Echinodermata Flashcards

Phylum Echinodermata

• Part of Marine Science I Honors.

Classifications of Life

• Kingdom Animalia includes:

  • Phylum Cnidaria

  • Phylum Porifera

  • Phylum Platyhelminthes

  • Phylum Nematoda

  • Phylum Annelida

  • Phylum Arthropoda

  • Phylum Mollusca

  • Phylum Echinodermata

  • Phylum Chordata

Learning Intentions & Success Criteria

• Learning Intention(s):

  • Learn about Phylum Echinodermata and its members.

  • Appreciate the ecological roles and diversity of its members.

• Success Criteria:

  • List and describe the general features and characteristics seen in members of the Echinodermata phylum.

  • Relate Echinodermata form to its functions.

Phylum Overview

• Echinoderms are recognizable by their five-point radial symmetry.

• Include starfish, brittle stars, sea urchins, sand dollars, sea cucumbers, and sea lilies.

• Adult echinoderms are found on the seabed at every ocean depth, from the intertidal zone all the way to the deepest part of the abyssal zone.

• Live in every ocean, including off the coast of Antarctica.

• The phylum contains about 7,000 living species and is the largest entirely marine phylum.

Early Echinoderms

• The first definitive echinoderms appeared near the start of the Cambrian Period (530-480 mya).

• The first fossil of an echinoderm was called an Arkarua and dated to have lived around 538 mya.

• The early echinoderms were sessile but evolved into animals able to move freely.

• These soon developed endoskeleton plates and external ciliary grooves for feeding.

• The Cambrian echinoderms were globular, attached to the substrate, and oriented with their mouths facing upwards.

• Examples of early echinoderms:

  • Protocintus mansillaensis (Class Homostelea, Middle Cambrian of Spain)

  • Ctenoimbricata spinosa (Middle Cambrian of Spain)

  • Ctenocystis utahensis (Class Ctenocystoidea, Middle Cambrian of Utah)

  • Gogia spiralis (Class Eocrinoidea, Middle Cambrian of Utah)

  • Rhenocystis latipedunculata (Class Stylophora, Lower Devonian of Germany)

Echinoderm Morphology

• Typical echinoderms have five-fold symmetry, usually with arms projecting outwards around a centralized axis.

• Even sea cucumbers, which show a more worm-like appearance, usually exhibit this body plan, known as pentameral symmetry.

• Most species have an arm number that is an increment of five—there are even echinoderms that display great numbers of arms, with some sea stars having up to 50.

• However, there are exceptions to this five-fold trend, and it is not uncommon to see a sea star with six or seven arms.

Water Vascular System

• All echinoderms have a unique water vascular system.

• While humans rely on a network of vessels filled with blood, echinoderms possess a complicated series of tubes which uses seawater to convey nutrients and gases throughout their body.

• This system uses water pressure to help give the animal shape and help them travel.

• In most echinoderms, water enters the body through the tube foot grooves and via a plate called the madreporite, which is often visible to the naked eye on the top side of sea stars.

• Sea lilies obtain water through many pores that cover their body.

Endoskeleton

• Echinoderms possess a unique internal skeleton (also known as an endoskeleton) composed of thousands to millions of calcium carbonate components known as ossicles.

• Similar to a complicated 3-D puzzle, these pieces are infused with tissue and covered by an epidermis, or skin like covering.

• Ossicles come in many shapes, which creates the diversity of complicated body types observed in echinoderms.

Ossicle Patterns

• In sea urchins (which includes sand dollars), ossicles are tightly fused into sphere or flattened cake-like shapes with articulated, projecting spines.

• The ossicles can be long and sharp—as you see in sea urchins—or short and hair-like as seen on sand dollars and their relatives).

• The opposite of this is observed in sea cucumbers, which only possess ossicles as small, separate pieces in a soft worm-like body, viewable only with a microscope.

• Groups such as sea stars display skeletons arranged in a fashion intermediate between these two extremes.

