sponges and placozoa

Sponges and Placozoans

Origin of Metazoa

Evolution of the eukaryotic cell was followed by diversification into many lineages including:Modern protozoansPlantsFungiAnimals

Multicellular animals are called metazoans.

PoriferaPorifera

ActinopodaActinopoda

CnidariaCnidaria

ApicomplexaApicomplexa

CiliophoraCiliophora

MolluscaMolluscaAnnelidaAnnelida

EuglenozoaEuglenozoa

BryozoaBryozoa

PlatyhelminthesPlatyhelminthes

RhizopodaRhizopoda

RotiferaRotifera

GranuloreticulosaGranuloreticulosa

NematodaNematodaPriapulidaPriapulidaKinorhynchaKinorhynchaLoriciferaLoricifera

PhoronidaPhoronidaBrachiopodaBrachiopoda

EchinodermataEchinodermata

HemichordataHemichordata

ChelicerataChelicerata

UniramiaUniramia CrustaceaCrustacea

ProtochordataProtochordata

VertebrataVertebrata

Dendrogram of Major Phyla

Choanoflagellates

Choanoflagellates are solitary or colonial protozoans with a flagellum surrounded by a collar of microvilli.

Choanoflagellates

Choanoflagellates resemble sponge feeding cells (choanocytes).

Scientists are studying colony formation and cell-to-cell communication in choanoflagellates in search of clues to the evolution of multicellularity.

Resulted to two origins of multicellularity: Syncitial ciliated and Colonial flagellated

Syncitial Ciliate Hypothesis

Syncitial ciliate hypothesis – metazoans arose from an ancestor shared with single celled ciliates.Recall multiple nuclei in reproducing ciliates.Later, each nucleus becomes partitioned.Trend toward bilateral symmetry as in

flatworms.

Syncitial Ciliate Hypothesis

Problems: In flatworm embryology nothing like

cellularization occurs.Does not explain flagellated sperm in

metazoans. Implies that radial symmetry is derived.histoincompatibility

Colonial Flagellate Hypothesis

Colonial Flagellate Hypothesis – metazoans descended from ancestors characterized by a hollow, spherical colony of flagellated cells. Individual cells became specialized for

different functions.Radially symmetrical, similar to a blastula.First proposed by Haeckel in 1874

Phylum Porifera

Sponges, Phylum Porifera, are multicellular heterotrophs.

They are asymmetrical. They lack true tissues and organs. Molecular evidence suggests they do share a

common ancestor with other animals. Kingdom Animalia is monophyletic.

Phylum Porifera

Sponges are sessile animals that have a porous body and choanocytes. Supported by a

skeleton of tiny needlelike spicules and protein.

They live in both fresh and marine waters.

Phylum Porifera

Sponges range in size and shape. Up to 2 meters in diameter! Encrusting, boring, finger, tube or vase

shaped.

Neighbors

Many organisms, including crabs, nudibranchs, mites, bryozoans, and fish live as commensals or parasites in sponges.

Skeletal Framework

The skeletal framework of a sponge may be fibrous or rigid.

The fibrous part comes from collagen fibrils in the intercellular matrix. Spongin

Rigid skeletons consist of needlelike spicules. Calcareous Siliceous

Suspension Feeders

Sponges are suspension feeders capturing food particles suspended in the water that passes through their body.

Suspension Feeders

Water flows in through incurrent pores called dermal ostia.

It flows past the choanocytes where food particles are collected on the choanocyte collar.

Suspension Feeders

Choanocytes take in small particles by phagocytosis. Protein molecules are taken in by pinocytosis.

Sponges can also absorb nutrients dissolved in the water.

Canal Systems

Asconoid – the simplest canal system. Choanocytes line the

spongocoel. Water enters through

the ostia and exit through the large osculum.

Usually tube shaped. Found only in the

Class Calcarea.

Canal Systems

Syconoid – tubular body and singular osculum like asconoids.

