The Spiny World of Echinoderms! | JONATHAN BIRD'S BLUE WORLD

Don’t get Skewered! Coming up! Jonathan  explores the spiny world of Echinoderms!

Welcome to Jonathan Bird’s Blue World! The Echinoderms are a group of  marine invertebrates with one famous and widely recognized member: the sea  star. The phylum Echinodermata , which contains about 7,000 species, gets its name from the Greek,  literally meaning "spiny skin.

" Most echinoderms, particularly sea stars and sea urchins, actually  do have fairly spiny skin, but of course, there are exceptions. This phylum exists  exclusively in the sea, and cannot be found on land or in fresh water. All echinoderms  have one thing in common: radial symmetry.

This means that the creatures have appendages that  point outward from the center of the body like the spokes on a bicycle wheel. Furthermore, these  appendages usually occur in multiples of five, although there are a few exceptions to that as  well. A north Atlantic common sea star has five arms.

Nearby, it’s cousin, the smooth sun  star has ten. But over in the north Pacific, a giant sun star has 22. Yes I counted them.

So  there are exceptions to the multiples-of-five rule.   The water vascular system is another  trait common to all echinoderms. By examining the underside of a sea star, you can see hundreds  of tiny feet, usually arranged into several rows on each arm.

These are called tube feet, and  are filled with sea water in most echinoderms. By expanding and contracting chambers within the  water vascular system, the echinoderm can force water into certain tube feet to extend them, like  the hydraulic system on a bulldozer! The animal has muscles in the tube feet, used to retract  them.

By expanding and retracting the right tube feet in the proper order, the animal can walk.  Many echinoderms can also form suckers on the ends of their tube feet, used to capture and hold  prey, or to hold onto rocks in a swift current. The phylum is broken up into three groups:  the sub-phylum Asterozoa, the Class Crinoidea, and the Sub-phylum Echinozoa.

The Asterozoa  are (as you might imagine) the star-shaped animals. Since they aren’t actually fish, the  correct term for these animals is sea stars, not starfish. Within this group, there are two  classes, the Asteroids and the Ophiuroids.

The Asteroids are the sea stars that have thick  arms and no recognizable central body. The Ophiuroids have thin arms and a distinguishable  central body part. Ophiuroids include the brittle stars and basket stars.

Brittle stars and basket  stars generally use their fine arms to catch plankton from the water, rather than hunt larger  prey like Asteroids generally do. Most tropical basket stars are nocturnal. They only come out  at night when the plankton is most abundant.

They are sensitive to light and hide the  minute I light them up with my camera. In California, immense beds of brittle stars  stick their arms into the current for food. This is a landscape dominated by brittle  stars!

The Crinoids, known as feather stars, are distant relatives of the sea stars. Like the  Ophiuroids, their arms are feathery, which makes them even better than brittle stars at catching  plankton, like a living net. Feather stars are quite common in the tropics, where they tend to  find a good place to perch and catch plankton.

The food is moved to the central mouth by cilia.   They’re also well represented in the deep sea, where they tend to have stalks to allow them  to get higher up off a flat sea floor. They go back almost 500 million years in the  fossil record, so crinoids have been around a long time.

The last sub-phylum is the  Echinozoa. This group includes two classes, the echinoids and the holothuroids. They  are related through common evolution, but they are very different.

The Echinoids, which  have the spiniest skin of all the Echinoderms, includes the sea urchins and their relatives, the  heart urchins and even the sand dollars, which are basically flattened sea urchins with super short  spines. Sea urchins also use their tube feet for locomotion like sea stars. They have a mouth on  the underside with teeth for chomping on algae.

And when the food supply is good, the urchins  multiply. Having all those sharp spines makes a good defense, not only for the urchin itself,  but for animals that can fit between the spines. Urchins often have fish, shrimp and other  small animals living with them for safety.

The class Holothuroidea is composed of creatures  called sea cucumbers. A sea cucumber is so named due to the fact that many members of this group  resemble the garden variety of cucumber. They also don’t really have spiny skin, so they are the  exception to the Echinoderm “spiny skin” rule.

But other anatomical features make it  obvious that sea cucumbers are clearly related to the other echinoderms.  For example, they have tube feet. Sea cucumbers are often somewhat football-shaped  and lay on their side on the bottom.

They have 5 rows of tube feet running lengthwise, like the  seams on that football. At one end of the animal is a mouth surrounded by tentacles. These  tentacles are also tube feet--specially-developed tube feet--used for feeding.

Some sea cucumbers  use them to feed on plankton. The tentacles collect food from passing water. Then, the  cucumber sticks each tentacle in its mouth, one at a time, and licks them off.

Mmm  mm, yummy plankton! Other kinds of sea cucumbers use the tentacles to sift through  sand on the bottom for particles of food. You might think that an animal like this would  be defenseless.

It has no spines like a sea urchin. It doesn’t even have tough spiny skin. It  just looks like a soft, squishy bite-sized meal.

But when threatened, some sea cucumbers can eject  a sticky disgusting goop called Cuvierian tubules to ensnare predators. Others can expel poisonous  chemicals to drive predators away. As a result very few animals will eat them.

And due to  that general lack of predators, sea cucumbers also make a safe home for symbiotic partners. An  emperor shrimp lives in safety on a sea cucumber, and in exchange for the protection,  keeps its host free of parasites. The echinoderms are a substantial  group of invertebrates, pretty easy to recognize, and only found  in seawater.

They aren’t particularly flashy but they are an important part of ocean  ecosystems. I see echinoderms on almost every dive from the tropics to Antarctica. They are  super fascinating animals of the Blue World!

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