External Appendages (Spines)

• Echinoderms possess an unusual suite of appendages on their skeletons, which are used for movement, interaction, and defense.

• In addition to spines, urchins and sea stars both possess pedicellariae (singular pedicellaria) which are jaw-like structures often at the end of stalks.

• Pedicellariae serve in a variety of functions, ranging from defense (with some species bearing venomous pedicellariae) to assisting in food capture in some species of sea stars.

Echinoderm Digestion

• In most echinoderms, there is a relatively straightforward digestive process with food entering the mouth and vacating via the anus.

• Brittle stars and certain groups of sea stars lack an anus, however, and vent food via the mouth once their digestion is complete.

Aboral and Oral Surface of the Sea Star

• Aboral Surface:

  • Eye at the end of each ray.

  • Spines.

  • Anus.

  • Madreporite or sieve plate.

• Oral Surface:

  • Eye at the end of each ray.

  • Tube feet.

  • Stomach.

  • Mouth (underneath the stomach).

  • Ambulacral groove.

Two-Part Digestive System

• Some echinoderms have a two-part digestive system found in certain sea stars, composed of a cardiac stomach, which extends out through the mouth, and the internal pyloric stomach.

• In many sea stars, the pyloric stomach can be easily observed while the animal is feeding, appearing as an amorphous translucent blob.

• The cardiac stomach digests the tissue of captured prey while it is still outside the body. The internal pyloric stomach further digests food and processes nutrients throughout the body.

Echinoderm Nutrition

• Echinoderms exist in all levels of the food chain.

• Some, like the sunflower sea star, are voracious predators that are quick and nimble when in pursuit of prey.

• Other predatory echinoderms eat shellfish like mussels and chitons.

• Sea stars rely on the strength of their arms and tube feet to pry apart the shells of bivalves, like mussels, to allow access for their eversible stomach to devour the tissue within.

Movement

• The water vascular system of echinoderms not only transports materials, it also allows echinoderms to travel.

• On the bottom of many echinoderm’s bodies are potentially thousands of water-filled “tube feet.”

• On the exterior, the feet appear as a fringe of appendages.

• Inside the body, a muscular and flexible bulb is attached to each tube.

• When the bulb is filled with water the tube foot is relaxed.

• To extend the foot the echinoderm forces water from the bulb into the feet which creates pressure.

• A synchronized coordination of all the tube feet allows the echinoderm to move—almost as if it were gliding.

Echinoderm’s Sticky Feet

• Sea stars and urchins walk on their numerous tube feet using more than friction to move about.

• At the end of each tube foot is a set of cells that secrete substances helpful for sticking to the seafloor (or any other surface).

• One cell secretes a glue-like substance that adheres to a surface, which allows the echinoderm to strongly attach to a surface temporarily.

• To detach, the second cell secretes another substance that dissolves the glue.

• As a sea star moves on, the secretions are left behind as a trail of “footprints.”

Lunules

• Movement is integral to survival, but in some cases so is staying put.

• For the lightweight sand dollar, staying in place in turbulent water takes a little ingenuity.

• That is why some sand dollars have visible slits in their body that radiate from their center.

• These slits are called lunules and they help keep the sand dollars from being lifted off the seafloor by wave currents.

Brittle Stars and Feather Stars Movement

• Most brittle stars are unusual in that they propel themselves along the seafloor using their arms in sort of a rowing or dragging movement.

• In the deep sea, there are recorded instances of unusual brittle stars which are able to swim for short periods.

• In contrast, basket stars and snake stars, which live on corals, move very rarely preferring to sit and feed as water currents bring food to them.

• Feather stars generally remain immobile for long periods of time but can periodically move using little “legs” called cirri which keep them attached to the sea bottom.

• Some species of feather stars are capable of using their arms to swim.

• Most feather stars which swim do so only for a short duration.

Sea Cucumbers Movement

• Sea cucumbers tend to crawl along the sea bottom but also can remain in one place for long periods of time.

• Exceptionally, there are some very bizarre sea cucumbers which live in the deep sea and are able to swim, mostly for short periods.

