The gastrotrichs, or “hairybellies” get their name from the Greek gaster or gastros meaning
stomach or belly, and thrix or trichos, meaning hair. This name comes from the fact that their
entire ventral surface, or belly, and only their belly, is often covered in visible cilia.
These are tiny bilaterally symmetrical animals usually less than one millimeter in length,
with a transparent bowling pin-shaped or strap-shaped body, that is dorsally arched
and ventrally flattened. They are divided into two clades, the Macrodasyida which are marine animals
except for two species, and the Chaetonotida, some of which are marine and some freshwater.
Like the flatworms of phylum Platyhelminthes, all hairybellies of phylum Gastrotricha
are triploblastic, meaning they have three unique germ cell layers,
and they are also unsegmented acoelomates, meaning they lack a true body cavity and segmentation.
Unlike the flatworms, however, the gastrotrichs have distinctive paired extensions on their
posterior ends called “feet” or “furca” which contain glands that aid in adhesion. Like the
flatworms, many gastrotrichs have a type of duo-gland system, where one gland secretes an
adhesive agent that allows the animals to attach themselves to the substrate,
and a second gland secretes a de-adhesive agent that breaks down the first adhesive.
In addition, gastrotrichs have a well-developed cuticle that may form plates, scales, and spines,
though it does not contain chitin and is not molted at any time throughout their lives. They
have a complete gut that begins with a terminal anterior mouth and ends with a ventral posterior
anus, in many species. Gastrotrichs eat detritus, algae, bacteria, and other organic material which
they pump into their mouths through suction from a muscular pharynx, which has a Y-shaped or
inverted Y-shaped lumen. From there, the food passes into their midgut and cylindrical
intestine, which is lined with digestive cells. Undigested material passes out of the anus.
Their excretory system consists of one or more pairs of protonephridia, which are equipped
with solenocytes instead of the flatworm “flame cells”, and function primarily for osmoregulation.
Their muscular system is unique and comprised of a system of separate longitudinal,
circular, and helicoidal muscles. Their nervous system includes a bilobed cerebral ganglion
at the head end and one or more pairs of nerve cords that extend to the tail. Sensory and motor
neurons project off the brain and nerve cords. Their sensory organs are comprised of ciliated
receptor cells. Some receptors line the body and function as mechanoreceptors that respond
to pressure, while others form ciliated pits on the head that function as chemoreceptors
and respond to various chemicals. A few species also have red or black-pigmented eyespots.
Like the flatworms, they lack any specialized circulatory or respiratory systems.
Gastrotrichs are considered to be cosmopolitan in distribution, meaning they are found all over
the world, though their patterns of distribution are not yet fully understood. They appear to be
locally abundant, sometimes reaching densities of more than 2.6 million per square meter. As one can
imagine, it’s a pretty small area for 2.6 million of any type of animal to live in, but gastrotrichs
are able to do it because their incredibly small size allows them to live in the spaces between
grains of substrate. Since the gastrotrichs are so small, they often make up an important
part of the meiofuana, or meiobenthos of marine and freshwater environments. Some species live
attached to plants and animals where they form an important part of the periphyton, which is a
collection of small animals, algae, cyanobacteria, microbes, and organic matter that adheres to many
submerged surfaces in aquatic environments, including live plants and animal shells.
Gastrotrichs are important detritivores, as the fifth most abundant group of sediment-dwelling
invertebrates on Earth. They are often an important part of the diet of free-living
flatworms and other slightly larger animals. In terms of defenses,
when threatened by something large enough to penetrate their outer covering, they can
withdraw their body and inch backwards using a combination of muscle contraction and adhesion.
In general, gastrotrichs are hermaphroditic, or monoecious, meaning they contain both male and
female reproductive organs. Most marine species are simultaneously hermaphroditic, meaning they
are born with both male and female reproductive organs and retain them throughout their lives.
Most simultaneous hermaphrodites take part in internal mutual reciprocal sperm transfer
where both partners are fertilized in unison. Most freshwater species have a sort of biphasic
form of reproduction, wherein they are born as parthenogens, meaning they can produce mitotic,
not meiotic eggs, in the absence of sexual reproduction. These eggs always hatch as females
and are genetic clones of the mother. After a period of time, these same parthenogenetic
mothers and daughters will become simultaneous hermaphrodites and develop sperm and meiotic eggs.
No one has ever witnessed copulation and sperm exchange between two such gastrotrichs.
Most gastrotrichs are oviparous, meaning they lay eggs. Some species can produce two different
kinds of eggs; thick-shelled dormant eggs that can survive in harsh environments for years
until conditions become more favorable, and thin-shelled eggs that hatch rapidly.
Newly hatched gastrotrichs also develop extremely rapidly and often reach sexual maturity within
three days. Some species survive for up to forty days, while many die much younger.
Since gastrotrichs somewhat resemble a cross between a flatworm and a rotifer, the latter of
which we will discuss later in the series, their classification has been in flux for some time.
At various points since their discovery, they have classified with the deuterostomes,
the rotifers, the nematodes, and the flatworms. The most recent analyses place them within the
clade Rouphozoa, under Platytrochozoa, making them the sister clade of the flatworms.
However it must be made clear that even this most recent classification is disputed by some studies,
and could very well change as research continues. Regardless, the phylum Gastrotricha itself
is considered to be monophyletic, and consists of about 840 described species
as of the recording of this tutorial. So let’s move forward and investigate other phyla.