Aggregata eberthi: a life cycle you won't believe
Most people have heard of the amoeba. Anyone who has done biology in High School has also heard of paramecium. Certainly everyone has heard of the debilitating disease called malaria. It is caused by an organism called Plasmodium. These are just three examples of an enormous assemblage of animals called Protozoa. Over 50,000 different kinds of protozoa have been described; the number keeps climbing. This number of species is greater than the total number of different kinds of birds, mammals, reptiles, amphibians and fish. Yet, most of us never give them a passing thought. Which is not surprising in some ways, because unless we have a good microscope, and know what we are looking for, we never lay eyes on these fellow tenants of mother earth.
Protozoa are single-celled animals. They are complete, self-sufficient entities that eat, move, reproduce and die. But they are able to pack all this normal living into one cell. Human beings consist of untold billions of cells, none of which can survive on its own.
In spite of their minute size, protozoans are extremely complex creatures, many of them leading unbelievably complicated lives. One of the greatest errors in the field of natural history is to apply the word “simple” to any creature or plant. But almost all zoologists commit this crime when it comes to protozoans. They do this because of their belief in the dogma of evolution, certainly not because they have been compelled into the conclusion based on the study of the creatures themselves. They reason that surely single-celled creatures must have crawled out of the bottomless pit of non-living matter way before many-celled creatures. And that therefore they must be simple. The philosophy is what has compelled the description “simple”, not the facts. And nothing could be further from the truth. Each one of the more than 50,000 kinds is a brilliant example of the infinitely inventive genius of the Almighty.
One thing is for sure with protozoans — once you have been bitten by the protozoa bug, you need never be bored again. More information is available on them than you have time to absorb in a hundred lifetimes. But God knows every one by name! He thought of and designed every single one of them. Let's consider the life cycle of just one of them — Aggregata eberthi.
Aggregata eberthi life cycle
Aggregata, like many other protozoans, enjoys a doubly complicated life cycle. If you would like to know just how complicated, read on. God has spared no pains on even the tiniest miracles that have come off His drawing board.
The first complication arises from the fact that Aggregata has to live in two separate hosts at different times in its life cycle. The second complication is that the creature itself takes a different form in each of the hosts. One form in one host reproduces sexually. The other form in the other host multiplies asexually. Asexual reproduction is perhaps best illustrated by considering the amoeba. When an amoeba reaches a certain size, it multiplies by "simply" dividing in two. (It's anything but simple). Under certain environmental conditions, amoebae reproduce sexually, in which two amoebae fuse together before then dividing into two daughter cells. Strawberries also multiply asexually (that is, without sex) by sending out runners from the parent plant that put down roots here and there, thus producing brand new strawberry plants.
Let's begin at the beginning. Which is frankly impossible. How do you find the beginning of a circle? But we must start somewhere, so let's begin with the final host . By definition, the final host is that host inside which a parasite multiplies sexually. Intermediate hosts are those in which a parasite undergoes no sexual reproduction, but often does increase asexually. In the case of Aggregata, the final host happens to be a cuttlefish. (H, below).
When a cuttlefish eats an infected crab, any mature protozoans in the crab (called merozoites; see k in diagram) break through the intestinal wall of the cuttlefish. Each individual merozoite then takes up lodgings inside an individual cell of the intestinal wall of the cuttlefish. Once established, and having enjoyed a hearty meal at the expense of the cell they inhabit, these merozoites (remember that's what they are called) transform.
Some of them develop through a series of stages into what are called microgametes. Others develop into macrogametes. Microgametes are extremely small, and are usually considered to be male gametes, or germ cells. Macrogametes, as the name suggests, are much larger, and are female germ cells. This process of the transformation of merozoites into either micro- or macro- gametes is given the enchanting name of "gamogony". (See l to p and q to s. l to p represent the formation of microgametes, and q to s represent the development of macro-, or female, gametes).
