THE MOON AND MARINE ANIMALS
Narayani Subramanian
A baby wolf goes to his mother and asks, "Mom, why do we howl at the moon?'
The mother wolf pondered for a minute, sighed, took a deep breath and answered, "Because the moon is just a giant white ball we cannot fetch! We howl in disappointment".
Jokes apart, we have all heard myths and folklore about why wolves howl at the moon. Researchers of behavioural ecology, however, dispute this. Contrary to popular belief, it is not the moon that they are howling at, they howl at each other! They use howling as a method of communication with other wolves! They have also not found any connection between the phases of the moon and the howling of wolves. 
The moon might not have an impact on the wolves and their howling, but it certainly has a tremendous influence on marine habitats. 
Oceanic tides:
The moon causes tides in oceans. Tides are the rise and fall of sea levels. Tides are caused by the effects of gravitational forces exerted by the moon, the sun and the earth's rotation. We all know that the sun is so much bigger than the moon, and the laws of physics dictate that the gravitational pull exerted by an object is influenced by its mass. So, the gravitational pull of the sun must be much higher than that of the moon. But, the force of gravity also depends on one more factor: distance. The distance between the moon and earth is much lesser than the distance between the sun and the earth. So the sun exerts only half the amount of gravitational force when compared to the moon!
Tides are influenced by the lunar cycle. In Indian coastal waters, we have "Semi diurnal tides", which means there are two high tides and two low tides every day. Every six hours, the tide changes. During the new moon and the full moon, the tides reach their maximum levels. At this time the height difference between the high tide and low tide is the maximum. Although this is not related to the waves, during these times, the sea tends be very rough.
Marine animals and lunar cycles
As we all know, the moon has distinct "Phases" that gradually change over a period of 29.53 days. This cycle exerts a significant influence on the behaviour of marine animals.  Coral "Mass Spawning", where all the corals in a reef habitat simultaneously release their reproductive gametes into the water, happens around full moon. The annual migration of Christmas island crabs for laying eggs is synchronised with the lunar cycle. The mating cycle of rabbitfishes is synchronised with the phases of the moon. The Pacific Palolo worm spawns exactly seven to nine days after the full moon. Many molluscs exhibit lunar periodicity in breeding activity.
Pacific Oysters:
A study in 2019 in the Southwest coast of France showed that the opening and closing of shells in Pacific oysters is synchronised with the lunar cycle. The scientists studied two different oyster species and concluded that, they are more open during the new moon period and more closed during the full moon. They have put forth the theory that this is probably due to the availability of algae during these lunar phases. Oysters are filter feeders, so if they keep their shells open during the time when there are more algae in the seawater, they will get more food to eat.
California Grunion:
Grunions are small fishes which are found around the coast of California. From Spring to summer (February to September), during full moon and new moon nights, grunions come to the beach! Just after the high tide, they gather all their energy, brave the crashing waves and reach the sandy, low water regions of the beach. Females lay eggs, males fertilize them. Popularly called "Dance of the Grunion", this event attracts a lot of local fishermen who catch them during these beach visits.
The grunions visit the beaches for four to eight times in a year, laying about 3000 eggs. The eggs are incubated in the sand and the residual water keeps them moist. Once the next high tide reaches them in a few days, they hatch in less than a minute and return to the sea!
How does this happen?
The above mentioned are some of the many examples of how the moon influences behaviour in the marine ecosystem. How does the synchronicity happen? A lot of theories have been put forward regarding this, but the mechanisms are not understood clearly yet. 
There are some studies which show that environmental triggers cause these behaviours. Deep in the ocean, there is a bristleworm species called Platynereis dumerili. After the new moon, this deep sea species swims to the surface, performs a dance like act, reproduces and goes back to the sea. Studies in 1950s had mentioned that moonlight is the trigger that causes this behaviour.
Scientists in 2013 decided to test this theory. They kept these worms in the laboratory - one worm was kept in complete darkness (no moon), one worm was kept in a box with a light source switched on at all times (perpetual full moon), and one worm was kept in a box where the light source was dimmed in accordance to the lunar cycle. Can you guess which of the worms swam to the surface and danced?
Yes! You guessed it right! The worm with the lunar light cycle synchronised itself to the light source, and swam to the surface after the "new moon" phase, when the light was fully switched off!
However, many other studies have shown that these periodic cycles are not just influenced by external triggers and that these periodicities follow a "Biological clock" which is genetically fixed. The authors of such studies argue that no matter where the animal is, its internal clock will work in accordance to the moon.
Science is still finding out answers to these questions and every new study is throwing more light into these fascinating patterns of nature. Do you have a pet at home? Does the behaviour of your pet change according to lunar cycles? What do you think causes these changes? Write to Jantar Mantar and let us know!

Image credits:
•	Tides - National Oceanic and Atmospheric Administration (NOAA), USA
•	Phases of the Moon - Fred Espenak, USA
•	Oysters - FAO
•	Grunion - Zuma Press, Inc. Alarmy stock photo.
•	Grunion2- Mark Conlin
•	Bristleworm - Martin Guhmann