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CHAPTER 12

Sponges and Placozoans

Powerpoint revised by Franklyn Tan Te

Copyright © 2013 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education.

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A Caribbean demosponge, Aplysina fistularis.

Origins of Multicellularity

A. Cells are the elementary units of life

1. Nature’s experiments with larger

organisms without cellular differentiation

are limited such that large, single celled

marine algae are rare

2. Sponges are the simplest multicellular

animals but their cell assemblages are

distinct from other metazoans.

3. Sponges have cells embedded in an

extracellular matrix supported by a

skeleton with needle-like spicules and

protein.12-3

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Origins of Multicellularity

◼ Increasing the size of a cell causes

problems of exchanging molecules with

the environment.

◼ Multicellularity prevents surface-to-mass

problems as smaller units greatly increase

surface area for metabolic activities

◼ Highly adaptive towards larger body size

◼ Sponges neither look like or behave as

animals but molecular evidence demonstrates

that they are phylogenetically grouped with

animals

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Origin of Metazoa

◼ Evolution of the Metazoa (animals)

◼ Eukaryotic cells evolved and diversified

into many lineages that led to modern day

descendants

◼ Includes all unicellular protozoans, colonial

and multicellular plants, animals, and fungi

◼ Multicellular organisms were collectively

called metazoans and are now also

termed “animals”

◼ Metazoans are placed in the Opisthokont

clade which include fungi,

choanoflagellates, and other groups

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Origin of Metazoa

◼ Choanoflagellates

◼ Solitary or colonial aquatic eukaryotes

◼ Each cell has a flagellum surrounded by a collar

of microvilli

◼ Flagellum beats and draws water into collar

where the microvilli collect particles like

bacteria

◼ Most are sessile but one species attaches to

floating diatom colonies and feed midwater

◼ Strongly resemble sponge feeding cells called

choanocytes, which have been argued to be

ancestral to choanoflagellates

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◼ Evidences of common ancestry between

Choanoflagellates and Metazoans

◼ By comparing the genomes or proteomes of

sponges with more complex taxa, scientists

can discover what cell transmitters or

morphogens the first metazoans possessed.

◼ Shared characteristics would have been

inherited from the most recent common

ancestor of animals.

◼ Molecular phylogeny indicates that colonial

bodies evolved early in the lineage.

Origin of Metazoa

Origin of Metazoa

◼ Recent research indicates proteins used by

colonial choanoflagellates for cell

communication and adhesion are

homologous to those that metazoans use in

cell-to-cell signaling.

◼ Sponge genome contains elements that

code for regulatory pathways of more

complex metazoans

◼ Sponges have proteins that code for spatial

patterning that specify anterior and

posterior pole of larvae.

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Origin of Metazoa

◼ Sponges today are less complex than their

ancestors

◼ Sponges have simple bodies composed of

aggregates of several cell types held together

by extracellular matrix

◼ Sponge bodies are not symmetrical and have

no mouth or digestive tract

◼ Placozoans share features with other

animal groups.

◼ Have small nuclear genome and the largest

mitochondrial genome in the animal kingdom

◼ Placozoan bodies are as puzzling as sponges:

they also have no heads or tails12-9

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Phylum Porifera

◼ General Features of Sponges

◼ Mostly sessile

◼ Body designed for efficient aquatic filter feeding

◼ Porifera means “pore-bearing”; sac-like bodies

are perforated by many pores

◼ Use flagellated “collar cells”, or choanocytes, to

move water to bring food and oxygen while

removing wastes

◼ Most of the 8600 sponges are marine, found in all

seas and all depths, while few live in brackish

water and 150 live in fresh water

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Figure 12.1 Some growth habits and forms of sponges.

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Figure 12.2 Sponge choanocytes have a collar of microvilli

surrounding a flagellum.

