history of animals and may represent the ancestral body plan
of all animals:
1) Fossil sponges are among the oldest known animal fossils, dating from the Late Precambrian.
Simple cell and spicule
of southern China
(~570 million years ago)
Archaeocyaths are an extinct group
(~530 million years ago) and contributed
greatly to the creation of the first reefs.
2) molecular evidence: more closely related to other metazoans than any other living group, but other phyla are more closely related to one another than to poriferans.
3) genetic/developmental evidence: Would you expect sponges to have Hox genes? If they did, what would this suggest about whether the poriferan body is primitive?
4) The porifera may be a paraphyletic group with some taxa of sponges being more closely related to the other animal phyla than to other porifera taxa.
What does this paraphyletic relationship suggests about the body plan of the ancestor of all animal phyla?
5) Body plan unique and simple among animals (see below)
Aristotle classified sponges as plants.
What is being accomplished by this movement of water through the sponge?
Metamorphosize when attaching to a
substrate (external flagellated cells migrate inward to
Class Hexactinellida - glass spongesClass Demospongia
Siliceous spicules (six intersecting rays) with secondary network forming trabecular net. Live at very great depths and osculum is notably large. Why?
Most diverse sponge group.
Spicules are of spongin (a collagen) and/or silicaeous, and includes 'bath sponges'
Variety of growth forms from encrusting sheets to branching stalks upright.
Includes the one freshwater exception, family the Spongillidae, the freshwater sponges (expel water by contractile vacuoles).
Commercial sponge harvest
Spicules of calcium carbonate and may be straight or composed of 3-4 rays.
Hexactinellids were widely viewed as an early branch within the Porifera, however there is controversy on the systematics of sponges based on recent molecular data.
Ideas to consider from the sponges: