Girardia tigrina

Girardia tigrina is a species of freshwater planarian or flatworm native to North America. It is also known as Dugesia tigrina or commonly the brown planaria. As of 2021 it was regarded as an invasive species in ten countries or islands, although it was not recorded as having an impact on people or the environment. At least four subspecies have been found and catalogued.

Several qualities of G. tigrina and other planarians make them useful in research. They are capable of regenerating all or even most of their bodies. Also, the lifespan of these flatworms is unknown. Scientists researching regeneration and aging frequently study this species to learn about tissue growth and development and aging.

Background

Platyhelminthes or flatworms are simple, bilaterally symmetrical, and triploblastic, meaning their bodies have three basic cell layers: endoderm, mesoderm, and ectoderm. The types of flatworms are planarians (Turbellaria), which are free-living organisms that live in freshwater, oceans, or moist habitats; and parasitic flatworms that need two or more host organisms to complete their life cycles. The latter include the classes Trematoda (flukes), Cestoda (tapeworms), and Monogenea (externally parasitic). Some varieties, such as many tapeworms, parasitize humans. Researchers have found more than twenty thousand flatworm species.

Most terbellarians are smaller than 0.2 inches (5 millimeters) long. Many are microscopic. Planarians are the largest flatworms, with some reaching 20 inches (0.5 meters) in length.

The head contains nervous tissue that serves as a primitive brain and sense organs. Two pigment spots, which are primitive eyes called ocelli, are usually found on the head. Most flatworms have one body cavity, the gut. The smallest flatworms may not have any body cavities. Larger flatworms may have a branched gut to ensure that food reaches all parts of the body. A single opening in the flatworm’s body is used to take in food and expel waste.

Flatworms are flat because of the single body cavity, which means they do not breathe. Instead, they respirate through diffusion, a process in which oxygen molecules pass through the outer membrane and are diffused through the body and into cells. The worm’s flat shape ensures that all cells are near the outer membrane and can receive oxygen; it also increases the surface area of the outer membrane, which increases diffusion.

Most flatworms are hermaphroditic, having the reproductive organs of both sexes. They usually have one pair of ovaries and multiple testes. The life cycle of planarians begins with fertilized eggs. These may be laid in clusters or singly and glued to an object. When the embryos have developed, larvae or worms emerge from the eggs. Turbellaria and Cestoda are also capable of regenerating tissue. A completely new organism can grow from very small parts, although the regenerated body may be imperfect. This quality has led them to be used frequently in medical research.

Most Turbellaria are carnivorous. Some can use threads of mucus to capture prey, but most seem to simply consume what they encounter. Usually the worm extends a mouth part called the pharynx. Some pharynxes have cilia that capture protozoans and other small organisms. Other Turbellaria have muscular pharynxes that allow them to feed on prey much larger than themselves by inserting the pharynx through the external membrane of prey such as crustaceans and removing the internal organs and fluids. Enzymes help to break down food into small particles that can be absorbed or enclosed by cells.

Overview

The species Girardia tigrina is classified in the Animalia kingdom, phylum Platyhelminthes, order Tricladida, family dugesiidae, and genus Girardia Ball according to the Global Biodiversity Information Facility, although researchers disagree about classification. The four subspecies identified are Euplanaria tigrina subspecies novangliae, Euplanaria tigrina subspecies tigrina, Girardia tigrina subspecies novangliae, and Girardia tigrina subspecies tigrina.

G. tigrina has a triangular head, which has two eyespots on the top that can sense light. Sensory lobes called auricles are located on each side at the back of the head, creating the triangular shape. Auricles sense touch and chemicals. The long body is usually brown with white and yellow spots and pointed at the posterior end. The average size is 0.35–0.59 inches (9–15 millimeters).

As free-living flatworms, they can live in most wet or damp locations including debris such as plant material. They prefer darkness, and in bodies of water including lakes and streams they are found at depths up to about 16 inches (40 centimeters). They are found in temperate regions, preferring water temperatures of 55–77 degrees Fahrenheit (13–25 degrees Celsius).

Reproduction can take place asexually or sexually. Asexual reproduction is achieved through fission, when one flatworm splits into two pieces. The original flatworm regenerates the missing parts. Sexual reproduction occurs between two individuals and usually takes place in summer. After encountering each other, they press the undersides of their posterior ends together and each inserts a penis into the other. After about one-and-a-half hours, they separate. Each can then create multiple cocoons from which about four offspring will hatch.

G. tigrina can swim in water. They can also produce mucus and use cilia that cover their bodies to propel themselves as they hunt for food. They consume small worms, insect larvae, small crustaceans, and other prey. In some places, their appetite for mosquito larvae has decreased the mosquito population. Common predators of this flatworm include amphibians, some insect larvae, and fish. They may be found alone but commonly forage in groups, which provides some protection from predators.

Researchers have been unable to determine the lifespan of G. tigrina. If they are injured, they regenerate missing parts. As long as food is available, they will continue to live and do not show signs of aging. When food is not available, starving G. tigrina will reabsorb their body tissue and shrink, returning to full size again when food is available.

Bibliography

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Dornelas, Aline S.P., et. al. “Effects of Two Biopesticides and Salt on Behaviour, Regeneration and Sexual Reproduction of the Freshwater Planarian Girardia tigrina.” Journal of Hazardous Materials, vol. 404, part A, 15 Feb. 2021, doi.org/10.1016/j.jhazmat.2020.124089. Accessed 10 June 2021.

“Girardia tigrina (Girard, 1850).” Global Biodiversity Information Facility, May 2021, www.gbif.org/species/5892120. Accessed 10 June 2021.

Macêdo, Luana Priscilla Rodrigues, et. al. “Comparative Ecotoxicological Evaluation of Peracetic Acid and the Active Chlorine of Calcium Hypochlorite: Use of Dugesia tigrina as a Bioindicator of Environmental Pollution.” Chemosphere, Oct. 2019, pp. 273–281, doi: 10.1016/j.chemosphere.2019.05.286. Accessed 10 June 2021.

Melo, A.S., and C.F. Andrade. “Differential Predation of the Planarian Dugesia tigrina on Two Mosquito Species Under Laboratory Conditions.” Journal of the American Mosquito Control Association, vol. 17, no. 1, Apr. 2001, pp. 81–83, pubmed.ncbi.nlm.nih.gov/11345426/. Accessed 10 June 2021.

“The Respiratory System and Direct Diffusion.” Libre Texts, 15 Aug. 2020, bio.libretexts.org/@go/page/14018. Accessed 10 June 2021.

Saccomanno, R. “Dugesia tigrina.” Animal Diversity Web, 2014, animaldiversity.org/accounts/Dugesia‗tigrina/. Accessed 10 June 2021.

Stocchino, Giacinta Angela, Ronald Sluys, Abdel Halim Harrath, Lamjed Mansour, and Renata Manconi. “The Invasive Alien Freshwater Flatworm Girardia tigrina (Firard, 1850) (Platyhelminthes, Tricladida) in Western Europe: New Insights into Its Morphology, Karyology and Reproductive Biology.” Contributions to Zoology, vol. 88, no. 2, 2019, pp. 236– 56, DOI: doi.org/10.1163/18759866-20191406. Accessed 10 June 2021.