Sea Urchin (Echinodermata)

1. Classification/ Diagnosis characteristics

Sea urchins are invertebrates part of the group known as Echinoderms. Sea urchins are eukaryotic organisms. Sea urchins belong to one of the two main groups of echinoderms that are motile called echinozoa. Echinoderms are marine animals. The animal morphyology is supported by gene sequences and cellular morphology. Sea urchins can be grouped in a deuterostomes clade meaning the blastopore develops into an anus. Sea urchins are in the same clade with sea cucumbers, sea stars, brittle stars, etc. They have several important diagnosis characteristics such as: radial symmetry, distinct organ systems, three embryonic cell layers, unique cell junctions, collagen and proteoglycans in extra-cellular matrix, coelomate, and multi-cellular. Sea urchins are eumertazoans and are classified with the trioblastic organisms. Eumetazoans have obvious body symmetry, a gut, nervous system, and tissues organized into distinct organs.

external image Seaurchin_bw.GIF

An overview of the different structural components of a sea urchin. While this organism is structurally limited in its survival, it persists through its abundance and sharp spines. (19) (SR)

2. Relationship to humans

Humans and sea urchins are both deuterostomes. Two major pieces of evidence show that deuterostomes share a common ancestor that are not shared with protostomes. First deuterostomes share a pattern of early development in which the mouth forms at the opposite end of the embryo from the blastopore. This is the opposite of protostomes. Second recent analyses of the DNA sequences of many different genes show a strong support for shared evolutionary relationships between deuterostomes. However these relationships are not apparent in the morphology of deuterstomes.
Some humans occasionally eat sea urchins. Sea urchins sperm and egg are also used to study fertilization because they produce so many.

The eggs of sea urchins, known as roe, are high in protein (up to 70%), amino acids, iron, and minerals, making the organism useful to regulate metabolism, lower cholesterol, and lower blood pressure. In addition to being used in the pharmaceutical industry, sea urchin roe is a delicacy in cultures around the world, and is often eaten raw with lemon juice. Despite the nutritious benefits of sea urchins, divers hate them because of their needle like spikes. The spikes can get lodged into human flesh easily, so divers are especially cautious (12). (SP)

Research into sea urchins physiology has also lead to new insights into cancer and genes. In 1875, Oskar Hertwig discovered the process of fertilization as the explanation behind sexual reproduction due to his observations of sea urchin reproduction. In 1982, TIm Hunt discovered the cyclin protein involved in cell division by experimenting on fertilized urchin eggs. This protein was later found in several other organisms, making is a possible evolutionary trait. Hunt's work also contributed to modern cancer research. Today, researchers experiment with sea urchins in the development of chemotherapy drugs that prevent mitosis by interfering with the replication of DNA.(20)(MT)

3. Habitat and niche

Sea urchins are important grazers on algae and live in the inter-tidal zones of the world's oceans. Most sea urchins eat algae, which they catch with their tube feet from the plankton or scrape from rocks with complex rasping structure. Sea urchins are salt water dwellers and are found in the ocean.

Sea urchins can be found all over the world in all oceans, warm or cold water and they live in a variety of environments. Examples include rock pools and mud, on wave-exposed rocks, on coral reefs in kelp forests and in seagrass beds in places such as Hawaii, the Caribbean, and Australia. (7) This organism lodges itself half way into the surface of sand, mud or holes to protect from large waves or currents. Sea urchins also live in areas where they can find sources of algae, sea grass, seaweed and other foods they can consume. (8) The sea urchin is also nocturnal, and usually hides in crevasses during the day and rise at night to feed. (7) (NU)

