Echinoderms Part Two

Echinoderm Environment:

Echinoderms are the largest phylum with no freshwater or terrestrial forms. Echinoderm environments must be marine, as in saltwater, for the echinoderm to survive. Within marine environments, the conditions echinoderms live in can vary greatly. Environments range in water temperature, water depth, water movement and the different organisms surrounding the echinoderms. Water temperatures can range from arctic temperatures to tropical temperatures. Water depth and movement somewhat tie in together in the idea that the more shallow the water, the more likely the water will be moving. Deeper water is sometimes less affected by rain and storms and therefore does not affect the echinoderms as much. Water movement could impact the echinoderms by moving them or destroying their habitats. Depending on the species the water depth and movement will vary. Echinoderms are generally found in shallow water near shores or in reef environments but can also live in great depths of water. Almost all echinoderms are benthic, meaning they live on the seafloor, but some sea lilies can swim for some distance while some sea cucumbers float throughout the water (Xiaofeng, Hagdorn, & Chuanshang, 2006). Below is a photo of a tank for marine animals. This tank contains brittle stars, sea cucumbers, and sea urchins along with non-echinoderm animals. This marine tank is kept in conditions to reflect the echinoderm’s natural habitat. The tank is salt water and is kept at 78°F. In the tank are rocks, plants and other things to help give shelter and protection to the animals.

picture 1

This picture is of a tank in a classroom at Ohio State University. In this tank there is a species of brittle stars, sea urchins and sea cucumbers. There are also other animals such as a shrimp and some small fish. Not to mention the plants and sponges present.

Brittle stars (Ophio coma) lead a cryptic lifestyle. They avoid predation by spending much of their day hiding within holes in coral and rocks found in their environment (Zubi, 2015). In the picture shown below, a brittle star is almost completely concealed under a sponge. Likewise, sea urchins’ main defense from predators is creating a burrow in the ground to leave less of their body exposed to predation. When sea urchins are found in environments not conducive to burrowing, like the marine tank shown above, they have been know to cover themselves with objects found in the environment for protection (Carefoot, 2010). The sea urchin shown below is exhibiting this trait, covering itself with shells from other organisms.

picture 2picture 3

The photo on the left is of a brittle star hiding out under a sponge in the tank at Ohio State. The photo on the right is of a sea urchin in the same tank carrying around a shell of another organism.



Though many Echinoderm species undergo sexual reproduction, a few species reproduce asexually. The bulk of echinoderm species are diecious, meaning that there are male and female individuals. The type of reproduction that an echinoderm goes through depends on the species and the environment (Mulcrone, 2005).

Sexual Reproduction:

Sexual reproduction refers to the combination of genetic material from two separate individuals, of different sexes, resulting in a new organism. The males provide the sperm and the females provide the eggs for reproduction. The sperm and eggs are propagated into the water column, and fertilization generally happens in the open water. Certain species of sea stars, brittle stars, and sea cucumbers go through internal fertilization. This is a recent observation and little information is known about this phenomenon. The release of the eggs and sperm is synchronized in two different ways. Some species have a breeding season where individuals congregate to increase the chances of fertilization. Some species’ breeding season may be affected by the lunar cycles. Commonly, echinoderms will leave their eggs and provide no parental care to the eggs (Young & Eckelbarger, 1994). Some species such as the Slender Sea Star (Leptasterias tenera) brood their eggs. This means that they carry their eggs with them until they are ready to hatch. This species of starfish carries the eggs in their stomach but other echinoderms carry the eggs in places such as in sacks under their arms, their coelom (main body cavity containing the organs) or in special egg cavities (Hendler & Franz, 1982).



When sexual reproduction occurs, the resulting eggs go through either direct or indirect development. When indirect development occurs, the fertilized eggs of echinoderms will develop into larvae known as planktonic larvae. In most cases, this stage occurs when the fertilized egg consists of a lower yolk volume.  The resulting larvae ends up assimilating into the surrounding plankton community. They will start feeding on numerous smaller organisms in an effort to meet the energy requirements needed to perform metamorphosis, and achieve their juvenile stage of their adult form. After a period ranging from one to three weeks, these bilateral symmetric larvae begin their metamorphosis, during which they will develop five water vascular canals that will eventually give them their characteristic pentaradial symmetry.

