Soil planarians that the world forgot about

A variety of planarians have been noticed in Hebbal forest department nursery, Big Banyan tree and Bangalore University, writes M Jayashankar.

Soil as microhabitat represents a dynamic world with rich diversity of soil borne organisms.

Soil fauna ranges from tiny microorganisms to comparatively bigger earthworms, arthropods and molluscs.

An accidental inventory of land planarians during a study revealed the secret life of the lesser known species.

The spotting of the land planarians was during a routine surveillance of the Giant African Snail, achatina fulica in the Hebbal Forest Department Nursery adjacent to the Hebbal Lake.

This led to a quest to understand their eco-biology.

The flatworms systematically placed under phylum platyhelminthes consist of three classes of animals, turbellaria (planarians), trematoda (flukes), monogenea, and cestoda (tapeworms).

They are mostly marine, some inhabit fresh water and some live in moist terrestrial habitats especially in tropical and subtropical regions.

Ignored species

They are the first group to have an organised brain, crudely though, in the form of a primitive nervous system.

Planarians are free living but flukes, monogenea and tapeworms are parasitic.

These poorly understood group of free-living flatworms containing somewhat over eight hundred species of slimy, flat or cylindrical animals can reach three-fourth to twelve and a half inches in length and 1/8 inch wide.

They move by a gliding motion and a thin mucous trail is left behind as it moves and can be mistaken for slugs, but as the name suggests, flatworms are more flattened dorso-ventrally.

Their bodies are usually very fragile, easily damaged by slightest touch.

The external appearance of these animals especially, the shape of the head, body form, general colouration, pigment patterns, bands, and stripes on both the dorsal and ventral sides of the body are secondary important taxonomic characters.

It was only in 1990 that a good revision of land planarians was made by turbellariologist Ogren and Kawakatsu and the internal morphology especially that of the copulatory apparatus, started to have a greater importance in describing a species correctly.

Interesting among the land planarians are bipaliid members of the family geoplanidae.

Bipaliidae have their head expanded in a spatula-like shape and multiple eyes, hence the common name hammerheads or arrowhead or shovelhead worm.

All the known bipaliid species have been reclassified into three genera, Bipalium Stimpson, 1857, Novibipalium Kawakatsu, Ogren & Froehlich, 1998 and Humbertium Ogren & Sluys, 2001.

The Indian bipaliid land planarian fauna was studied mainly by Von Graff (1894, 1899), Whitehouse (1919) and De Beauchamp (1930).

The independent era of India has seen very few reports of these slimy creatures.

With the photographs of few species recorded during initial surveys, a communication was established with Dr Masaharu Kawakatsu, a well known turbellariologist from Japan.

In a series of communications under the guiding spirit of Dr Kawakatsu, the findings from Bangalore took form of a research article entitled A List of Known Bipaliid Land Planarian Species from India, with a Record of Three Diversibipalium.

They were also spotted in other nurseries and horticultural fields near the Big Banyan Tree, Hesaraghatta and Bangalore University Campus.

Ferocious predators

Land planarians are the apex predators of the soil and feed upon a variety of soil organisms such as earthworms, isopods, insects and snails and are cannibalistic.

They follow the scented trail of their prey.

They hunt, attack and capture using physical force and the adhesive and digestive properties of their mucus.

All planarians feed through a muscular and reversible pharynx located in ventral side of the body.

All pharynxes are equipped with glandular secretions that externally digest and dissolve their prey.

Some Invasive Alien Species (IAS) of flatworms may pose a threat to local biodiversity and negatively impact agriculture, for example, through a decline in earthworm species.

However, the number of land planarians in an area usually is small and control is not recommended. 

During favourable season (rainy), planarians move outside their protective areas at night to hunt for earthworms that come to the soil surface.

They prefer dark areas, cool temperatures and require high humidity.

However, they do not have mechanisms for water conservation and are thus unable to withstand desiccation. 

Flatworms are useful as bioindicators because of their high sensitivity to environmental changes and low dispersal capacity.

Their fragile nature is being used as a model organism for fine-scale phylogeographic studies in the Atlantic rainforest in southern Brazil.