Origins of Multicellularity and Metazoa

Origins of Multicellularity

• Cells are the fundamental units of life
• Nature's experiments with organisms lacking cellular differentiation face limitations
• Large, single-celled marine algae are rare
• Sponges are the simplest multicellular animals, yet their cell arrangements differ from other metazoans
• Sponge cells are integrated into an extracellular matrix, strengthened by needle-like spicules composed of protein and skeleton.
• Multicellularity addresses challenges of cell size by enhancing surface area for metabolic activities.
• Increasing cell size hinders molecule exchange with the environment
• Larger body size is highly adaptable through multicellularity
• Molecular evidence groups sponges phylogenetically with animals despite their differing appearance and behavior.

Origin of Metazoa

• Eukaryotic cells diversified into lineages leading to current descendants
• Includes unicellular protozoans, colonial and multicellular plants, animals, and fungi
• Multicellular organisms, known as metazoans, are now termed animals
• Metazoans belong to the Opisthokont clade and include fungi, choanoflagellates, and additional groups
• Choanoflagellates are aquatic eukaryotes existing as solitary or colonial forms
• Each choanoflagellate cell possesses a flagellum encircled by a microvilli collar
• The flagellum beats, drawing water into the collar where microvilli capture bacterial particles
• Most are sessile, but one species fastens to diatom colonies while feeding in midwater
• Choanocytes of sponges resemble choanoflagellates and are believed to be their ancestors
• Comparative genomics of sponges helps determine cell transmitters or morphogens in early metazoans
• Shared traits were passed down from the most recent shared ancestor of animals
• Molecular phylogeny suggests colonial structures developed early within the lineage
• Recent research shows proteins used by colonial choanoflagellates for cell communication and adhesion are homologous to those used by metazoans in cell-to-cell signaling
• The simplest multicellular animals are sponges.
• Sponge genome contains code elements for regulatory pathways found in complex metazoans
• Sponges possess proteins governing spatial arrangement, defining anterior and posterior axes in larvae
• Present-day sponges exhibit less complexity compared to their ancient counterparts
• Modern sponges possess simple bodies composed of collections of various cell types bound by an extracellular matrix
• Sponge bodies lack symmetry and contain no mouth or digestive system
• Placozoans share traits exhibited in other animal classifications
• Placozoans exhibit reduced nuclear genome size accompanied by increased mitochondrial genome relative dimension within animal kingdom
• Body plans are equally enigmatic as with sponges; absence distinguishes the presence or absence of a definitive head/tail distinction within specimens exhibited

Phylum Porifera: General Features

• Sponges are generally sessile
• Efficient aquatic filter feeders
• "Porifera" means "pore-bearing", referring to their perforated sac-like bodies
• Choanocytes move water for feeding and respiration
• Most of the 8600 sponge species are marine, inhabiting varying depths of the seas, with some found in brackish water and 150 in freshwater

Phylum Porifera: Size, Color, and Symmetry

• Size ranges from millimeters to exceeding 2 meters in diameter
• Pigments in dermal cells lead to species being brightly colored
• While embryos swim freely, adults remain attached.
• The shapes can vary from radially symmetrical to irregular depending on growth conditions.
• Growth patterns depend on the substratum, water direction and flow speed, and space

Phylum Porifera: Ecological Interactions and Chemical Composition

• Numerous animals are commensals or parasites residing either on or within sponges
• Sponges can grow on various living organisms, providing some crabs with camouflage and protection
• A noxious odor and bioactive compounds are common in both sponges and resident microorganisms
• Sponge extracts have applications and effectiveness in medicine and pharmaceuticals