Sea Urchin Habitat

4. Predator avoidance

Appendages assist sea urchins with avoiding predators. Sea urchins have spines that harm any predator that endangers its safety. Some sea urchins are poisonous to protect themselves.
Sea urchins have spines that help protect them from predators. They have two sets of spines: one long, one short. These spines are embedded in a hard outer shell constructed from close fitting plates. In some urchins, these spines are filled with poison (3). As soon as the animal senses danger, it begins to wave the spines back and forth. Other small animals take advantage of this protection and will retreat into the spines to avoid predators (1). Sea urchins can also use these sharp spikes to dig burrows in soft rock in order to partially hide themselves. Once inside, they can pile rocks and shells on top of themselves to blend in with the surrounding area (2). (RG)
Some sea urchin have biting jaws known as pedicellariae which they will use as a last defense against attackers. Pedicellariae are found on moveable stalks and are found between the spines and the tube feet. These defense systems seem to act independently of the main nervous system of the sea urchin and have their own responses to external stimuli. When the sea urchin is in danger, the pedicellariae is revealed and upon contact with the predator, bites and releases a poison to deter the foe (6). (AG)
Sea Urchin "Test"
Sea Urchin "Test"

Sea Urchin Tests

Sea urchins react at once if their shell is touched by a sharp object, and they will point all their spines at that area. They also have the able to regenerate any spines that are lost or damaged by predators. Sea urchins are nocturnal as a means of avoiding predators that hunt during the day. (JM 15, 16)

5. Nutrient acquisition

The water vascular system of the echinoderm helps it feed. The water vascular system is a network of water-filled canals leading to extensions called tube feet. The tube feet help the sea urchin catch algae from plankton or scrape it from rocks. The endoderm of sea urchins help produce the linings of the digestive tracts. Sea urchins typically eat kelp, but they can also eat sponges and other invertebrates (21). (SM)

Sea urchins are omnivores and rely on both plant and animal matter for food. Their spines are versatile and not only serve as armor but also as a way to move and mechanisms through which the sea urchins are able to obtain food; sea urchins can trap food particles that float in the water with their spines. Similarly, sea urchins' five pairs of tube feet, found among the spines, help in both motility and food collection. Because the feet have suckers, the sea urchin is able to capture food along the ocean floor as it moves alone. Additionally, there exist pedicellarines, small claw-shaped structures among the spine, which are used as a defense mechanism as well as another way to obtain food. The sea urchin has a toothy mouth, called the Aristotle's lantern, located on its underside. While the Aristotle's lantern may be venomous to warn predators off, it is primarily used to scrape algae and other food from the rocks that the sea urchins travel along. (AC) (9)

Click Here For Video Of Sea Urchin Feeding

6. Reproduction and life cycle

Echinoderms can reproduce by regeneration. Regeneration is the development of a complete individual from a piece of an organisms. Sea urchins reproduce by sexual reproduction meaning the joining of two haploid sex cells or gametes to form a diploid offspring.

Most sea urchins have separate genders and release eggs and sperm at a specific breeding time in the spring. There are five gonads, reproductive glands, that communicate with five gonopores that release the sperm and eggs.The fertilization of sea urchin eggs is external, meaning it doesn't happen within their bodies. Female sea urchins release millions of jelly-coated eggs at a time and male sea urchins release sperm. After the egg is fertilized and the sea urchin becomes a larvae, it feed on phytoplankton for 5-8 weeks. After, adult features start to develop internally and externally as spines and tubefeet become visible. The time from fertilization to a fully reproductive adult is 2-5 years. (5) (6) (LK)

