Sea Urchin (Echinoderm)By David Goodman
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Classification/Diagnostic Characteristics:
The two main groups of motile echinoderms are the echinozoans (sea urchins and sea cucumbers); and asterozoans (sea stars and brittle stars).
Members of one major extant clade, the crinoids (sea lilies and feather stars), were more abundant and species-rich 300 to 500 million years ago then they are today. There are some 80 described living sea lily species, most of which are sessile organisms attached to a substrate by a stalk. Feather stars grasp the substrate with flexible appendages that allow for limited movement. About 600 living species of feather stars have been described. Unlike the crinoids, most of the other surviving echinoderms are motile. Echinoderms are part of a group called the deuterostomes. Deuterostomes share a pattern of early development in which the mouth forms at the opposite end of the embryo from the blastopore, and the blastopore develops into the anus. Recent phylogenetic analyses of the DNA sequences of many different genes offer strong support for the shared evolutionary relationships of deuterosomes. These pieces of evidence indicate that deuterosomes share a common ancestor not shared with the protosomes. (2)
external image Seaurchin_bw.GIF
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Relationship to Humans:
All deuterosomes have skeletal support features, where present, are internal rather than external. In 2006 the sea urchin genome project showed that there were many similarities between sea urchins and humans. Most commonly the California purple sea urchin is the most similar, of the different species of urchins, to humans. They have similar gene families as well as similar sensory proteins that are found in humans. (3)

Sea urchins are also consumed as food in many places, especially as component of some sushi in Japan. Sea urchins are highly valuable on the food market, being potentially worth $450 per kilogram in Japan. (4)

This happens to be a rare variety of land urchin.
This happens to be a rare variety of land urchin.

Habitat and Niche:
The tube feet of the different echinoderm groups have been modified in a great variety of ways to capture prey. Most sea urchins eat algae, which they catch with their tube feet from the plankton or scrape from rocks with a complex rasping structure. Sea urchins are important grazers on algae in the inter tidal zones of the world's oceans. They live on the rocky seafloor in both shallow and deep depths of water. (5)

Population densities vary by habitat, with a larger concentration of sea urchins appearing in barren areas of the sea floor. (6)

Predator Avoidance:
The sea urchin is commonly eaten by crabs, snails, some birds, fish and even people. They have small feet which allow them to move, and they also have quills which protect them.
In addition to these quills, the sea urchin also has pedicellariae, which are three-jawed pincers on the top of its stalks. In some species, they also contain poison glands. The movable spines in some species are solid, while are hollow in others and [[#|filled]] with toxins and paralyzing neurotransmitters. (7)

Sea urchins will use their teeth and spine to carve out holes in the ocean floor, in which they will hide for protection, often arranging the rocks, shells, and sea weed around the hole for camouflage. (8)

The sea urchin has four types of pedicellariae. Three are smaller and only respond to physical stimuli and ignite a physical response. The fourth type, the globiferous pedicellaria, is significantly larger and three-jawed, having tips that connect to a poison glands that inject its attacker with toxins. (9)

Nutrient Acquisition:
Most of the 2000 species of brittle stars ingest particles from the upper layers of sediments and assimilate the organic material from them, although some species filter suspended food particles from the water, and others capture small animals.

Sea Urchins eat plant and animal matter, including kelp, decaying matter, algae, dead fish, sponges, mussels, and barnacles. (2)

Echinoderms are either filter feeders, which sieve their filters through food particles; substrate eaters, which eat organic matter that accumulates on the bottom of the ocean; or carnivores, which feed on other animals. Members of the clade Crinoidea have a U-shaped gut, or digestive tract, with the mouth and the anus being on the same surface. In other groups, the gut is straight, with the mouth and anus being located on approximately opposite sides of the body. The intestinal tract of the sea urchin, Strongylocentrotus purpuratus, consists of two loops, one being clockwise and the second being counter-clockwise. Both loops of the intestine are equipped with glands that secrete enzymes such as amylase, which catalyzes the breakdown of starch into sugars, and proteinase, which breaks down proteins into smaller polypeptide chains. Although there is no evidence for enzymes that can digest entire algae or agar, the second loop of sea urchin intestines contains agar-digesting bacteria, which are thought to provide the sea urchin with nutrients. (10) (11)