• In one instance there is a true swimming sea cucumber (Pink Sea Cucumber) which appears more like a jellyfish and lives in the water column alongside other swimming animals.

Sight

• Since echinoderms lack well developed nervous systems, it would be easy to assume they are unable to see, but a few special cases show that there are exceptions.

• Many sea stars and sea urchins show evidence of being able to perceive light and dark.

• Some have small, but visible eye spots at the end of each arm.

• Their ability to “see” varies—some species can only detect lights and shadows, while other sea stars likely can make out basic images.

• The spiny sea urchin, Diadema africanum, can make out low resolution images—when approached they can point their spines toward the looming object.

Urchin Anus

• The orange ring at the center is NOT an eye but instead the urchin’s periproctal cone, a structure commonly referred to as the urchin's anus.

Urchin Defenses

• Urchins are well known for their spines, which are used to deter predators and some are venomous.

• Get stabbed by the long spines of a Diadema setosum (long spined urchin) and you will feel a noticeable (but mostly just inconvenient) sting.

• The most toxic urchin is the deceptively unassuming Toxopneustes, or the flower urchin.

• Flower urchins have spines like every other urchin, but they are not as noticeable and they look reminiscent of a flower head, but instead of petals they are covered in claw -like pedicellariae.

• It is the many pedicellariae that are extremely toxic!

Flower Urchin Sting

• Dr. Tsutomu Fujiwara, 1935: “I felt as if the toxin were beginning to move rapidly to the blood vessel from the stung area towards my heart. After a while, I experienced a faint giddiness, difficulty of respiration, paralysis of the lips, tongue and eyelids, relaxation of muscles in the limbs, was hardly able to speak or control my facial expression, and felt almost as if I were going to die. About 15 minutes afterwards, I felt that pains gradually diminish and after about an hour they disappeared completely.”

Sea Star Defenses

• Within their skin is a chemical called saponin, a bitter-tasting compound commonly found in plants. The bitterness deters fish and other hungry predators.

• When sea stars are consumed by an unsuspecting animal (like a dog) it often results in nausea and vomiting.

• Sea stars and sea cucumbers are the only animals besides sea sponges and octocorals known to produce saponin.

• Some sea stars take their chemical defense one step further and produce a highly toxic chemical called tetrodotoxin, which is also found in some pufferfish and the blue-ringed octopus.

• This toxin disrupts nerve signals which can lead to death for those that ingest it.

Special Regenerative Properties

• Echinoderms have a pretty amazing ability—they can lose an appendage and simply grow it back.

• The process, called regeneration, differs from species to species.

• When the body or arm of a sea star or brittle star splits, it initially covers the exposed wound with a protective layer of specialized skin cells.

• In general, regeneration involves the transformation of cells.

• When a sea star is in the process of regeneration it causes its cells to lose their unique characteristics and these cells then migrate toward the growing part of the body.

• When a sea star or brittle star spontaneously splits, it is called fission. This is often a means of asexual reproduction, or reproduction involving only one individual.

• Regeneration can take months in some species, and this is why it is common to see sea stars or brittle stars with only partially grown arms. It is still a mystery how they are able to do this!

The Sea Cucumber’s Unique Defense

• Sea cucumbers don’t have arms, but they are just as skilled when it comes to rebuilding lost body parts.

• Sea cucumbers often lose body parts on purpose.

• They expel their innards, including their respiratory organs, digestive organs, and gonads, as a means to distract an oncoming threat as a defense mechanism!

• They will also do this on a seasonal basis, though scientists are still unclear as to why.

• Once the innards are gone, they are regenerated, albeit more quickly than the arms of the starfish.

Echinoderm Reproduction

• Many echinoderms begin their life floating in the water column.

• After spawning of sperm and eggs by the adult parents, eggs and sperm will meet and combine to form a fertilized egg.

• Initial development creates a tiny larva that then spends the first part of its life swimming in surface waters.

