Purple Sea Urchin

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The largely unnoticed creatures of kelp forests and tidepools often look like something out of a sci-fi movie. Strange sea anemones, odd fish and brightly colored Sea Slugs are fixtures in an underwater landscape. The Purple Sea Urchin is right at home in this environment; with its bright color and sharp spines. It’s hard to tell whether it’s a plant or an animal, let alone a species of this planet.

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Despite its extraterrestrial look, it is an animal related to a starfish. Like the starfish, it lives along the rocky shores and sea floors on the Pacific Coast in shallow areas affected by the crashing waves. Also, like starfish, sea urchins have tube feet, but unlike the five legs of a starfish, tube feet cover the whole spiny external part of the sea urchin and enable it to move slowly using hydraulics.

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Sea urchins feed mostly on algae and decaying organic matter, although they are known to eat kelp and sponges as well. The mouth of the sea urchin, located on the underside of its body, has five calcium carbonate teeth.

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The sea urchin actually uses these teeth to “dig” into rocks, where it lodges itself for extended periods of time. In fact, if a sea urchin carves out a hideout for itself at a young age, it can grow too large over time and get stuck in its own hole.

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Unfortunately, large groups of sea urchins can actually be detrimental to kelp forests. Sea urchins feed upon kelp and often eat the stems of the plant found near the rocky ocean floor. This effectively kills the entire kelp plant. It is often sea otter predation on the sea urchins that keeps sea urchin populations in check and preserves the kelp forests.

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However, the sea urchin is not defenseless against these hungry predators. Its first line of defense is its sharp spines, which many divers can tell you are no joke. The next line of defense is the tiny stinging structures found in their spines, called pedicellarines.

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Purple Sea Urchins are considered an indicator species. They are very sensitive to changes in their environment, and usually are one of the first in their ecosystems to show signs of distress when the water quality starts to decline. Stress in sea urchins often shows itself as drooping spines and a lack of movement.

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Aggregating Anemone

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This organism was quite abundant and frequently seen on my quest for Pacific Ocean tidepool creatures.

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Their green color comes from an endosymbiotic (living within the anemone mutually benefiting both organisms), photosynthetic algae in their tentacles and body.

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The tentacles can be retracted inside the body cavity or expanded to catch passing prey. When not submerged in the water, they pretty much look like blobs covered with fragments of shells from things they’ve eaten.

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Aggregating Anemones catch prey that comes within reach of their tentacles and immobilize it with the aid of their venom-filled stinging cells within their tentacles.

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The tentacles are triggered by the slightest touch, firing a harpoon-like filament into their victim and injecting a paralyzing neurotoxin. The prey is then transported to the anemone’s mouth and engulfed.

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Despite the potency of its venom to its prey, sea anemones are harmless to humans. I stuck my finger near this one and it wrapped around it, trying to pull my finger into its mouth!


The closest relatives of these amazing creatures are jellyfish and corals. This was a fun animal to make an acquaintance with on my trip to California.

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Ochre Sea Star

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While tidepooling near San Francisco, I came across a number of these cool creatures. They are relatively large sea stars, with five arms and a rough texture. Their short spines on the upper surface are arranged in wavy patterns.

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Most of them are either orange (ochre) or purple, and there is no explanation as to why they come in two dramatically different colors. The bottoms of each of its five stout arms are covered with thousands of tiny tube feet tipped with suction cups.

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Ochre Sea Stars are classic intertidal animals. They have been called a “keystone species,” as their presence or absence significantly affects the entire community in the intertidal zone.

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One of this animal’s favorite foods is mussels. Although slow-moving, it does wander about, searching for prey. When it finds a mussel, it wraps a its arms around it and is able to pull the mussel’s shell slightly apart.

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To eat, sea stars extrude their stomach out through their mouth. They then lay their stomach on the soft tissue of their prey. Digestive juices from the stomach dissolve tissue and the stomach absorbs the dissolved material.

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Also commonly known as the Pacific Sea Star, it can regrow a lost arm, though it may take a year to do so. It was a great experience to encounter these impressive, colorful tidepool residents.

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Pacific Red Octopus

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One of the unquestionable highlights of tidepooling in California was seeing this cool creature – the first octopus I’ve ever encountered in the wild. The Pacific Red Octopus doesn’t seem to be as picky when it comes to its diet as many other species of octopus and will consume whatever food they can find, like crabs, clams, barnacles and scallops.

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An octopus usually collects food at night and then retreats to its den to eat at its leisure. It kills its prey with venom secreted from its salivary glands, then cracks the shell with its sharp beak.

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It can also drill a hole in the snail’s shell and inject a chemical that separates the snail’s flesh from its shell. An octopus deposits empty shells outside its den in a pile – commonly called an “octopus’s garden.”

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Although it has excellent eyesight, The Pacific Red Octopus uses touch and smell to find food; thousands of chemical receptors and millions of texture receptors line the rims of its suckers for the purpose of detecting food.

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Like all octopuses, it can change its color and texture, making its appearance highly variable. Color can vary from a deep brick red, to brown, to white, or mottled mixtures of the three.

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These are thought to be among the most intelligent of all invertebrates. The presence of individual personalities is a hallmark of intelligence, and the Pacific Red Octopus was the first invertebrate in which individual personalities were demonstrated.

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