Brief video depicting nutrient acquisition in several types of sea urchins

Reproduction and Life Cycle:
Sea urchin ova release species-specific chemical attractants that increase the motility of conspecific (same-species) sperm and cause them to swim towards the ova. When a sperm reaches an ovum, it must get through two protective layers - a jelly coat and a proteinaceous vitelline envelope - before it comes into contact with the ovum's plasma-membrane. The success of a sperm's assault on the ovum's protective layers depends on the acrosome, a structure at the tip of the sperm head that contains enzymes and other proteins enclosed by a membrane. When the sperm makes contact with the protective layers surrounding the egg, substances in those layers trigger an acrosomal reaction in the sperm that begins with the breakdown of the plasma membrane covering the sperm head. The acrosomal enzymes are then released and digest a hole through the jelly coat. A structure called the acrosomal process, produced by polymerization of the protein actin, then extends out of the head of the sperm. The sea urchin acrosomal process is coated with species-specific recognition molecules known as bindins. Bindins recognize and bind to receptors on the vitelline envelope and on the egg membrane. This final recognition process brings about the slow block to polyspermy and egg activation. Plasma membranes of sperm and ovum fuse, forming a fertilization cone that engulfs the sperm head and brings it into the egg cytoplasm.

Blocks to polyspermy have been studied extensively in sea urchin ova. Within seconds after the sperm and ovum plasma membranes make contact, an influx of sodium ions changes the electric charge difference across the ovum's plasma membrane. This fast block to polyspermy prevents the fusion of any other sperm with the ovum's plasma membrane. The change in membrane charge lasts only about a minute, but that is enough time to allow an additional block to sperm entry to develop. The slow block to polyspermy converts the vitelline envelope into a physical barrier that sperm can't penetrate. Just under the ovum's plasma membrane are vesicles called cortical granules that contain enzymes and other proteins. Sperm entry stimulates the release of calcium ions from the ovum's endoplasmic reticulum; this wave of calcium ions causes the cortical granules to fuse with the plasma membrane and release their contents. Cortical granule enzymes break the bonds between the vitelline envelope and plasma membrane, and cortical granule proteins attract water into the space between the two layers. As a result, the vitelline envelope swells and rises from the surface of the ovum to form a fertilization envelope. Cortical granule enzymes also degrade the sperm-binding molecules on the surface of the fertilization envelope and cause it to harden. All of these mechanisms prevent additional sperm from reaching the surface of the ovum.

Some species can reproduce by regeneration, the development of a complete individual from a piece of an organism. Echinoderms (sea stars), for example, have remarkable abilities to regenerate. If sea stars are cut into pieces, each piece that includes an arm and a portion of the central disc can grow into a new animal.

Growth and Development:
During early development in animals, polarity is specified by an animal pole at the top of the zygote and a vegetal pole at the bottom. This polarity can lead to determination of cell fates at a very early stage of development. Sea urchin embryos can be bisected at the eight-cell stage in two different ways. In the two halves (with four cells each) of these embryos are allowed to develop, the results are dramatically different for the two different cuts: For an embryo cut into a top half and a bottom half, the bottom half develops into a small sea urchin and the top half does not develop at all. For an embryo cut into two side halves, both halves develop normally, though smaller. In sum, the top and bottom halves of an eight-cell sea urchin embryo have already developed distinct fates. These observations led to the model of cytoplasmic segregation. This model states that certain materials, called cytoplasmic determinants, are distributed unequally in the egg cytoplasm. During the early cell divisions of the embryo's development, the progeny cells receive unequal amounts of these determinants. The amount of determinant received determines each cell's fate and its pattern of gene expression.

The sea urchin blastula is a hollow ball of cells only one cell thick. The beginning of gastrulation is marked by a flattening of the vegetal hemisphere as the individual blastomeres change shape. These cells, which are originally rather cuboidal, become wedge-shaped, with smaller outer edges and larger inner edges. As a result, the vegetal pole bulges inward, or invaginates, as if someone were poking a finger into a hollow ball. The investigating cells become endoderm and form a primitive gut called the archenteron. Meanwhile, some cells of the vegetal pole break away from neighboring cells and migrate into the central cavity. These cells become mesenchyme - cells of the middle germ layer. Mesenchymal cells act as independent units, migrating into and among the other tissue layers. Changes in cell shapes cause the intitial invagination of the archenteron, but eventually it is pulled by additional mesenchyme cells that form at the tip of the archenteron and send out extensions called filopodia that adhere to the overlying ectoderm. When the filopodia contract, they pull the archenteron toward the ectoderm at the opposite end of the embryo from where the invagination began. The mouth of the animal forms where the arachenteron makes contact with this overlying ectoderm. The opening created by the invagination of the vegetal pole is called the blastopore; it will become the anus of this deutrosome animal. Only cells from the vegetal pole are capable of bulge in inward to initiate gastrulation, probably because of uneven distribution of regulatory proteins in the egg cytoplasm. As cleavage progresses, the regulatory proteins are localized in different groups of cells. Thereafter, specific sets of genes are activated in different cells, determining their different developmental capacities

