BIOL 201: Invertebrate Zoology

Chapter 5: Porifera & Placozoa

Rob SwatskiAsst. Prof. Biology

HACC-York1

Phylum Porifera

Porifera = the “pore-bearing” sponges

Not considered to be eumetazoans

Posses epithelioid & rudimentary

connective tissue

Lack true muscle & nervous tissues

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PoriferaOverview

8000 species, most marine, 150 FW

Primitive, sessile filter feeders

Most asymmetrical, but some display radial symmetry

Erect, branching, or encrusting on

substratum3

Sponge Body Plans

Asconoid Syconoid Leuconoid

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AsconoidSponges

Simplest body plan: hollow tube with base attached to

substrate

Ex: Leucosolenia

Have 1 large spongocoel (atrium) lined with 1 layer of

choanoderm

Choanocytes beat & draw water through

ostia (aquiferoussystem)

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AsconoidSponges,

cont.

Water exits spongocoelthrough single large

osculum

Smallest & thinnest of all sponges (1 mm

diameter)

Growth imited by spongocoel diameter

If they had larger diameters, their body volume would exceed pumping capacity of

choanoderm6

SyconoidSponges

Body wall has incurrent canals & outpockets

(radial canals or choanocyte chambers) lined with choanoderm

Increases choanoderm SA & decreases spongocoel

volume

Larger than asconoidsponges (several cm), with

thicker body walls

Grantia & Sycon (Scypha)7

Syconoid sponge (Sycon)

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LeuconoidSponges

Contains 1000’s of choanocyte chambers,

further increasing choanoderm SA

Smaller excurrent canals further reduce

spongocoel diameter

The largest sponges (several cm – 1+ m), with the thickest body walls; indeterminate growth

May have more than one osculum

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Leuconoid sponge10

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Structure of Sponge Body

Wall

Classified as either cellular or syncytial

Cellular spongeshave 2 primitive primary tissues:

epithelioid & mesohyl

Epithelioid tissue resembles epithelium

Mesohyl: middle CT layer of fibrous ECM

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Epithelioid Tissue

Pinacoderm

Covers outer body & lines inner chambers (around

choanoderm)

Pinacoytes Porocytes

Choanoderm

Flagellated cells with a collar of microvilli

Choanocytes

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Mesohyl Tissue(all cells are dynamic, totipotent & amoeboid)

Archeocytes

Can differentiate into any sponge cell; aide in digestion (phagocytosis)

& internal transport

Lophocytes

Secrete & maintain collagen fibers

Spongocytes

Produce thick skeletal spongin

fibers

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Mesohyl Tissue, cont.

Sclerocytes

Secrete spicules: skeletal elements made

of silica or calcium

Myocytes

Muscle-like cells around osculum that constrict or dilate to control water flow

Germ cells

Reproductive cells: oocytes & spermatocytes

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FW Leuconoidsponge

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Syncytial Sponges

Contain simpler, reduced cells in a

syncytium, a continuous

cytoplasm that lacks membranes

No pinacoderm & choanoderm; have

collar bodies instead

Collar bodies are located

individually in collar body

chambers (not in epithelioid sheets)

Mesohyl with archeocytes,

sclerocytes, & germ cells

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SyncytialSponges,

cont.

Body wall resembles 3-D

cobweb-like pattern: trabecular

syncytium

Each strand of the trabecular

syncytium encloses an axis of mesohyl

Contains collagen & spicules

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Porifera Taxonomy

SP Symplasma(Hexactinellida)

Glass sponges: syncytial

Subphylum Cellularia(Cellular Sponges)

Class Demospongiae: 80% of all sponges; siliceous spicules &

spongin

Class Calcarea: calcareous

spicules

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Glass Sponge(SP Symplasma)

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SP Cellularia(Class Demospongiae)23

SP Cellularia(Class Calcarea)

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SP Cellularia(Class Calcarea)

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Sponge Skeleton

Diverse mesohyl acts as an endoskeleton

Some skeletons have fine collagen fibers

only

May contain spicules, spongin, or both

Incredible diversity of spicules: some project

through mesohyl to protect outer sponge

body26

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Water Pumping

Most pump a water volume equal to their body volume every 5

seconds

Large SA & flow regulation slows down

velocity

Contract or relax myocytes to adjust osculum diameter

Can also close ostia & adjust flagellar beat of choanoderm & collar

bodies 30

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Locomotion

Some have limited ability to move (1-4

mm per day)