7. Growth and development

One of the important differences between a protostomes group and a deuterostomes group, like sea urchins, is the that in a deuterstome the blastopore becomes the anus and the mouth forms later. In a protostomes the blastopore becomes the mouth and the anus forms later. Invagination of vegetal pole characterizes gastrulation in the sea urchin. Gastrulation is the process where the blastula is transformed by massive movements of cells into an embryo with multiple tissue layers and distinct body axes. The sea urchin blastula is a hollow ball of cells only one cell thick. In the beginning of gastrulation a flattening of the vegetal hemisphere occurs as the sea urchin blastomeres change shape. The original cuboidal shaped cells become wedge-shaped with smaller outer edges and large inner edges. This results in the vegetal pole bulges inward or invaginates. The invaginating cells become endoderm and form a primitive gut called the archenteron. Some cells of the vegetal pole break away from neighboring cells and migrate into the central cavity becoming mesenchyme, cells in the middle germ layer. These cells will act as independent units and migrate into and among other tissue layers.
Initial invagination of the archenteron is caused by changes in cell shape, but is eventually pulled by additional mesenchyme cells that form at the tip of the archenteron and send out filopodia which are extensions. The filopodia adhere to the ectoderm and when the filopodia contract the filopodia pull the archenteron towards the ectoderm at the opposite end of the embryo from where invagination began. The mouth of the sea urchin forms where the archenteron makes contact with the ectoderm. The opening created by the invagination of the vegetal pole is the blastopore and it becomes the anus of the sea urchin.
Only cells from the vegetal pole can bulge inward to initiate gastrulation because of the uneven distribution of regulatory proteins in the egg cytoplasm. As cleavage continues, the regulatory protein are localized in different groups of cells. After that specific sets of genes are activated in different cells. This determines their different developmental capacities.
Gastrulation forms three layers called endoderm, ectoderm, and mesoderm. Endoderm is the innermost germ layer and it forms from blastomeres that migrate to the inside of the embryo. Endoderm produces the linings of internal spaces like those of the digestive and respiratory tracts and the urinary bladder. It contributes to the structure of some internal organs such as the endocrine glands, pancreas, and liver. Ectoderm is the outer germ layer and it forms from those cells remaining outside the embryo. Ectoderm gives rise to the nervous system and produces the epidermal layer of skin. Mesoderm is the middle layer and it is made up of cells that migrate between the endoderm and the ectoderm. The mesoderm gives rise to the heart, blood vessels, muscles, bones, and several other organs.
During the growth of a radial symmetric organism the body parts are arranged around a single axis point at the body's center. The regulatory and signaling of genes that govern the development of body symmetry, body cavities, appendages, and segmentation are widely shared among different animal groups. Echinoderms evolve their adult forms with unique symmetry where the body parts are arranged along five radial axes much later than other deuterostomes who have bilateral symmetry. Echinoderms undergo a radical change as they develop into adults changing from a bilaterally symmetrical larva to an adult with pentaradial symmetry, which is symmetry in five or multiples of five. Echinoderms have no head and they can move well in many directions. Echinoderms develop an oral side and an opposite aboral side.
Skeletal support features are present internally in deuterstomes and some deuterostomes have segmented bodies, but they are not as obviously segmented as protostomes. Adult echinoderms develop an internal skeleton and a water vascular system.

Sea Urchin Devel.gif
(SL 10)

8. Integument

Sea urchins have a internal skeleton. Echinoderms have a system of calcified internal plates covered by thin layers of skin and some muscle. The calcified plates are mostly thick and they can fuse inside the inside entire body forming the internal skeleton. Sea urchins have an ectoderm which gives rise to the nervous system and produces the epidermal layer of skin. Sea urchins have spines that cover their bodies.

Organisms such as sea urchins are distinguished by a calcareous exoskeleton (or a rigid structure protecting another softer, more delicate structure) in the form of a protective armor. This layer of the organism supports itself with a plasma membrane capable of amassing nearby food particles in the seawater. Furthermore, many sea urchins have spines as self-defense mechanism against predators. (18) (SR)

9. Movement

Sea urchins are motile. Due to the fact that sea urchins are coelomate animals it is known that they have a coelom, a body cavity that develops within the mesoderm. A coelom is lined with a layer of peritoneum, muscular tissue, and this tissue also surrounds the internal organs. A coelomate animal has more control over the movement of the fluids in its body cavity than a pseudocoelomate like the round worm. The structure of a sea urchins body cavity influences the way in which it moves. Coelomate animals have a gut internal organ as the endoderm, a peritoneum as the mesoderm, a coelom as the cavity, muscle as the mesoderm, and an ectoderm. Appendages of the sea urchin assist with movement.
Due to the fact that sea urchins have radial symmetry, sea urchins have no head , and they move well but usually slower than other deuterstomes in many directions. The water vascular system helps with locomotion.

