Download Video Notes: Shape of Life VII – Echinoderms What would the

Video Notes: Shape of Life VII – Echinoderms
What would the ultimate survivor be like? Surely, we would think it is something like ourselves.
Life has been evolving for more than half a billion years, and we think we know its ultimate
achievement. Only we have ability to reflect upon ourselves and our world. But is it correct to
place ourselves at the top of the evolutionary ladder?
Our success as a species is due to a combination of brain and brawn. Our basic body design is
bilaterally symmetrical, with cephalization (having a head). For example, our nostrils, eyes, and
ears are arranged symmetrically on either side of our head, allowing us to have stereo senses.
Our large brain (which consumes about 25% of the energy our body needs every day) processes
stimuli rapidly, and controls a response using powerful muscles that manipulate an intricate
skeleton. Our heart beats 100,000 times per day, and supplies the cells of our body with the
necessary oxygen and food. All large active animals are built according to this basic design.
However, do organisms have to be big, brainy, and fast to be successful? Are other animals that
either barely move or don’t move at all, have radial symmetry and don’t even appear to be
animals, less successful than we are?
While many of these animals appear less impressive, they are very well adapted for the
conditions in which they live. The question of which is better becomes mute, and is replaced by
another question - how do other kinds of animals live and work in their environments? Consider
echinoderms, which are found worldwide in the oceans. Many of these animals bear a
resemblance to plants on land (in fact, many are named after flowers).
To learn about the echinoderm body, consider a sea star – all echinoderms are a variation of the
obvious five-part radial symmetry. If you join the arms of a sea star upward to form a sphere,
you wind up with a sea urchin. Turn the sea urchin on its side and stretch it to make a five-part
tube, you wind up with a sea cucumber. The endoskeleton of a sea star is a latticework made up
of thousands of platelets. Special proteins in the muscles allow them to be locked in place,
something our muscles could never do. A ring of nervous tissue (the nerve ring) coordinates the
movements of the tube feet and the arms (more about tube feet below).
For more than ½ billion years echinoderms have lived successfully, meeting the same basic
requirements for life we face in a way very different from us.
Although relatively small and slow moving, sea urchins can dominate and even destroy
environments. Sea urchins graze on algae, including kelp, and if they become too numerous can
destroy entire kelp forests (these “deforested” areas are called “urchin barrens”). Urchins don’t
have eyes, but do have special sensory tube feet that taste the water for traces of food. Their
regular tube feet are used for anchoring to the bottom and locomotion, and can also snag drifting
bits of kelp for consumption. The mouth is surrounded by a star-shaped feeding apparatus called
the Aristotle’s Lantern, which is so powerful that it can shatter a grain of sand. Kelp is cut into
bite-sized pieces by the Aristotle’s Lantern, and an urchin can consume a lettuce-sized piece
every hour (also, urchins nibble away at the stipes and holdfasts of kelp thalli, weakening them
and making the kelp susceptible to breaking away from the bottom). In this way, an entire kelp
forest can be mowed down in a matter of months by an army of urchins.
Sea cucumbers also have a powerful impact on their world. Sea cucumbers can be very
numerous, and in the deep sea make up the vast majority of animals on the sea floor. They are
deposit feeders; as they slowly lumber over the sea floor, special tube feet that have been
modified into tentacles that surround the mouth shovel sediment into it. Organic materials in the
sediment are digested, inorganic sediments pass through and are ejected out the anus. Legend
has it that all of the sand in the oceans has passed this way at least once. How do these
soft-bodied animals (the endoskeleton is reduced to small, spicule-like hard parts that are
embedded in soft flesh) defend themselves? Some of them simply taste bad, some can eject
sticky filaments out of their anus that expand on contact with water - these contain a potent mix
of poison and glue (these filaments represent a portion of their breathing apparatus and/or guts).
Brittle stars are the smallest and fastest of the echinoderms, and can eject their arms as decoys
when threatened. They often remain in hiding, but can also occur in vast numbers on the sea
floor. They are typically suspension feeders (some are deposit feeders).
Echinoderms have been evolving a unique and successful approach to life for more than 500
million years. Because they live in slow motion, patience and unique methods are required to
study them. Indeed, for a long time scientists have considered them to barely be animals, largely
lacking social behavior. Do echinoderms fight for food and space like other animals? Do they
have any social lives?