In contrast, during direct development, fertilized eggs will contain a larger yolk volume. Due to this, they are able to be sustain their life cycle development solely on their yolk, saving them from expending energy feeding, and also allowing them to bypass the larval stage of development. In this developmental strategy, often times the female parent will be involved in rearing the juveniles.  In these instances, the female will hold the young juveniles near its mouth, on the underside of its body, or even in specially developed pouches that are found on the upper side of the body. This allows the female to provide predation protection to its young until they fully developed. (Pawson, 2014)


Asexual Reproduction:

Echinoderms are capable of performing asexual reproduction, and the most common way they achieve this is through division of body parts or body sections, which is called fragmentation.  What happens in this reproductive strategy is that an echinoderm may deliberately or accidently detach a limb or body section. This may happen due to possible predation, an unforeseen accident, or in some cases with species of starfish, it separates itself by pulling opposing limbs in opposite directions until the individual splits. (Pawson/Encyclopedia Britannica, 2014)


The reason this works without permanently harming the individual is because of a unique process in which they are able to seal areas of exposed tissue where a section has been detached, and then immediately it begins regenerating new tissue to replace that section. Not all classes accomplish this strategy in the same manner, as multiple solutions seems to have arisen within each class. In species of starfish and brittle stars, this process requires that part of the central disk be included with each separate segment in order to regenerate into a complete individual. In species of sea cucumber, not only do they seal off the “wound” from where they split in half, the resulting two regenerating parts have to go through a tissue reorganization in order to successfully complete the process. (Speer and Waggoner 1999) In sea lilly and other starfish species, they can simply detach a limb or body section regardless of what part is present and either regenerate the missing limb, or in the limbs case, regenerate an entire new body.  While impressive, this asexual reproductive method has shown itself to be pesky for habitat management teams, as complete removal of sea stars and sea lilies can prove to be very difficult.  (Pawson/Encyclopedia Britannica, 2014)  


Echinoderms throughout evolutionary time:


The first true echinoderms appear in the fossil record in the Cambrian period, which began about 560 million years ago. During the pre-Cambrian period, echinoderm ancestors are not believed to have had radial symmetry (Zamora, 2012). Echinoderm ancestors in the pre-Cambrian are also thought to have been soft-bodied with unmineralized plates, which contrasts with all true echinoderms, starting in the Cambrian, who have hard mineralized skeletons which provide structure and protection (Speer, 1999). The earliest echinoderms are thought to have been deposit feeders, which intake and extract nutrients from the sediment on the floor of the ocean. This varies from the current method of feeding found in echinoderms today, suspension feeding, in which echinoderms eat particles found suspending in their environment (Steele). The first echinoderms in the Cambrian period had both radiate and asymmetric forms and a similar water vascular system to those found today, including one coelom (Zamora, 2012).


During the mid-Cambrian, the species richness, or the number of species, increased by fifteen percent. In the Ordovician period, starting around 510 million years ago, the earliest echinozoans, starfish, and brittle stars appear (Baumiller, 2015). After the Great Ordovician Biodiversification Event (GOBE), suspension-feeding organisms, like the echinoderms dominated ocean floor habitats. Three echinoderm classes from the Cambrian went extinct during this period, six classes survived from the Cambrian, and eleven new classes appeared after the GOBE. No new classes of echinoderms have emerged since the GOBE (Baumiller, 2015).


Since the Ordovian period, the number of classes has steadily declined. Though the classes of sea lilies and blastoids dominated the echinoderm phylum through most of the Paleozoic period, by the end of the Permian period, which ended 250 million years ago, all of the blastoids and almost all of the sea lilies were extinct. From the Mesozoic period, which directly followed the Permian period, to today, there have been only five classes of echinoderms, the Sea Lilies (Crinoidea), Starfish (Asteroidea), Brittle stars (Ophiuroidea), Sea Urchin (Echinoidea), and Sea Cucumbers (Holothuroidea) (Baumiller, 2015).




