origins of multicellular animals. three hypotheses syncytial ciliate hypothesis –ancestor is...

Origins of Multicellular Animals

Three Hypotheses

• Syncytial ciliate hypothesis– Ancestor is single celled ciliate with multiple

nuclei

• Colonial flagellate hypothesis– Ancestor is colonial flagellate like Volvox

• Polyphyletic hypothesis– There may be multiple ancestors

THE PHYLUM PORIFERA

INTRODUCTION TO PORIFERA

• unusual animals

• originally thought they were plants

• Are primarily marine, mostly in shallower waters

• Are sessile and attached to substrate or objects- occasionally on other animals such as crabs

• Are the most primitive metazoans and have neither true tissues or organs

Morphology of Sponges

Sponge Cell Types

• Pinacocytes- outer cells; equivalent of epiderm

Sponge Anatomy

Pechenik, 1996

Pinacocyte

Choanocyte

AmoebocytePorocyte

Sponge Cell Types

• Pinacocytes- outer cells covering sponge; equivalent of epidermis

• Choanocytes- – similar to choanoflagellates– collared cells with flagella - create water current and collect

food matter.

Sponge Anatomy

Pechenik, 1996

Pinacocyte

Choanocyte

AmoebocytePorocyte

Sponge Cell Types

• Pinacocytes- outer cells covering sponge; equivalent of epiderm

• Choanocytes- – similar to choanoflagellates– collared cells with flagella - create water current and collect

food matter

• Amoebocytes- – amoeba-like cells– store, digest and transport food, excrete wastes, secrete

skeleton – give rise to buds in asexual reproduction

Mesophyl(=Mesenchyma)

• Beneath the pinacocytes - a gelatinous protein layer

• it contains the skeletal material (ie. spongin and spicules) and amoebocytes

Sponge Anatomy

Pechenik, 1996

Pinacocyte

Choanocyte

AmoebocytePorocyte

Types of Spicules4 general types

• Monaxon- needle-like or rod-like; straight or curved

Types of Spicules4 general types

• Monaxon- needle-like or rod-like; straight or curved

• Tetraxon- has 4 prongs

Types of Spicules4 general types

• Monaxon- needle-like or rod-like; straight or curved

• Tetraxon- has 4 prongs

• Triaxon or Hexaxon- 3 or 6 rayed

• Polyaxon- multiple short rods radiating from a common center; burr shaped, star shaped or like a child's jack.– Some species have a mixture of types

SponginGive phylum its common name

• Some species have no spicules, but do have spongin

• spongin is a type of hardened secreted protein

• Some species have both spicules and spongin

Three Basic Sponge Types

• Asconoid

• Syconoid

• Leuconoid

Asconoid Sponges

• most primitive and simplistic in structure

• have radial symmetry

• are tube shaped

Asconoid Spongetwo basic openings

• Ostia- – incurrent pores that open into a central cavity

called the spongocoel– it is lined with choanocytes or collar cells

• Osculum– the opening of the spongocoel to the outside– water leaves the sponge

Asconoid Sponge Design

• Imposes definite size limits to sponges due to the problem of water flow

• The spongocoel contains such a large volume of water that it is hard to push it out rapidly

Syconoid Sponges

• next level of complexity – walls are invaginated– allowing for greater surface area over

which water can pass

• typically vase shaped like the asconoid sponges

• radial symmetry

Syconoid Structure

• helps to rectify some of the water movement problem – increasing the surface area

• so there are more choanocytes to water volume

– decreasing the spongocoel volume

• these sponges able to get bigger than asconoid

Leuconoid Sponges

• highest level of complexity in sponges

• lost radial symmetry and are very irregular in shape and may attain large sizes

• invaginated canals are even further invaginated and folded to from small flagellated chambers

• further increase in surface area makes these sponges highly efficient in moving and filtering water

• spongocoel is gone except for canals that lead to the osculum- or there may be a series of excurrent openings

• the largest sponges; most hydrologically efficient

Leuconoid Sponge Design

Sponge Reproduction

• Sexual

• Asexual

Sexual Reproduction in Sponges

• gametes formed by amoebocytes• there are both hermaphroditic and dioecious

species– most hermaphroditic species produce eggs and sperm

at different times so they do not self fertilize

• sperm is released into environment via osculum and is brought in by another sponge via ostia

• fertilization takes place in parent sponge• zygote is expelled - it drops to bottom and begins

to develop

Asexual Reproduction in Sponges

• two types:• Budding- fragmentation of body wall, buds

appear as outgrowth on sides of sponge• when they reach a certain size they drop off and settle to

bottom to form a new sponge

• Gemmules- occurs only in freshwater sponges

• gemmules are groups of food laden amoebocytes that deposit a hard covering of spicules around them

• formation is triggered by environmental conditions such as decreased temperatures

• they allow the sponge to pass the winter or periods of drought

• after which the outer covering breaks open and a new sponge develops

Osmoregulation and Excretion in Sponges

• no special organs

• main waste is ammonia – it is removed by water currents within the

sponge

HIGHER CLASSIFICATION OF SPONGES

4 classes of sponges

Class Calcarea

• spicules composed of calcium carbonate

• spicules are monaxons or tri or quadraxons

• all three types of sponges exhibited

• All less than 10 cm high– ex. Leucosolenia and Grantia

• found in shallow coastal waters

• all are marine

Class Hexactinellida(glass sponges)

• Spicules of Silica and fused to form a lattice like skeleton

• cup or vase shaped with well developed spongocoel

• most beautiful example is Euplectella - venus flower basket

• chiefly live in 500-1000 meter depth• are syconoid sponges• all are marine• may have commensal relationship with shrimp -

where a male and female live inside the sponge; get trapped inside when they out-grow the pores of sponge

Class Demospongiae

• Largest class - 95% of sponges in this class• spicules are silicious if present otherwise

skeleton is made of spongin• variously shaped some are huge • all are leuconoid• all but one family is marine- Spongillidae- is

freshwater about 150 freshwater species• this is the group from which we get our

commercial sponges

Class Sclerospongiae

• proposed in 1970 to include 6 species from Jamaica

• have silicious spicules and spongin

• also have an outer covering composed of calcium carbonate

• are leuconoid sponges