Sea urchins, or simply urchins, are globular echinoderms of the phylum Echinodermata (consisting of starfish, sea cucumbers, sand dollars, brittle stars, and crinoids), forming the class Echinoidea.
These animals begin their life as bilaterally symmetrical larvae but develop into adults with pentamerous or five-fold symmetry. Their bodies are encased in a hard shell called the test, which contains numerous spines and pores that allow the tube feet to extend outwards.
Currently, about 950 species of sea urchins are found across the tropics to the polar regions, typically settling on the seabed. They usually crawl on their tube feet and search for plankton, algae, and seaweed. However, they also feed on slow-moving invertebrates, like sponges, mussels, and even other echinoderms. Though difficult to defeat because of their spiny armor, these animals are preyed upon by sharks, crabs, spiny lobsters, sea otters, wolf eels, triggerfish, and wrasses. About 18 species of these echinoderms are edible and consumed as a delicacy in countries like Japan, Vietnam, and South Korea.
Since the 19th century, they have been widely used as model organisms in developmental studies for the ease of studying their embryos.
Most sea urchins measure 3 to 10 cm (1 to 4 in) across their test, though the largest species, the red sea urchin (Mesocentrotus franciscanus), grows up to 18 cm (7 in) in test diameter, with a spine length of 8 cm (3 in). In contrast, the smallest species, Echinocyamus scaber, has a test diameter of barely 5.5 cm (0.22 in).
These animals generally possess rigid, spherical bodies covered in numerous spines. While their larvae exhibit bilateral symmetry, the adults develop five-fold symmetry, also known as pentamerism. The mouth is positioned on the lower surface (oral side), and the anus is located on the upper surface (aboral side).
In some groups, such as sand dollars (order Clypeasteroida), the adults have flat, oval-shaped bodies that retain some degree of bilateral symmetry. Their upper (dorsal) surface is slightly domed, while the lower (ventral) surface is flat.
A sea urchin’s body is covered by a rigid shell or test comprising fused plates of calcium carbonate. It is encased in a thin epidermis, followed by a dermis and a thin layer of muscle and skin. Hence, despite serving as an external covering, the test is also often referred to as the endoskeleton of these animals.
The test has five longitudinal ambulacral grooves separated by five wider interambulacral grooves. These ten columns are composed of two sets of ambulacral plates (thus, 20 columns in total). The plates feature tiny holes (or pores) that allow the tube feet to extend outward, and rounded tubercles serve as attachment points for the spines. These spines, often venomous, serve as their defense by puncturing the bodies of their enemies.
In most sea urchins, the spines are arranged in two series: a long primary and a short secondary. The spines are typically cylindrical and hollow, with the shortest located at the poles and the longest at the equator.
Nestled among the spines are small claw-shaped structures with jaws known as pedicellariae. These structures play an important role in defense, capturing food, and removing unwanted substances from the surface of their bodies.
As echinoderms, sea urchins have an elaborate water vascular system that leads downwards from a calcareous opening called the madreporite. This opening leads to a longitudinal stone canal, followed by a ring canal, which surrounds the esophagus.
Radial canals spread out from the ring canal into each ambulacral groove, leading to lateral canals that end in bulbous ampullae. The ampullae extend into two tubes that pass through the pores on the test and extend outwards as slender tube feet.
This system works under hydraulic pressure and aids in both locomotion and feeding.
The hemal system comprises an intricate network of vessels in the mesenteries around the gut. These vessels circulate coelomic fluid to the body cavity. This fluid comprises specialized cells called phagocytic coelomocytes, which help transport nutrients, gases, and waste products. These wastes are further expelled through the gills and tube feet.
The mouth is composed of five teeth or plates made of calcium carbonate. Within these plates lies a tongue-like structure, which helps procure prey. Together, the mouth and the teeth were initially referred to as Aristotle’s lantern, though this name is currently used to refer to the body shape of certain sea urchins.
The mouth is surrounded by an area of soft, membranous tissue called the peristome, which includes multiple tiny bony pieces, five pairs of modified tube feet, and five pairs of gills (when present).
