A bit earlier in the series we talked about the origin of multicellular life.
We discussed choanoflagellates and sponges,
noting that these represent the emergence of animal life around 600 million years ago.
Since organisms resembling modern sponges are likely the ancestors of all the animals on Earth,
it is with such organisms that we will begin our exploration of the animal kingdom.
Sponges are members of phylum Porifera. The name “Porifera” means “pore bearer,” which is very
fitting for sponges since they’re full of pores, or ostia. Tiny little holes cover their bodies.
But this isn’t the only unique aspect of sponges. Not only are sponges almost all asymmetrical, with
just a few exhibiting radial symmetry, but their cells do not organize into tissues or organs. They
don’t even have a mouth or anus, so there is no digestive tract. Instead, their unique body shapes
are adapted to allow for maximal efficiency of water flow through their central cavity, where the
water deposits nutrients and then leaves through a hole on their top end called the osculum,
meaning “little mouth.” Some species might have several oscula, though most have just one.
The bodies of sponges are made of two thin layers of cells with a jelly-like mesohyl sandwiched
between them, which is primarily composed of collagen. The outer layer of a sponge is covered
in a single-layer of external “skin” composed of flat plate-like cells called pinacocytes.
Pinacocytes at the base of the animal also anchor it to its substrate, the thing it is stuck to,
while those that form its “skin” can serve not only to protect it, but also to digest organic
material. However, the cells most responsible for feeding are choanocytes. These are cylindrical
cells with a single flagellum. It is the whip-like motion of these flagella that are responsible
for creating a current to drive water through a sponge’s body. So an important thing to understand
is that sponges don’t “suck in” water, instead their insides are lined with tiny flagella-bearing
choanocytes that literally create a current around them. This current isn’t strong enough to bring in
large animals, but tiny living creatures and organic material are easily swept into it.
So, the flagella whip back and forth causing a current around the sponge. The water around the
sponge is drawn into its pores, which are lined and controlled by porocytes that create closable
valves. In addition, there are other cells found within the mesohyl, like the sclerocytes,
or spongocytes in the demosponges, which excrete spicules, the structural element of sponges
that also deter predators. Many sponges can shed their spicules, forming a dense carpet of spines
around them. Other species produce toxins that prevent other sessile organisms, such as bryozoans
or sea squirts, from growing on or near them. Many species of sponge also have a variety of
other cells that move through their mesohyl. They have oocytes and spermatocytes,
which are the reproductive cells, grey cells, which act like immune cells in other animals,
amoebocytes, which are amoeba-like totipotent cells that can transform into any other type
of cell, and others like myocytes, which conduct signals and allow parts of the
animal to contract, as well as several types of collagen-producing cells. Many of these cells,
excluding the sclerocytes, can actually move and drift freely in the sponge mesohyl.
If you look closely at some of these cells, they look and behave almost like individual protozoa,
which supports what we learned about the emergence of multicellular life in an earlier tutorial.
Now, something fascinating about sponges is that since they lack any true digestive system,
almost every one of their cells consumes organic matter.
So as water is flowing into their bodies, any tiny organic material such as phytoplankton and
detritus that is trapped by a part of the sponge is consumed. This is a type of “filter feeding”
since the sponges only consume organic material suspended in the water column. However,
there are sponges, like the ping-pong tree sponge, that are carnivorous.
This uses tiny, hooked spicules to capture small crustaceans and slowly digest them alive.
This carnivorous behavior is fascinating for sponges since as adults, these organisms are
sessile, meaning they anchor themselves to a substrate and grow from there. Some sponges
have taken this behavior to the next level, and actually house photosynthesizing endosymbionts
within their bodies. These few species produce more food and oxygen than they consume,
like plants. But remember, they’re still animals. Their bodies capture the endosymbionts,
they can’t grow them on their own. Also, they, like all species of sponge, are mobile as larvae.
Speaking of larvae, or baby sponges, let’s talk about sponge reproduction.