Result of collective amoeboid movement of cells: very dynamic mesohyl environment

(remodeling)

Osculum contraction by myocytes

Whole body contraction

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Nutrition

Filter feeders able to phagocytize food <

50µm (dinoflagellates, bacteria, viruses,

debris)

Some are carnivores & don’t filter; trap small

animals such as crustaceans

Choanocytes transfer particles to vacuoles

for digestion

Archeocytes remove wastes & inorganics

from system34

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Symbiotic Relationships

Some have PSN endosymbionts: cyanobacteria, dinoflagellates,

chlorophytes

Symbionts help create the bright colors of

sponges

Sponges must live in shallow water for PSN

Some obtain up to 80% of nutrients from

photosynthate36

Internal Transport

Gas & waste transport via simple diffusion

Sponges are “leaky,” so water penetrates their

entire body

Sheets of cells are only 1 cell layer thick

Mobile amoeboid cells

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Internal Transport,

cont.

Ammonia is the main metabolic waste

Common nitrogenous waste in aquatic

environments

Archeocytes transfer wastes & nutrients

Some individual cells posses contractile

vacuoles38

Nervous Tissue

Lack nerve cells

Some have localized action potentials responsible for

myocyte contraction

Glass sponges generate AP’s that

travel rapidly across their syncytium

Used to stop flagellar beating

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Sponge Ecology

Many produce toxins to prevent

predation

Food for spongivores:

nudibranchs, fish, turtles

Hawksbill turtle feces can be up to

95% siliceous spicules!

Some release chemicals that kill

sessile competitors

(corals )40

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Sponge Ecology,

cont.

Habitat for animals (shrimps & brittle

stars)

Decorator crabs place sponges on

their carapaces for defense & gliding

Cliona breaks down calcareous shells

Bores into shells for protection

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Reproduction

Clonal reproduction via fragmentation &

budding

Sponges display incredible powers of

regeneration

Reproduce as a response to wave damage or grazing

Gemmules: spore-like masses of nutrient-

rich archeocytesenclosed by a shell; undergo diapause

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Reproduction, cont.

Sponges frequently reproduce sexually

Hermaphrodites(monoecious)

Germ cells occurthroughout mesohyl

Choanocytes can also release sperm & form

eggs46

Reproduction, cont.

Sperm are broadcasted into water column

Choanocytesphagocytize incoming

sperm, but don’t digest them

Choanocytesdifferentiate into an

amoeboid cell & deliver sperm head to

egg

Most eggs fertilized via phagocytosis (most

sponge sperm lack an acrosome)

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Reproduction, cont.

Some sponges are oviparous: release zygotes into water

column

Most are viviparous: retain zygotes within

body

Release larvae at a later period

Sponge larvae are very diverse:

coeloblastula, amphiblastula, parenchymella 48

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Reproduction, cont.

Larvae are short-lived, settle in a few days, & creep across substrate

until suitable spot is found

Metamorphose into a juvenile

Varied lifespan: live several years in

temperate waters; 200+ years in tropics or

deep sea

Some only grow 0.2 mm/yr & could be

5000 years old with constant growth rate

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Phylum Placozoa

Trichoplax: superficially resembles large ameba

(2-3 mm diameter)

Upper & lower cell layers, 25 µm thick

Enclosed by single layer of epithelioid cells

Densely ciliated ventral surface for locomotion

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Phylum Placozoa, cont.

Fiber syncytium: CT layer of watery ECM &

syncytial network; contractile

Resembles ameba in form & locomotion as it glides across substratum

Feeds on algae via extracellular digestion

Reproduces asexually via budding &

fragmentation; sexual repro & larvae unknown

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PlacozoanPhylogeny

Early evolutionary line of simple metazoans

Intermediate between sponges & cnidarians (RNA sequence data)

Resembles hypothetical protometazoan

Epithelioid layer is more similar to true epithelial

tissue53

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