The colonial theory is the classic theory of metazoan origin.

It was originally proposed by Ernst Haeckel in 1874 and later modified by Metschnikoff 1883 and later by Libby Hyman in 1940. Libby Hyman is one of the premiere invertebrate zoologists in the country. She spent over 27 years writing a multi-volume compendium on invertebrates between 1940 and 1967 which is still used today.

The theory proposes that flagellated protozoans are the anscestors of modern metazoans.

The anscestral protozoon in this theory was a hollow sphere of colonial flagellated cells that developed anterior and posterior locomotor orientation.

Such a protozoon also evolved cell specialization into somatic and reproductive functions. Haeckel referred to this protometazoan as a BLASTEA

This isn’t hard to imagine because there are several living colonial protozoans that exhibit these characteristics. choanoflagellate protozoans in particular.

The BLASTEA then underwent invagination (like gastrulation) and gave rise to a double-walled gastrula-like body (Haeckel’s GASTREA)

The support for this colonial theory is this:

1) As stated earlier, there exist numerous present day choanoflagellate protozoans that resemble this condition now.-making the protometazoan more plausible

2) Recapitulation of this form in the gastrula of existing in many modern metazoa.

3) The body walls of many lower metazoa such as Poriferans and Cnidarians, bear flagellated (or monciliated) cells.

4) flagellated sperm

5) Allows for the development of mesodermal tissue later without a secondary loss of that germ layer-which would be the case among Cnidarians and Ctenophorans.

SHOW OVERHEAD (page 114 Brusca-top)

Hyman and others have modified Haeckel’s original theory by suggesting that the transition to a layered body construction-like a gastrula, occurred through INGRESSION rather than gastrulation and that the primitive metazoa were solid-not hollow. Ingression is observed among some Cnidaria larvae (planula) and currently, it is believed to be the anscestral condition among Cnidaria-no one really knows right now however.

Such a scheme suggests that bilateral symmetry arose later and is derived relative to radial symmetry.

Draw on board:

colonial flagellate---->ingression of cells--->solid ball or ovoid mass of cells with flagellae on outside (gastraea)------->planuloid anscestor----->motile bottom dwellers which gave rise to flatworms and then sessile bottom dweller---->gave rise to Cnidaria.

SHOW OVERHEAD (page 114 Brusca-bottom)

Otto Butschli in 1883 suggested that the proto-metazoan was bilaterally symmetrical and crawled about ingesting food. It had only two cell layers-a top layer and a bottom layer-eventually the two layers hollowed allowing invagination of the larger nutritive cells on the bottom.

This allowed for increased surface area and a double walled body.

Most evolutionary theorists have attempted a monophyletic hypotheses about metazoan origins-but there have been suggestions that metazoa are polyphyletic.

Not surprisingly, this is third theory is known as the polyphyletic theory and is really just a combination of the other two.

It suggests that colonial flagellates gave rise to the Cnidaria and Poriferans and that syncytial ciliates gave rise to acoel flatworms and higher metazoa.

SHOW OVERHEAD (page 115 Brusca)

All of the ideas concerning the evolutionary origins of the metazoan condition exist a common problem. --What are these intermediates between protozoans and metazoans?

Here are some "real" organisms that have served as inspiration and examples of intermediates:

A) Volvox-a colonial flagellate.

B) Paramecium,-a multinucleate protozoan similar to what the protoacoel flatworm would resemble and has been used as an argument for the sycytial theory.

C) Sphaeroeca and

D) Proterospongia-colonial choanoflagellates-which are very close to resembling the simplist of sponges. They even possess collar-cells that are virtually identical to those seen in sponges. These colonial protzoans are even attached by a gelatinous substance that resembles the mesohyl of poriferans. It is almost certain that sponges evolved from these types of protozoans. The questions is did all other invertebrates also arise from these.

E) Trichoplax-is generally regarded as the most simple metazoan or an intermediate

between protozoans and metazoans. These are enigmatic animals. They are so strange that they make up their own Phylum-Placozoa.

F) Salinella-an animal described by J. Frenzel in 1892. (Phylum ) found in salt beds in Argentina-no one has found one since he first described it.

