Phylum Gastrotrichia Chaetonotida, Macrodasyoidea
Meiobenthos covered with scales and spines

Microscopic cleaner on the bottom of the water: Gastrotrich
Lyricsand,　Vocal: Shin Kubota　Music, Composition : Juri Goto

　
　The Gastrotrichia, which includes marine macrodasyoidea and fresh water chaetonotida, is aquatic meiobenthos with the maximum body size of only few millimeters. Many species of the Gastrotrichia have bilaterally symmetric and stumpy bodies. Unlike the Kinorhyncha, Gastrotrichs have no segmented body. This phylum is named after its unique characteristics that are flat ventral side and short hair covering the body in a unique pattern (from Latin, gaster=stomach, thrix=hair).
　A notable characteristic of this phylum is that the body of any species is completely covered with scale-like cuticles or spines. A lot of hair grows on the ventral side of the body, and the Gastrotrichs move the hair to glide on the aquatic plants, on the surface of corals or in the sand. Because only a single hair grows on each cell, their hair is actually flagella, not cilia. One of the unique characteristics of the Gastrotrichia is tiny chimney-like adhesive tubes, which secrete “glue”, all around the body to attach temporary to various surfaces on the sea bottom.
　Normally, the outer layer cells of an organism have one nucleus per cell, but strangely, the outer layer cells of the Gastrotrichia have many nuclei per cell. They are so called “mulitnulear cells”, but the reason is not known yet. The Gastrotrichia has no circulatory or respiratory organs. Many species of the Gastrotrichia use their pharynx like a pump to suck in and eat microbes and organic materials.
　All the marine species of Gastrotrichs, such as chaetonotida, are hermaphroditic. However, the male and female elements mature at different rates. Thus, an individual becomes male first and then becomes female later. Compared to such hermaphroditic marine gastrotrichia, no male individual has been found among the fresh water species at all. However, reproduction can be done in a unique way. The method is without fusing a sperm and an egg, but the offspring grows from a matured egg. This method is called parthenogenesis that is not unique to this phylum, and thus, they do not have monopoly on this method. Of course, there are Gastrot.
　Of course, there are Gastrotrichia species whose male and female copulate by a normal method, and the offspring develops from the fertilized egg. Another unique method is also known as well. Some species of male Gastrotrichia bears a capsule, called spermatophore, filled with sperms from their body to the surrounding, and let the female find and collect it. This indirect method is without the copulation procedure.
　Some fresh individuals just hatched from the eggs are known to mature only in three days. They do not go through the developmental stage of peculiarly shaped planktonic larvae which are unique to marine invertebrates. The larvae do not transform or molt. Their life span is expected to be about 16 days, but not many species are studied. Thus, culturing experiments is necessary to confirm the life span.
　Currently, 749 species of the Gastrotrichia have been recorded, but many more unrecorded species are presumably hidden. As a matter of fact, many new species are discovered and recorded from a few meters deep sandy/muddy bottom of the south Beach in front of the Seto Marine Biological Laboratory (Kyoto University) as indicated in Figure 14. Progress in the basic taxonomic study is expected in the future.



Microscopic life on the bottom of the water Gastrotrich
We are the cleaners who eat garbage

Sucking in the garbage with the pharynges like a pump
My trademark is a lot of downy hair on my belly

With the hair on the belly, the stumpy body less than 1 mm slides on
All the downy hairs are flagella
We grow only one hair on each cell

Many chimney-like projections everywhere on the tiny body
Mucus is secreted from the projection to stick to the sand grains and to resist the current

A skin cell on the body has multiple nuclei
The reason is unknown
They are protected by numerous scales and thorns

Microscopic life on the bottom of the water Gastrotrich
The macrodasyoidea looks like a belt living in the sea

They can shift the time to become the male or the female
They try hard copulating to make babies

All the chaetonotidae in the freshwater are females
They have a mysterious ability to make babies without the presence of the male

The children are miniatures of the adults. They are only smaller
The growth is very rapid without molting or metamorphosing, just grow bigger and bigger

Microscopic life on the bottom of the water, less than 1 mm long
The chubby body gliding along
On the planet in the sea and freshwater, 430 species are living


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