Sea+Urchin


 * By Vincent Miao**

**1. Classification/Diagnostic Characteristics **

Sea urchins are echinozoans, which are members of the echinoderms, which encompass around 13,000 species divided among 23 major groups. Only six of these 23 groups have extant species present today, and almost all of them live solely in marine environments. Echinoderms are themselves part of a larger grouping of species, deuterostomes, which, like humans, develop a mouth and anus at opposite ends of the embryo during early development. In deuterostomes, the blastopore is the beginning of the anus and the mouth forms afterwards, opposite that of the development of protostomes. Echinoderms themselves are defined by three unique characteristics: pentaradial symmetry, a calcified internal skeleton, and a water vascular system. During their development into adults, larvae undergo a unique change from being bilaterally symmetrical, two-sided symmetry like in humans, to pentaradially symmetrical, or symmetry in five around a central axis of symmetry. The internal skeleton of echinoderms are a complex of fused-together internal plates with skin and muscle cells. Finally, the water vascular system assist echinoderms with gas exchange, locomotion, and nutrient acquisition.

Kingdom: //Animalia// Phylum: //Echinodermata// Class: //Echinoidea//



**2. Relationship to Humans **

Both humans and sea urchins are deuterostomes as both develop an anus from the blastopore and a mouth on the other side. Other common traits include being triploblastic and coelomate. Triploblastic refers to the three layers of germ cells present in the blastula of a developing organism: the ectoderm, mesoderm, and endoderm. Coelomate refers to the presence of a coelom, or mesodermic cavity present within the organism.

Sea urchins are used as one of many biomedical research models in cell and developmental biology. Because they share 7700 genes with humans, they provide a comparison to our own genome and those of other deuterostomes. They are also used for food by the indigenous people of California, who eat the yellow egg mass of the urchin raw and thus are important in the fishery industry along the west coast of the United States.

Sea urchins are often used in aquariums to monitor water quality in tanks. As sea urchins stop moving and relax their spines immediately when stressed by their surrounding conditions, they are great indicators of anomalies in their habitats.



**3. Habitat and Niche ** Sea urchins, like almost all of their echinoderm relatives, live solely in marine environments, the oceans and seas. They can survive in a wide range of water temperatures and are most commonplace in unpolluted intertidal zones. They are able to do this by burrowing part of themselves into the sand to stay in one place while the waves crash over them. So-called "urchin beds" can cover huge chunks of the intertidal zone, extending for acres at a time and providing a living area for tens of thousands of sea urchins. Sea urchins thrive in environments with a pH level between 6.0 and 9.0. They are primary consumers, eating almost exclusively algae and seaweed they find on the ocean floor with their unique tube feet or on rocks which they scrape with their spines and tube feet. However, sea urchins have also been known to be carnivores, even eating individuals of the same species at times. They tend to live in areas with lots of algae, sea grass, and other sources of food. Sea urchins can be commonly found among coral reefs, especially those in the Caribbean Sea and off of the coast of Australia. This type of environment is suitable for sea urchins because they are nocturnal, and reefs allow them to hide in the day and then come out at night to feed.

**4. Predator Avoidance**

The multitude of sharp spines that are often poisonous that encompass sea urchins serves as their first line of defense against predation. Some sea urchins even have spines designed to break off and splinter within the predator, making it harder and more painful to remove. Usual poisonous spines produce burning pain, numbness, or paralysis. With such a potent defense mechanism that could wound potential predators, sea urchins possess a powerful means of avoiding being consumed. When a sea urchin senses that a predator is nearby, it begins to wave its spines around, creating an impenetrable barrier for any organisms in search of prey. Sea urchins can move their spines as they are attached by a ball and socket joint. Some sea urchins have another method of avoidance: camouflage. Using their tube feet, these sea urchins pick up flecks of rock, shell, and seaweed and cover their bodies. The coverings blend in with the environment so that predators cannot detect the sea urchin beneath. Finally, with their sharp spines, sea urchins are able to dig burrows into the sea floor, effectively hiding them from predators.