G)A rhombozoa- a symbiont-probably commensal on nephridia of cephalopod molluscs.

H) Orthonectid-a parasite within gonads of several different marine invertebrates-usually rendering the host sterile.

I’d like to talk for a bit about three of these phyla. The Placozoa just mentioned and two other groups the Phylum Rhombozoa and the Phylum Orthonectida. Although these are not diverse or economically important, they constitute only 50 species they are very important for understanding how the simplist metazoans may have lived and so are very important from an evolutionary standpoint.

Collectively, the Rhombozoa and Orthonectida make up the Mesozoa. Mesozoa, like Metazoa is not a taxonomic classification but a level of complexity (like asconoid or syconoid sponges). Metazoa refer to multicellular animals with a blastula and some stage of development. Mesozoa are multicellular but seem to lack this developmental stage-or in the case of the Placozoa, the adult form pretty much IS a flattened out blastula.

Lets start with Placozoa

SHOW OVERHEAD OF TRICHOPLAX

This looks a lot like the hypothetical plakula larva of Butschli.- in fact it is-just add some free floating mesenchymal ameboid cells in here and picture this as a gelatinous matrix-and Wa La. -its a real animal.

Trichoplax adhaerens, the sole member of the phylum Placozoa was discovered in 1883 on the side of an aquarium. That is still the primary place for finding individuals.

It consists of several thousand cells arranged in a double layer plate. The ventral cells are distinct from the top and it always orients itself in the same plane so it possesses specialization of somatic cells on the most simple level possible. There is no anterior-posterior orientation.

The dorsal cells are flattened and bear a single flagellum. The ventral cells are more columnar in shape and not all bear a flagellum.

The ventral process may temporally become invaginated for increasing surface area while feeding. This is sort of like a temporary gastrulation process that is reversible. Thus the bottom is functionally distinct from the top cells. The top serves as a simple epithelium, whereas the bottom is specialized for feeding.

Between the two sheets of cells is a mesenchymal layer of ameboid cells embedded in a gelationous matrix. Some researchers consider Trichoplax to be a true diploid animal with the bottom representing endoderm and the top representing ectoderm.

There is no basal membrane between the two layers which is typical of a true germ layer. Based on this fact, it may be more closely related to Poriferans than animals with true germ layers.

The animal feeds by phagocytosis and secretes digestive enzymes out of the ventral cells while the animal is invaginated-like a temporary gut.

It reproduces asexually by fission and budding. Eggs have been found and are believed to be derived from specialized ameboid cells in the mesenchyme. This means that there are probably cells specialized for reproduction as well.

Another interesting fact, Trichoplax has less DNA than any other multicellular animal-about the same as a protist or bacterium.

Now the other two groups-the Phylum Rhombozoa and Orthonectida. Both are very simple organisms with very complicated life cycles. One, the Orthonectids are parasitic, the other Rhombozoa, are probably commensals that evolved from a parasitic habit- and as you will see as this course continues-parasitism and complex life cycles seem to go together.

SHOW OVERHEAD OF ORTHONECTID and life stages (p. 256 Barnes)

Members of the Phylum Orthonectida have been found in flatworms, nemerteans, polychaete annelids, bivalve molluscs, and some echinoderms. where they absorb dissolved nutrients.

Orthonectids are incredibly simple. A mass of usually ciliated cells surrounds either sperm or egg cells depending on the sex. Some members have muscle fibers, others do not. Most movement is by ciliary action rather than the muscles moving.

I will spare you all the terminology associated with the life cycle. I will just say a ciliated larva form exists after males fertilize females after being released from the host. There is a plasmodium stage that is just a giant amoeba-like mass of one syncytial cell and a few mitotically dividing cells that give rise to a sexual adult.

SHOW RHOMBOZOA OVERHEAD

Rhombozoans have one of the most specific habitats of any multicellular animal. They are found in the nephridial cavities of squid, cuttlefish, and octopods. Nepridia are the functional equivalent of kidneys. They absorb nutrients from the urine of their hosts and appear to live commensally since no pathology is associated with their presence.

