Sea Urchin

Sea urchins are small, spiky creatures found in various habitats, from shallow coral reefs to deep-sea trenches, and they play important roles in the ecosystems in which they live. Approximately 950 species of these creatures live on the seabed of every ocean and inhabit almost every depth zone.

These spiny, globular animals are part of the phylum Echinodermata, which includes starfish and sand dollars. They are best known for their round, spiny, spherical shells called ‘tests.’ These sea animals have spines, which they use to protect themselves from predators and cling to rocks and other surfaces as they crawl along the ocean floor.

Sea urchins, particularly the well-studied purple sea urchin (Strongylocentrotus purpuratus), play a pivotal role in marine ecosystems by controlling algal growth and serving as prey for various predators. This article looks at some key features and characteristics that will enrich the reader’s understanding.

Appearance

Sea urchins look like porcupines, some people call them sea porcupines. They are small and measure between 3 and 10 cm (1 to 4 inches), weigh an average of one pound, and are covered by sharp spines like porcupines.

These spines vary in length and color depending on the species, but they are typically short and black or brown. Some species have longer, more colorful spines.

Some mature species have spherical bodies with five equally sized parts. Several sea urchins are oval with distinct rear and front ends, which gives them a degree of bilateral symmetry. Due to this ‘irregular’ body form, these sea urchins can easily burrow through soft materials like sand.

Sea urchins move using a water vascular system that powers their numerous tiny, flexible tube feet that extend from the pores on the test. Each tube foot ends in a suction cup-like structure. Tube feet are used for locomotion, clinging to surfaces, and capturing food particles.

It also has a small mouth on the underside of its body and five rows of tube feet for movement and feeding. The head lies towards one end of the oral surface in irregular urchins or the center in regular urchins. Lips of softer tissue, with embedded bony pieces, surround the mouth.

They do not have true eyes with lenses or complex structures. Instead, they possess light-sensitive cells distributed over their bodies, particularly on their tube feet. These light-sensitive cells allow sea urchins to detect changes in light intensity, shadows, and movement, helping them avoid predators and navigate their environment.

Habitat

They are found in many marine environments, from shallow coral reefs to deep-sea trenches. However, they are commonly found in rocky areas, where they can find shelter and food, and in sandy or muddy areas. Some species are found in the intertidal zone, where they are exposed to the air during low tide, while others are found at deeper depths.

They can inhabit any depth zone from the intertidal seashore to over 5,000 meters. A few species, such as Cidaris abyssicola, can live at depths of several kilometers.

Diet

Most of them are omnivorous, meaning feeding on various plants and animals. They are known to graze on algae, which they scrape off rocks and other surfaces using their small, tooth-like structures called ‘Aristotle’s lantern.’ Some species also feed on small invertebrates, such as worms and snails, and even dead plant and animal matter.

Reproduction

Sea urchins are dioecious. In simple terms, they have separate female and male sexes even though no distinguishing features are visible externally.

They reproduce sexually, with males and females releasing their gametes (sperm and eggs) into the water. When the gametes meet, fertilization occurs, and the resulting zygote becomes a larva, which settles on the ocean floor and develops into an adult urchin.

Please note that the female’s eggs often float freely in the sea. However, some species hold them with their spines to increase their security.

Some species can regenerate lost or damaged body parts, including their spines.

Threats

Sea urchins face several threats, including habitat destruction, pollution, and overfishing. In some areas, urchins are harvested for their edible roe or eggs. It’s often served raw, with rice, or tucked into dumplings in Japanese restaurants. In Mediterranean cuisine, you’ll mostly find it blended into sauces, tossed with pasta, or spread on crostini, and this alone is enough to deplete their populations over a short period.

Some species are also vulnerable to many diseases, which can affect their survival. Climate change is also a concern, as rising ocean temperatures and acidification can negatively impact urchin populations in days.

The sharp spines protect the urchin from predators, increasing their safety. However, human beings have invented ways to catch and kill these animals. Some health conditions like the bald sea urchin disease can also cause loss of the spines and lead to an increase in exposure to the threats.`

A species Cidaris abyssicola lives at depths of several kilometers and faces fewer threats than their counterparts that struggle to deal with overfishing and other similar threats.

7 More Fantastic Facts about Sea Urchins

1. Sea urchins have a hard, calcified skeleton made of plates called ossicles.
2. They are found in every ocean in the world.
3. They can move slowly using their tube feet but are mostly stationary creatures.
4. Sea urchins are an important part of marine ecosystems, as they help control algae growth and provide food for other animals.
5. They have a long history of being valued as a source of nutritious human food.
6. Some species can live for incredibly long periods, with individuals estimated to be over 100 years old!
7. Sea urchin roe, known as “uni” in Japanese cuisine, is considered a delicacy in many cultures around the world!

FAQs