Practice Tests
IELTS Reading (AC) Task 1 #95

A. In the profound darkness of the deep ocean, where sunlight cannot penetrate, a remarkable natural phenomenon illuminates the gloom: bioluminescence. This is the process by which living organisms produce and emit light. While most commonly associated with marine life, it is also found in some terrestrial creatures like fireflies. However, it is in the vast, dark expanses of the world''s oceans that bioluminescence is most prevalent, with estimates suggesting that the vast majority of deep-sea species exhibit this capability in some form. This "living light" is not merely decorative; it serves a range of critical functions essential for survival in an environment of perpetual darkness.

B. The mechanism behind this natural glow is an intricate biochemical reaction. Essentially a form of chemiluminescence, it involves a light-producing substrate, broadly called luciferin, and an enzyme, luciferase, which acts as a catalyst. When luciferin reacts with an oxidising agent, typically oxygen, the luciferase enzyme facilitates a reaction that releases energy in the form of light. Unlike incandescent light bulbs, this process is incredibly efficient and is often referred to as ‘cold light'''' because very little energy is wasted as heat, with nearly all of it being converted directly into light. The specific chemical composition of the luciferin and luciferase can vary significantly between species, resulting in different colours of light, most commonly blue and green as these wavelengths travel furthest in water.

C. Organisms have evolved to use bioluminescence for a host of ingenious strategies, primarily for offence and defence. For predation, some creatures, like the infamous anglerfish, use a luminous lure dangling in front of their mouths to attract unsuspecting prey. Defensively, a sudden flash of light can startle and deter an approaching predator. Another sophisticated defensive tactic is counter-illumination, where animals on their undersides produce light that matches the intensity of the faint light filtering down from the surface, effectively camouflaging their silhouette from predators lurking below. Some species of squid will even release a cloud of bioluminescent ink instead of dark ink to distract and confuse their attackers.

D. Beyond simple survival tactics, bioluminescence plays a vital role in communication and reproduction. In the featureless void of the deep sea, light signals can be used to identify potential mates. Different species have developed unique, species-specific patterns of flashes, akin to a secret code, to attract suitable counterparts and engage in courtship rituals. These intricate signals, varying in duration, intensity, and rhythm, ensure that individuals can find and recognise their own kind. This visual communication is fundamental for the continuation of many species that would otherwise struggle to locate each other in such a vast and dark habitat.