Without going into details about the life cycle, I’d like to mention the morphology of the adult or nematogen as it is called. These are about half a millimeter long. The entire animal is only 20-30 cells. There is a long central cell, and a single layer of ciliated cells that wrap around it. The axial cell or central cell serves as a reproductive cell and some of the other cells serve as attachment.

The question is how do these simple animals, the Placozoa, Rhombozoa, and Orthonectida relate to other Metazoans- or to each other.

Trichoplax, the sole member of the Placozoa, appears to be the most agreeable among taxonomists and evolutionary biologists-no small task.

It has been suggested that the invagination of the ventral surface may have been the origin of gastrulation among animals and that Trichoplax is very likely an extant representative of an intermediate between the protozoa and the metazoans. Woo Hoo-go tell your friends.

The evolutionary affinities of the Rhombozoa and Orthonectids are another story. Cladists have been name-calling each other about what they are for decades now.

The lines are drawn up along two camps:

Some believe the simple body forms are primitive and thus may serve as a possible body plan from which higher organisms have evolved.

The other camp thinks the simple body plan is degeneration from some more complex metazoan ancestor due to parasitic habits.

Stunkard (1954, 1972) suggested that Rhombozoa and Orthonectids came from flatworms-especially the flukes. This contention is based on shared extremely complex life cycles between groups and general morphological features.

Stunkard may have been right in one regard. Both Phyla have recently been shown to possess mesodermally derived cells along the central axis. They have multiciliated outter cells like that found in flatworms.

My personal view is that Rhombozoa and Orthonectids are extremely derived flatworms with highly reduced body plans geared almost exclusively for reproduction-not primitive organisms.

The Placozoa however are not.

That’s it.

Friday, I’ll be covering Poriferans to get us back on track. I don’t think I’ll hurt a sponges feelings with less time spent on them since they don’t have nervous systems.

Poriferans are by many zoologists, believed to be the simplest of metazoans

Out of 5000 species total, 150 are freshwater sponges found in 2-3 families.

Because of their inability to move, modular, vegetative growth form and apparent lack of responses to external stimuli, they were long thought to be plants.

Sponges are built around a single purpose.

Since sessile organisms can’t go after food, they must make food come to them by creating water currents and filtering minute particles from them.

As seen in lab, sponges are constructed around a system of water canals through which water is pumped and minute food is filtered and cells are specialized for different functions.

Cell specialization was one of the ways that distinguishes multicellular animals from a colony of unicellular animals.

Lets review these different cell types that you saw in lab.

SHOW OVERHEAD OF SPONGE CELL TYPES

Choanocytes- these are the feeding and respiratory cells of the sponge. Sponges only feed intracellularly (phago and pinocytosis). The choanocytes take in food particles that are about 2-5 micrometers. The fact that most ostia or incurrent pores are 50-175 micrometers limits the size of potential food items in the sponge too.

DRAW CHOANOCYTE ON BOARD

Thus sponges eat organic detritus, some protozoa, unicellular algae and stuff like that-and they may filter upwards of 1,200 times their own volume per day to get enough food.

Porocytes

Pinacocytes

amoebocytes

sclerocytes -are specialized amoebocytes that create spicules using calcium carbonate or silica

spongocytes-produce fibrous supportive spongin-a collagen.

archaeocytes are the totipotent cells-will differentiate into any other cell type are used in repair and regeneration.

it is the aracheocytes that are contained within the gemmules of freshwater sponges.

There are other cell types that create the polysaccharide ground matrix in which the spicules are present.

The fact that the ground matrix or sandwich layer between the choanocyte layer on the inside and the pinacoderm on the outside is acellular is important.

Keep in mind that sponges have no respiratory system other than flushing water through its channels.

The mesohyl does not require oxygen or gives off metabolic waste so that surface to volume ratio limitations can be overcome. The same is true of the mesoglea layer of Cnidarians

Another characteristic that differs between true multicellular animals from a colony of protozoans are developmental features.

SPONGE REPRODUCTION

Sperm are shed directly into the water column. When this occurs at specific times of the year, this is called smoking sponges.

The sperm are taken up by other sponges and caught in the choanocytes. The sperm is enveloped by the cell in a special vacuole. The choanocytes then lose their collars and flagella and move in an amoeboid fashion to the areas where eggs are maintained. It is yet unknown how the sponge recognizes a sperm cell of its own versus that of other sponges or invertebrates.