With 7,000 living species, echinoderms are very diverse (Mulcrone, 2005). Echinoderms vary in physiology and morphology such as shape, size (width and length), color and locomotion. Amongst the five main classes of echinoderms, probably the most obvious difference is the shape of the body. For example sea stars have a star looking shape while sea urchins are spherical, sea biscuits and sand dollars are round, and sea cucumbers are longer and more caterpillar-like. Below is a photo, from left to right, of a brittle star skeleton, sea cucumber and sea urchins remains, followed by live specimens of sea cucumbers, sea urchins, sea biscuits, and sand dollars. The diversity in the body shape can clearly be seen.

sea cucumber


urchin1 urchin2 urchin3 urchin4

In the photo above you can not only see the diversity in body shape but also the color and size. The brittle star in the photo is an Orphiarachnella ramsayi and was found on a reef in Sydney Australia. These brittle stars can range in colors from black and white to having some pink, red and green as well (O’Hara, 2015). Compared to the sea cucumber that is bright orange, the color obviously varies. The size between these three specimens can range from 1-10 inches. Diversity in echinoderms is not only between the five classes but between the species in each class. For example sea stars vary in size, color and number of appendages. Below is a photo of three very different sea stars.


This photo shows the variance in size, color, and number of limbs that can be on just sea stars alone. The starfish on the left was about seven or eight inches across while the starfish on the right was less than an inch.  The color variation is also shown.

blue starfish

Shown above is a blue sea star, Linckia laevigata.


As seen in the photo the sea star on the left is larger and has only five appendages while the sea star on the right is much smaller and has six appendages. The sunflower starfish (Pycnopodia helianthoides) can have between 16 and 24 arms and can be up to a meter in diameter (Sustainability Species Identification). This just shows the diversity of appendages in sea stars. The sea cucumber and sand dollar have no appendages at all. One other notable difference that has occurred between echinoderms is their locomotion, or how they move. For example sea stars move by using their tube feet to attach to the ground and sort of walk. Brittle stars on the other hand use their appendages to push themselves from place to place. Finally, sea cucumbers can pull themselves along somewhat like a snail and even some species can swim (Mah, 2012). Sea urchins, primarily move using their tube feet, however, they can also use their spines for fast movement, as shown in the video below. The next two videos show a brittle star, a chocolate chip sea star, and a sea urchin moving.



This video shows how a brittle star uses its limbs, as opposed to tube feet, to move around the tank.


This is a timelapse of a starfish moving around on top of an oyster.

This video shows a long-spined sea urchin, Diadema setosum, moving using its spines. Sea urchins primary mode of locomotion is their tube feet, but they occasionally use their spines for movement.

This is just a short overview of the diversity of echinoderms that has developed over millions of years.


Human impact:


Human beings as we continue to grow in numbers have a lasting impact on many different animal species, however, it is becoming more evident of our impact on the oceans and the animal species that call the oceans home. This section will specifically cover the effects humans have on echinoderms but many of these effects have a cascading effect on many different animals.


The fishing industry has a large impact on the survival of echinoderm species. The fishing industry removes substantial amounts of the echinoderms’ diet from the oceans including, clams, mussels, and oysters. This reduces the food available to the echinoderms. The pet trade also has several direct effects on many species of echinoderms. A big part of the pet trade industry involves collecting corals and live rock for sale in many countries all over the world. The coral and live rock make up the habitats of numerous echinoderms and removing them effectively leaves the echinoderms unable to protect themselves. Also, humans pull all echinoderms (starfish, sea urchins, sea cucumbers, sea lilies and brittle stars) out of their natural habitat to be directly sold in the pet trade. Both of these problems affect the number of echinoderms in the wild. Taking rock from reefs removes a lot of habitat that these animals rely on, and removing them from areas completely diminishes the number of breeding animals.


Humans play a large role in the habitat destruction affecting many echinoderm species. Human have a large carbon footprint, and as we are releasing large amounts of carbon dioxide into the atmosphere, the pH of the oceans is slowly rising. This slight increase in pH is not detrimental to adults in all species currently, but it has a significant impact on the more sensitive species of echinoderms. Also, these environmental changes do affect all echinoderms during their developmental stage when they are the most susceptible to environmental changes. This effect can be severe; a change of just 0.2 units of pH leads to a 100% mortality rate in 8 day old common brittle stars (Ophiothrix fragilis) (Dupoint, 2009).



