Planaria torva

by Roy Cosner

Taxonomic Classification

Kingdom: Animalia

Phylum: Platyhelminthes

Class: Rhabditophora

Order: Tricladida

Family: Planariidae

Genus: Planaria

Species: Planaria torva

Description

P. torva are free living flatworms belonging to the class Rhabditophora (previously Tuberllaria). When fully mature, an individual may grow to 15 mm in length and 4 mm in width with a body that is dorsolaterally flattened with the mouth and pharynx centrally located on the ventral surface (Ball, Reynoldson & Warwick, 1969). The anterior end is rounded and lacks the cephalic lobes that are present in other species. Ocelli are positioned dorsally on the anterior end and give the appearance of true eyes. P. torva varies in coloration but is generally brown or gray and may have some mottling. Planarians are acoelomate animals with an endoderm, mesoderm, and ectoderm. P. torva are benthic predators, feeding on gastropods, arthropods, insect larvae and nematodes (Kreuzinger-Janik, Kruscha & Traunspurger, 2018). The digestive system ends in a branched gastrovascular cavity that extends throughout the body. These flatworms lack a respiratory system, meaning that gases are diffused across the body wall.

Habitat and Distribution

The various species of planaria occupy both fresh and saltwater, as well as some terrestrial habitats. P. torva is found in both calm, slow moving and fast running freshwater ecosystems (Ball, Reynoldson & Warwick, 1969). This species occurs only in Europe, while several other species are distributed almost worldwide. It is believed that historical events are responsible for its distribution within Europe, most likely being introduced through the timber trade, as P. torva has a strong association with the canal systems (Reynoldson & Piearce, 1979).

Reproduction

Planarians are capable of both sexual and asexual reproduction. Mature organisms are hermaphroditic, and reproduce sexually through reciprocal sperm transfer, after which the fertilized eggs are laid in cocoons on the underside of rocks and plants (Beveridge, 1982). When the eggs hatch, the juvenile is a small version of the adults lacking functional gonads, which develop as the organism matures. Although they are hermaphrodites, self fertilization is not known. Asexual reproduction can occur through both fragmentation and fission. Fragmentation occurs when contractions of the circular muscles cause the organism to "break" into several pieces, with each piece developing into a fully formed individual. Architimous fission is the common mode of fission used, which results in a transverse division of the organism below the pharynx, with the resulting halves regrowing the missing head or tail (Beveridge, 1982).

The Amazing Ability of Regeneration

Planarians have long been the object of scientific study due to their regenerative abilities. If you were to slice the organism in half, each half would regenerate into a complete individual. Amazingly, an individual can be cut into 279 pieces, with each piece regenerating into a complete worm (Shok, 2014). Amputation of part of the organism or injury is the major stimulus for regeneration. When injury is detected, the organism reduces the surface area of the wound through strong muscle contractions at the site while specialized cells called rhabdites create a protective covering over the wound (Resinger & Kelbetz, 1964). Special adult stem cells called neoblasts are responsible for regeneration after wounding and are the only mitotically active somatic cells present in the mature flatworm. With the exception of the region anterior to the ocelli and the pharynx, neoblasts are found throughout the body of the organism( Baguna, 1989). Following amputation, a blastema forms at the site of the injury, which helps develop the missing body parts of the organism. The new cells that are used for the formation of the blastema are provided by the neoblasts, which proliferate after injury (Reddien & Sanchez, 2004). Scientists continue to work with these organisms in hopes of being able to apply the concepts of regeneration towards modern medicine. An interesting aspect of this regeneration is the affect that light exposure has on the organism. Although P. torva lacks image forming eyes, the ocelli are able to detect the presence of light. An important aspect to this relationship between light exposure and regeneration is that of negative phototaxis, which is the movement away from light sources (Agarwal, 2014).

Based on the act of negative phototaxis, I established an experiment to test the impact of light exposure on regeneration in planaria. In this experiment, I manipulated the amount of light exposure received by the flatworms after they had been cut in half. Three groups were established for the test, with a group receiving 12 hours of light, one receiving 2 hours of light a week, and a control in which the organisms were kept in complete darkness for the duration of the test. Throughout the experiment, when a section of the cut flatworm had completely developed into a full organism it was cut in half again, leaving all cut sections in the dish. Each week of the experiment I recorded the total number of complete individuals present in each treatment (Table 1). At the end of the experiment I compared the total number of complete planaria for each treatment.

Table 1: Number of completely regenerated planaria per group, per week.

The control, which received no light, ended the experiment with the highest number of individuals, followed by the 2-hour and 12-hour treatments. These results supported my hyposthesis that increased exposure to light would negatively affect their regenerative abilities. This shows that light exposure is correlated to regeneration but does not show causation.

References

Aggarwal, Sahil. “A Study of Planarian Phototaxis, Tactile Response and Regeneration.” Inquiries Journal, vol. 6, no. 3, 2014, pp. 1-2.

Baguñà J, Saló E, Auladell C. "Regeneration and pattern formation in planarians. III. Evidence that neoblasts are totipotent stem cells and the source of blastema cells." Development. 1989:107.

Ball, I., Reynoldson, T., Warwick, T. "The taxonomy, habitat and distribution of the freshwater triclad Planaria torva (Platyhelminthes: Turbelleria) in Britain." Journal of Zoology, vol. 157, 1969.

Beveridge M. "Taxonomy, environment and reproduction in freshwater triclads (Turbellaria: Tricladida)." International Journal of Invertebrate Reproduction, 5, 1982: 107-113.

Kreuzinger-Janik, B., Krushca, S., Majdi, N., Traunspurger, W. "Flatworms like it round: nematode consumption by Planaria torva and Polycelis tenuis." Hydrobiologia. vol. 819, issue 1, 2018: 231-242.

Reddien, P., Sánchez, A. "Fundamentals of planarian regeneration." Annual Review of Cell and Developmental Biol. 2004; 20:725-57.

Reisinger, E., Kelbetz, S. "Fine structure and discharge mechanism of rhabdites." Z. Wiss. Mikrosk, 65, 1964:472–50.

Reynoldson, T., Piearce, B. "Predation on snails by three species of triclad and its bearing on the distribution of Planaria torva in Britain." Journal of Zoology, 1979.

Shok, Seeder. “Unraveling how Planaria Regenerate.” nature.com, 21 April 2014, http://www.nature.com/scitable/blog/accumulating-glitches/unravelling_regeneration_in_planaria.