Time-lapse photography is starting to reveal how echinoderms (especially sea stars) behave.
Seas stars have complex behaviors, but interact in a slower time frame. They strike bodily poses,
chase one another, etc. They fight for dominance through ritualized wrestling bouts, which start
with the opponents apparently sizing one another up with their arms raised; then they lock arms,
and wrestle for position. It appears that the one that winds up on top and pins its opponent wins,
and enjoys a dominant position in the social hierarchy. If a food source is placed on the bottom,
sea stars will flock to it from a wide area. Most often, it is the first sea star at the food source
that wins the wrestling bouts. As with other animals, it appears that possession confers an
advantage over competitors.
Sea stars are interesting for many reasons. They don’t seem to age; death appears to result from
extreme physical damage or disease. Furthermore, sea stars can survive tremendous amounts of
injury, and have amazing powers of regeneration. For example, from an arm and a portion of the
disk, a whole sea star can grow back.
The typical sea star body has small spines, with “skin gills” for gas exchange located in between.
Sea stars move and feed in unique fashion using their hydraulically powered tube feet. The
water vascular system is inflated by water drawn into the body through the sieve plate, or
madreporite. From a central ring canal, radial canals run into the arms, and ultimately into
muscular bulbs (called ampullae) and the tube feet. Muscles around each bulb can contract,
squeezing water into the associated tube foot below it, and extend the foot outwards. Each tube
foot is also controlled by muscles that can bend it in different directions. Special sensory tube
feet at the ends of the arms can taste the water, and sea stars also have an organ at the tip of each
arm that can sense light and darkness.
Sea stars are fearsome predators, and bivalve mollusks are among their favorite prey. As sea
stars move up into shallow mussel beds from deeper areas, they search for openings in the
mussel beds that would allow them to get a good grip. Once a victim is selected, the sea star
grips the mussel with its tube feet and pulls the valves apart with its powerful arms. Only a small
opening needs to be created, because the sea star’s stomach is everted out through the sea star’s
mouth and slipped into the mussel’s shell. Digestive enzymes are released by the stomach, and
the mussel’s flesh is digested within the shell. The digested nutrients are absorbed by the sea
star’s stomach, which is then withdrawn back through the mouth. Without speed or brains, sea
stars hunt with an efficiency that would be the envy of any lion. They can also feed on the
carcasses of dead animals, and actively compete with one another for food.
Pycnopodia is a giant among sea stars, with more than 20 arms and growing to the size of a
manhole cover. A top predator in the kelp forest community, everything tries to get out of its
way. Pycnopodia dines on everything it can catch; its numerous tube feet overpower its victims
(snails, abalones, and worms, for example). Other tools at its disposal include tiny pincers
(called pedicellariae) that come in various sizes, and that are available for different jobs –
keeping the aboral surface clean, for example. Pedicellariae can also be used to snag prey (e.g.,
worms). Echinoderms certainly show us that there are other ways of being very successful.
How did they evolve in the first place?
Echinoderms first show up in the fossil record during the Cambrian Explosion, with an unusual
body plan that later led to modification and diversity within the group. Recall that there were
three basic kinds of animals prior to the Cambrian Explosion; sponges, cnidarians, and animals
resembling flatworms. The Cambrian Explosion then led to the evolution of large active
animals, a design that would eventually lead to us. Echinoderms are a group that evolved radial
symmetry from bilateral symmetry with cephalization (in other words, this group exemplifies an
evolutionary reversal or loss event – they have essentially lost their heads). The modern crinoids
closely resemble the first echinoderms. Rather than becoming faster and more active, the first
echinoderms opted for a sedentary life as suspension feeders anchored to the ocean floor, using
their tube feet for feeding. Some of these early echinoderms were enormous, and grew to a
height three times that of a Tyrannosaurus rex. With the passage of time, the group then
diversified, and eventually spawned more active animals that moved, grazed, and hunted.
Echinoderms have survived numerous mass extinctions, and have diversified and specialized to
live successfully in a wide array of marine environments. They are winners of the game of
survival, and they do it without moving fast, or having ears, eyes, or even a brain. Echinoderms
demonstrate the futility of asking the question what kind of animal is best; all surviving branches
of evolution represent uniquely successful forms of life.