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Phylum Porifera

◼ Sponges vary in size from a few

millimeters to over 2 meters in diameter

◼ Many species are brightly colored because

of pigments in dermal cells

◼ Embryos are free-swimming while adult

sponges always attached

◼ Some appear radially symmetrical but many

are irregular in shape

◼ Some stand erect, some are branched, and

others are encrusting

◼ Growth patterns depend on shape of

substratum, direction of water, speed of flow

and availability of space

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Phylum Porifera

◼ Many animals like crabs, nudibranchs,

fish, and other species do live as

commensals or parasites in or on sponges

◼ Sponges can also grow on a variety of

other living organisms with some crabs

using sponges for camouflage and

protection

◼ Sponges and microorganisms living on

them often have a noxious odor and

produce a variety of bioactive compounds

◼ Certain sponge extracts have manifested

medical and pharmaceutical effectiveness.

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Phylum Porifera

◼ Skeletal structure of a sponge can be fibrous

and/or rigid consisting of calcareous or

siliceous spicules

◼ Fibrous portion comes from collagen protein

fibrils in intercellular matrix

◼ There are several types of collagen, which vary in

chemical composition; sponges contain spongin

◼ Composition and shape the spicules form the

basis of sponge classification

◼ Modern materials science view spicules for

possible nanoparticle products

◼ The simplistic exterior of sponges often mask

their chemical and functional sophistication

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Figure 12.3 Diverse forms of spicules that support a sponge body.

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Phylum Porifera

◼ Sponges date back to the early Cambrian and

maybe even Precambrian period

◼ Traditionally grouped in three classes based on

spicules and chemical composition

◼ Calcarea: calcium carbonate spicules with

one, three, or four rays

◼ Hexactinellids: glass sponges with six-rayed

siliceous spicules

◼ Demospongiae: siliceous spicules around an

axial filament, spongin fibers, or both

◼ Homoscleromorpha, was formed to contain

sponges without a skeleton or with siliceous

spicules without an axial filament

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Figure 12.4 Cladogram depicting evolutionary relationships among the

four classes of sponges.

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Phylum Porifera

◼ Form and Function

◼ Body openings consist of small incurrent

pores or dermal ostia in the outer layer of

cells called pinacoderm

◼ Sponges feed by collecting suspended

particles from the water through internal

canal systems

◼ Water is directed past the choanocytes, which

are flagellated collar cells that keep the current

flowing via beating of flagella

◼ Microvilli in the collar trap and phagocytize

food particles that pass by.

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Phylum Porifera

◼ Sponges non-selectively consume food

particles (detritus, plankton, and bacteria)

◼ The smallest particles (80%) are taken into

choanocytes by phagocytosis

◼ Protein molecules may be taken in by

pinocytosis

◼ Two other cell types, pinacocytes and

archaeocytes, facilitate feeding

◼ Dissolved nutrients can also be absorbed by

sponges

◼ Efficiency of food capture is dependent on

water movement through the sponge body

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Phylum Porifera

◼ Three types of sponge body designs

◼ Asconoids

◼ Simplest body organization

◼ Small and tube-shaped to allow water to flow

directly across cells so no “dead space”

◼ Choanocytes are in a large internal chamber, the

spongocoel

◼ Choanocyte flagella pull water through the

pores and extract food particles

◼ Used water is expelled through a large single

osculum

◼ All Calcarea are asconoids

◼ Leucosolenia sp. and Clathrina sp, for example

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Figure 12.5 Three

types of sponge

structure.

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Figure 12.6 Clathrina canariensis (class Calcarea) is a common

Asconoid on Caribbean reefs.

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Phylum Porifera

◼ Syconoids

◼ Resemble asconoids but larger and with a

thicker more complex body wall

◼ Body wall is folded outwards with choanocyte-

lined radial canals that empty into spongocoel

◼ Water enters through dermal ostia and move

into tiny openings called prosopyles into the

radial canals

◼ Food is ingested by choanocytes and used

water is pumped through internal pores

called apopyles then outwards via osculum

◼ Spongocoel is lined with epithelial cells rather

than choanocytes as in asconoids

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Phylum Porifera

◼ Developmental evidence of being derived

from asconoid ancestors

◼ Syconoids pass through an asconoid stage in

development but do not form highly branched

colonies

◼ Flagellated canals form by evagination of the

body wall

◼ Syconoid body plan is not homologous among

all sponges that have it

◼ Classes Calcarea and Hexactinellida have

syconoid species (ex: Sycon sp.)