Sea urchins are able to move using their tube feet. To move laterally, paired muscles in the feet and extension and retraction of muscles in the hydraulic 'bulb' behind each foot that operates due to the movement of water under pressure. Individually, the 'feet' are not very strong, but because there are often tens of hundreds of them, the sea urchin is able to move. (11) (CM)


10. Sensing the environment

Appendages help sea urchins sense and react to their environment. Sea urchins can see with their spines and sea urchins have a simple nervous system.

Before scientists had mapped the sea urchin's genome, it was known that they could sense light. However, it has now been determined that sea urchins have thousands genes that code for eye-related structures, including 6 genes that code for opsins, or light sensitive proteins. It was found that these genes are activated in the urchin's "tube feet," or the structures that sway amid its spines. Each foot has a group of light-sensitive cells, meaning that about 200,000 of these cells are present in each animal. It has also been determined that light-sensing cells are located in the actual cell of the animal. In a sense, the sea urchin uses its whole body as a sort of "eye." (SS-4)

11. Gas Exchange

Sea urchins have a water vascular system that helps with gas exchange. The water vascular system is a network of water-filled canals leading to extensions called tube feet. The endoderm produces the lining of the respiratory tract.

Sea urchins conduct gas exchange in primitive gill-like organs called papulae, of which they have five external pairs, positioned near the mouth. These gills have hollow, fluid filled cavities called diverticula, which transfer oxygen to the water vascular system and carbon dioxide out of the organism. When sea urchins reach low oxygen levels, muscles near the gills pump fluid over them to help increase gas exchange. Some sea urchins do not have gills at all, but rather perform gas exchange in their tube feet. (YR) (13) (14)

12. Waste removal

The endoderm of sea urchins produce the lining of the urinary bladder. Sea urchins excrete primarily ammonia as the nitrogenous waste. Ammonia is highly soluble in and diffuses rapidly, so excreting it is simple for marine animals. Ammonia is highly toxic and must be excreted continuously to prevent its accumulation. Waste is excreted through the anus at the top of the body (21). (SM)

13. Environmental physiology (temperature, water, and salt regulation)

Sea urchins are osmoconformers and they live in sea water. Sea water is abundant in salt, so sea urchins allow their extra-cellular fluid to equilibrate with sea water. Sea urchins, unlike osmoregulators, keep extra-cellular fluid osmolarity equal to that of the sea water environment. This prevents it from living in extremely salty environments or too dilute environments.

14. Internal circulation

Sea urchins have a water vascular system that helps with internal circulation. The water vascular system works by pressuring the tube feet. Although one tube foot may not be able to do much, many tube feet are able to preform the function intended (21).The main circulatory fluid is generated by the coelom which suspends its internal organs. It also has a circulatory system driven by contractions of its heart.

15. Chemical control (endocrine system)

Sea urchins have an endoderm which produces the linings of internal spaces like those of the digestive and respiratory tracts and the urinary bladder. It contributes to the structure of some internal organs such as the endocrine glands, pancreas, and liver. Osmoconformers are ionic regulators meaning sea urchins regulate the ionic composition of their extracellular fluid; however, sea urchins may not regulate the overall concentration of osmolarity. Sea urchins usually regulate the concentration of certain ions.

Review Questions

1. How do sea urchins acquire nutrients? What feature allows them to ward off predators? (RS)
2. What is a significant difference between protostomes and deuterosomes? How does this difference become apparent during development? (RS)
3. What can sea urchins trap food particles with? (SM)


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