The sea urchin's predators include crabs, sunflower stars, snails, sea otters, fish, birds, and people.

**5. Nutrient Acquisition ** Sea urchins’ primary source of nutrition are algae and seaweed, which they intake through a mouth containing five teeth and a tongue-like structure that faces the seafloor. This mouth is found on the oral surface of the sea urchin on the bottom, as opposed to the aboral surface on the top where the anus is located. Food is moved to the sea urchin's mouth, often called Aristotle's lantern, through use of its tube feet. The sea urchin's teeth are extremely strong; they self-sharpen as they eat and have been documented to chew through both concrete and iron bars of piers. Echinoderms such as the sea urchin also possess simple digestive systems of a stomach, intestine, and anus.

**<span style="font-family: Arial,Helvetica,sans-serif;">6. Reproduction and Life Cycle **

<span style="font-family: Arial,Helvetica,sans-serif;">Sea urchins can reproduce by sexual reproduction, where females release millions of eggs through their genitals on the top of them that fuse with the sperm of males. Sea urchins begin reproducing between the ages of two and five years old, and most sea urchin species have two separate sexes They release their gametes (sperm or egg) into the sea during the spring mating period. Sea urchins produce sexually by combining two haploid sex cells into one diploid cell. After the initial contact between sperm and egg, the sperm releases acrosomal enzymes that digest the jelly coat of the egg. As soon as the two cells' membranes fuse together, blocks to polyspermy, the fertilization of an egg by more than one sperm cell, appear. Sodium ions provide a charge difference over the plasma membrane to stop any other sperm in the short term while an envelope forms to protect the cell from polyspermy in the long run. After a four arm larva known as an echinopluteus forms, more arms grow over a period of a few weeks while the sea urchin feeds on phytoplankton, which allows the metamorphic stage to finish at the sea urchin's adult stage. It is interesting to note that reproduction affects the physical location of the different sexes of sea urchins; males are more likely to be found on higher, exposed locations that allow sperm to be better dispersed by currents while females prefer lower, sheltered areas where young have some protection from predators.

<span style="font-family: Arial,Helvetica,sans-serif;">Regeneration is another pathway of reproduction in other types of echinoderms. In regeneration, a complete echinoderm develops from a piece of another echinoderm that was cut off. Sea urchins, however, have a limited regeneration capability where broken spines can be regrown.

**<span style="font-family: Arial,Helvetica,sans-serif;">7. Gr ****<span style="font-family: Arial,Helvetica,sans-serif; font-size: 13px; line-height: 1.5;">owth and D ****<span style="font-family: Arial,Helvetica,sans-serif; font-size: 13px; line-height: 1.5;">evelopment **

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 13px; line-height: 1.5;">Sea urchin embryos are triploblastic; they contain three germ layers: the ectoderm, endoderm, and mesoderm. During gastrulation, the blastospore, the indentation in the blastula, the hollow sphere of cells, appears to eventually form the anus. The mouth forms on the opposite side of the embryo. As larvae called pluteus, sea urchins are bilaterally symmetrical, with one side of the organism the same as the other; however, as they develop, they transform into pentaradially symmetrical organisms with five distinct repetitions of body structure around a central, main axis in the course a of few weeks. The pleteus starts with four arms and gradually grow out to eight arms; at that time, sea urchins develop spines and tube feet in about a week of metamorphosis. Sea urchins become reproductively-capable between two and five years of age and can live more than twenty years.

