Coastal creatures

Maneka Sanjay Gandhi

The silver yellow full moon lights up the sky and makes me instantly happy. We have forgotten how the moon can change the landscape, because we have so many artificial lights. But, as the moon spins round the planet, it alters the Earth’s ecosystems, transforming the lives, food, shelters and reproduction cycles of hundreds of species.

Tides are the result of the gravitational pull of the moon. When the moon is full, or new, it is directly in line with Earth and the Sun, exerting a strong pull on the ocean and causing stronger tides. When the moon is half-full, it is out of alignment with Earth and the sun, producing weak neap tides.  The life cycle of coastal creatures depends on these tides. From March to August the sand is carpeted in California with small silvery grunion fish, who dig into the sand, mate on top of the holes and release eggs. Ten days later the eggs hatch and are swept out by the high tide to sea, the entire event choreographed by the moon. Some species of sea turtles wait for the full moon's high tide to ride waves onto shore, and lay their eggs far up on the beach.

The marine annelid worms, Platynereis dumerilii, regulate their reproduction cycle according to the phases of the moon with amazing precision. They live in the sea bed. Under the full moon, the sex cells in adult worms mature; then, exactly 14 days later under the new moon and four hours after sunset, they float to the surface and synchronously release eggs and sperm. After spawning the worms die, and a new generation begins to grow in the seabed.

Many ocean animals have biological clocks finely tuned to the cycles of the moon. Zooplankton are tiny animals who swim up from below the ocean depths toward the surface every night to feed on algae. They are preyed on by larger animals that hunt by sight so, as soon as it is dawn, zooplankton head back down. This rhythm is dictated by the sun. But, in the Arctic, where the winter sun cannot be seen for months, zooplankton also have an internal clock that is set to the moon. When the winter moon is full over the Arctic, it stays above the horizon for a handful of days and during this time, zooplankton dive to take cover from predators. But the moon also rises and sets—and the zooplankton respond, rising and diving over the course of this cycle.

Oysters, who open their shells to eat and spawn, also fall under the moon’s influence. French researchers monitored how widely oysters opened their shells during new moons, and more closed when the moon was full. In addition, the oysters could tell the difference between the first quarter moon and the third quarter moon, and were significantly more open (by nearly 20 percent) at the latter.

Speckled sea lice burrow in the sand in the intertidal zone, which is covered by water at high tide, and dry when it’s low. They have an internal lunar clock, and are more active during the full and new moons, with their stronger currents, and more sedate during the weak neap tide.

Sandhoppers, tiny crustaceans that live buried in sandy beaches, can tell the difference between sunlight and moonlight, and so are able to move in the right direction between the sea and the shore regardless of the time of day.

The marine bristle worm, Platynereis dumerilii, looks like an amber centipede and lives on algae. Bristle worm populations swim to the ocean surface just after the new moon, and dance in circles while mating. Any change in the moon’s light changes the spawning ritual.

Studies of fiddler crabs have shown that even when kept in the lab under constant light and temperature, the animals are still most active at the times that the tide should be out. This internal lunar clock, running in synchrony with the Moon helps animals anticipate tide movements; a skill that might give some creatures an edge. Galapagos marine iguanas, with the most accurate lunar clock, are more likely to survive tough times, presumably because they are best at reaching feeding spots on the beach first.

On a full moon night, each December, corals around the world, but most spectacularly off the coast of Australia, synchronize a massive release of egg and sperm packed together in round, small, pink buoyant bundles. While environmental factors, like temperature, salinity and food availability, help in triggering the event, researchers have found that levels of moonlight seem to play a major role: If the sky is too cloudy, and the moon obscured, the corals will often not spawn. Sometimes they delay until the next full moon. Researchers reveal that not only do corals have light-sensitive neurons tuned to the dim blue wavelengths of moonlight, they also have genes that change their activity level in sync with the waxing and waning moon, regulating reproduction.

Palolo worms, which live in warm ocean waters worldwide, also release sperm and eggs together in a precisely timed explosion. They live on the seafloor, or in coral, and feed for most of their lives. But, for two days in October when the moon is full, their rear bodies turn into sacks of egg or sperm, break off from the rest of the worm and swim toward the surface—and the light of the moon. Exactly one month later, they repeat this feat in even great numbers.

For many animals the moon is essential to migration and navigation. 

During certain phases of the moon, Japanese Sesarma crabs collectively scuttle across mountain slopes toward sea-flowing rivers, where they release their eggs and sperm. The annual migrations of Christmas Island crabs, which move in waves of crimson from forest to sea to mate and lay their eggs, also seems to be linked to moonlight’s shifting intensity.

For other species, the moon’s light is more important as a navigational cue. Migrating chum salmon swim more quickly, and at shallower, depths during a full moon, likely because they are using its light as a lodestar. Albatrosses and streaked shearwaters often fly more frequently, and for longer periods of time under a full moon, perhaps because they can see farther.

Barau’s petrel is a seabird that breeds on the island of Réunion in the Indian Ocean. The petrels time their pre- and post-breeding migrations by the length of the day, waiting until it reaches 12.5 hours before setting off.  They always arrive at the breeding grounds on the full moon, suggesting that they use the moon to synchronise their migrations.