The embryo may develop within the sponge or be sent out via the osculum.

There are three larval types

SHOW OVERHEAD (page 202 Brusca)

Blastula larvae

Parenchymula

Amphiblastula

SHOW SPONGE DEVELOPMENT OVERHEAD

Here is a crude drawing of one of the basic developmental schemes in sponges.

A blastula larvae grows flagella inside and undergoes an inversion process unique to sponges. The embryo turns inside out.

This inside out blastula is free swimming and referred to as an amphiblastula. You probably saw one in lab. The large macromeres or polar cells give rise to the pinacoderm and the micromeres or flagellated cells give rise to the choanocyte layer.

The amphiblastula settles on a substrate and a second sort of inversion takes place that resembles gastrulation.

Remember that sponges reproduce by fission, fragmentation and, in the case of freshwater sponges, gemmules too.

What else besides reproduction is required for a constitutient cells to represent a whole animal versus a colony of cells?

1) independence-can the cell live on its own?

2) Does the cell communicate chemically or through a nervous system with other cells? Does it recognize self from non-self (Does it have an immune system?)

3) If the cells are hacked up into tiny pieces, do the individual cells reformulate themselves into another colony or just die?

4) does the animal or group of cells behave as a unit? Is there coordination between groups?

5) Are some cells used for sexual reproduction and others not?

These questions address the degree of independence of groups

of cells from each other.

Do sponges respond to external stimuli?-as a unit.

Yes, they regulate flow rates through their bodies in three ways

This has been tested with the collection of known volumes of fluid from sponges as mentioned in Ruppert and Barnes, thermistor flowmeters, and dyes.

What they have found is that current flow changes by the following means:

1) contraction or expansion of cells around osculum (oscula)

2) contraction or expansion of porocyte cells around ostia

3) choanocyte activity can be started or stopped.

It has been shown that in many sponges, contraction of cells spreads out from the point of stimulation.

Other times sponges respond to stimuli some point from the stimulus.

If the base of a sponge is touched, it frequently results in the osculum contracting or flow rate stops.

This is interesting since sponges have no nerve cells. How are they capable of responding to external stimuli this quickly?

A number of studies have been conducted on sponges to get insight into the origin of nervous systems in metazoa.

Cells must be able to communicate with each other.

Some amoebocytes resemble neurons in that they have long extensions which are in contact with other cells.

DRAW AMOEBOCYTES

Until recently there has been no evidence of any kind of conduction sytem in sponges.

Magnesium Chloride is frequently used to inhibit neuromuscular transmission in many physiology experiments-but using MgCl2 doesn’t chage their behavior. Touch an end, still get response at osculum.

These results suggest that cells stimulate each other by physical expansion or contraction. Several researchers have observed waves of contraction of pinacocytes in flagellated chambers which reduce flow by stopping pumping action of chanocytes.

This type of system is too slow to explain some observations

Reiswig has shown that a disturbance of one area of a sponge can cause a local deformation of cells and shut down choanocytes.

Oxygen levels drop with lack of pumping

Cells at osculum respond to decreased oxygen by contracting, whic makes oscular opening smaller, speeding up flow.

So the cells are responding to changes in their environment-no coordination of response; no conduction of stimulus.

However, research (Lawn, Mackie, and Silver, 1981) has shown that some kind of electrical conduction system exists in sponges.

Working with a large hexactinellid sponge from coastal British Columbia-that cannot change the shape of its osculum. It does change its pumping rate by stopping choanocytes. Pumping rates change both spontaneously and due to physical disturbances.

DRAW SPONGE AND THERMISTER FLOWMETER POSITION OVER OSCULUM

(a flowmeter and thermister were used to measure pumping rates)

Most importantly, pumping rates responded to electrical stimulus.

DRAW TWO ELECTICAL STIMULUS POINTS

Two electrical stimulus points one close and one distant from point of current measure

Response takes longer when shock comes from distal electrode.

DRAW ELECTRODE RESPONSE

Conduction of stimulus over a distance through tissue of body wall is at a rate of .17 to .3 cm/sec, which is slow when comppared to nervous transmission, but too fast for conduction by chemical diffusion and exceeds (by an order of magnitude) the speed of contraction waves in sponges.

