﻿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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