Owen's Weed:
the surf grass environment of Cattle Point
Above: bread crumb sponge, Halichondria
Left: Surf grass at Cattle Point
Along the most seaward stretch of the
exposed tidal rocks, dense mats of surf grass (Phyllospadix scouleri)
dominate the substrate. The mats form at the periphery of the tidal zone,
and except at the lowest tides of the year they are not exposed nor accessible.
Unlike the other tidal areas of Cattle Point, the surf grass beds are dense
enough to reduce the number of barnacles to nearly zero. The dominant algae
forms seen in the upper areas are also absent. When the tide is out, the
surf grass lays horizontally in thick mats covering and protecting the
life underneath. These mats capture moisture and protect many of the tidal
areas when exposed to air. When the tide is in, surf grass floats vertically,
providing a rich three dimensional environment for creatures that rely
on both the sea floor and the vertical shafts of the grass as a substrate
on which to live. The mats of surf grass are only found at shelves lying
in parallel to the sea surface, experiencing significant surge forces when
the tide comes in but little direct wave action.
At the lowest tide, surf grass
beds are fully exposed for approximately an hour, and thus the animals
found there do not necessarily need to be adapted to life in the air as
in other Cattle Point habitats. Though the air temperature is greater than
the water temperature, the mats of grass keep the habitat dark and cool,
and thus the temperature and moisture remain constant. The dominant animals
in the environment are sponges (Halichondria spp.) and top snails
(Calliostoma spp.). Though once thought to be plants, sponges are
one of the oldest and most successful lineages of multicellular animals.
They are simple in structure and therefore feed and process food in a way
fundamentally different than most other animals. Halichondria, the
bread crumb sponge, is an aggregation of cells that does not form tissue
layers but is differentiated to perform particular tasks. For feeding,
they use specialized Choanocyte cells. By beating their cilia, or microtubule
tail, these cells create a flow of water that moves through a porous collar.
This collar acts as a net and traps food particles floating in the water
column. Unlike other animals that digest food extracelluraly in stomachs
or guts, sponge cells phagocytize, or eat, small pieces of food and digest
them intracellularly in bubbles called food vacuoles.
In this habitat, devoid of strong
wave action, not all defenses are about thick shells and strong claws.
Sometimes it's best just to blend into the background and hold still when
predators swim above. Like its relative the decorator crab, the kelp crab
knows all about holding still. Using claspers on its rostrum, the crab
holds a blade of kelp grass above its mud brown carapace, as if it were
just another leaf growing along the shore.
Also found in Owen's weed is an
unusual anemone known as the proliferating anemone. The anemone Epiactis
prolifera is known for brooding its young on its pedal disk. Epiactis
are hermaphroditic, containing eggs and sperm at all times. Self fertilization
is the most common form of reproduction, and cross fertilization is relatively
rare. The eggs are fertilized in the digestive cavity, and the larvae crawl
out of the mouth and embed in double or single rows in the grooves around
the column. The young grow into little anemones, where they remain until
they are fully formed (about 5 mm tall) and move off. It is possible to
see 40 young in different stages of development on one specimen.
Animals found in Owen's Weed (Surf
grass, Phyllospadix scouleri )
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