Echinoderms are found all over the world and play a major role in the animal kingdom and the environment. They help to keep algae growth down, feed other animals, and feed people in certain countries. People need to understand the importance of echinoderms. If too many are captured or we keep destroying their habitats, it could affect more than just the echinoderm’s future. As a keystone species in many ecosystems, the loss or elimination of species of Echinoderms could drastically and permanently affect numerous marine habitats around the world, as well . They are a diverse family, with 7,000 currently known species of echinoderms ranging from sea stars to sea cucumbers, and with the scientific community making new discoveries using ever developing technology, we could see many more species discovered in our lifetime.


Work Cited:


Baumiller, T. (2015). Fossil record of Echinoderms. Retrieved from


Carefoot, T. (2010). Learn About Sea Urchins: Predator and Defenses. Retrieved from


Dupoint, S., & Thorndyke, M. (2009). Impact of CO2 -driven ocean acidification on invertebrates early life-history – What we know, what we need to know and what we can do.Biogeosciences Discuss, 6, 3109-3131. accessed 29 March, 2015.


Echinodermata. (n.d.). Retrieved from


Echinoderms. (n.d.). Retrieved from


Hendler, G and Franz, D. (June, 1982). Washington, DC: Smithsonian Institution and New York, NY: Brooklyn College.


Mah, C. (2012, Sept. 18). Deep-Sea Swimming Sea Cucumbers and the “most bizarre holothurian species in existence”! Retrieved from


Mulcrone, R. (2005). Echinodermata (sea stars, sea urchins, sea cucumbers, and relatives). Retrieved from


Pawson, D. (2014, July 24). Echinoderm. Retrieved from


O’Hara, T. (2011). Brittle Star. Retrieved from


Speer, B., & Waggoner, B. (1999, September 15). Echinodermata: Fossil Record. Retrieved from


Speer, B., & Waggoner, B. (1999, September 15). Echinodermata: Life History and Ecology. Retrieved from


Steele, M. (n.d.). Ecology of Soft-Sediments. Retrieved from ecology.pdf


Sustainability Species Identification. (n.d.) Sunflower sea star. Retrieved from


Xiaofeng, W., Hagdorn, H. and Chuanshang, W. (2006). Pseudoplanktonic lifestyle of the Triassic crinoid Traumatocrinus from Southwest China. Retrieved from;jsessionid=9342400ED743C5DFC4E653363D1CDAF0.f02t02


Young, Craig M.; Eckelbarger, Kevin J. (1994). Reproduction, Larval Biology, and Recruitment of the Deep-Sea Benthos. Columbia University Press.


Zamora, S., Rahman, I., & Smith, A. (2012). Plated Cambrian Bilaterians Reveal the Earliest Stages of Echinoderm Evolution. PLoS ONE, 7(6).


Zubi, T. (2015). Invertebrates: Echinoderms. Retrieved from




Introduction: What are Echinoderms?

Echinoderms are a group of marine animals consisting of well known organisms such as the starfish, sea cucumber and the sand dollar. The phylum Echinodermata consists of about 7000 living species and the phylum is divided into five smaller classes. Echinodermata is Greek for “spiny skinned.” This is clearly seen on echinoderms such as the brittle star and the sea urchin. The most well-known echinoderms are the species of five-armed sea stars. However, other sea stars species have been found to have up to 40 arms (National Geographic). Many species of echinoderms also have unique features in their bodies which allow them to regenerate a lost limb, spine, or even intestine if it is lost, for example, to predation (Mashanov, 2014). Some echinoderms can regenerate a whole new body from a severed arm (National Geographic). This process has important consequences for scientist studying regeneration in vertebrates, like humans (Mashanov, 2014). Echinoderms are very important in both the environment and to people as well. Sometimes these effects by the echinoderms can be positive or negative. Without echinoderms, many areas of the ocean would be greatly affected and therefore, echinoderms are an important animal phylum to learn about.