The walls of the sponge are folded to form choanocyte lined canals. Increased area for

feeding. Class Calcarea.

Canal Systems

Leuconoids – most complex, permits an increase in sponge size.

Choanocytes line the walls of small chambers where they can filter all the water that flows through.

Most sponges.

Types of Cells

Absence of tissues & organs means that fundamental processes occur on the cellular level.

Respiration and excretion occur by diffusion in each cell.

Mesohyl is the gelatinous matrix containing skeletal elements & amoeboid cells.

Types of Cells

Choanocytes, flagellated collar cells, generate a water current through the sponge and ingest suspended food.

Types of Cells

The choanocytes pass food particles to archaeocyte cells for digestion.

Digestion occurs entirely within cells, there is no gut.

Other cell types secrete spicules (sclerocytes), spongin (spongocytes), & collegen (collenocytes).

Types of Cells

Pinacocytes are thin, flat, epithelial-type cells that cover the exterior and some interior surfaces of the sponge.Almost a true

tissue.

Reproduction

Sponges have remarkable regeneration capabilities.

Regeneration following fragmentation is a form of asexual reproduction.

External buds can break off to form new sponges.

Internal buds (gemmules) in freshwater sponges can remain dormant in times of drought.

Reproduction

Most sponges are hermaphrodites meaning that each individual functions as both male and female.MonoeciousGametes are derived from choanocytes or

sometimes archaeocytes.

Reproduction

Most sponges are viviparous.After fertilization, the zygote is retained

and is nourished by the parent. Ciliated larvae are later released.

Some are oviparous releasing gametes into the water.

Reproduction

Sponges in the class Calcarea and a few Demospongiae have an unusual developmental pattern where the embryo turns inside out. Flagellated cells become choanocytes & archaeocytes. Larger cells become pinacocytes.

Class Calcarea

Calcareous sponges (Class Calcarea) have spicules composed of calcium carbonate.

Small, usually vase shaped. Asconoid, syconoid, or leuconoid in structure.

Class Hexactinellida

Glass sponges (Class Hexactinellida) are mostly deep sea forms. Spicules are six-rayed and made of silica.

Hexactinellids lack a pinacoderm or gelatinous mesohyll.

Chambers appear to correspond to both syconoid and leuconoid types.

Class Hexactinellida

Some advocate placing hexactinellids in a subphylum separate from other sponges.

Trabecular reticulum made of a fusion of archaeocyte pseudopodia - forms the chambers opening to spongocoel. Trabecular reticulum is largest

continuous syncytial tissue known in Metazoa.

Choanoblasts are associated with flagellated chambers.

Collar bodies do not participate in phagocytosis – this is the function of the primary and secondary reticula.

Class Demospongiae

Class Demospongiae contains most of the sponge species.

Spicules are siliceous, but not six-rayed.

Spicules may be bound together by spongin, or absent.

All leuconoid, mostly marine.

Cladogram of Sponge Classes

Phylogeny and Adaptive Diversification

Sponges appeared before the Cambrian. Glass sponges expanded in the Devonian.

One theory - sponges arose from choanoflagellates. However, some corals and echinoderms also have collar

cells, and sponges acquire them late in development Molecular rRNA evidence suggests a Common

ancestor for choanoflagellates and metazoans. Sponges and Eumetazoa are sister groups with Porifera

splitting off before radiates and placozoans.

Phylum Placozoa

Trichoplax adhaerens is the sole species of phylum Placozoa (marine). No symmetry No muscular or

nervous organs Placozoans glide over

food, secrete digestive enzymes, and absorb nutrients.

Phylum Placozoa

Cell layers Dorsal epithelium Thick ventral epithelium of monociliated cells and nonciliated gland cells. Space between the epithelia contain fibrous “cells” within a contractile

syncytium. Grell considers it diploblastic.

Dorsal epithelium represents ectoderm and ventral epithelium represents endoderm.