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 microphagous, feeding on microbes like bacteria, algae, protozoans, as well as detritus.
They are one of the few invertebrate groups exhibiting eutely, a biological phenomenon in which each species has a fixed number of somatic cells upon reaching adulthood.
Around 800 species of gastrotrichs have been described and grouped under two orders: Macrodasyida (Macrodasyidans) and Chaetonotida (Chaetonotids). While most macrodasyidans are marine, chaetonotids primarily inhabit freshwater environments.
They range between 0.002 and 0.118 in (0.06 and 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 bowling pin with a curved dorsal side and a flattened ventral surface. Their anterior end is conspicuously bulbous in appearance.
In chaetonotids, the posterior end of the body bears two prominent 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 contrast, macrodasyidans possess multiple adhesive tubes along the anterior, lateral, and posterior body surfaces.
The paired gonads lie lateral to the intestine in the posterior half of the body. In macrodasyidans, the gonads are arranged longitudinally, with testes anterior to the ovaries, while in chaetonotids, which are typically parthenogenetic, a single ovary is located medially or laterally near the intestine.
Their bodies are encased in a cuticular layer, beneath which lie an epidermis and layers of muscle fibers organized in circular, spiral, dorsoventral, and longitudinal arrangements. The ventral surface features numerous cilia arranged in different patterns, like continuous layers, rows, patches, or transverse bands, often varying across the regions of the body.
In some species, the cuticle thickens to form scales, hooks, and spines that help protect and anchor the animal to the substrate.
As acoelomates, these animals lack a body cavity or coelom. Instead, loose cellular tissue called the mesenchyme fills the space where the coelom would have been. While gastrotrichs lack a well-defined circulatory or respiratory system, they have other essential organ systems.
The anterior mouth connects to an elongated, muscular pharynx with a Y-shaped lumen lined by myoepithelial cells. In some macrodasyidans, the pharynx bears pores opening to the ventral surface.1These pores, equipped with valves, expel excess water ingested with food.
The pharynx is followed by a cylindrical intestine lined by glandular cells that secrete digestive enzymes and digestive cells that assimilate the digested nutrients. 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 released through the body wall during the passive diffusion of gases.
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. Each protonephridium comprises a specialized cell called a cyrtocyte, which contains a filter-like structure formed by cytoplasmic rods surrounding a central flagellum. The flagellum beats and draws fluid through this filter, helping to excrete excess water. All cyrtocytes drain into a single outlet cell 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 give rise to 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 the head and specialized fleshy appendages for chemoreception, along with ciliary photoreceptors in the cerebral ganglion that detect changes in light.
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. The name of the phylum was coined by the Russian zoologist Élie Metchnikoff in 1865.2
Though the common name ‘hairybellies’ hints at their ventral cilia, the name ‘hairybacks’ most likely stems from mistaken interpretation of the name gastrotrich.
Morphologically, these animals seem most closely related to members of the groups Rotifera, Gnathostomulida, and Nematoda. However, molecular phylogenetic analyses reveal that their closest 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 have a cosmopolitan distribution and are typically found in marine and freshwater habitats.
In marine ecosystems, they are primarily benthic, dwelling on the sandy bottom sediments. In such environments, they have been recorded at population densities reaching 364 individuals per 10 cm², making them third in abundance among the meiofaunal invertebrates, after nematodes and harpacticoid copepods.3
In freshwater habitats, gastrotrichs are the fifth most abundant invertebrate group, with densities of up to 158 individuals per 10 cm².4 In both marine and freshwater environments, they reside within the periphyton, a biologically rich layer that supports diverse aquatic organisms and detritus.
These animals also occupy interstitial spaces between sand and sediment grains, beneath submerged objects, and within the thin water film surrounding soil particles on land. They also thrive in stagnant pools and muddy areas with anaerobic conditions and can tolerate the presence of toxic gases, like hydrogen sulfide.
They are microphagous, feeding on microscopic food particles. Freshwater gastrotrichs typically consume bacteria, algae, protozoans, and fungal spores, whereas marine species feed on all of these as well as detritus.
They feed by generating water currents, either through rhythmic contractions of the muscular pharynx or by the lashing movement of their cilia. Once the prey is trapped in the current, it is drawn into the pharynx.
Though occasionally sedentary, gastrotrichs typically display various 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. Some species 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.5
They have a brief lifespan, ranging from 3 to 21 days in the wild. However, under a controlled environment in the laboratory, some species, like Lepidodermella squamatum, have survived for as many as 40 days.
Gastrotrichs reproduce both sexually and asexually. While most macrodasyidans reproduce sexually, 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. In many species, the males often pack the sperm in packets called spermatophores and typically use an 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: To date, only Urodasys viviparus has been found to give birth to live young (viviparous).
Many species of this group reproduce asexually through parthenogenesis. The males in these species either have degenerate, non-functional reproductive organs or lack them altogether. 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 invertebrates, such as turbellarians, heliozoans, and certain protozoans, like large ciliates, amoeboids, and flagellates. Some larval midges may also feed on them.6
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