In the beginning:

It is estimated that there are up to 13,000 extinct species of echinoderms and that the very first echinoderm was alive in the Lower Cambrian period. This period of time would range from 490-540 million years ago. The oldest fossil available is called Arkarua. This species was small, round and disc-like with five grooves extending from the center (Echinoderm Fossils).  The first echinoderm was thought to be very simple (Knott, 2004). The organism was motile and bilateral in symmetry. Bilateral symmetry means the organism can be cut right down the middle and be split into two equal halves. The echinoderm ancestry later developed radial symmetry as it was thought to be more advantageous to the species. The bilateral symmetry can still be seen in the larvae of echinoderms but once they reach adulthood, they develop radial symmetry. The first picture below shows an echinoderm larvae and the bilateral symmetry is clearly shown. The concept of radial symmetry is clearly illustrated in starfish including the Horned starfish (Protoreaster nodosus), shown below. Species of starfish, like the common starfish, have five radially symmetrical projections projecting from a central disk. These feet have symmetrical outer and inner structures (Zubi, 2013).

Bilateral Symmetry in Starfish Larvae

Martin_starfish larvae_photo_1 

This picture represents the bilateral symmetry of the echinoderm larvae. The red line dissects down the middle and divides the larvae into two equal halves. Throughout development the bilateral symmetry is lost and becomes radial symmetry.

Radial Symmetry in an adult Starfish


This picture clearly shows the radial symmetry of starfish. Specifically this starfish has pentaradial symmetry.


  • Phylogeny

The extant echinoderms are divided into five clades including the Sea Lilies (Crinoidea), Starfish (Asteroidea), Brittle Stars (Ophiuroidea), Sea Urchins (Echinoidea), and Sea Cucumbers (Holothuroidea). Out of these it is clear that they form a monophyletic group, however there is doubt as to their phylogenetic relationship within the tree itself. This debate is based on whether Brittle Stars (Ophiuroidea) and Starfish (Asteroidea) form a sister clade, i.e. they are each others closest relative, or not (Wray, 1999). Today there are only really two well supported hypotheses those are as follows:

1. Asterozoan Hypothesis: In this hypothesis it is believed that Brittle Stars and Starfish form a sister clade, and just like in the Cryptosyringid hypothesis Sea Urchins and Sea Cucumbers form another sister clade and Sea Lilies is the most basal group. This hypothesis is based off of molecular phylogenetic studies which help to show that even though Brittle Stars has a pluteus-type larva which is the larval form of both Sea Urchins and Sea Cucumbers this could just be a result of convergent evolution or that Starfish reverted to an older form of larval form (Telford, 2014).

Asterozoan Hypothesis


2. Cryptosyringid Hypothesis: Similar to the previous hypothesis, Sea Lilies is the most basal group, however in this hypothesis Brittle Stars and Starfish do not form a sister clade. This hypothesis has support in the development of the organism so that Brittle Stars are sister to Sea Urchins and Sea Cucumbers. This is because they all share a common larval state during early development which could imply that Brittle Stars are more closely related to the sister group containing Sea Urchins and  Sea Cucumbers than Starfish (Telford, 2014).

Cryptosyringid Hypothesis


Now that their placement among themselves is better understood, where do Echinoderms in general fit in with other animals and other organisms? Echinoderms fit in the superphylum deuterostomes of which composes animals who during development the anus forms first unlike the protostomes which have mouth first development. Humans also fall into this superphylum whereas snails and insects develop mouth first. they are within the supergroup unikonts which is also composed of many animals.


The above figure represents the phylogenetic tree of the Echinodermata back to the supergroup Unikonts (Keeling, 2009). The associated divergence dates, or estimated time periods a group split from a common ancestor, are included above in millions of years (MYA) (Hedges, 2006).


  • Fossil record and molecular clock

The oldest echinoderms found to date are from the Cambrian period. This period was about 540 million years ago. Some fossils have been found that may be an ancient echinoderm, but there is no definite proof at the moment. The ancient phyla of echinoderms was divided into classes based on body geometry, type of plating, body symmetry and the absence or presence of appendages. Three basic body plans emerged during the Cambrian echinoderms (Scripps Institution of Oceanography, 2011).

  •   Ctenocystoids: with or without appendages, tessellate plate type and a lateralized and symmetrical/asymmetrical body plan.
  • Helicoplacoidea: no appendages, imbricate type plates, ellipsoidal shaped body and helical symmetry.
  • Edrioasteroid: no appendages, tessellate and imbricate plate type,  disc shaped body and pentaradial symmetry.

