Eccentric Sand Dollar Notes: Dendraster excentricus
Background Information:
 http://www.wallawalla.edu/academics/departments/biology/rosario/inverts/Echinodermata/Clas
s%20Echinoidea/Dendraster_excentricus.html
 http://en.wikipedia.org/wiki/Dendraster_excentricus
NOTES:
If students stop paying attention as intently or begin talking let them know that their will be a test at the
end of class, it will be sort-of hard, and each student will have to take their own. This is a play on
words as they will learn later a test is the technically accurate term for a sand dollars “shell” and each
student will be taking it back to their seat. However this humorous point does get students attention
instantly until they realize the test isn't over the content.
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Sand Dollars are members of the phylum Echinodermata. This name means spiny (hedgehog)
skin.
Show echinoderm display box.
◦ Background Information: http://en.wikipedia.org/wiki/Echinoderm
Distribute Sand Dollars
◦ Look at spines
▪ Long spines on bottom (ventral) side
▪ Short spines on top (dorsal / Arboral) side
◦ Extend wrist and place sand dollar on wrist ventral (flat) side down. Wait.
▪ As the sand dollar warms up it will begin moving faster attempting to get away from the
heat and tickling the soft skin on the wrist. Flip sand dollar back over to see if the
spines are now moving faster.
◦ Observe Ventral side:
▪ The center is the mouth
▪ the lines leading to the mouth are called feeding tracks.
 Eccentric sand dollars filter feed / scavenge food from the water column. While
standing on edge they capture food with their spines as it floats by and pass it along
the feeding tracks using pedicillaria (microscopic pincers), tube feet, and spines.
Once the food reaches the mouth it is passed into the animal and ground up in the
tooth-like structure of the aristotles lantern.
◦ Observe the Arboral side:
▪ Petalidium:
 On the top (curved) side of the sand dollar one of the most prominent features is the
flower shape. This is called a petalidium. There are five flower petal-like lobes to
the petalidum. These are outlined by small slits that pass all the way into the softer
interrior of the sand dollar and soft tissue extends from these slits when underwater.
These tentacle-like extensions are skin gills. As with other echinoderms sand dollars
absorb oxygen directly through the soft skin gills in their backs.
 The sand dollar has two shades to it's spines. Approximately 1/3 of the animal is
light colored and the rest is a darker shade. When eccentric sand dollars are
underwater they “stand” on edge with the light part of the body anchored in the sand.
Several things are worth noting regarding this behavior:
◦ Have students place sand dollar between fingers standing on edge with the light
side down as if the sand dollar were under water and the fingers represent the
sand layer covering only the light part of the sand dollar. (the dark color likely
comes from algaes and other components of the water.
◦ It allows the sand dollar to turn in the current. When acres of sand dollars are
turning together it is likely they can influence the speed and direction (like a boat
rudder) of ocean currents directly over the colony and maximize the amount of
food collected.
◦ The petalidium structure and skin gills are off center on the eccentric sand dollar
giving the animal its name. Eccentric means off center or off balance. Eccentric
sand dollars are the only sand dollar species in Oregon. This is important
because the sand dollar stands on edge to feed, unlike other sand dollar species
and the petalidium must be exposed to moving/oxygen rich water instead of half
buried in the sand.
▪ Other Notes:
 Sand dollars likely live around 10-15 years, though no one seems to know.
 There are male and female sand dollars, but there is no easy way to tell them apart.
 When a sand dollar spawns the eggs and sperm are shot into the water column where
the eggs are fertilized and begin developing into sand dollar embryos and larvae.
These become part of the zooplankton for a time. As larvae they look like
microscopic ghosts and nothing like the adults. After sensing an adult colony the
sand dollar larvae swallows a grain of sand (or something else to make it denser) and
sinks to the bottom where it will grow into an adult within the colony.
◦ Collect live sand dollars and hand out paper towels, string, vials, and tests.
▪ Review the parts of a sand dollar worksheet and have students fill it in.
◦ Sand dollar dove necklaces:
▪ Talk about the internal structure of a sand dollar focusing on the aristotle's lantern mouth
parts.
▪ Demonstrate to students how to break a sand dollar shell open without destroying the
sand dollar doves inside. Chip off the edges all the way around, then open the two
halves like an Oreo cookie.
 Life Lesson: patience and persistence pay off in life and this is also true while
searching the the sand dollar and sand for the doves.
 Collect doves in the vial, place the string over the hinge of the vial, close the vial on
the string, tie the ends together and a necklace has been created to take home. Stress
the importance of not misplacing this necklace as they all look the same. It needs to
be around their neck, or in their bag so it doesn't get lost or mixed up with other
necklaces.
 Clean up area by folding sand and test scraps up in the paper towel and throwing
them away.
 After giving directions release students to begin (provide a time limit to aim for
(usually 5-10 minutes)
 Line up at the door to leave. If extra time is available students may look around the
room at the aquariums, etc.