LECTURE 03 - Porifera and Placozoa Flashcards
How is the flagellum considered a power source for Porifera?
Movement of the body through a fluid or movement of fluid through the body is amplified by contractile elements powered by ATP, or which there are two main kinds in eukaryotes
- Flagella and cilia, based on dynein/tubulin
- Muscle cells, based on actin/myosin
Flagella operate at low Re = UL/v < 10^-2 and motion is therefore dominated by viscosity; inertia plays little part
A free cell body is propelled through the fluid in the same direction as the waves passing along the flagellum
- an anchored flagellum develops a thrust which accelerates a stream of water to a velocity of several body lengths per second, i.e., about 100 µm/s
In what organisms is the structure of cilia and flagella conserved in?
The structure of cilia and flagella is conserved in eukaryotes and it thought to have evolved before LUCA
What is the choanocyte?
- the choanocyte is a uniflagellate cell with a collar of microvilli surrounding the base of the flagellum
- Movement of the flagellum draws a stream of water through the collar, in which the microvilli are spaced at 0.2µm intervals
- Hence, particles larger than 0.2µm, such as bacteria, are retained by the collar
- They are then ingested by phagocytosis, and subsequently transferred to another cell, the archaeocyte, for digestion
What is the rate of water flow in the small sponge Haliclona?
About 300 ml/ml of sponge tissue/hour
- this is equivalent to about 2 x 10^8 bacteria/ml sponge/hour, or about 0.02 mg of organic matter
What is the density of a choanocyte?
the choanocyte density is about 10^9 cells/ml sponge, so each choanocyte captures about 3-4 bacteria per day
How does water circulation work for Porifera?
- Water is drawn in by choanocytes through pores in the wall of the sponge, created by doughnut-shaped porocytes, and enters the interior of the sponge body, the spongocoel
How is the water stream strengthened in Porifera?
The water stream is strengthened by arranging many choanocytes in a chamber, creating a thrust that greatly exceeds that of a single cell
What are spicules and how are they distributed?
- microscopic needle-like structures that many sponges use as a structural skeleton and as a defence against predators
- Spicules are distributed in the mesohyl and give shape to the sponge body because they are connected by spongin (the sponge version of collagen, found in demosponges) or are cemented by silica (hexactinellids) or interlock
- They are often unique characters of species
What is the sponge struture?
- As water flows out of the opening of the spongocoel (osculum), it creates a pressure difference like the draught of a chimney that increases the efficiency of the water stream
- ASCONOID
- The simplest body plan is to arrange the choanocytes on the wall of the spongocoel
- SYCONOID
- Folding the wall provides more internal surface area for choanocytes, increases the power of the water flow through the sponge, and thereby supports larger individuals
- LEUCONOID
- Folding the folds provides even more surface area and enables sponges to grow up to a metre or more in size
What is Bernouilli’s Principle?
When the velocity of a fluid increases its pressure decreases
What is one useful corollary of Bernouilli’s Principle?
- Water velocity is inversely proportional to the cross-sectional area of the flow
- The cross-sectional area of any given pore is considerably smaller than the channel, chamber or spongocoel it opens into. Consequently, water slows down and flows smoothly over the feeding surface
- But the cross-sectional area of the osculum is smaller than the COMBINED areas of all the pores, so that exit velocity is high enough to remove wastewater from the immediate vicinity of the sponge
Describe sponge reproduction.
VEGETATIVE PROPAGATION
- Sponges can proliferate through budding, branching or fragmentation; they have remarkable powers of regeneration
- They may also form reduction bodies, each consisting of a ball of archaeocytes with a covering of pinacocytes and spicules
- Freshwater sponges survive winter as reduction bodies called gemmules
SEXUAL CYCLE
- There are no discrete gonas and the germ line remains open
- Ova and sperm develop from somatic cells: ova primarily from archaeocytes and sperm primarily from choanocytes, although there are exception (e.g., oocytes from choanocytes in Calcarea)
- Most sponges are hermaphroditic
- The sperm are released en masse into the water column and enter the water canal system of neighbouring individuals
- Here they are phagocytosed by choanocytes, which then de-differentiate into amoeboid carrier cells
- These migrate into the mesohyl and transfer the sperm to oocytes
- The fertilized egg then begins development in the mesohyl
Describe the development of a sponge.
- Sponges exhibit a wide range of modes of development
- The genes regulating development are similar to those in other metazoans, except that there is no Hox/ParaHox set
- Cleavage in sponges is usually total and equal, but some calcareous sponges produce blastulae with micromeres and macromeres
- Late blastulae experience extensive cellular reorganizations
- These embryonic reorganizations have been regarded by some authors as a gastrulation process equivalent to that occurring during the embryogenesis in other animal phyla, but the homology of development processes in sponges with gastrulation and germ layer formation in other metazoans is obscure and uncertain
- Most sponges have a biphasic pelagobenthic life cycle with a small ciliated larva that metamorphoses into the benthic adult
- sponge larvae are lecithotrophic (feeding on maternally supplied yolk; as opposed to planktotrophic) and hence have only a short period of time in the plankton
Describe sponge symbionts
- Sponges harbor a very wide variety of symbiotic microbes, living mostly in the mesohyl
- They can amount to 30% or more of the wet weight of the sponge
- Many are much more abundant in sponge tissue than in the surrounding seawater, and sponges appear to avoid eating them
- In some cases, the symbionts seem to be vertically transmitted
- Some symbionts are mutualists, for example cyanobacteria, where mutual transfer of resource has been demonstrated
- In other cases, it is not clear whether or not the microbes are mutualists
- The sponge together with its community of microbes has been interpreted as a single evolutionary unit, the “holobiont”
Describe carnivorous sponges
- Demosponges in the family Cladorhizidae are able to capture living prey such as shrimps with hooked spicules
- Most live in deep water, below 1000m, or in caves
- Some retain chambers and choanocytes, such as the Chondrocladia; prey are captured within the inflated chambers
- In others, such as Asbestopluma, the water canal system and choanocytes are lost
