Question

This passage is adapted from an article about the discovery and ecological significance of Movile Cave in Romania.

In $1986$, while surveying the barren, limestone-rich plains of Dobrogea near the Black Sea, Romanian geologist Cristian Lascu was tasked with testing the ground for a new power plant. Instead of finding solid bedrock, Lascu stumbled upon a geological marvel that had remained sealed from the outside world for over five million years: Movile Cave. This subterranean cavity, situated just twenty meters below the surface, would soon rewrite our understanding of how life can thrive on Earth.

Unlike typical caves, which are carved by surface rainwater filtering through limestone, Movile Cave was formed by sulfidic thermal waters rising from deep within the Earth's crust. This unique genesis created a highly hostile environment. The air inside the cave is thick with hydrogen sulfide and carbon dioxide, while oxygen levels are less than half of what is found in the outside atmosphere. To the human lungs, the air is highly toxic and suffocating, and the extreme humidity makes physical exertion incredibly difficult. For decades, scientists believed that sunlight and photosynthesis were the absolute foundation of all food chains on Earth. Movile Cave, however, shattered this biological dogma.

The base of the food web in Movile Cave is not sunlight, but rather a thick, floating mat of autotrophic bacteria that covers the water in its lower chambers. These bacteria engage in chemosynthesis, a process where they oxidize the abundant hydrogen sulfide to produce organic matter, releasing sulfuric acid as a byproduct. This metabolic process provides the primary energy source that sustains a complex and diverse community of organisms, all of which have adapted to live in total, permanent darkness. The water in the cave is warm and mineral-rich, providing a stable, tepid bath that fosters the rapid growth of these bacterial colonies.

Since the cave's discovery, biologists have identified forty-eight distinct animal species residing within its chambers. Remarkably, thirty-three of these species are completely endemic to Movile Cave, meaning they are found nowhere else on the planet. Among these unique inhabitants are blind water scorpions, wingless beetles, and specialized leeches that feed on the local microflora. Over millions of years of isolated evolution, these organisms have discarded unnecessary traits like pigment and eyesight, instead developing elongated antennae, hypersensitive chemical receptors, and advanced sensory organs to navigate their pitch-black world. Many of these creatures are translucent, showing their internal organs through their thin outer membranes.

The discovery of Movile Cave has had profound implications beyond terrestrial biology. Astrobiologists, who study the potential for life on other planets, frequently point to Movile Cave as an analogue for environments on Mars or Jupiter’s moon, Europa. If life can thrive in the dark, toxic depths of Romania without any reliance on solar energy, it is highly plausible that similar chemotrophic organisms could exist beneath the icy surfaces of distant celestial bodies. This has shifted the focus of search-for-life missions toward underground aquifers and subterranean caves rather than surface searches.

To preserve this delicate and invaluable ecosystem, the Romanian government has tightly restricted access to the cave. Only a handful of researchers, equipped with clean suits and specialized breathing apparatuses, are permitted to enter each year. Today, Movile Cave remains a testament to the resilience of life, proving that even in the most hostile and isolated environments, evolution finds a way to flourish. By studying this closed ecosystem, scientists hope to unlock the secrets of Earth's earliest life forms, which likely existed in similar, oxygen-depleted, chemical-rich environments billions of years ago.

According to the passage, of the forty-eight animal species identified in Movile Cave, how many are found nowhere else on Earth?