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Figure 12.7 Cross section through wall of sponge Sycon

sp., showing choanocytes in canals within the wall but do

not line spongocoel.

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Phylum Porifera

◼ Leuconoids

◼ Most complex and larger, for more food-

collecting regions

◼ These regions have choanocytes lining in small

chambers that effectively filter all water present

◼ Clusters of flagellated chambers are filled from

incurrent canals and discharge to excurrent

canals which lead to osculum

◼ After food is removed, used water is pooled to

form an exit stream that leaves through an exit

pore at very high velocity

◼ This high rate of exit flow prevents the sponge

from re-filtering used water and wastes

◼ Most sponges are leuconoid type

Phylum Porifera

◼ The leuconoid system has high adaptive

value to efficiently meet high food

demands of larger body size

◼ Has the highest proportion of flagellated

surface per volume of cell tissue

◼ More collar cells can filter more particles

◼ Water flow slows down inside due to greater

surface area within the chambers

◼ Large sponges filter 1500 liters of water per

day for maximum food collection

◼ The leuconoid system has evolved

independently many times in sponges

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Figure 12.8 This orange demosponge, Mycale laevis, often grows

beneath plate-like colonies of the stony coral Montastrea annularis.

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Phylum Porifera

◼ Types of Cells in Sponges

◼ Sponge cells are arranged in a gelatinous

extracellular matrix called mesohyl or

mesenchyme

◼ The connective “tissue” of sponges found in

fibrils, skeletal elements, and amoeboid cells

◼ Absence of organs requires that all

fundamental processes occur at the

individual cell level

◼ Respiration and excretion via diffusion and

water regulation via contractile vacuoles in the

archaeocytes and choanocytes

Phylum Porifera

◼ Visible activities seen in sponges include

slight alterations in shape, local

contraction, propagating contractions, and

closing and opening of incurrent and

excurrent pores

◼ Sponges can close their osculum due to

heavy sediment load

◼ Movements occur very slowly but they suggest

a whole body response in organisms lacking

complex organization above the cellular level

◼ Apparently excitation spreads from cell to cell

by mechanical stimuli and signaling molecules

like hormones or via electrical impulses12-31

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Figure 12.9 Small section through sponge wall, showing

four types of sponge cells.

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Phylum Porifera

◼ Choanocytes

◼ Oval cells with one end embedded in mesohyl

and exposed end has one flagellum surrounded

by a collar

◼ Collar consists of microvilli connected to

each other by fine microfibrils

◼ Forms a fine filtering device to strain food

◼ Particles too large to enter collar are trapped

in mucous and slide down to base to be

phagocytized

◼ Food is passed to archaeocytes for

intracellular digestion with no need for gut

cavity

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Figure 12.10 Food trapping by sponge cells. A) Cutaway section of

canals showing direction of water flow. B) Two choanocytes, and C)

structure of the collar.

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Phylum Porifera

◼ Archaeocytes

◼ Amoeboid cells that move about in the

mesohyl with many functions

◼ Phagocytize particles in the pinacoderm

◼ Receive particles for digestion from

choanocytes

◼ Can differentiate into many other more

specialized cell types

◼ Sclerocytes: secrete spicules

◼ Spongocytes: secrete spongin

◼ Collencytes: secrete fibrillar collagen

◼ Lophocytes: secrete large amounts of

collagen

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Phylum Porifera

◼ Pinacocytes

◼ Thin, flat, epithelial-like cells that cover the

exterior and interior surfaces of sponges

almost like real tissues

◼ Form pinacoderm with a variety of intercellular

junctions but no basal membrane for most

sponges

◼ Ingest food by phagocytosis and are

contractile to regulate surface area of sponge

◼ Form myocytes that are circular bands around

oscula and help regulate flow of water

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Phylum Porifera

◼ Cell Independence: Regeneration and

Somatic Embryogenesis

◼ Sponges have a great ability to regenerate

lost parts and repair injuries

◼ Complete reorganization of the structure

and function of participating cells or bits

of tissue occurs in somatic embryogenesis

◼ Process of reorganization differs in

sponges of differing complexity

◼ Regeneration following fragmentation is

one means of asexual reproduction

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Phylum Porifera

◼ Types of Asexual reproduction

◼ Fragmentation

◼ Sponge breaks into parts that are capable of

forming a completely new sponge

◼ Bud formation

◼ External buds

◼ Small individuals that break off from

parents that have reached a certain size

◼ Internal buds or gemmules

◼ Formed by archaeocytes that collect in

mesohyl and coated with tough spongin and

spicules that can survive harsh

environmental conditions

Phylum Porifera

◼ How gemmules work?

