Poriferans - natureboy

Poriferans

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The first metazoans

Phylum Porifera (Sponges) • ~5,500 extant species •  98% are marine, 2% freshwater • Lack nerves and have no true musculature • No specialized organs • Adults are asymmetrical or superficially radially symmetrical • Cells are totipotent

• Use choanocytes (unique flagellated cells) to drive water through canals and chambers which constitute the aquaferous system 2

Choanoflagellates (protozoan) – can operate in a colonial way.

How are these guys any different?

Sponges can differentiate self from non-self

The first metazoans

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Poriferan Body Plan

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Choanoderm •  inner layer of cells •  Choanocytes (collar cells)

–  Cylindrical arrangement villi surrounding single flagellum

Poriferan Body Plan

Functions: 1.  Circulation – generate currents to circulate

water through sponge

2.  Feeding - Capture of food particles

3.  Reproduction- Capture incoming sperm

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•  Mesohyl –  Gel like matrix –  Acellular and non-living –  Contains archaeocytes –  Contains spicules

Poriferan Body Plan

Archaeocyts: • Amoeboid like cells

• Digest food particles captured by choanocytes • Give rise to sperm and eggs • Role in self-recognition • Eliminate wastes • Differentiate to sclerocytes or spongocytes

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Sclerocytes, spongocytes and spicules • Derived from archaeocytes • Produce spicules – calcium carbonate, silica, or collagenous protein

Importance •  Act as support elements for sponge structures and possibly to deter predation • Systematists use them for species identification

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Pinacoderm (epidermis) –  Continuous layer on the external

surface of sponge –  Lines the incurrent and excurrent

canals

–  Basement membrane is absent* –  Separates sponges from all

other metazoa epithelia which have a basement membrane

Poriferan Body Plan

Pinocytes – thin flattened cells that make up the Pinacoderm

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Poriferan Body Plan

Three basic body types: 1.  Asconoid 2.  Synconoid 3.  Leuconoid

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Poriferan Body Plan

Three basic body types: Asconoids

•  Choanoderm is one cell thick, simple and continuous

•  Single osculum •  Small simple forms

•  Rarely exceed 10 cm in height

Leucosolenia sp.

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Poriferan Body Plan

Three basic body types: Syconoids

•  Folding of pinacoderm and choanoderm •  Simple and complex forms •  Choanocyte chambers

•  Open to atrium via apopyle •  Single osculum

•  Complex forms •  Incurrent canals •  Prosopyles – openings from inccurent chambers

into choanocyte chambers Path of water flow in complex forms: Dermal pore incurrent canal prosopyle

choanocyte chamber osculum atrium apopyle

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Scypha sp.

Sycon sp. 12

Poriferan Body Plan

Three basic body types: Leuconoids

•  Largest and most complex sponges

•  up to 1 m in height •  Complex folding of choanoderm •  Thickening of mesohyl •  Discrete choanocyte chambers

•  Increase in number •  Decrease in size

•  Atrium is reduced to a series of excurrent canals which lead to oscula

Dermal pore incurrent canal prosopyle

choanocyte chamber osculum excurrent canal apopyle

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Clathria sp

osculum

Excurrent canal

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Poriferan feeding •  most sponges are filter-

feeders •  rely on intracellular digestion

–  Phagocytosis –  Pinocytosis

•  Coordinated efforts of choanocytes and Archaeocytes

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Poriferan feeding

•  most sponges are filter-feeders

•  water is moved by current action and choanocyte flagellae

•  choanocyte collars capture food particles

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The rate of water flow is important for efficient filter-feeding in sponges

The increased surface area created by the compartmentalization in leuconoid sponges reduces the velocity of water flow to enhance particulate capture by choanocytes.

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Poriferan feeding •  Predatory Sponge!!

–  Only one species (Abestopluma)

–  Discovered 1995 –  Mediterranean sea caves

•  Lack choanocyte lined aquaferous system

•  Trap prey on external spicules

•  Feeding cells migrate to envelope and digest prey

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Reproduction Can have both asexual and sexual reproduction Asexual Reproduction 1.  Budding

•  Small piece of sponge buds or breaks off •  Carried by current •  Anchors to new substrate and begins to grow a new individual

2.  Reduction bodies •  Marine sponges

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Reproduction Can have both asexual and sexual reproduction Asexual Reproduction 1.  Gemmule formation

•  Occurs mainly in freshwater sponges of family Spongillidae

•  Occurs in response to harsh conditions (temp or environmental)

•  Remarkable example of poriferan cell totipotency

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Reproduction Gemmule formation • Archaeocytes aggregate in mesohyl and udergo rapid mitosis

• Trophocytes (nurse cells) stream to archaeocyte mass and are engulfed by phagocytosis

• Provide nutrition during dormancy

• Mass of cells surrounded by three layers of spongin

• Amphidisc spicules are transported to spongin envelope

• Micropyle forms (absence of spicules in this region)

• Gemmule begins “hibernation” while parent sponge dies and disintegrates

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Reproduction Gemmule hatching • Favourable environmental conditions

• Micropyle opens up • First archaeocytes released flow over gemmule and substratum

• Form pinacoderm and choanoderm framework

• Second wave of archaeocytes colonize this initial framework

• Growth of a new sponge!

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Reproduction Sexual Reproduction • Most sponges are hemaphroditic

• Produce egg and sperm at different times

• Cross fertilization is required

• Sperm – arise from choanocytes

• Oocyte – forms in meshyle surrounded by follicle cells and nurse cells

•  Sperm are released via the aquaferous system and broadcast by the current

• Choanocytes capture sperm, phagocytise sperm (differentiate into amoeboid form) and deliver to the oocyte located in the mesohyl

• Choanocytes digest the sperm of other sponge and invertebrate species but not their own (species recognition)

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