Cytokinesis usually begins with a furrowing of the [[#|plasma]] membrane, as if an invisible thread were cinching the cytoplasm between the two nuclei. This contractile ring is composed of microfilaments of actin and myosin, which form a ring on the cytoplasmic surface of the plasma membrane. These two proteins interact to produce a contraction (just as they do in muscles), pinching the cell in two. The microfilaments assemble rapidly from actin monomers that are present in the interphase cytoskeleton. Their assembly is controlled by calcium ions (commonly used in cellular signaling) that are released from storage sites in the center of the cell.

drawings of developmental stages of a sea urchin
drawings of developmental stages of a sea urchin

The above image illustrates the growth of a sea urchin from its stage as an egg to a mature adult. (12)

Adult echinoderms have a system of calcified internal plates covered by thin layers of skin and some muscles. The calcified plates of most echinoderms are thick, and they fuse inside the entire body, forming an internal skeleton. They have hard chalky plates called the test and a spiny hard shell. (13)

Feather stars grasp a substrate with flexible appendages that allow for limited movement. These appendages are tubular, and have a suction that function that aids both in feeding and in locomotion (movement). Sea urchins are able to "walk" in a sense using these feat along rocky areas while their spines provide balance. In some species of sea urchins, the spines act as the principle structure for movement. (14)

These spines are light sensitive and react to shadows causing sea urchins to be nocturnal. To avoid being pushed away by currents, sea urchins often lodge themselves in holes. (15)

Although sea urchins have no brain, they do have a large nerve ring with five attached nerves. These nerves, in turn, branch out into smaller nerve endings. This nerve system allows for movement in reaction to stimuli.

Sensing the Environment:
Although the sea urchin does not have an eye, its entire body serves as its "eye" since the urchin can visually and chemically sense its environment. (16)

Sea Urchins sense the environment through their sense of touch, light, and chemicals. They can feel around their environments, see bright lights as either beneficial or harmful and can sense chemicals in the environment. (16)

Gas Exchange:
One unique feature of adult echinoderms is that they have a water vascular system, a network of water-filled canals leading to extensions called tube feet. This system functions in gas exchange, locomotion, and feeding.

Waste Removal:
Echinoderms do not have kidneys, so they remove their waste using diffusion. Their wastes usually consists of ammonia gas. (17)

Sea urchins have a simple gut. The food passes through their esophagus, suspended with fine membranes, and reaches the hind gut. Nutrients then run up the center vertically to the anus on the middle of the upper surface. (18)

The location for [[#|waste removal]] is at the top of the head. (19)

Environment Physiology (Temperature / Water / Salt Regulation):
Studies have shown that a sea urchins temperature regulation methods can vary between the specific breeds of the organism and the times in which those breeds are reproducing (temperature regulation is very dependent on the embryo). Depending on the geographic habitat the sea urchin is living in, they can adjust their optimal temperature accordingly. (20)

Internal Circulation:
The cytoskeleton contributes to the asymmetrical distribution of cytoplasmic determinants in the egg. An important function of of the microtubules and microfilaments in the cytoskeleton is to help move materials in the cell. Two properties allow these structures to accomplish this: Mictrotubules and microfilaments have polarity - they grow by adding subunits to the plus end. Cytoskeletal elements can bind specific proteins, which can be used in the transport of mRNA. For example, in the sea urchin egg, a protein binds to both the growing (+) end of a microfilament and to an mRNA encoding a cytoplasmic determinant. As the microfilament grows toward one end of the cell, it carries the mRNA along with it. The asymmetrical distribution of the mRNA leads to asymmetrical distribution of the protein it encodes, a regulator of gene transcription.

Chemical Control (i.e. Endocrine System):
There is a thyroid hormone in sea urchin larvae that is supplied by food. However, the function of throxine in their metamorphosis process, like stimuli, receptors, and effectors, are unknown. (21)

Review Questions:
1. Adult echinoderms have a unique vascular system of gas exchange. Explain the system's structure and function.
2. What surround the ovum, and how do sperm bypass these layers?

1. Hillis, David M., David Sadava, H. C. Heller, and Mary V. Price. Principles of Life High School Edition. Sudnerland, MA: Sinauer Associates, 2012. Print.