**<span style="font-family: Arial,Helvetica,sans-serif;">8. Integument ** <span style="font-family: Arial,Helvetica,sans-serif;">Sea urchins are usually 2-4 inches long in diameter and can be brown, black, red, white, purple, or green. <span style="font-family: Arial,Helvetica,sans-serif; font-size: 13px; line-height: 1.5;">They have long spines, the longest of any echinoderm, which aid in locomotion and protection and cover its whole spherical body. Some species have solid spines and others have spines filled with poison for defense from predator attacks. Along with the spines, sea urchins contain pedicellariae, or slender stalks that function as miniature jaws, which helps cleanse the body through the removal of crustaceans and other small organisms. Their integument also consists of long hollow projections with suckers at the end called tube feet. These flexible stalks usually extend above the spines and aid in attachment, movement, and nutrient acquisition. Tube feet also aid in camouflage as the sea urchin is able to grab rocks, shells, and decaying matter from the seabed to cover itself and avoid enemies. Sea urchins have an internal skeleton made of fused calcified plates, called ossicles, which contain small holes to allow spines and tube feet into the environment. This rigid internal skeleton is called a "test," which comes from the Latin "testa," or shell.

**<span style="font-family: Arial,Helvetica,sans-serif;">9. Movement ** media type="custom" key="24528814" align="center"

<span style="font-family: Arial,Helvetica,sans-serif;">Sea urchins move by using water pressure differences in their tube feet, a system of pouches of water connected to radial canals that can interact with the outside environment. The radial canals are connected to a central ring canal. Additionally, sea urchins are coelomates, that is, they have a coelom, a cavity within the body that forms out of the mesoderm during early development. The coelom is surrounded by muscles called peritoneum, which give the sea urchin more control over the fluid within its body cavity. This method of locomotion, while slow, is sufficient enough for the sea urchin to move around and find food.

**<span style="font-family: Arial,Helvetica,sans-serif;">10. Sensing the Environment ** <span style="font-family: Arial,Helvetica,sans-serif;">Sea urchins possess a simple nervous system without a central brain and are able to sense the environment around them with their spines and are very sensitive to light, chemicals, and touch. The nervous system of the sea urchin is made of a central nerve ring that encompasses the mouth and nerves that radiate outwards into the sea urchin's other body areas. Recent research suggests that although sea urchins don't have eyes, their entire body may function as one compound eye. Indeed, the gene Pax6, which dictates the development of eyes in many organisms, from flies to humans, is found in the sea urchin genome. Additionally, sea urchins have six genes that code for light-sensitive opsin proteins. Each tube foot on a sea urchin contains about 140 light-sensitive cells, which total to about 200,000 light-sensitive cells in a sea urchin. Sea urchins also have statocysts which help the organisms orient themselves vertically.

**<span style="font-family: Arial,Helvetica,sans-serif;">11. Gas Exchange ** <span style="font-family: Arial,Helvetica,sans-serif;">Sea urchins intake oxygen and remove carbon dioxide through both their gills, called papulae, and tube feet. Tube feet are needed as a surface through which gases are exchanged because the sea urchin's gills are quite rudimentary and cannot exchange gases quickly enough to support the whole organism. The five gills are located around the mouth of the sea urchin.

**<span style="font-family: Arial,Helvetica,sans-serif;">12. Waste Removal ** <span style="font-family: Arial,Helvetica,sans-serif;">Waste is removed from the sea urchin through the anus, which is located on the aboral, or top-facing surface. Ammonia, which is highly toxic to many organisms, is the main substance that is excreted by sea urchins; other substances, most containing nitrogen as well, are also secreted. Phagocytic coelomocytes in the coelom collect waste products excreted from the individual cells and actively transport them outside the sea urchin through the gills and tube feet.

**<span style="font-family: Arial,Helvetica,sans-serif;">13. Environmental Physiology ** <span style="font-family: Arial,Helvetica,sans-serif;">Similar to many other sea-dwelling invertebrates, sea urchins are osmoconformers, which allow the salt concentration inside and outside of the organism to equilibrate. This is opposite that of marine vertebrates, which are usually osmoregulators that keep the concentration of salts lower within themselves than in the seawater surrounding them.

