Echinodermata: Those With the Spiny Skin

Time to examine some echinoderms! This phylum was our last group of marine animals to examine. We made it through algae, annelids, mollusks, and arthropods. Echinoderms are a pretty unusual and beautiful group. Starfish, brittle stars, sea urchins, and sea cucumbers make up this phyla. They’re named after their rather pokey exterior,  coming from the Greek term for “spiny skin.”

One of the identifying characteristics, most clearly seen on some starfish, is pentamerous radial symmetery. This simply means that the body is composed of 5 identical sections, and these sections radiate outward in a circle (like the arms on the clock). Pixar demonstrated this quite well in their creation of the character Peach. See how she has 5 identical arms? Starfish can have more than 5 arms, but you will usually be able to subdivide the body into 5 repeated sections.

They also have a rather interesting system to assist with movement, called the water vascular system.

Basically, water enters their body through the medreporite. From there, it flows into an interior closed system of canals. These canals originate from the central ring canal and radiate out into each of the appendages. The lateral canals connect with the tube feet.

The tube feet are controlled by the pressure within the water vascular system. It’s a bit like filling a water balloon with water. When the balloon is empty, its limp and floppy, but when it’s filled with water, it’s tight and contracted. Protractor muscles control the amount of water pressure (found in the balloon-like structure in the image below). In order to control the motion of each individual tube foot, echinoderms have postural or orienting muscles (located at the top of the foot).  These work to swing the entire foot from one side to another (though still in a straight line). In order to bend the foot retractor muscles are used (found at the bottom of the foot).

In the end, this system allows echinoderms to use their tube feet for locomotion, feeding, and respiration.

Another cool feature is the ossicles that cover their bodies. These are calcareous, bony plates that make up the echinoderms internal skeleton. Depending on the species, their structure can be fused together into a solid “shield-like” structure (called a test) or hinged together to function like joints. On the image of the sea urchin test below, the spiny bumps you see are created by the ossicles.

Okay, now that we’re familiar with the basic characteristics, we can start looking at the individual classes of this interesting phyla.

Class Asteroidea

That’s starfish in plain English! They classically represent the echinoderms. They have a central disc with 5 arms (though they can sometimes have more), tube feet used for movement (usually 2 or 4 rows), and spines. Some features, however, are rather unusual. For example, when starfish feed, they extrude one of their 2 stomaches. The first stomach is pushed out through their mouth, and envelopes their prey (or a portion of the prey) and digestive enzymes are used to break down the food. Once they’re done slurping up their soup of a meal, the stomach is pulled back in and the second stomach continues digesting the meal. Check it out:

Starfish also have  pedicillariae, which are super tiny claw-like structures that cover the outside of the body. Their use is largely a mystery in some species, but for the most part, they have 2 types: cleaning and one for feeding. The image below shows a close-up of a starfish. The little pinkish-white bumps you see are the pedicillariae.

And this is an example of what it looks like up close (though it varies from species to species):

They also have papulae. (Has anyone else noticed that science has waaaaay too many terms that start with “p”? Pedipalps, papulae, pedicillariae, pleopods… By time I’m done with this exam, saying supercalifragilisticexpialidocious backwards will be a cinch!) Papulae are used for gas exchange and to get rid of wastes. They’re little tube-like structures covering echinoderms body surface, increasing surface area. This increased surface area helps expose them to more water at a time, allowing their bodies to absorb oxygen at an increased rate.

We also looked at various stages of starfish larval development under a microscope. This video illustrates the development, showing a flow of each of the stages:

We first examined the gastrula, which looked a lot like this:

You can see at the top of the image how the body has sort of caved inwards. The technical terminology for this is the “invagination of the blastopore.” This particular part of the body will become the anus.

a labeled image fo the gastrula Image from

a labeled image fo the gastrula
Image from

The next stage is the free-living larval stage. At this point, the starfish has started to develop coelomic sacs, oesophagus, mouth, stomach, intestine, and anus.

The large, purple central core is the stomach. At the top of this structure is a tube (oesophagus) and opening (mouth) that leads to the exterior of the body. At the bottom of the stomach is also a tube (intestine) and opening (anus). The little sacs that look like deflated balloons at the top of the stomach are coelomic sacs. These will basically become the starfish’s guts.

After this, the starfish larvae moves into a planktotrophic stage (meaning it lives among and eats algae in the water column). The first of these planktotrophic stages is the bipinnaria.

The darker boarders around the larvae are the ciliated bands. The starfish has also begun to develop stubby little arms (the points on the bottom of the triangular region of the larvae). The round ball that the black arrow is pointing to is the stomach, and the tubes branching off of it make up the digestive system. This next image helps to identify the individual parts. You can see the mouth is made up by loop-like structure at the top that connects to the stomach. The anus is also visible.

From the bipinnaria stage, the starfish develops into the second planktotrophic stage, known as brachiolaria. It’s somewhat difficult to believe that these guys eventually end up turning into starfish.

They start to sprout stringy 3 more stringy appendages. These all develop around a sucker. Here’s a labeled diagram to help orientate you:

With its 3 arms and sucker, the brachiolaria larva is able to attach itself to the substrate. This is where the real magic begins. It starts to break down the larval tissues and undergo metamorphosis. The structures are rearranged and remade (digestive system, arms, tissues, etc.).

And a juvenile starfish is formed…

It’s interesting to note that the brachiolaria larva has bilateral symmetry, meaning that one half is identical to the other half, while the adult starfish has the classic echinoderm radial symmetry. The body plan completely changes during metamorphosis. You can see that the tube feet have begun forming on the arms along the canals of the water vascular system. Overall, the process from egg to reproductive starfish takes about a year, depending on the species and water temperature.