The most common form of reproduction in sponges is sexual. They release sperm cells into the water.
In some species, ova, or eggs, are also released into the water,
while in others they are held within the bodies of the sponge, hoping to be fertilized by a sperm
cell from a different organism belonging to the same species. It’s important to note that while
most sponges are hermaphrodites, that is, they are male and female at the same time, they don’t have
gonads, or reproductive organs. Sperm and eggs are instead produced by individual cells.
Once an egg cell is fertilized it develops into a zygote and then into one of four types of larvae.
All of them are basically tiny cell balls with flagella that line their epidermis. These larvae
will then swim for a few days. Fascinatingly, many of these larval sponges possess “pigment
ring eyes” that are sensitive to light, and can mediate their locomotion. Eventually though,
they will sink in the water column and then crawl until they find a place to settle.
From there they will slowly grow into adults. Many sponge species can also asexually reproduce
using one of three different methods, those being budding, fragmentation, and gemmules.
Contrary to popular belief, only a few species of sponges can reproduce asexually by budding,
where a new sponge develops from an outgrowth, or bud, on the parent sponge, which then drops off.
Fragmentation, however, is a bit more common. All known living sponges can remold their bodies.
Fragmentation is when a sponge regenerates from fragments that are broken off,
although this only works if the fragments include the right types of cells, like the totipotent
amoebocytes. It’s true that some sponges can completely regenerate from a single cell,
but it must be the right cell. Not all sponge cells, and not all sponge species can do this.
The third method of asexual reproduction is through gemmules. When environmental
conditions become less hospitable to the sponges, like if temperatures drop,
many freshwater species and a few marine ones produce gemmules, which are like tiny
survival pods of unspecialized cells that can remain dormant until conditions improve.
Then they either form completely new sponges or recolonize the skeletons of their dead parents.
To finish up, let’s go over the most common body structures, also known as aquiferous systems
of sponges, and their extant taxonomic classes. There are three main sponge structures. First, the
simple tube or vase shape lined with choanocytes known as asconoid. Second, the syconoid,
or pleated body wall, that has inner pockets lined with choanocytes, which connect to the outer
pockets. And third, the complex leuconoid pattern, with mesohyl that contains a network of chambers
lined with choanocytes, connected to each other and to the water intakes and outlet by tubes. This
third body type is actually the most common. Less common body structures are the sylleibid, which is
often considered to be a transitional structure between syconoid and leuconoid conditions,
and the solenoid, which is characterized by complex tubes lined with choanocytes.
We should point out that the tiny black things lining the inside of the sponge are choanocytes.
In terms of taxonomy, the four extant classes of sponges: are Demospongia, Calcarea,
Hexactinellida, and Homoscleromorpha. The Demospongiae are the most common and diverse class
by far, with over 8,800 identified species, making up 76% of all sponge species. They are soft-bodied
leuconoid sponges with a hard, sometimes massive skeleton made of spicules composed of silica.
Some species of demospongia can live for hundreds or even thousands of years and grow to enormous
sizes. The Calcarea, or calcareous sponges, include species from all of the three main
body structures, including the solenoid. They have spicule skeletons made of calcium carbonate
and are typically small and drab in color. They include about 400 species. The Hexactinellid
sponges are all leuconoids that are found in the deep sea, and commonly called “glass sponges.”
They are unique among the sponges because they can rapidly conduct electrical impulses across
their bodies. This makes it possible for them to respond quickly to external stimuli.
Lastly, the Homoscleromorpha are either massive or encrusting
leuconoid or sylleibid sponges with very little variation in spicule
form. They have recently been considered to be phylogenetically distinct from the Demospongiae.
And that’s it for sponges. As much as we discussed, there is a considerable amount
of information that we didn’t cover, but as the purpose of this series is an introduction to
all the major phyla of kingdom Animalia, let’s move forward and investigate the Cnidarians next.