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A Guide Through the Tidepools

Updated: May 9, 2023

By Angela Velazquez

Grab your warmest layers, waterproof boots, and apply some reef-friendly sunscreen. We are going tide pooling! The intertidal zone is the area where the ocean meets the land between high and low tides. As you look down on the rocky intertidal, it looks like an impossible habitat for life to survive in. But upon further exploration, you will find many creatures living in the pools, between the crevices, hiding under rocks, and clinging onto the rocky substrate for dear life. Here in Southern California, we have the most biodiverse intertidal zone in the country!

Picture Credits: Angela Velazquez

The following six marine invertebrates are found in our local tide pools: Spanish shawl, ochre sea star, sea hare, black sea hare, striped shore crab, and sunflower anemone. To better admire these beautiful animals in their natural environment, let’s learn how to properly identify them and learn some interesting facts about them!

Picture Credits: Angela Velazquez

1. Spanish Shawl (Flabellinopsis iodinea)

The Spanish shawl is a strikingly beautiful nudibranch, a very colorful sea slug that is almost impossible to miss. The most notable characteristic of this nudibranch is their neon-colored bodies: the body is purple, the cerata are orange, and the two rhinophores on top of their heads are scarlet. The pair of rhinophores are sensory tentacles that detect smell and taste. Whereas the fringe on their backs are a set of organs called cerata, which function as gills, are extensions of the digestive system, and harbor stinging cells.

This nudibranch exhibits aposematic coloration. Aposematism is the use of warning coloration to inform predators that the animal is poisonous, venomous, or otherwise distasteful. Being colorful in the animal kingdom is an effective predator deterrent for these creatures!

This easy to see species, by nudibranch standards, can grow up to 70 mm in length. That is equivalent to three U.S quarters placed in a row. When it comes to their diet, they are quite the picky eaters. They enjoy eating sea anemones, bryozoans, and the eggs of other mollusks. Bryozoans are microscopic marine invertebrates that live in large colonies, which might encrust an entire kelp blade. To get around, they use the muscular foot on their undersides to crawl along the seafloor or they can swim, unlike other nudibranchs!

2. Ochre Sea Star (Pisaster ochraceus)

The ochre sea star is in the phylum Echinodermata and in the class Asteroidea. This sea star species is often clustered under shady ledges or in crevices where they can avoid desiccation from the sun. They are highly variable in color, most are purple, however some can also be orange, yellow, reddish, or shades of brown. They are relatively large and consist of five rays (arms) arranged around a central disk. The aboral surface of the sea star contains many small spines called ossicles that form an interlaced pattern. On the undersides of the arms, they contain tube feet with suckers which are used for feeding, locomotion, and allows them to attach to hard substrates in high wave activity shores. On the tip of each arm a single eyespot is present. Their eyespots are photosensitive and allow them to detect shapes and shadows.

The ochre sea star is a famous carnivore which feeds on mussels, chitons, and sea urchins, which it slowly pries open and devours. Sea stars can evert or turn their stomachs inside out over its prey and digest it outside of their bodies. Moreover, this sea star species is highly important and is commonly known as a keystone species. A keystone species is a species on which other species in an ecosystem largely depend, such that if they were removed the ecosystem would change dramatically. Studies have shown that ochre sea stars help control mussel populations which will expand to quickly exclude and crowd other species. Through their predation of mussels, sea stars balance structure and species diversity in specific communities.

Picture Credits: Angela Velazquez

3. Sea hare (Aplysia californica)

The California sea hare is a large mollusk in the phylum Mollusca. The overall body texture is delicately fleshy and can grow as long as 40 cm. The head region contains four tentacles. Two are on the top of the head behind the eyes and the other two are slightly above the mouth. The species' common name (sea hare) is due to the upper tentacles resembling rabbit ears.

Like all Aplysia species, the California sea hare is herbivorous. Its diet consists mainly of red algae, which gives the organism its reddish-brown coloration. Usually, sea hares resemble the food on which it grazes and cannot be distinguished easily from its surrounding environment. It uses a form of cryptic coloration to camouflage so well. One of the more notable behavioral aspects seen in sea hares is their ability to release a deep purple ink when threatened. The ink has been shown to have a foul taste and acts as a defense mechanism against predators.

Picture Credits: PedroPVZ

4. California Black Sea Hare (Aplysia vaccaria)

The black sea hare is a very similar species as the California sea hare. Color is normally uniform black to dark brown, sometimes with fine white speckles. The California black sea hare is the world’s largest gastropod, weighing in at about 31 lbs. Unlike the California sea hare, it does not produce a purple ink-like secretion when disturbed. Like others of its kind, this remarkable animal also has an entirely herbivorous diet. In fact it only feeds on kelp and brown seaweed.

Photo Credits: Amber Phil

5. Striped sea shore crab (Pachygrapsus crassipes)

The striped shore crab is a small crab in the phylum Arthropoda. Commonly, this shore crab will have brownish-purple or black carapace with green stripes. The carapace refers to the shell on the back of a crab that is composed of a hard bone called chitin. The shape of the carapace is squared and typically ranges between 4 to 5 cm in size. The claws are reddish-purple with a striped pattern on the upper surface, whereas the lower surface has a whitish-gray coloration. Its walking legs are purple and green.

Shore crabs are opportunistic predators, meaning they feed on a variety of prey and are able to adapt to whatever food becomes available. Prey items include green algae, worms, mussels, hermit crabs, and small decaying organisms. Crabs have a hard exoskeleton that surrounds the outside of their bodies; however, this hard shell cannot expand as the crab grows and matures. To compensate, crabs periodically must shed their shells and develop a new and bigger shell in a process called molting. When they molt, they fast to lose weight so they can slide out more easily. When they start developing their new shell they fill up with water to make a bigger shell to grow into.

Photo Credits: Genny Anderson

6. Sunflower anemone (Anthopleura sola)

The sunflower anemone is a species of sea anemone in the phylum Cnidarian. It is a solitary anemone that is commonly 12 cm but can grow up to 25 cm wide. The column of the anemone is green to white in color and is twice as long as its width when extended. The oral disc is radially striped and has five short rings of thick, pointed feeding tentacles. The tentacles are pale with tips colored in pink, lavender, and light blue. The sunflower anemone is often covered with shells, rocks, and sand, which not only serve as protection from desiccation and solar radiation, but also provide camouflage against predators.

Sunflower anemones are carnivores and feed on almost anything, including copepods, mussels, crabs, and other small animals that encounter their tentacles. Anemones capture prey with their tentacles, which harbor stinging structures called nematocysts that paralyze their prey.

We have only begun to scratch the surface with these highlighted animals. There are thousands more to discover and learn about. Especially here in Southern California, we have access to incredible tidepools. Remember the next time you're exploring, respect the animals you encounter. It's important to not touch or take any animals. Instead let's admire them with our eyes! To learn more about the marine animals that live in the intertidal, join us on our next Marine Protected Area hike!

Image Credits: OC Coastkeepers

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