Class Ophiuroidea

Have you ever been to an aquarium and seen these guys?

They look sort of like starfish, but their arms are quite alien looking. They’re brittle sea stars, belonging to the class Ophiuroidea. They’re related to starfish, but slightly different. They have a central disk with 5 distinct arms. These arms, unlike that of starfish, are jointed, giving them a squiggly appearance. They sort of remind me of Medusa’s hair.

Another difference is the location of the madreporite (the spot where water is taken in for the water vascular system) and the presence/absence of abulacral grooves. In starfish, the madreporite is located on the aboral surface (the side opposite the mouth) and they have abulacral grooves.

Brittle sea stars, however, have the madreporite on the oral surface (the same side as their mouth) and do not have abulacral grooves.

Brittle sea stars are also different in the fact that they do not use their tube feet for movement. Instead, their jointed arms and spines can have rapid movement. They also have bursae, areas in the body wall that cave inward, forming a sac.

If you look on either side of the arms, it looks a bit darker and shaded. These are the bursal slits. They’re used in reproduction. The mouth is the star-shaped structure in the center of the body, and is surrounded by skeletal plates. They also have a ophiopluteus larva, meaning they have 4 pairs of extra-long arms with cilia and supported by calcium-based rods.

This video, albeit a bit long, helps to demonstrate how quickly the brittle stars can move. It’s of two brittle stars fighting over a dead shrimp at the bottom of the ocean:

Class Echinoidea

Sea urchins! These spiny guys are rather interesting (and smelly to dissect!). Their body is centralised around a sphere or disk shape (sand dollars are also echinoids), and they have no arms. The bony-like ossicles that make up their body are fused together into a solid test. Like starfish, they have moveable tube feet in order to get around. They can also move their spines to aid in this respect. The spines are attached in a ball-and-socket-like joint, and are flexible but to a point. Just like our arms and legs.

photo 1

Urchins also have pedicillariae. This video gives you a good idea of what they are and what they look like:

For an even better view of the translucent pedicillariae, check out this link: Predators and Defenses. About halfway down the page, there’s a link to a video of the little pedicillariae in a pincer action!

We examined dried and preserved sea urchins, looking for the ambulacral and interambulacral areas. This image gives you a good idea of where those areas are (click on the image to see a larger view):

In the ambulacral zone, you can see the small holes where the tube feet come out.

They also have a rather strange mouth surrounded by 5 pairs of gills (the mouth is the white thing in the center of the body).

photo 2

The unique structure is known as Aristotle’s Lantern, and when removed from the urchin, it looks like this:

We were also given an urchin from the bait shop to dissect. When we did it, we were instructed to use our scissors to cut around the “equator” of the urchin and pull it apart. Of course, my intestinal tract (and everyone elses) ended up quite emaciated as a result.

photo 3

After doing a bit of a YouTube search, this seems like a better method (she also explains the location of the madreporite and anus):

The orangey stuff you see in my photo above is roe, or urchin eggs. It’s also known as “uni” and typically eaten in sushi and Korean food. The black stuff is mainly the guts. Since mine had burst open, I ended up having lots of green specks bursting from the intestine. The specks were algae… it was basically urchin poo. I’m quite sure I won’t forget the smell of urchin guts for quite some time.

This video gives you a good look at the intestines if you’re interested (plus, I’m pretty sure she’s wearing a Mickey Mouse shirt…):

Sea stars have much shorter intestines, since they are feeding upon animal prey. Urchins, on the other hand, eat algae and need the longer digestive system in order to take the time to break down the tough cell walls of their dinner.

Urchin larvae is echinopluteus. They have 3-4 arms with cilia.

This BBC video clip talks about all 3 of the classes we’ve discussed so far (Asteroidea, Ophiuroidea, and Echinoidea) and a bit about how they interact in the ecosystem:

Class Holothuroidea

Sea cucumbers are, in my opinion, some of the oddest creatures on the planet.

They have an elongated body with no arms. Their skeleton is largely reduced, so they have a more fleshy appearance. To feed, they use tentacles that can be found around their mouths. These tentacles are actually modified tube feet. The tube feet that are used for movement run in 5 horizontal rows along the body. Internally, they have “respiratory trees” that are used to collect oxygen. The madreporite (where they take water into the water vascular system) is found floating free in the coelom (body cavity).

If the cucumbers themselves aren’t strange enough, check out the pearl fish:

Class Crinoidea

This class consists of feather stars and sea lilies. Sea lilies are  made up of a stalk (which may have cirri, little appendage-like structures) and a crown (which is a reduced central disk and arms). Feather stars are lacking the stalk portion.

The arms are branched with pinnules, or secondary branches.

The tube feet have a mucus on them and are found on the pinnules. They’re used in filter feeding.

The basal calyx, or the bottom of the flower-like part, is where the mouth is located. This image helps to illustrate that (click the photo for a larger view):

They  have a lecithotrophic vitellaria larval stage. Lecithotrophic means that the larvae have a yolk that they feed upon for nutrients, instead of relying upon plankton. Vitellaria is the group of glands that secrete the yolk.

And that’s it for echinoderms and the end of our Marine Biodiversity labs for the semester! Now it’s time to study the groups of  algae, annelids, mollusks, arthropodas, and echinoderms for our final exam. But first, which phylum was your favorite? Vote in the poll below!

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