What do werewolves and ocean tides have in common? They are both controlled by the Moon! However, werewolves are mythical, while tides are one of the most well-proven natural phenomena. Tides are the regular, vertical rise and fall of the ocean water level. 

This rhythmic rise and fall is caused by the gravitational forces exerted by the Moon and the Sun on the Earth. Although the Moon is much smaller than the Sun, the Moon exerts a stronger force on the Earth because it is closer to us! These gravitational forces pull the ocean water towards the Moon or Sun, causing the ocean to swell in some areas and creating a higher water level across from the Moon or Sun. The Earth, however, continues to rotate, causing different continents to pass under this swell, creating tides across the globe. Because of the Earth’s rotation, tides are semidiurnal in most places, meaning they occur twice a day. 

During new moons and full moons, the Moon, the Sun, and the Earth are aligned in a row. At these times, the gravitational forces of the Moon and the Sun work together to pull the ocean water farther, creating an even larger swell, known as a spring tide. Spring tides show the greatest changes in water level. On the other hand, when the Moon and the Sun pull the ocean at 90° angles, their gravitational forces work against each other, creating the smallest swell, known as a neap tide.

The intertidal zone, or the area between the tidal high and tidal low, is one of the most diverse ecosystems on the planet! There are many different species in the intertidal zone that have developed unique adaptations to survive being under water at high tide, but also exposed to the sun and wind at low tide. Creatures in the intertidal zone include barnacles, crabs, seaweed, sea anemones, sea stars, and sea slugs, to name a few! 

It is also super important for humans to understand tides as well, not just animals. Low tides can expose the shore and make it impossible for ships to enter harbors without running aground. On the other hand, high tides can raise the water level so high that ships cannot pass under bridges. Thus, understanding the tides has a huge impact on our ability to travel by sea. The largest change in tides occurs at the Bay of Fundy in Nova Scotia, Canada, where the tides cause a change in water level from 11 to 53 feet! More recently, scientists have been working to harness the energy created by tides as a source of renewable energy. Can you think of any other ways that tides may influence human activities?

Here are some fun tide riddles you can try and stump your friends and family with!

I rise and fall, a rhythmic dance, I pull the sea with every chance. What am I?

I ebb and I flow, with rhythms so grand, I’m a watery traveler, sculpting the land. What am I?

I am both the height and the low, I follow the moon, wherever it goes. What am I?

Intertidal zones play host to some of the marine world’s most unique organisms, and perhaps the most popular is the sea star. Made truly famous by a certain cartoon character, sea stars are more widely known as starfish, though they are not actually a “fish.” Sea stars are echinoderms, meaning that they lack a backbone. Sea urchins and sand dollars are other examples of echinoderms. Sea stars have been around for around 450 million years and live solely in saltwater environments. They are incredibly diverse and widespread, with nearly 2,000 species living at every level of the ocean, from tide pools to deep sea floors. Color, texture, and size vary from species to species, as does the number of arms a sea star may have. The largest sea star in the world, the Sunflower Sea Star, grows up to three feet in diameter and has between 16 and 24 arms! The smallest, known only as Parvulastra parvivipara, meanwhile, measures just 9 millimeters wide. Among the more common species of intertidal sea stars are common sea stars, ochre sea stars, northern sea stars, which are both native to North America. These species tend to exhibit the most common physical features of sea stars—orange in color with five limbs. 

When looking at a sea star, you may wonder: “how does this thing move, eat, see, and reproduce?” The answers to these questions are what make sea stars such fascinating creatures. Hundreds of suction-cupped tube feet lie on the undersides of their arms. These tube feet are connected by internal canals, called a water vascular system, that extend and contract to propel a sea star along the sand or rock it’s on. Having a water vascular system means that seawater essentially acts as a sea star’s blood! And while intricate and interesting, this method of movement does little in terms of speed—it may take a week for a starfish to move a mile. Tube feet also allow sea stars to cling to rocks and play a role in feeding. Sea stars eat mollusks and bivalves (animals with two shells, like clams and mussels) and use their feet to open their prey’s shells. They catch food in their mouths, which is under the center of its body, then project their stomach out through their mouth, digest their prey, then pull their stomach back inside their body. A sea star’s eyes appear as tiny red dots at the end of each arm and are limited to perceiving only light and dark. Reproduction among sea stars is both conventional and unconventional. Most common is mating between a male and a female, who then lays eggs. However, sea stars are able to reproduce asexually, or without mating, through a process called fragmentation or fission. A sea star will detach an arm or lose a central disk, both of which will grow into an entirely separate sea star! 

Next time you’re at the beach, look out for sea stars lounging around on the shoreline and rocks. Maybe challenge yourself to find one of their eyes!

 

Questions 

1. Echinoderms do not have a ____

 

2. What is the largest species of sea star called?

 

3. How do sea stars move?

 

With summer in full swing, you might be interested in visiting a few beaches. But have you ever noticed how some beaches are silky smooth, some are strewn with rocks and pebbles, and others are conglomerations of small pools dug into outcroppings? However diverse, all of these habitats are known as the intertidal zone. The intertidal zone is commonly defined as the transitional habitat between land and ocean. Life in these transitional zones can be calm, violent, sparse, and flourishing. To survive in such transient conditions the biota, or flora and fauna, of these regions have developed incredible adaptations.

Some of the most common creatures you’ll come across in intertidal zones are mollusks, soft-bodied animals that typically have a protective shell covering, such as mussels, gastropod snails, and, surprisingly, octopuses! To deal with harsh tidal conditions, their shells protect from battering against sharp rocks and provide an enclosed refuge during low tide, which keeps them from drying out or getting too hot in the sun. Some shelled creatures have developed a heat-shock protein, which is only expressed in response to extreme temperatures and aids in the protection of functional proteins within organisms exposed to sunlight. Many intertidal creatures have adapted to stick themselves to their environment, as if glued in place. Limpets suction themselves to surfaces with their “foot,” or the muscular portion of their body, and barnacles secrete a “glue” from their antennae that cements them in place. Similarly, growing algae can form holdfasts, which are like anchors that keep the plant from breaking away with the waves. In sandy intertidal zones, creatures tend to be flatter and camouflaged, like flounders and some crabs, or have stronger muscles for burrowing into the sand. What kinds of adaptations do you think you would need in order to survive the intertidal zone?

 

Questions

1. What is an intertidal zone?


2. What kinds of adaptations help mollusks survive in intertidal zones?


3. Can you think of a species not mentioned in the article that lives in the intertidal zone? (Hint: many arms!) What adaptations do you think they have?