High Pools: the most exposed intertidal areas of Cattle Point
The high intertidal is the harshest of Cattle
Point tidepool environments. Though desiccation, wave action,
and surge doesn't pose a threat in the upper tide pools, these
pools do experience the most extreme environmental changes. The
most limiting factor for life in these pools is the fluctuating
temperature and salinity throughout a tidal cycle. Our characterized
pool (approximately ten by fifteen feet in size) has within it
surf grass, crevices, and exposed areas -- all of which are found
in other environments, but the flora and fauna is almost completely
different. Why? Temperature, that's why. The pools heat up seven
to ten degrees during the average day, and though for air breathers
like us seven degrees doesn't feel like much, in a marine environment
water acts as a heat sink, draining the heat from an animal twenty
five times faster than in air.
The dominant animals here are not barnacles or
leather chitons, which are noticeably absent, but instead a vast
number of small gastropods, including both periwinkle snails (Littorina
scutulata and L. sitkana) and limpets (Lottia digitalis).
On average, the mean body size of the animals in the high tide
pool is smaller than those found elsewhere, and the diversity
is very low. The dominant chiton in the area is the hairy chiton,
Mopalia spp.. The dominant crab is the shore crab, Hemigrapsis
nudus--in our target pool we found eight individuals. There
were also numerous, small hermit crabs, all using very small periwinkle
shells as homes.
The sea anemone Anthopleura elegentisma
is found exclusively in the high intertidial at Cattle Point.
This often abundant animal as been pushed to the periphery of
the environment, because it is unable to compete for space elsewhere
with the barnacles and rock weed. In the high intertidal,
we found clones (large colonies of anemones derived by asexual
reproduction from a single founder) ranging from 20 to 30 individuals,
often limited to crevices and cracks that remain moist all day.
Anthopleura feed on a diversity of species and take advantage
of the occasional unlucky barnacle knocked off rocks by logs or
wave action. Like most anemones, Anthopleura remove particles
from suspension using their tentacles. The tentacles have numerous
nematocysts, a cellular organelle that fires a sticky threat used
to puncture or entangle prey. Once Anthopleura captures
prey, each tentacle curls and forces the food item through the
mouth into the gastrovascular cavity. Anthopleura
also derives nutrition from symbiotic algae housed within the
tentacles. These algae, either zooxanthelle or zoochlorellae,
provide the anemone with its characteristic green color.
Anthopleura take advantage of a large portion of the sugars
generated by symbiont photosynthesis. Anthopleura elegantissima
reproduces sexually and asexually. Sexual reproduction occurs
when male and female anemones broadcast spawn gametes during late
summer to early fall. The larvae have a brief planktonic existence,
until they settle to form adults. Asexual reproduction occurs
when a large specimen undergoes longitudinal fission, dividing
in half and regenerating the lost parts. These offspring
will repeatedly divide in half until a colony is formed. An individual
can reproduce sexually and asexually within a year, but fission
typically takes place only once a year.
Like many species in the rocky intertidal, spiral
tube worms found in this habitat build tubes made of calcium carbonate
to protect them from predation and desiccation. If you look along
the bottom of rocks in the upper intertidal, you will see small
white spirals, secreted by the worm as it grows. When the tide
is in, spiral tube worms use their tentacles to pluck food from
the water column. But when then tide goes out, these small marine
worms retract their tentacles into their shells, which blend into
the rocks like a spattering of bird droppings.
We used data recorders to monitor temperature fluctuations over two days. The recorders were left in two different tide pools (one high; one mid-level, aka Joe's crab den). The peaks represent the temperature reached while the tide pool was exposed and separated from the ocean. The valleys represent the temperature of the pool while sumerged as part of the ocean. The peaks correspond exactly with the daily low tides.
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