When they are mature, the male (micro-) gametes rupture the host's cell wall, and start thrashing around in the cuttlefish's gut. Their swimming, made possible by a couple of whip-like hairs, is not aimless. They are seeking cells that contain mature female, or macro-, gametes. How they can recognize such a cell is a very good question. This is the sort of question that entire articles are written about in specialist scientific journals. Believe it or not, there are a number of journals devoted solely to protozoans. Anyway, when they do find such a cell, they somehow pierce a cell wall, and make their way to the waiting macrogamete. (See s in the diagram, which shows a microgamete just coming into contact with a macrogamete). Here the male germ cell fuses with the macrogamete. The result is called a zygote.
This is by no means the conclusion of the story. Now another remarkable development occurs. It is a process called "sporogony". That is, spore making. Still inside the host cell the zygote develops into a "sporont" (t). In the sporont, the single nucleus keeps dividing, thus producing numerous very tiny "sporocysts" (u). (Figure A in the diagram shows a single sporocyst). To make matters more difficult to follow, each sporocyst develops a thick wall around itself, and then divides up into three "sporozoites" (See fig. B, which shows a sporocyst rupturing, and releasing its three sporozoites). The whole process from zygote to sporozoite is called sporogony. Remember in all of this that we are discussing what are considered to be among the simplest forms of life. It should be quite clear that to ever use the word “simple” in describing any aspect of God's handiwork is nothing short of an insult to God.
If sporocysts occur in large numbers in a cuttlefish's intestine, they can do considerable damage. Sometimes, sadly, it brings about the death of said cuttlefish. This is one means of enabling the complicated life cycle of Aggregata to continue, for in order for the life cycle to be completed , the sporocysts must somehow be swallowed by a crab. This can occur when a crab eats a dead infected cuttlefish. Alternatively, it can take place when a crab eats the feces of an infected cuttlefish. Crabs will eat just about anything.
Upon entering the gut of a crab, the sporocysts break open and release their complement of three sporozoites (b and B). These sporozoites then penetrate cells of the crab's intestinal wall (a). Exactly how they do this is debatable. After spending billions of dollars on research on the AIDS virus, it was not until 1993 that the Pasteur Institute worked out how the AIDS virus was able to penetrate cells, (raising the hope that a drug that would hinder this would be the panacea). How the sporozoites invade cells may be still be a mystery, I'm not sure. But repeated observations indicate that it takes only a few seconds for penetration to be effected. Here they change form slightly and become what is called a "trophozoite" (d). (Tough going, isnt it.)
They don't spend a long time in the first cell after penetration, but soon migrate to the next deepest layer of the gut. Here they undergo an amazing series of developments which has been given an equally painful name — "schizogony" (e to i). In schizogony, the protozoan begins to enlarge in size, presumably at the expense of the host cell. The nucleus undergoes repeated division (another miracle in itself), and the new nuclei arrange themselves around the inside rim (g). These nuclei then push through their cell wall, taking just a little of other cell material with them. They then attach to the outside of their mother cell (h), and form a cell wall around themselves, thus becoming completely independent spores known as merozoites (i). The merozoites are the new infective stage of Aggregata. For the life cycle to be completed, the crab now has to be eaten by a cuttlefish. Which brings us back to the beginning of the article.
This article provides just one example of the thousands of different life styles of single-celled protozoans. God had to think up each and every one. An impossible task for man, easy for God!
A, Mature sporocyst with full complement of three sporozoites. B, Sporocyst open and liberating sporozoites.C, Cross section of gut of crab with four schizonts containing merozoites. D, Mature sporocyst showing two valves and three sporozoites. E, Crab intermediate host becomes infected by eating sporocysts and harbors schizogenous phases of life cycle. F, Segment of gut of crab, showing schizogony. G, Infected crab harboring extra-intestinal merozoites (as in C, F) infective to final (definitive) host. H, Cuttlefish final host. I, Segment of gut of cuttlefish in which gametogony and sporogony take place. (From "Animal Parasites". Unfortunately, since I copied this diagram years ago for personal use, its author and publisher are unknown to me.)