Animals are extremely diverse in anatomy, life cycle, adaptations… Our planet is full of imaginative wonders (many of them will be covered in future issues): survivors in the hardest conditions, ultraspecialized parasites, sun-powered animals, overcomplicated mating strategies… the list just has no end.
Amongst the uniqueness of animal diversity we found some animals that are completely different to anything else. These animals have been puzzling scientists for decades and some continue to do so. We are talking about the monotypic phyla or, as I call them (and I suppose I have taken the term from somewhere), microphyla.
What is a phylum (or, in plural, phyla)? They are basically the main groups in which animals as a group have been divided by scientists. Each of these groups have a series of characteristics that clearly derive from the other groups.
To give an analogy, imagine a box full of tools. You may have different sized screwdrivers, tweezers for different options… you may have a high variety of instruments, but you can all group them in smashing tools, cutting tools… You know that all smashing tools are resistant, in spite of their shape. Cutting tools, on the other hand could be resistant, but what clearly defines them is that they are sharp. These groups represent the phyla.
For example, the phylum called chordata (where we are) groups all animals which have (or had) a back-bone structure, slits in pharynx and a tail, amongst other morphological characteristics (they have, of course, genetic and biochemical similarities, too). Currently, there are about 34-36 of these main groups (some are still controversial, as defining a phylym is not so easy), most of them already discovered by the XIXth century.
While most of them have between hundreds to thousands of species, there are some other phyla with just 1 or 2 species. This means that some animals are so unique and so different to anything else that scientist have been forced to create, at some point in history, new phyla just for them. Not all scientists agree on having new phyla just for these isolated animals and have tried to fit them in other groups. In many cases, therefore, the controversy about what are these animals is still on the table.
We inaugurate a new series where we will have a look at these unique, odd-looking and sometimes very unknown animals. Today: Cycliophorans and Micrognathozoans
Cycliophora, habitants of a lobster mouth
First, we encounter the phylum Cycliophora, which means carriers of small wheels, a fitting definition. Symbion pandora and Symbion americana, the only described species for this phylum, are 1mm sized organism that live either on the sea or attached to the mouth parts of a lobster, where they were originally discovered.
But not any lobster, in fact, they have been only found in the mouth of Nephrops norvegicus, the common lobster, who inhabits the coasts of Europe (and is a usual in fish markets and restaurants) and Homarus americanus, an American lobster.
Returning to its etymology, what does “small wheels” refer to? Well, their mouths. As many filter feeders, this structure is indispensable for the animal survival. Therefore, it has a replacement system that consists on little mouths growing inside the animal that eventually protrude from the main body.
As many sessile marine animals, its life cycle is overcomplicated (just to name a few examples: the colonial Thaliacea or the parasitic cycles of some Plathyelminthes). In our case, the feeders are only the ones living inside the lobster. Other free-living stages do not feed and are basically fast reproducing stages. The male, in fact, it’s reduced to its minimal expression, lacking any part of the digestive system. Every reproductive larvae, including the asexual ones (conveniently named Pandora, given the chaotic life cycle of these animals), grow inside the main body and are afterwards protruded. They scape, then, to liberty, even if it just momentaneous.
Although the reproductive cycle is not completely known, everything points to one complex system (that I’ll try to summarize in one sentence): one feeder can produce either a female or male stage inside, males escape and attach to feeders with growing females inside, then females escape and attach with sticky cilia to the mouth of the lobster, meanwhile the male produces two secondary males inside its body that fertilize the female, afterwards inside the female a new larvae (choroid) grows, eventually bursts its way out of the female and escapes and finally it attaches to the lobster (again), which gives a new son feeding stage (you are allowed to breathe, now).
To sum up, Ciclyophorans are still quite enigmatic, although its position in the animal tree of life seems to be quite clear (sisters to Entoproctans, also a rather obscure clade of tiny marine animals). As a moral of the story, never underestimate your neighbor lobsters.
Micrognathozoa, one of the most complex mouth on Earth
Continuing with mouth issues, now we confront Micrognathozoa, with Limnognathia maerski as the only representative. This animal was discovered in an island near Greenland (Disko Island) during a university field trip, back in 1994, and also has been also found in Crozet Islands (Anctartica), so the completely opposite part of the world. Limnognathia maerski wasn’t formally described until 2000, although for the first moment was placed as a unique pyllum in the Gnathifera, a clade that gathers a plethora of strange microscopic phyla. It is believed that these animals could be relicts from the Cretaceous, when their actual habitat was a tropical reef.
Limnognathia maerski usually lives swimming on the surface of mosses, where it hunts algae and bacteria. Despite this basic diet, one of the most prominent features of this animal is its mouth (take a look at the first image in the post!). It’s composed of almost 15 pieces that measure about 14 µm and are united by muscles, working in a similar way as Arthropod and Vertebrate jaws. Interestingly it doesn’t have a fully functional anus (just temporarily functional) and usually protrudes the jaws, in a kind of “vomit” behavior in order to expel its residues.
Concerning the reproductive cycle… well, only female specimens have been found so far (should I change all the “its” for “she”, then?) and it is believed to reproduce by parthenogenesis, as many other freshwater animals of the meiofauna.
In the next issue….
We will meet one of the simplest animals in the world that could give us clues for the origin of this kingdom and we will encounter a recent finding that could potentially be a new phylum.
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Thanks for reading !
Kristensen, R. M. (2002). An introduction to Loricifera, Cycliophora, and Micrognathozoa.Integrative and Comparative Biology, 42(3), 641-651.
Sorensen, M. V., Funch, P., Willerslev, E., Hansen, A. J., & Olesen, J. (2000). On the phylogeny of the Metazoa in the light of Cycliophora and Micrognathozoa. Zoologischer Anzeiger, 239(3-4), 297-318.
Obst, M., Funch, P., & Kristensen, R. M. (2006). A new species of Cycliophora from the mouthparts of the American lobster, Homarus americanus (Nephropidae, Decapoda). Organisms Diversity & Evolution, 6(2), 83-97.
Funch, P., & Krlstensen, R. M. (1995). Cycliophora” is a new phylum with affinities to I. Nature, 378, 14.
Giribet, G., Sørensen, M. V., Funch, P., Kristensen, R. M., & Sterrer, W. (2004). Investigations into the phylogenetic position of Micrognathozoa using four molecular loci. Cladistics, 20(1), 1-13.
Brusca, R. C., & Brusca, G. J. (2003). Invertebrates. Sunderland, Massachusetts: Sinauer Associates.
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