• The larva tends to look very different from their adult version and typically have bilateral symmetry.

• After a time of feeding on plankton, they undergo a metamorphosis, turn into their adult form, and sink to the seafloor.

Brooding Sea Stars

• Some sea stars brood a clutch of eggs either within their body or by sitting atop them.

• In brooding species, the young fertilized eggs develop directly into an adult without a planktonic larval stage.

• These baby sea stars feed off of a supplied yolk and may even eat other eggs in the brood pouch.

• Brooding species often occur in cold environments that are harsh places for larvae.

Echinodermata Species Divisions

• There are two major subdivisions recognized in the Phylum Echinodermata:

  • Eleutherozoa

    • Characterized by being motile (able to move)

    • Includes starfish (1,745 species), brittle stars (2,300 species), sea urchins and sand dollars (900 species), and sea cucumbers (1,430 species)

  • Pelmatozoa

    • Characterized as being sessile (stuck in place) but some are motile

    • Consist of feather stars and sea lilies (580 species) and the extinct blastoids

Sea Stars (Asteroidea)

• Sometimes called starfish, sea stars are the most well-known group of echinoderms.

• There are close to 2,000 species of sea stars.

• Some dwell in the intertidal and are frequently bashed by waves while others live deep in the sea.

• Sea stars are often predators that are both agile and relatively quick.

• The fastest known sea star—a sand star called Luidia ciliaris—can move at speeds up to 3.3 meters per minute.

Brittle Stars (Ophuiroidea)

• A brittle star looks much like a sea star, though its center body is in the shape of a disk and its extending arms are often serpentine.

• Some look like a true star with radiating arms while others, like the basket stars, have branching arms that look like a web of tree roots.

• There are over 2,000 species of brittle stars. Many live in the deep sea.

Sea Cucumbers (Holothuroidea)

• Sea cucumbers come in many colors including black, brown, green, and red. Many are multicolored.

• Unlike most other echinoderms, sea cucumbers have a rotund body shape that looks like a large slug, but despite this unique appearance, they still have the five-fold symmetry found in all echinoderms.

• Unlike their close relatives, sea cucumbers breathe through their anus.

• There are about 1700 sea cucumber species known today.

• Sea cucumbers are a very successful group in the deep sea. It is estimated that they account for over 95 percent of the total weight of animals on the deep-sea floor.

Sea Urchins (Echinoidea)

• Sea urchins and sand dollars are classified within this group, however can be divided even more between urchins with “regular” skeletons and those which are irregular.

• “Regular” urchins show five-part symmetry with a skeleton which is typically round but with long spines that project away from its body.

• Spines can vary in shape, ranging from thick and pencil-like to sharp and needle-shaped.

• Some unusual urchins, such as the helmet urchins (Colobocentrotus) look slightly bizarre compared to most other urchins since their body is covered in a smooth, plated exterior.

Irregular Urchins: the Sand Dollars and Sea Biscuits

• In contrast, “irregular” urchins have undergone an unusual change in their skeleton as an adaptation to living in sand and sediment.

• Although their skeletons have the same five-part symmetry, their bodies have an added bilateral symmetry—that is they have a shape with a forward and back as well as right and left sides, which helps facilitate digging.

• Spines in irregular urchins, such as sand dollars, are present but are so short as to appear as a fuzz on the animal’s surface.

• Some sand dollars have noticeable holes on their bodies, which are called lunules. Lunules assist sand dollars by preventing them from being caught and swept away by currents.

Sea Lilies and Feather Stars (Crinoidea)

• Those that have a stalk are commonly referred to as sea lilies, while those without a stalk are called feather stars.

• The distinction has no bearing on how closely related they are to one another.

• While feather stars are common in shallow water, sea lilies remain at great depth, with the shallowest species living 100 meters down.

• Both have frilled arms with branches called pinnules that give them their feathered appearance.

• Often, they will have more than five arms, and some will have as many as 180.

• All are suspension feeders, meaning they eat drifting particles in the water.

Any Questions?