<span style="font-family: Arial,Helvetica,sans-serif;">Sea urchins are extremely sensitive to shifts in the composition of their surroundings and will show signs of stress, including lack of movement and drooping of spines, even after a small change in the composition of their environment.

**<span style="font-family: Arial,Helvetica,sans-serif;">14. Internal Circulation ** <span style="font-family: Arial,Helvetica,sans-serif;">Nutrients are circulated around sea urchins through the water vascular system that is present throughout a sea urchin’s body. In addition, circulatory fluid circulates inside the coelom, the lage cavity within the sea urchin’s body. This circulatory fluid has phagocytic coelomocytes which are essential in blood clotting. These particles also collect waste products and remove them from the body. Sea urchins' blood circulation goes through the hemal system; however, the blood does not contain any pigments such as hemoglobin that specialize in bringing oxygen molecules to individual cells. Oxygen is transferred across cellular membranes to the sea urchin's cells from its water vascular system and hemal system.

**<span style="font-family: Arial,Helvetica,sans-serif;">15. Chemical Control ** <span style="font-family: Arial,Helvetica,sans-serif;">Even though sea urchins are osmoconformers and cannot control the overall concentration of ions within their extracellular fluid, they are ionic regulators and can control the concentrations of certain selected ions.

**<span style="font-family: Arial,Helvetica,sans-serif;">Review Questions ** <span style="font-family: Arial,Helvetica,sans-serif;">1. What kind of system do sea urchins have to circulate nutrients? <span style="font-family: Arial,Helvetica,sans-serif;">2. Describe a typical habitat of a sea urchin. Why is this advantageous? <span style="font-family: Arial,Helvetica,sans-serif;">3. Describe three in ways that sea urchins protect themselves from predators. <span style="font-family: Arial,Helvetica,sans-serif;">4. Explain why sea urchins are such good indicators of the pollution levels in a body of water. What will likely happen to sea urchins as humans continue to pollute the oceans? <span style="font-family: Arial,Helvetica,sans-serif;">5. What evidence is there that sea urchins may have primitive eyes and how does this evidence link sea urchins with other species?

**<span style="font-family: Arial,Helvetica,sans-serif;">Resources ** <span style="font-family: Arial,Helvetica,sans-serif;">[] <span style="font-family: Arial,Helvetica,sans-serif;">http://www.asnailsodyssey.com/LEARNABOUT/URCHIN/urchRepr.php <span style="font-family: Arial,Helvetica,sans-serif;">[] (1) <span style="font-family: Arial,Helvetica,sans-serif;">http://en.wikipedia.org/wiki/Sea_urchin <span style="font-family: Arial,Helvetica,sans-serif;">http://www.princeton.edu/~achaney/tmve/wiki100k/docs/Sea_urchin.html <span style="font-family: Arial,Helvetica,sans-serif;">http://tolweb.org/treehouses/?treehouse_id=4881 <span style="font-family: Arial,Helvetica,sans-serif;">[] <span style="font-family: Arial,Helvetica,sans-serif;">[] <span style="font-family: Arial,Helvetica,sans-serif;">[] <span style="font-family: Arial,Helvetica,sans-serif;">[] <span style="font-family: Arial,Helvetica,sans-serif;">[|http://blogs.discovermagazine.com/notrocketscience/2011/05/02/sea-urchins-use-their-entire-body-as-an-eye/#.Upv7l8RDssJ] <span style="font-family: Arial,Helvetica,sans-serif;">[] <span style="font-family: Arial,Helvetica,sans-serif;">[] <span style="font-family: Arial,Helvetica,sans-serif;">[] <span style="font-family: Arial,Helvetica,sans-serif;">[] <span style="font-family: Arial,Helvetica,sans-serif;">[] <span style="font-family: Arial,Helvetica,sans-serif;">[] <span style="font-family: Arial,Helvetica,sans-serif;">[] <span style="font-family: Arial,Helvetica,sans-serif;">[] []