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Ochre Sea Stars
You will most likely encounter sea stars in the low tide zone when the
tide is out. However, they
commonly migrate up to the middle tide zone to feed when the tide is up.
Sea
Star at water's edge, zero tide level (G. Anderson)
One of the most common sea
star species in the tidepools is Pisaster ochraceus,
also called the ochre sea star.
Ochre
Sea Star, the 'keystone species' of our California rocky shores (G.
Anderson)
Each ochre sea star can eat
up to 80 adult mussels each year and thousands of barnacles. This is the 'keystone
predator' in our rocky intertidal ecosystem. Without it, the mussels
would dominate, and other species, such as the aggregating anemone, would
be crowded out. There would not be the great diversity of species that we
see in the Low Tide Zone because mussels would overgrow everything. This keystone predator helps maintain the
high diversity of the low tide zone.
Ochre Sea Star in feeding position on top of a
mussel (G. Anderson)
Sea stars are uniquely
adapted to hold onto solid shell with hundreds of sucker-tipped tube
feet that are found under each leg.
Ochre sea star attacking a mussel with its
tube feet (K. Meldahl)
The tube feet function by water
pressure that enters the sea star through a special sieve plate on the
upper surface of its body. This
sieve plate (also called a madrepore) can
usually be seen if you look closely at the back of a sea star just
off-center.
Central
disk of ochre sea star, note seive plate (above
left), Seive plate (madrepore)
close up (above right) (G. Anderson)
The tube feet can pull two
pieces of shell apart for hours (or days, if needed) until their prey
tires. Sea stars never tire of pulling open a shellfish because they have
hundreds of tube feet - always resting a few. Once there is the tiniest
crack (a tenth of a millimeter is all that is needed) the stomach of the
sea star can emerge, ooze into the crack and digest the prey. In general,
it takes more than six hours to consume a mussel. The upper limit of
where sea stars prey on their favorite food, the mussel, is the middle
tide zone. The sea stars do not
venture higher than this because they cannot tolerate that much dryness.
Few things prey on sea stars. Even if a sea star is attacked, the
predator usually just bites off an arm or two, the sea star has amazing
regenerative abilities, and can often regrow missing arms. They can
sometimes even regrow an entirely new animal from just one leg.
Sea stars have sexual
reproduction mostly during spring and summer. This occurs when the
separate-sexed adults release their eggs and sperm from five openings on
their top surface. Often when one sea star spawns this causes those
nearby to also spawn, creating a concentrated mass of eggs and sperm in
nearby waters - increasing the chance for fertilization. This broadcast
spawning is well known by aquariums that quickly remove any spawning sea
stars so as not to cloud the water for their visitors.
Solitary Anemones
(variously
known as Starburst Anemones or Green Anemones)
Anthopleura is the genus name for several species of
large, solitary anemones, including the Starburst Anemone and the
Green Anemone.
Solitary
Starburst Anemone (above) showing radiating lines on the oral disk. (G. Anderson)
Starburst
Anemone in open position showing feeding tentacles (above left),
versus Starburst Anemone in closed position (above right).
Anemones will generally be open when they are covered by water, and closed
when they are exposed to the air.
(G. Anderson)
The starburst anemone has fighting tentacles, called acrorhagi, just like the aggregating
anemone. These special tentacles show up white when the anemone is in a
fighting mood.
Starburst
Anemone with a few acrorhagi inflated on left
(G. Anderson)
The anemone fights with its neighbors using these acrorhagi. The
purpose of the fighting is to maintain territory and assure adequate
space. If two starburst anemones happen to touch, they inflate their acrorhagi (which are generally deflated and hidden
between the feeding tentacles and the side of the anemone). They then fight until one of them
moves.
Acrorhagi fully inflated on Starburst Anemone (G.
Anderson)
In this manner, they maintain even spacing. You may see these anemones in
the middle of a fight, with their white, blunt acrorhagi
inflated. As they fight (touching each other with their acrorhagi) the white areas of the acrorhagi
become tattered. Eventually one
anemone moves away from the tentacle-reach of its neighbor to stop the
fight.
It is interesting to look at the different color
patterns on the tentacles and oral disks of these starburst anemones. The
various shades of green come from a combination of the natural color of
the anemone and from green-colored symbiotic algae that grow in their
tissues. Anemones found under rocks or in the shade have little symbiotic
algae so are generally very pale. The various striping on their tentacles
is genetic and serves to show how each is unique (unlike the clones of
aggregating anemones where each clone member is identical).