◼ When parent sponge dies, gemmules survive

and remain dormant during the harsh

situations

◼ Live cells within gemmules escape through

special opening called micropyles and develop

into new sponges

◼ Gemmulation is a adaptation to changing

seasons and for colonization of new habitats

◼ Gemmules are controlled by weather, internal

chemicals, and by remaining inside the parent

sponge.

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Figure 12.11 Section through a gemmule of a freshwater

sponge (Spongillidae).

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Phylum Porifera

◼ Sexual Reproduction

◼ Most are monoecious (both male and

female sex cells in one body)

◼ In some Demospongiae and Calcarea

◼ Gametes develop from choanocytes

◼ Some gametes from archaeocytes

◼ Most sponges are viviparous where zygote

is retained within parent and provided with

nourishment until it is released as a

ciliated larva

◼ One sponge releases sperm which enter

the pores of another sponge

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Phylum Porifera

◼ Different types of fertilization and zygote

formation exits in sponges

◼ Viviparous sponges have choanocytes that

phagocytize the sperm and transform into

carrier cells that transport sperm through the

mesohyl and to oocytes to form zygotes

◼ Oviparous sponges release both sperm and

oocytes into water for external fertilization

◼ The free-swimming larva of most sponges

is a solid-bodied parenchymula; six other

larval forms exits.

◼ The outwardly directed flagellated cells of

the parenchymula become choanocytes

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Phylum Porifera

◼ Unique development patterns in Calcarea

and some Demospongiae

◼ Hollow stomoblastula develops with flagellated

cells oriented toward the interior

◼ Blastula then turns inside out (inversion) and the

flagellated cells now turn outside

◼ Small flagellated cells or micromeres located at

anterior end while larger non-flagellated

macromeres located at posterior end

◼ Macromeres overgrow invaginating micromeres

during metamorphosis and settlement

◼ Micromeres become choanocytes, archaeocytes,

and collencytes while macromeres give rise to

pinacoderm and sclerocytes

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Figure 12.12 A) Development of demosponges, B) Development of the

calcareous syconoid sponge Sycon sp..

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Phylum Porifera

◼ Class Calcarea (Calcispongiae)

◼ Calcareous sponges with spicules of

calcium carbonate

◼ Spicules are straight (monaxons) or have

three or four rays

◼ Most are small with tubular or vase shapes

◼ Many are drab in color, but some are bright

yellow, green, red, or lavender

◼ Leucosolenia sp. and Sycon sp. are marine

shallow-water

◼ Asconoid, syconoid and leuconoid body

forms

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Figure 12.13 Some sponge body forms.

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Phylum Porifera

◼ Class Hexactinellida (Hyalospongiae)

◼ Glass sponges with six-rayed spicules of

silica bound together to form network

◼ Nearly all are deep-sea forms

◼ Most are radially symmetrical with vase or

funnel shaped bodies attached by stalks of

root spicules onto the substrate

◼ Have syncytial cell structure that have many

nuclei with a large cell which were produced

by the fusion of many cells or division of

nuclei without dividing the cytoplasm

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Phylum Porifera

◼ Most Hexactinellids have trabecular

reticulum that is bilayered, sheet-like and

tubular with collagenous mesohyl cells

◼ Cytoplasmic bridges connect choanoblasts

and archaeocytes with trabecular reticulum

◼ Choanoblasts are unusual cells that make

flagellated outgrowths called collar bodies

whose flagella beat to move water like

choanocytes

◼ Food is collected by directing water

through the primary and secondary

reticulum layers

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Figure 12.14 Diagram of part of a flagellated chamber of hexactinellids.