The jaws comprise five strong arrow-shaped plates known as pyramids. Each plate bears a toothband on the ventral surface, containing a hard tooth pointing towards the center of the mouth. There are specialized muscles that control this jaw apparatus, thereby controlling the action of the teeth.
The pharynx leads to the esophagus, which loops down the mouth to connect to the small intestine and a single caecum. The small intestine loops around the inside of the test and joins the large intestine, which, in turn, loops around in the opposite direction. The large intestine leads to the rectum, followed by an anus. An additional tube, the siphon, runs along the intestine and probably helps in water resorption.
In the aboral pole of a sea urchin lies a membrane called the periproct, which surrounds the anus. This membrane is composed of numerous hard plates, five of which comprise the genital plates and contain the genital openings or gonopores.
Gaseous exchange in most sea urchins occurs through five pairs of external gills attached to a peristomial membrane around the mouth. However, some groups, like heart urchins and sand dollars, lack gills and instead breathe through their tube feet.
They have a simple nervous system lacking a true brain. Instead, they have a large nerve ring surrounding the mouth. Five nerves radiate from this ring into the five arms of a sea urchin, which further branch into finer nerves that innervate the tube feet and spines.
Though they lack eyes or eye spots, sea urchins have numerous sensory cells embedded in the epithelium, particularly in the spines, tube feet, and around the mouth. Hence, their entire body could be referred to as a single compound eye.
Most species, except pencil urchins of the order Cidaroia, have stalked balancing organs called statocysts nestled in their ambulacral areas.
Regular sea urchins possess five gonads lying underneath the interambulacral areas of the test, whereas irregular forms typically have four gonads. Heart urchins of the order Spatangoida have two or three gonads.
These gonads have a single duct that drains into an opening called the gonopore, which lies in one of the genital plates around the anus. A muscular lining above the gonads helps these animals squeeze their gametes through the gonopore.
Together with sea cucumbers (class Holothuroidea), sea urchins (class Echinoidea) form the subphylum Echinozoa. The name of the class Echinoidea is obtained from the Greek word ekhinos, meaning spines, a reference to the spiny bodies of its members. Similarly, the terms sea urchin or urchin originate from the Old French word herichun and the Latin ericius, both meaning hedgehog, reflecting the resemblance of these echinoderms to hedgehogs. As a result, they were also historically referred to as sea hedgehogs.
Around 950 species of sea urchins are currently divided into 2 subclasses and 13 orders.
In an alternate taxonomy, a separate infraclass called Irregularia is placed under the subclass Euechinoidea. As the name suggests, this infraclass includes all sea urchins with an irregular body shape, such as sand dollars and heart urchins.
Since these echinoderms are equipped with a hard test, they are easily preserved and flaunt a rich fossil record. The earliest fossil specimens of sea urchins date back to the Middle Ordovician Period (around 465 million years ago). From this period onwards, fossil samples for this group have been abundant.
Fossils from the Palaeozoic Era, particularly from the Devonian and Carboniferous Periods, are found only in parts, mostly comprising isolated spines and scattered plates. Specimens from the Ordovician and Silurian Periods unearthed from the shallow-water limestones in Estonia suggest that early sea urchins with thin tests probably lived in relatively quieter waters than modern forms.
Only six species are known from the Permian Period, suggesting this group almost went extinct at the end of the Paleozoic era. However, two groups survived into the Triassic Period: ancestors of modern pencil urchins (order Cidaroida in the subclass Perischoechinoidea) and modern euechinoids (subclass Euechinoidea). By the time they reached the Upper Triassic Period, sea urchins started to increase in number once again.
In the Jurassic and Cretaceous Periods, the euechinoids diversified, giving rise to multiple lineages, including the Atelostomata, the group of first irregular sea urchins. Eventually, in the Paleogene and Neogene Periods, sand dollars (order Clypeasteroida) originated. Members of this group had flattened tests and tiny spines and gradually adapted to loose sand in shallow water.
Echinoderms inhabit a wide range of environments, from warm seas to polar oceans. However, they are most commonly found along temperate and tropical coasts, thriving in shallow waters up to a depth of ten meters where resources are abundant.
