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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