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Phylum Porifera

◼ Class Demospongiae

◼ Contains 95% of living sponge species

include most large sponges

◼ Spicules are siliceous but not six rayed

and may be absent or bound together by

spongin

◼ Leuconoid body form for all species

◼ All marine except for Spongillidae, the

freshwater sponges

◼ Marine demosponges are highly varied in

color and shape, with some growing to

several meters in diameter.

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Phylum Porifera

◼ Freshwater demosponges

◼ Widely distributed in well-oxygenated ponds

and streams

◼ They encrust plant stems and submerged

wood

◼ Look like wrinkled scum, pitted and porous

with brown and green colors

◼ Flourish in summer and in early autumn

◼ Reproduce sexually, but existing genotypes

may also reappear annually from gemmules

◼ Sponges die by late autumn and asexually

release gemmules to prepare for next year’s

population.

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Figure 12.15 Marine Demospongiae on Caribbean coral reefs. A)

Pseudoceratina crassa, B) Aplysina fistularis, C) Monanchora unguifera

Phylum Porifera

◼ Class Homoscleromorpha

◼ Mostly marine with a variety of colors but

live in cryptic habitats

◼ Generally found near shore but have deep

water forms

◼ Separated from Demospongiae due to

presence of true basement membrane

under pinacoderm or extracellular matrix

◼ Also have adherens cell junctions that

from true tissues unlike other sponges

◼ Divided into two clades based on absence

or presence of spicules12-53

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Phylum Porifera

◼ Phylogeny and Adaptive Diversification

◼ Sponges appeared before the Cambrian and

two calcareous sponge-like organisms were

in Paleozoic reefs.

◼ Sponges share many traits with other

animals and are considered sister taxon

◼ Proteins for cell adhesion and cell-signaling are

homologous to other animals

◼ Some sponges have basement membrane with

collagen and adherens junctions with cadherin

molecules that connect epithelial cells

◼ Sponge have blastula and some form gastrula

stages like many animals

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Phylum Porifera

◼ Adaptive Diversification

◼ Poriferans are a highly successful group

with thousands of species in diverse

habitats

◼ Diversification centers on their unique

water-current system and its degree of

complexity

◼ New feeding mode has evolved for sponges

found in deep water caves with low

nutrients

◼ Illustrates the non-directional nature of evolution

Phylum Porifera

◼ Unique features of deep water sponges

◼ Many tiny hook-like spicules cover highly

branched body

◼ Spicule layer can entangle the legs of

crustaceans that come near sponge

◼ Filaments of the sponge body grow over prey,

slowly enveloping it and later digesting it

◼ Most of the group are carnivores and not

suspension feeders

◼ Some have symbiotic methanotrophic

bacteria

◼ Contain siliceous spicules, but lack

choanocytes and internal canals so very

different than regular sponges12-56

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Figure 12.16 The carnivorous sponge, Chondrocladia lyra , is

commonly called a “harp sponge.”

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Phylum Placozoa

◼ Proposed by K. G. Grell (1971) based on

a single species- Trichoplax adhaerens

◼ Tiny (2-3 mm) marine form that is plate-like

and has no symmetry

◼ No major organs, no muscular or nervous

system

◼ Lacks basal lamina beneath epidermis and

no extracellular matrix but has genes for it

◼ Body has dorsal epithelium to cover cells

and have thick ventral epithelium of

monociliated cells and nonciliated gland

cells

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Phylum Placozoa

◼ Space between the epithelia contain

multinucleated fibrous “cells” within a

contractile syncytium

◼ Placozoans glides over food, secretes

digestive enzymes, and absorb nutrients

◼ Divide asexually and produce “swarmer”

stages by budding.

◼ No sexual stages have been seen but have

isolated eggs in the laboratory

◼ Considered diploblastic with dorsal

epithelium representing ectoderm and

ventral epithelium representing endoderm

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Figure 12.17 A) Trichoplax adhaerens is a marine placozoan, B)

Section through Trichoplax adhaerens, showing histological structure.