From the middle of the Cambrian period to the mid to late Ordovician period, the class diversification of the echinoderms occurred twice. According to the fossil record, the diversification decreased at the end of the Cambrian period but this may be due to the lack of artifact preservation. No diversification is more significant than the time known as the Great Ordovician Biodiversification Event (GOBE). The class level during this period was as high as 21. From the Cambrian period to the Ordovician period, eleven new classes originated. Since this peak of diversification, the amount of class diversity gradually decreased. Eventually the amount of classes decreased to eight. With the Blastoids, Ophiocistiods and Isorophid edrioasteroids going extinct in the Permian period, there were only five classes that survived the Mesozoic. These five classes are the same classes that are around today, including, Starfish (Asteroidia), Sea Lilies (Crinoidea), Sea Urchins and Sand Dollars (Echinoidia), Sea Cucumbers (Holothuroidea), and Brittle Stars (Ophiuroidea)(Fossil record of Echinoderms).


Key evolutionary innovations:

Echinoderms developed many key evolutionary characteristics that define all species within the phylum, making them one of the most unique animal phyla.  Four major synapomorphies are identifiable within all species of the Echinoderms that distinguish all members of the phylum. A synapomorphy are traits or characters recognized specifically with that species.


Radial Symmetry: Unlike chordates, like humans or sharks, echinoderms possess a radially symmetrical body plan. In almost all situations involving echinoderms, the species exhibits pentamerous radial symmetry (pentaradial), or five sided radial symmetry.  What this means is that observed head on, an observer will be able to distinguish five separate, interconnected segments that are all similar in shape, appearance, and anatomy (Morris, 2009). The best group of animals to show this radial symmetry are the starfish.


Water Vascular System: In Echinoderms, the water vascular system is their key to everyday living.  It provides Echinoderms with many functions, including gas exchange, locomotion, feeding, and respiration.  The system allows sea-water to be facilitated through an external pore located on the upper portion of the organism called a madreporite, which acts as like a filtered water pump to bring in and excrete water. This system also provides Echinoderms their locomotion through specialized tube feet.  Tube feet provide locomotion for most Echinoderms by expanding and retracting from an individual when water is pushed into or syphoned out of these structures, allowing them to move within their environment to hunt for food and locate shelter. These tube feet also provide Echinoderms with their primary sensory perception as they possess numerous nerve endings, giving them a “view” of their surrounding environment (Class Notes, Knott, 2014). One species which takes advantage of tube feet locomotion is the pincushion sea urchin (Lytechinus variegatus). They posses many tube feet which provide them with sensory information about their environment and assist with locomotion. Below is a video of the starfish using its tubed feet to walk along the tank.

 Sea Urchin Tubed Feet

 This video shows how the Sea Urchin uses its tubed feet to attach to the wall of an aquarium. They suction cup onto the glass for attachment and movement. 

Mesodermal Skeleton: Echinoderm’s skeleton is unique to the animal kingdom.  It is made up of many tiny plates or spines called ossicles, which are comprised of calcium carbonate. In a typical animal, this would lead to the organism having a heavy skeleton, but in the case of Echinoderms, they remain light through a sponge like material called stereom.  Instead of having a rigid skeleton, the stereom is porous, being comprised of a network of calcium crystals that give an echinoderm its shape and rigidity without carrying extra mass (Manton, 2014). Below is a photo of an exposed skeleton of the common starfish (Asterias rubens).


This is a photograph of an exposed skeleton of a starfish, as indicated by the arrow. The network of porous ossicles is evident in this structure.


Mutable Collagenous Tissue: Echinoderms possess special type of tissue that in effect can very rapidly change from a rigid state to a free moving, or loose, state using its nervous system. These tissues are key to connecting ossicles together as ligaments made up of primarily collagen.  This allows Echinoderms to achieve a wide variety of body positions with very minimal, to no muscular effort, and then instantly lock into place. This provides a unique feeding advantage as well, as in the case of sea stars where they can envelop a selected prey species in a loose tissue state, and then incapacitate them by quickly changing to a rigid state (Knott, 2004).




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 Echinoderm Fossils. Recieved from

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