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But first, a discussion on the origin of metazoa

Metazoans (= animals) - Criteria based on:

• Multicellular
• Motile at some stage of its lifecycle
• Heterotrophic
• Diploid organisms with development proceeding via a blastula

Theories origin of metazoans

Syncytial theory - Metazoa evolved from multinucleated ciliate based on:

• 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

(Syncytial refers to protoplasm that contains numerous nuclei not separated from each other by plasma membrane).

Problems with Syncytial Theory

• Radial symmetry occurs in "Radiates" (sponges, cnidarians, ctenophores, and placozoans)
• No evidence of syncytial cells in basal metazoans.

Colonial Theory -Metazoa evolved from hollow, spherical colonial flagellates

Flagellate protozoans formed a colonial ring of organisms, termed a blastaea then increasing division of labour including invagination to form cnidarian-like guts.  This would be similar to the embryology of many metazoans:
 

Support for Colonial Theory
 

• Flagellated sperm widespread in animals
• Colonial tendency in flagellates
• Shape of mitochondrial structures similar to flagellates
• Molecular evidence indicating a close relationship between flagellates and 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)

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. and that choanoflagellates are sister to the monophyletic Metazoa. Does the evidence of monophylogeny suggest that two or more of these theories might be correct concerning the origin of metazoans?
 
 
 
 

Phylum Porifera: The Sponges
'Pore-bearing' animals.

Diverse: ~5,000 living species described.
 

General Characteristics of Poriferans

Aristotle classified sponges as  plants.

Sponges have typically been considered an early branching event in the history of animals and distinct from the other metazoans:
 

1) Fossil sponges are among the oldest known animal fossils, dating from the Late Precambrian.

 

Simple cell and spicule  associations from  Doushantuo phosphorites  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) Body plan unique and simple among animals:

• unidirectional water flow through pores (ostia) in outer walls and out other larger opening(s) (oscula) driven by the beating of flagella and passively.  How is water driven through sponges passively?


 Allowing for:
• filter food particles
• waste removal
• gas exchange

A variety of growth form that can be loosely assigned into various body types:  

Asconoid

Syconoid

Leucanoid

 
 
• Body supported by a hard skeleton composed of spicules.

     growth of spicule

• Sponges separated into cells can re-assemble themselves.

• Lack of characteristics more typical of most other animals:
• No specialize physiological systems. How do they get by without them?
• No true musculature
• Often no symmetry

Poriferans have recently been placed in a diploblastic assemblage along with Placozoans, Cnidarians and Ctenophorans:

two tissue layers with cells in between not organized as tissues (mesohyl with acrheaocytes)

molecular (RNA) similarities.

 

Life History

• Mostly marine and sessile (intertidal to the deep-sea).  Sponges also provide a haven for a variety of parasites or commensals.

 


Environmental conditions can partially influence sponge morphology.
 
 
 

•  filter-feeders on very small (e.g. bacteria), some are symbiotic with alga, and one small group is considered carnivorous

No recorded extinctions, but threatened particularly by sedimentation  (why?) and destruction of their habitat.
 
 
 
 
 

Reproduction

• Asexual. buds or, in many freshwater species, gemmules (why are gemmules more common in freshwater species?)
  

 
• Sexual. Most are hermaphroditic
Sperm may be "broadcast"
Development usually internal.
Larvae free-swimming, and come in a variety of forms.


 

Metamorphosize when attaching to a substrate (external flagellated cells migrate inward to become choanocytes).

Class Hexactinellida - glass sponges

Siliceous spicules  (six intersecting rays) with secondary network forming trabecular net.   Live at very great depths and osculum is notably large.  Why?

Class Demospongia

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
 

Class Calcarea

Spicules of calcium carbonate and may be straight or composed of 3-4 rays.

Other classes:

• Some traditional classes have been separated into other sponge classes

• Others are represented in mostly found in the fossil record

  

Phylogenetic relationships within Porifera
Hexactinellids were widely viewed as an early branch within the Porifera, however there is some controversy based on recent molecular data.

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.  Why should we be concerned about whether this group is mono- or paraphyletic?


This paraphyletic relationship suggests that the ancestors of all animal phyla aroze from a sponge-like ancestor.
 
 

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