Most species are found on seabeds, ranging from the intertidal zone to significant depths. Cidaroids, particularly those in the family Echinothuriidae and the genus Dermechinus, inhabit the abyssal zone at depths of 13,000 to 20,000 feet. Meanwhile, members of the family Pourtalesiidae are among the deepest-living sea urchins, confined to the hadal zone, reaching depths of up to 6,850 meters in the Sunda Trench.
The population densities of sea urchins vary according to the habitat. For example, they have the maximum density in barren areas compared to those rich in kelps. Some species, like the shingle urchin (Colobocentrotus atratus), are resistant to wave action and are one of the few species that can even survive without water.
Sea urchins are primary consumers in their ecosystem, as they have a herbivorous diet, including plankton, algae, and seaweed, such as kelp. However, they often consume various invertebrates such as mussels, conches, polychaetes, sponges, and even other echinoderms like sea cucumbers, brittle stars, and crinoids and thus could be omnivores.
These animals typically move by walking on their tube feet, much like starfish. They rely on hydraulic pressure, pumping water in and out of the tube feet to generate the force required to move their bodies. Occasionally, their spines assist by pushing the body along the substrate or lifting it off the surface. Some species, like the purple sea urchin, use their jaws to burrow into the seabed.
In habitats where resources are plenty, species like red sea urchins barely move about 7.5 cm per day. But, in areas with scanty food availability, they can travel up to 50 cm a day.
As a broad group, the lifespan of these animals varies considerably with species. For instance, red sea urchins survive over 100 years, while purple sea urchins live for about 20 years on average. Interestingly, red sea urchins in Canada have been found to survive up to 200 years.
In contrast, the edible sea urchin lives for only 5 to 10 years.
Most echinoderms are dioecious, meaning they have separate male and female sexes. They reproduce sexually, with both males and females releasing their gametes (sperm and eggs) into the surrounding water through their gonopores (broadcast spawning).
In most cases, the unfertilized eggs float freely in the water, though some species may cling to them using their spines. As the egg meets the free-floating sperm, they fuse and fertilize externally, giving rise to the single-celled zygote. Gradually, the zygote undergoes repeated divisions to form a multi-cellular embryo.
The cells of the embryo start to undergo multiple divisions and rearrangements before giving rise to the larva.
In approximately 12 hours, the embryo undergoes ten cycles of cell division, forming a ball-like structure called the blastula. This stage includes a blastocoel, a fluid-filled cavity, and a single layer of epithelial cells surrounding the blastocoel.
The blastula divides further, undergoing gastrulation to form the gastrula. The development of gastrula includes:
At the end of gastrulation:
This stage, known as the echinopluteus, is characterized by 12 long arms (in most species) lined with bands of cilia for capturing food. However, in some species, like Heliocidaris erythrogramma, the blastula is so rich in yolk that the larvae do not need to feed.
The larva undergoes the following sequential stages of development:
The fully developed echinopluteus sinks to the bottom and metamorphoses into the juvenile in as little as one hour. The juvenile then develops tube feet, spines, and a protective test.
With time, the juveniles grow in size, transforming into adults having pentaradial body symmetry. It attains sexual maturity in a few years, depending on the species.
They fall prey to a number of marine animals, including crabs, spiny lobsters, sea otters, wolf eels, cods, triggerfish, and wrasses, such as the California sheephead. Sometimes, other echinoderms, like sea cucumbers, also kill them. Larger animals, like sharks, are also found to feed on sea urchins.
Sea urchins play an important role in the environment by maintaining the balance of marine ecosystems.
The European edible sea urchin (Echinus esculentus) is currently listed under the Near Threatened (NT) category of the IUCN Red List of Endangered Species. Similarly, populations of the species Paracentrotus lividus living in the Mediterranean are also plummeting.
One of the major causes behind the rapid decline of these animals is climate change. Rising global temperatures and increasing ocean acidification (high pH levels) accelerate the dissolution of calcium carbonate. As a result, the shells of sea urchins are becoming progressively thinner, leaving them more vulnerable to extinction.
Other factors affecting the numbers are habitat loss, pollution, and overharvesting for commercial purposes.
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