Gastrotrichs are microscopic, aquatic invertebrates that constitute the phylum Gastrotricha. They are acoelomates (lack a body coelom) and are also referred to as hairybellies or hairybacks due to the presence of multiple brush-like cilia on the ventral side of their bodies. Additionally, they possess cement glands at the posterior end, which produce a sticky, adhesive substance that helps gastrotrichs cling to a substrate.
These animals are found in both marine and freshwater environments, as well as semi-terrestrial habitats like sandy shores. They are primarily microphagous, feeding on microbes like bacteria, algae, protozoans, and even detritus.
Currently, around 800 species have been described and grouped under two orders: Macrodasyida (Macrodasyidans) and Chaetonotida (Chaetonotids). While members of the former are primarily marine, some chaetonotids inhabit freshwater.
They range between 0.002 to 0.118 in (0.06 to 3 mm) in length.
These animals have a simple body with an indistinct head housing a simple brain, a muscular pharynx, and an elongated trunk with an undifferentiated gut and gonads.
They exhibit bilateral symmetry, resembling a translucent strap or bowling pin with a curved dorsal side and a flattened ventral surface. However, their anterior end is swollen and lobe-like in appearance.
The posterior end of the body bears two conspicuous projections, each having a cement gland. In this double-gland system, one gland secretes an adhesive, glue-like substance while the other produces a de-adhesive agent. In macrodasyidans, there are multiple adhesive glands extending to the anterior end of the body.
The gonads lie lateral to the gut towards the posterior end of the body.
Their bodies are encased in a cuticular layer, beneath which lie an epidermis and layers of muscle fibers organized in circular, helicoidal, dorsoventral, and longitudinal arrangements. The ventral surface features numerous cilia, hence their nickname ‘hairybellies,’ which are arranged in various patterns, including continuous layers, rows, patches, or transverse bands.
In some species, the cuticle thickens to form scales, hooks, and spines.
They are acoelomates and thus lack a body cavity or coelom. Instead, the space within their bodies is occupied by loosely organized connective tissue. While they lack a well-defined circulatory or respiratory system, they do have other essential organ systems.
The anterior mouth connects to an elongated, muscular pharynx with a Y-shaped lumen lined by myoepithelial cells. In certain species, the pharynx bears pores opening to the ventral surface. These pores, equipped with valves, expel excess water ingested with food.
The pharynx is followed by a cylindrical intestine lined by glandular and digestive cells. Further, the intestine leads to the anus, located ventrally at the posterior end of the body.
These animals expel excess water and some metabolic wastes (osmoregulate) through excretory organs called protonephridia. Most nitrogenous wastes, however, are indirectly released through the body wall during the passive diffusion of gases (respiration).
In chaetonotids, a single pair of protonephridia opens through separate pores on the lateral side of the body, while in macrodasyidans, multiple pairs of these organs open along each side.
The cells of protonephridia (called cyrtocytes) surround a series of cytoplasmic rods that enclose a central flagellum. These cells drain into a single outlet cell, which is connected to the outgoing protonephridial duct.
They have a simple nervous system comprising two primary ganglia, which serve as the brain. The two ganglia, connected by a commissure, are located on either side of the pharynx and radiate a pair of nerve cords running along the longitudinal muscle bands.
The bristles and ciliated tufts on their body surface are the primary mechanoreceptors for sensing their surroundings. They also possess ciliated pits on their head, ciliary photoreceptors in the cerebral ganglion, and specialized fleshy appendages that serve as chemoreceptors.
The term ‘gastrotrich’ originates from the Greek words gaster (stomach) and thrix (hair), referring to the ciliated epidermal layer on the ventral (belly) surface of these animals. As a result, they are commonly called ‘hairybellies’ or ‘hairybacks,’ as a possible mistranslation of the name gastrotrich.
Morphologically, these animals seem to be most closely related to members of the groups Rotifera, Gnathostomulida, and Nematoda. However, genetic studies reveal that their closest phylogenetic relatives are members of the groups Platyhelminthes, Ecdysozoa, or Lophotrochozoa.
About 800 species of these invertebrates are classified under two orders: Macrodasyida and Chaetonotida. These orders are further subdivided into 17 families.
Gastrotrichs are cosmopolitan, found abundantly in marine and freshwater habitats, as well as in semi-terrestrial environments like sandy seashores.
In marine ecosystems, they are primarily benthic, dwelling on the bottom sediments, with populations reaching densities of 364 individuals per 10 cm². They are the third most abundant invertebrates in marine sediments, after nematodes and harpacticoid copepods.
In freshwater habitats, gastrotrichs rank as the fifth most abundant invertebrate group, with densities of up to 158 individuals per 10 cm². In both marine and freshwater environments, they reside within the periphyton, a biologically rich layer that supports diverse aquatic organisms and detritus.
They also occupy interstitial spaces between sediment grains, beneath submerged objects, and within the thin water film surrounding soil particles on land. Remarkably, they can also thrive in stagnant pools and muddy areas with anaerobic conditions and can tolerate even the presence of toxic hydrogen sulfide gas.
They are microphagous animals that usually feed on microorganisms, like bacteria, protozoans, algae, and diatoms. Additionally, they consume dead and decaying detritus (detritivores).
They generally feed by creating water currents, either by pumping their muscular pharynx or by lashing their cilia. Once the prey falls into the current, it is sucked into the pharynx.
Although generally solitary and sessile, gastrotrichs display varied hydrostatic movement patterns. Chaetonotids usually glide smoothly using the cilia on their ventral surface, whereas macrodasyidans employ a creeping motion, resembling looper caterpillars, by utilizing the numerous adhesive glands on their bodies.
Certain species, such as those in the chaetonotid genus Stylochaeta, exhibit a unique jerky movement by rhythmically moving their long spines toward the sides of their body, creating a distinctive locomotory pattern.
They have a brief lifespan, ranging between 3 to 21 days. However, in the laboratory, the freshwater species Lepidodermella squamatum has survived up to forty days.
Gastrotrichs reproduce both sexually and asexually, depending on the order in which they are classified. For instance, macrodasyidans reproduce sexually, while many chaetonotids undergo asexual reproduction by parthenogenesis.
They are sequential hermaphrodites with a single pair of gonads, the anterior part of which produces sperm while the posterior end produces ova. The males often pack the sperm in packets called spermatophores and use their intromittent organ to transfer them to the seminal receptacle (sperm-storing site) of the female.
The gametes fertilize internally and form an embryo within the fertilized egg. The fertilized eggs are finally released into the surrounding water through the rupture of their body wall.
The embryo undergoes direct development into miniature adults or juveniles (true for all gastrotrichs) through holoblastic cleavage. The growth of the embryo is determinate, with each cell predestined to develop into a specific part of the body.
Exception: Only Urodasys viviparus is viviparous and gives birth to live young.
Many species of this group reproduce asexually through parthenogenesis. The males of these species have degenerate and non-functional reproductive parts (if present at all). The females produce eggs of a diameter of less than 50 μm, which grow directly into juveniles without the need for fertilization by sperm. These juveniles attain sexual maturity in about 3 days.
They are preyed upon by other benthic invertebrates, such as turbellarians, heliozoans, sarcodines, and some midges.
These animals exhibit eutely, a biological phenomenon in which every species has a fixed number of somatic cells as adults.
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