The magnificent Lakshadweep archipelago, made up of coral reefs and atolls, lies around 420 km west of the coast of Kerala. Its clear waters offer exceptional underwater visibility, and on any ordinary dive, you will find its reefs rife with activity. Schools of paddletail snappers, several hundred strong, nudibranchs grazing on their favourite patch of algae, butterflyfish roaming in pairs, nibbling on coral, or large groupers swimming boldly through the reef. What is less frequently seen is the flip side — the reef after dark. As the sun sets, the vibrant visuals of the reef subside, and many fish cease activity and nestle into coral crevices for shelter. Moonlight trickles through the sea, sieved by its surface, creating a shifting pattern on the shallow reef. In this dim light begins the nightlife of the reef.
As you dip your head into the water after dark, the sound is jarring. The reef at night is bustling with activity. Snaps, clicks, pops, and croaks — a cacophony of calls. Sound travels much further and faster in water than in air. Fish grunt and squeak to communicate with others of their species, to coordinate while hunting, or to find mates. Juvenile fish, crustaceans and coral larvae use these sounds to home in and settle in the reef. Roaring over all the others are the sharp snaps of millions of snapping shrimp. Their chorus is loud enough to hinder undersea exploration efforts as their incessant snaps render sonars ineffective.
Then, as you switch on your torch, a new marine world appears. Nocturnal crustaceans like crabs and cowries come out of their crevices in search of food. Polychaete worms swim through the water in peristaltic motions, looking like floating centipedes. Marine predators emerge, using the cover of darkness to hunt. Corals come alive, and predators are on the prowl.
While nocturnal predators force diurnal reef fish deep into coral crevices, well-defended echinoderms (like starfish and urchins) come out to feed at night. (1) The burrowing urchin (Echinometra mathaei), well-armed with short, thick spines, comes out of its burrow at night to graze on algae. Predators foolish enough to try to eat it suffer a mouthful of spines, a lesson for them to learn not to try again. (2) Collector urchins (Tripneustes gratilla), already laden with a thick mat of spines, search for additional cover to disguise themselves. As they graze within the seagrass meadows of the lagoon, they drape themselves with coral rubble, pieces of shell, and even the seagrass blades.
(3) Sea cucumbers (Holothuroidea) come out in the open, feeding on sediment or detritus. Yet they are largely ignored by predators due to their reputedly nasty defence tactics. If threatened, sea cucumbers will spill out their guts and respiratory tubes, which are coated in a sticky substance, thoroughly entangling the predator. Yes, as a defence tactic, these tubular animals “eviscerate” (yes, that is the scientific term) themselves! Over time, they grow these expelled internal organs back and return to scooping through the sediment. Photos: Wenzel Pinto (1,2), Vardhan Patankar (3)
Reef fish can change colour dramatically at night. Often, vividly coloured butterflyfish and unicorn fish take on dull or brown hues, as though the absence of light has leached the colour out of them. This is because colour is not as crucial for camouflage at night. Visually dependent predators shift to black-and-white vision in dim light, rendering colours meaningless.
This longbarbel goatfish (Parupeneus macronemus) does not bother changing its colour, resting motionless on the sand despite being a bright red. Instead, it changes its pattern to a patchy mix of reds and whites. This mottled appearance breaks up its outline, making it difficult for predators to distinguish it from the background. Photo: Wenzel Pinto
Bioluminescence is not the only light at night. Large floodlights installed at the jetties brighten the surface waters of the reef’s edge, illuminating hundreds of fish schooling below. This artificial light has had some unintended consequences for marine life.
Some plankton are naturally attracted towards light, and this attraction becomes even stronger after dusk. So when the jetty floodlights illuminate the water’s surface at night, swarms of plankton are attracted to the light, which in turn attracts the fish who hunt them. As this effect moves up the food chain, high-value fish (1) like needlefish (Family: Belonidae) dart through the surface waters, hunting for prey. This concentration of fish is so beneficial to fishermen that jetties are the most frequented nighttime fishing locations in Lakshadweep, landing fishermen their best catch.
(2) The jetty lights sometimes also attract hawksbill and green sea turtles (in picture) who may come for a midnight feed. While beneficial to fishermen, bright light sources can also have another effect, discouraging female turtles from approaching the beaches where they nest. Photos: Umeed Mistry (1), Wenzel Pinto (2)
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Wenzel Pinto
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