Phylum Porifera: Skeletal Structure and Classification

• The fibrous and/or rigid skeletal structure is composed of calcareous or siliceous spicules
• The collagen protein fibrils in intercellular matrix form fibrous portions.
• Sponges contain several collagen types which vary in chemical composition, including spongin
• Sponge categorisation relies on spicule composition and shape.
• Spicules are viewed as potential nanoparticle products in modern materials science
• The outer simplicity of sponges belies their chemical and functional complexity
• Their origins trace back to the early Cambrian and possibly the Precambrian era
• Classification occurs traditionally into classes, based on spicules and chemical structure
• Calcareous sponges (Calcarea) feature one, three, or four rayed calcium carbonate spicules
• Spicules in Hexactinellids (glass sponges) possess six-rayed siliceous components
• Demospongiae includes siliceous spicules, spongin fibers or both arranged around an axial filament
• Homoscleromorpha includes members lacking skeleton or exhibiting siliceous spicules without axial fillament

Phylum Porifera: Form and Function

• Tiny incurrent pores, or dermal ostia, form sponge body openings contained in the external cellular layer (pinacoderm)
• Sponges gather suspended particles to feed through the use of internal canal systems
• Water is channelled via flagellated collar cells whose water currents are modulated and maintained by flagella excitation via beating mechanisms.
• Microvilli traps particles and passes food on in the collars

Feeding, Digestion and Efficiency

• Sponges ingest multiple food particles, including detritus, plankton, and bacteria non-selectively
• Phagocytosis is how choanocytes consumes of smallest particles (80%)
• Protein molecules are ingested by pinocytosis
• Digestion is facilitated by pinacocytes and archaeocytes
• Nutrients are also absorbed
• The efficacy of food capture depends on water movement through the body

Phylum Porifera: Body Designs of Sponges - Asconoids

• Asconoids have a simple design
• The body is small and tube-shaped so water flows through cells with no dead space
• Choanocytes lie in the spongocoel, a big internal chamber.
• The choanocytes pull water through and extract food, while the single osculum expels water
• All Calcarea are asconoid, including Leucosolenia sp. and Clathrina sp.

Phylum Porifera: Syconoids and Leuconoids

• Larger in size and more complex relative to asconoids because of a thicker and relatively more complicated body
• Body walls folding outward with choanocyte-lined canals opening into the spongocoel
• Via dermal ostia, water flows into tiny canals called prosopyles.
• Choanocytes then absorb the food.
• The water pumped through internal pores is called an apopyle before being discharged through the osculum
• Epithelial cells comprise the spongocoel lining, instead of choanocytes, unlike other sponge types.
• Leuconoids are the most complex and largest, which enables more efficient food collection
• Small chambers containing choanocytes filter water
• Flagellated chambers release water collected via incurrent canals into excurrent canals towards the osculum
• After removing food, water gathers forming a stream that exits through an exit pore
• The high speed discourages re-filtering of used water.
• Most sponge species are leuconoid

Phylum Porifera: Adaptive Benefits of Leuconoid System and Cell Types

• High adaptive value, for efficiently meeting food demands of larger body sizes
• Possesses highest flagellated-surface-area-to-cell-tissue volume
• Due to greater surface area inside chambers, water flow lowers
• Sponges filter 1500 liters of water per days for maximum food capture
• The type has evolved in many species
• The connective "tissue" of sponges features skeletal elements, fibrils, and amoeboid cells
• Breathing and excretion occur via diffusion, and, in choanocytes and archaeocytes, contractile vacuoles perform water regulation.
• Slight changes in shape are seen, alongside local contraction, propagating and contractions and closing along with opening of incurrent / excurrent pores
• Osculums close in response to significant sediment load
• The movements imply organism wide response, regardless of complexity above the cellular level of said organisms.
• Cell excitation is stimulated with mechanical cues and communication molecules e.g., via electrical impulses

Phylum Porifera: Choanocytes, Archaeocytes, and Pinacocytes

• Oval in shape and embedded at mesohyl location(one end). The flagellum has its collar when in an exposed location.
• Fine micro fibrils connect the filaments of the collar.
• Filtering mechanism that efficiently removes food.
• Particles trapped by the collar, or in the mucus, are transferred to the base for phagocytosis
• Nutrients transfer to archeocytes, for cells with no requirement / for digestion, or other gut activity.
• Archaeocytes move around and perform different functions in the mesohyl
• Inside the dermal layer engulf particles via phagocytosis
• Also absorb particles that need digestion from the choanocytes
• Differentiate between a variety of cell types.
  • Sclerocytes, for speicule secretion.
  • Spongocytes, secreting spongin
  • Collencytes that releases fibillar collagen
  • Lophocytes that make a whole load of collagen.
• Thin epithelials - pinacocytes, protect and cover the surfaces.
• Junctions attach cells that don't have a membrane
• Pinacoderms - inter cellular junctions, but no actual membrane.
• Ingest food.
• The surface area can be changed by contraction or surface regulation.