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Green
Starburst Anemone (from symbiotic algae) above left, White Starburst
Anemone (in the shade) above right. (G. Anderson)
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Other Low Tide Zone species
The Low Tide Zone has the highest diversity of any of
the tide zones. Here are some
other interesting species that you may encounter.
Octopuses
Extremely shy by nature, octopuses generally hide, so
it is only the most watchful and observant tidepooler
who discovers this interesting animal.
(G. Anderson)
Our common species, called the two-spotted
octopus, changes from matching its environment to standing out in
contrast to it. Octopods are
masters at blending into their environment by changing color and skin
texture in seconds.
(G. Anderson)
The name (two-spotted octopus) comes from the two fake eyespots, below
its inconspicuous real eyes. These fake eyespots can be turned on or off.
A close look at the pattern of the fake eyespot is needed to distinguish
the two species of the two-spotted octopus (Octopus bimaculoides and Octopus bimaculatus).
Octopus bimaculoides' eyespot has a blue
chain instead of a starburst.
The octopus can bite with a beak, found in the middle
of its eight legs. A drop of poison is generally delivered with this bite.
The poison is enough to paralyze small fish or crabs, but usually does
not hurt humans (unless you happen to be allergic to it). It is best to
avoid any octopus bites as one never knows if you could have a reaction
(similar to a bee sting).
When stressed, the animal may shoot out a cloud of ink, as a smokescreen,
and jet away. It is always a good idea to look around the area where an
octopus is discovered because female octopods attach their strings of
eggs to a sheltered area in the rocks, often an overhang or small cave.
They guard these eggs until they hatch. If you do find an octopus and
move it, be sure to replace it in the same tidepool
... in case it was a female guarding eggs.
Brittle Stars
Brittle stars, scientific name Ophiothrix
spiculata, may be found under rocks and in
rock cracks. Brittle stars are very different from sea stars, such ast the ochre sea star. They are filter feeders,
often found in great numbers on our ocean bottom, under the sand and in
tight spaces. They are called brittle stars because they can drop their
legs (or parts of their legs). They regrow these dropped legs but it
takes a few months to do this and return to normal.
(G. Anderson)
Sea Hares
Looking a little like a rabbit, sea hares can be quite common in our
local tidepools. They can weigh up to 16 pounds
but are usually more like three to four pounds in the lower pools.
Although these slugs appear to be just a big blob, they have a hidden trick
… beautiful purple ink that can be released if you reach inside the skin
flaps on the top and tickle them. In nature this acts as a smoke screen
(similar to the octopus's ink).
(G. Anderson)
Sea Slugs or Nudibranchs
The finest treasures of the low tide zone may be the nudibranchs. There are over 100 species in
California. Many are brightly colored, but some blend into their
surroundings or match their prey (upon which they may live). The most
common and most flashy of the nudibranchs is
the Spanish shawl, scientific name Flabellina
iodinea. It is easy to see why it is called
the Spanish shawl, as its colors remind one of flamenco dancer shawls.
This species also 'dances' occasionally by letting go of the substrate
and wildly thrashing its body back and forth, creating the same look as a
flashy flamenco dancer's skirt.
(G. Anderson)
These are beautiful creatures, but some species are toxic. Many of the slugs (including the Spanish
shawl species) feed on stinging animals, like jellyfish and sea anemones.
They are capable of keeping the stinging cells alive in their bodies at
the tips of all those 'furry' processes, known as cerata.
Then, when a predator (like a fish) comes by for a bite of this slug the
stinging cells fire and the fish is repelled. The predator is rarely
wounded, but it is believed that the predator remembers the flashy colors
and never again bothers what it thought was a tasty morsel. So, the
flashy color is thus a type of 'warning coloration.'
Marine slugs also have interesting reproductive habits. They are
hermaphroditic, but must mate with another individual. Their reproductive
pore is on the right side of their body so they must position themselves
just right. Eventually they get together, cross-fertilize and then
separate to lay their fertilized eggs. The eggs hatch as planktonic
larvae.
There are many other beautiful nudibranchs and
a few related slugs, like Janolus barbarensis, Hermissenda crassicornis, Triopha catalinae, Anisodoris nobilis, Acanthodoris rhodoceras, Diaulula sandiegensis, and Berthellina
engeli.
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