Conduction is unpolarized; no directionality was seen

All regions of sponge seemed equally responsive.

No electrical activity was recorded by intracellular electrodes, however, they conclude that electrical or tactile stimuli evoke some kind of propagated signal which travels from site of stimulus to widely dispersed areas of the sponge, where its effect is noted.

No contraction of inhalant or exhalant openings could be observed. (contrary to what was found by Reiswig in other sponges).

Current changes were likely caused by choanocyte flagellar activity.

Recent findings suggest that all surfaces of hexactinellid sponges may be syncytial thus the primitive conduction system may be acting as a single neuron-which is another possible adaptive function of a syncytium-it allows for quicker communication between cells-since each individual cell is so much larger.

As these experiments continue, we may have to change our definition of a nervous system.

I’ve been talking about how sponges can control water flow rates in various ways. But that is a two-way street. Water also shapes the morphology of sponges.

The oscula and ostia of sponges tend to be smaller in stronger currents and tend to orient away from the current than sponges of the same species in weaker currents.

DRAW PICTURE ON BOARD

I may have given the impression that the flow through sponges is mostly active on the part of the sponge. Which it is-but sponge architecture takes advantage of surrounding water flow.

SHOW OVERHEAD page 71 Pechenik

Water flow out the osculum still increased when choanocyte activity was halted by freshwater immersion as external water flowed. This translates into sponge morphology serving as a passive means of regulating water flow

Another fundamental question concerning whether or not an animal is a colony of cells or a functional individual is whether the cells can function on their own.

In 1907 H.V. Wilson discovered that when sponges are fragmented into individual cells by passing them through a silk screen, they reaggregate.

The cells would move around on their own for a few hours then reorganize and form new sponges.

Reaggregation is a topic of interest to cell biologists because of its implications for cell membrane fucntions, develoment and immunobiology.

The cells of Microciona have amoebocytes that explore with extension of pseudopodia when encountering choanocytes. Then their membranes "zip" together and they form aggregations.

Apparently electrical resistance drops at the point of adhesion, which allows rapid transfer of materials through membranes.

There is some debate over whether aggregation is species specific or even individual -specific. Can cells from the same sponge recognize each other? Can cells of different species form a new sponge?

Withn the genus Microciona, two species have recognition sites on cell mmbranes. When these sites are selectively coated with antibodies, aggregation will not occur.

Hildemann, Johnson and Jokiel (1979) did a much simpler study to look at Histoincompatibility of sponges.(Science).

To qualify as having an immune system the following have to be demonstrated:

1) the organism must show anagonism against foreign substances

2) the antagonism must be specific for that substance

3) Future responses should be altered by the first response-memory.

20 years ago, invertebrates were thought to lack immune systems

This is no longer the case

-including sea urchins, insects, star fish, clams and snails, sea anemones, and coral. What about sponges?

SHOW OVERHEAD (Pechenik page 67).

One of the reasons this species was studied is that it was noted that the sponges would have tissue fusion between adjacent sponges-or individuals from the same colony-but never different colonies.

So sponges only respond to clone-mates-but more research needs to be done.

SPONGE ECOLOGY

I’d like to talk a bit about sponge ecology

They are sessile in their habits, but exhibit several different growth forms.

SHOW OVERHEAD OF GROWTH FORMS (page 74 of R&B)

Here is a nice SLEAZE of sponges.

Generally sponges colonize hard surfaces although some species can live on soft substrates as well.

You can see that there are many shapes to sponges and they occupy different microhabitats. Often times, Poriferans compete for space on surfaces-particularly on coral reefs. Some of their competitors include bryozoans, corals, sea anemones, and other sponges.

They have evolved to exploit all available surfaces including:

Vertical sponges-use very little surface area and exploit the vertical areas above the substratum (branching forms)

Encrusting forms exploit crevices and non-horizontal surfaces that are more difficult for other sessile organisms to exploit.

Boring sponges can exploit hollow areas within the substrate itself such as empty shells, or holes formed by old, degraded coral.

Sponge competition is usually for space more than anything else.