Phylum Porifera: Cell Independence, Asexual Reproduction, and Gemmules

• Sponges regenerate lost area, can fix injury
• Cells create somatic embryogenisis, a total transformation/ reconstruction.
• Regeneration - the regeneration of asexual reproduction through fragments
• Fragmentation creates new sponge
• external form an offspring with set dimensions
  • Archaeocytes assemble at mesophyl , these will have tough spongin and spicules for harsh conditions.

Phylum Porifera: How Gemmules Work, and Sexual Reproduction

• Upon death of parents, gemmules continue at adverse locations
• Specialized cell openings make a new sponge through micropyles
• Adaptations that change with seasons encourage the colonization with environments
• Controlled by multiple aspects from weather to internal sponge factors
• Mostly monoecious which gives both sexes in one body.
• Demo and Calcerea gametes come out with archaecoytes
• The majority are viviparous, therefore the nourishment, and zygote stay put and come out if they are ciliated.
• One transfers sperm through pores of something else

Phylum Porifera: Fertilization, Larval Forms, and Development

• Viviparous possess choanocytes, carrier cells, and transport
• Both types leave out sperm
• Freeswimming larva - and come out with forms or another route
• Flagellated parenchyma goes and turns, creates development patterns of calcarea and demo.
• Hollow stomoblastula forms anterior small flagella location, and the macromer is now posterior
• macromeres cover micromeres and begin the metamorphosis.
• Micro -> arecho, collencytes
• Macro-> pinacoderm/scelrocytes

Phylum Porifera: Class Calcarea (Calcispongiae)

• Calcareous sponges contain calcium carbonate spicules
• Spicules feature straight shapes or have 3-4 rays.
• Most exist as either small or vase outlines
• Range in color from dull tone patterns, to yellow lavender
• Leucosolina sp are marine-shallow
• Types are asconoid/syconoid forms

Phylum Porifera: Class Hexactinellida (Hyalospongiae)

• Glass sponges possess six-rayed spicules composed of silica for forming networked pattern
• Almost entirely deep-sea based individuals make up species population samples.
• Radial symmetry exhibits vase funnel design, fastened root stalks
• Have cytial body, that has loads of nuclei without divding the membrane

Phylum Porifera: Class Hexactinellida - Reticulum

• Most hexactinellids show trabecular reticulum (2 layered) mesh sheet
• The trabecular, archaeocytes, are bridged due to cytoplasm
• Unusual types of cells called blast bodies that are like water flow through chyanocytes
• Direct water through main or secondary layer

Phylum Porifera: Class Demospongiae

• 95% constitute the sponge
• Lack spicules, and silica may be around
• They are bonded with spongin
• Marine life can also be in ponds, lakes, e.t.c
• High color shape variations with high diameter expansion

Phylum Porifera: Class Demospongiae - Freshwater

• Well mixed lakes and ponds are where they derive from
• Wrinkled green / brown scum is its likeness

Phylum Porifera: Phyologeny and Characteristics

• Appeared some time before.
• Sponges have adaptive properties traits like animal cells
• Proteins that allow joining.
• Some sponges are basement collagen, cell structure adhesion
• Some blastula can form to like stages in creatures

Adaptive Diversification

• Lots of species
• Water - unique system of degree complexity

Unique Features

• Hook spicules cover main branch pattern
• The crustaecn will meet a spicule entrapped limb
• Filament slowly digest preym
• Symbiotic symbiotic bacteria
• Contains chyanocete interal so its diff