Geetha Iyer

The Nipah virus is in the news again in Kerala and attention has once again brought fruit bats, of the Family Pteropodidae, more specifically species belonging to the genus Pteropus, in focus. There are over 1400 species of bats and many of them are friends of farmers. Worldwide, over 300 species of fruit plants depend on bats for pollination. Figs, mangoes, banana, cocoa and many more are pollinated by them. They have few predators and their deadly enemies are disease causing microorganisms, such as fungi, virus, etc. While large trees are resting places of some species of bats, they are among those few animals that live in caves.

Caves as a habitat for biological diversity is only beginning to be studied in India. They have been explored for geological features but not for biological diversity except to think of them as resting places for mammals large and small. The scientific study of caves is called speleology; biospeleology is the study of cave biology. Speleology is different from spelunking, which is just exploring caves for fun – a recreational activity. Speleology is serious study.

The importance of caves as habitats, which may house organisms with specialized features, has begun to attract scientists from India. That they are likely to provide evidence even for evolution has spurred the interest of scientists worldwide. Some believe that it might also provide clues for understanding climate change.

Characteristics of a cave
According to cave biologist David Culver, a biological definition for caves is a natural opening in solid rock, larger than a few millimetres in diameter with areas of complete darkness. Rock shelters, or natural tunnels that are called caves by geologists do not count as they have no zone of complete darkness.

There are many ways by which caves are formed; for example, by dissolution of rocks or from lava. There are different types of caves the world over and how they are formed defines them and the habitats they provide for organisms. Thus, we have solution caves, sea caves, lava caves, talus caves, erosional caves, glacier caves, etc. Whatever be the means by which they are formed, there are some features that are common to all caves. There is no sunlight in these caves. They are dark environments. Hence, photosynthesis is not possible and the creatures living here must look to other sources for obtaining nutrition. Therefore, cave dwellers also depend on nutrients brought in from outside by natural activities, such as flooding.

Since the animals have to adapt to conditions such as total darkness, high humidity, uncertain nutrient resource availability, the number of species living here are fewer and have adapted to dwell in harsh environments.

Biodiversity in caves
Organisms living in caves can be classified into three groups – Trogloxenes, troglophiles, and troglobites. Troglos is the Greek word for caves. The organisms are so classified based on which part of the cave they occupy and their way of life.

Trogloxenes: [Troglos-caves; xenos-guests] These can be termed as part-timers, who enter and leave the cave, not using it as a permanent home. Such animals use caves for hibernation, nesting, giving birth, or as a temporary shelter for resting. They don’t spend their lives in these caves. They are found at the entrance zone – a habitat that is closest to the environment above ground. It receives sunlight and has variable temperatures, and green plants. Bats, bears, owls, racoons and even some moths and snails are examples of trogloxenes.

Troglophiles: [Troglos; phileo-love] If one moves a bit farther into the cave then one enters the twilight zone. As there is very little light in the twilight zone, plants don’t really grow here. The temperature remains a bit more constant but may still fluctuate in conjunction with the weather above ground. Organisms living in the twilight zone need moisture and coolness to survive. These organisms like to stay inside caves even though they can survive outside. They leave the caves only to go in search of food. Specific species of beetles, crickets, worms, frogs, salamanders, etc., are animals that prefer to stay inside caves.

In 2020 scientists discovered a species of frog called Amolops siju from the limestone caves of Siju in South Garo Hills of Meghalaya. This species, new to science, was found both near the entrance of the cave and in the total dark zone. Since it showed no specific cave-adapted features, they concluded that it is a troglophyllic species. This is the second species of frog discovered in caves, the earlier one being Micrixalusspelunca in 2014 from a cave in Tamil Nadu. Frogs are not the only new species found in the caves of Meghalaya. A micro snail species, Georissamawsmaiensis was found by researchers in a limestone cave in Mawsmai, nearly 175 years after the last such genus was discovered in the area. Five species of cave dwelling snails have been reported from the caves of Meghalaya.

Troglobites: [Troglos-bio] These are organisms that live their entire life within the cave. These live in the darkest areas of the cave and are totally adapted for the dark. They can never live outside the caves.

Their eyes are either reduced or totally absent and visual pigments poorly developed. Instead their senses of touch, smell, and taste are well developed. So they have better olfactory systems, longer legs or fins, antennae, allowing them to move and locate food more efficiently in the dark.

Their metabolisms are adapted to allow them to go for a long time without food. They have fewer offspring than their relatives living outside the caves. They accumulate lipids to store more energy. Many troglobites are ambush predators and do not stalk their prey, thereby saving energy. Their circadian rhythm i.e., the day and night cycle, can function independent of sunlight.

Troglobites include cave fish, as well as some salamanders, insects, and arthropods such as springtails, cave crickets of the family Rhaphidophoridae, pseudoscorpions, tailless whip scorpions, cave crayfish, shrimp, and millipedes.

Within a limestone cave in a remote forested part of Jaintia Hills, scientists found a nearly-blind species of subterranean fish, similar to the Golden Mahseer. There are close to 250 species of subterranean fish with an average length of 8.5 cm. Compared to this average size, this newly discovered species measuring 40cm, is the world’s largest subterranean fish.

For the classroom
Food chains and food webs studied in class usually have examples from land or water ecosystems of commonly seen species. Caves sport detritus food chains and food webs. Invertebrates like earthworms, snails, beetles, crickets, and spiders adapted to live in the dark are found in caves, there are microorganisms too, as also large and medium sized mammals, reptiles, amphibians, and fishes.

1. Construct cave food webs and food chains and find out why this habitat is under threat?
2. Research and find out about the abiotic features of limestone caves of Siju from South Garo Hills, Meghalaya. Try to predict what groups of animal and plant species are likely to survive in them. Compare your answers from data published by researchers on these caves.

India has thousands of caves located in places like Chhattisgarh, Kerala, Meghalaya, Madhya Pradesh, Maharashtra, Tamil Nadu, and the Andamans. Each has its own microclimate, darkspaces, and fragile ecosystems that host a range of species, some of which are possibly rare or endemic species of fauna. Most of them are now facing threats that range from poaching, habitat destruction to climate change.

References
• https://untamedscience.com/blog/cave-biology-101-the-cave-biome-in-real-life/
• Cave Biology Life in darkness by Aldemaro Romero. Cambridge University Press

The author is a consultant for science and environment education. She can be reached at scopsowl@gmail.com.