In this regard, sponge competition is more like plant competition than animal competition in some ways.

The sponge must find suitable area to develop more individuals or reach a size where reproduction is possible.

Once the sponge has found a suitable site, it must prevent other competitors from encroaching on its space. The sedentary animal is vulnerable to predators and also a constant rain of other dispersing invertebrates that may settle on the sponge itself and grow.

To generalize many years of habitat selection and community ecology of sponge assemblages-especially intertidal areas-this generalization may be stated:

selective sponge predation is rare.

the influence of the generalists predators (like starfish and opistobranch molluscs) is minimal but may prevent the dominance of individual sponge species.

Sponge diversity is high especially where substrate types is high

Physical Disturbance is minimal

Competitive dominance is rare.

In other words, they seem kind of like rainforests in their ecological characteristics.

Considering the importance of substrates-what criteria do sponges choose sites?

The distribution of sponge species is influenced by primarily two factors

1) availability of substrates as stated earlier. Species compete for substrates often by growing over other individuals. In the case of filtering invertebrates, surface area constitutes a more limiting resource than food usually.

2) Water turbulence/turbidity

where water is turbulent with gravel and sand as sediment, sponges are ground up.

calm areas with silt for sediment, stagnant water. The pores become clogged.

Intermediate sites are best with some turbulence, rocky heterogeneous substrates have more species.

Of course it is the larvae that must settle. There have been very few studies that have examined if or how larval poriferans select suitable substrata.

One result of habitat selection in sponges (seems like an oxymoron doens’t it?) is that they all prefer sites with an algae or bacterial mat on the surface-this makes sense since this material constitutes about 20% of the diet of most sponges.

Some sponges like the boring sponge Cliona, require bivalve shells to survive since they burrow into them. The larvae of these sponges have a preference for calcium carbonate surfaces, but not necessarily bivalves.

It is also known that larvae have responses to several different stimuli.

Most respond to gravity-having an early negative geotaxic response (that means they swim up) and then acquire a positive one. This is believed to function in increasing dispersal distance from the parent sponge.

Most sponges respond to light. Some are positively or negatively phototaxic. Crevice living sponges usually have a negative phototaxic response while others do not.

Larvae respond to high turbidity by crawling rather than swimming. Thus dispersion distances, contrary to intuition, tend to be shorter rather than longer.

Sponges are important ecologically.

Boring sponges like Cliona are important because they contribute to erosion of reef systems. In high density, up to one millimeter of reef can be eroded per year (like in Bermuda). This may not seem like much, but it is enough to create a distinct subtidal notch around many islands.

Sponges are also important for their filtering capacity.

Some researchers have measured particle collection efficiency in sponges and found the following:

96% of bacteria were retained

90% of algal cells

70% of dinoflaggellate protozoa

All of this plankton contitutes only 20% of the diet-the other 80% is organic detritus

Besides the habitat types and growth forms, the niches of sponges are also defined by the particle sizes that they consume. The canal widths and pore size (ostia) determine what can be fed on.

Some sponges like Microciona have associations with bacteria. They concentrate the bacteria then regulate growth of cultures in the surface of canals by antibiotic substances-these sponges may be "farming" bacteria.

Verongia that you saw in lab has up to 38% of its tissue volume composed of bacteria (Pseudomonas and Aeromonas). The mesohyl volume is only 41% and cellular volume is only 21%. In this case, the sponge is really a bag for holding bacteria rather than filtering. Other sponges like Haplosclerida have very very low levels of bacteria inside.

It is known in Dictyoceratida that when sponge matrix is added to bacteria, there growth is greatly enhanced and that the bacteria is well established even on dispersing larvae.

Sponges are also known to be in association with blue-green algae and the sponge actively phagocytises the algae.

Some sponges harbor zooxanthellae and zoochlorellae much like coral do. Some of the bright sponge colors may be attributed to these symbionts.

Some crabs actively collect sponges of certain species and place them on their backs.

As seen in lab spheciospongia harbors commenal shrimp. One individual sponge was found to contain 16,000 shrimp.

Characteristics of Phylum

Porifera

8. Skeletal elements composed of calcium carbonate, silicon dioxide, or collagen fibers