Barb's Wet Channel
During the lowest tides of the year, from -2.5 to 3.4 feet, the water depth in the channel is only about 2.5 feet, exposing many animals that wouldn't be seen above water during more typical low tides of -1.3 to 2.3 feet. Tides were exceptionally low during our observations, allowing us to investigate the diversity of both the surge channel and surrounding areas. The low tide left the east and west walls of the channel exposed to the air, sun, and wind, subjecting the animals to prolonged desiccation and predation.
Algal diversity seemed to play an important role in animal diversity of the east and west walls. The three dominant species were leather strap algae, sea lettuce, and Hedophyllum. By covering animals, algae can decrease exposure to desiccation and predation and may promote the establishment of some rare animals relative to typically more dominant animals, such as barnacles.
Algae may also have a negative effect by decreasing the amount of habitat available to animals. We found that the east wall face had little to no algal cover, allowing colonial ascidians and encrusting sponges, such as the bread crumb sponge, to dominate. Leafy bryozoans were also present, often dominating cracks and crevices.
The top part of the vertical west wall is dominated by thatched barnacles, with most diversity located below the dense barnacle mats. Barnacles appear to settle on exposed areas, in regions that experience greater wave action and exposure to the sun and air. Thick calcareous shells keep these crustaceans closed, which can reduce desiccation during periods when the animals cannot feed. At high tide, barnacles emerge to feed and reproduce. They are hermaphrodites, so finding a mate is a little easier if at least one other barnacle is nearby. Fertilization is internal and may involve more than one mate. Larvae are brooded inside the shell; upon release, they settle and attach with the help of a cement gland on their antennae. Thatched barnacles are rarely found less than several feet from the bottom of the surge channel, suggesting that they don't fare as well in lower levels.
Beneath the barnacle swath, the wall showed greater diversity, including sea slugs (Discodoris and Rostanga), sea cucumbers, worms, hermit crabs, sea anemones, and others. The abundant orange encrusting sponge, Ophlitaspongia pennata, shows a unique relationship with the red sea slug, Rostanga. Individuals of Rostanga, which are only about 5mm long, live in association with the sponge, feeding and reproducing on it. Like barnacles, the slug is hermaphroditic; copulation results in internal fertilization simultaneously for the paired animals, which mate normally from March through July or October, depending on the year. Rostanga lays its camouflaged eggs in ribbons, which form spirals on the surface of the sponge. The eggs don't trigger a mucous reaction, which is often a form of protection on the sponge's behalf. The camouflage color of the sea slug and its eggs provides protection from predators, since few animals feed on Ophlitaspongia.
Other interesting animals that may be seen in the surge
channel are blood stars, which appear to be relatively abundant not only
here but in other lower intertidal zones on rock walls, large colorful
Urticina anemones in shallow waters or pools, Pycnopodia
sunflower stars under algae and at the bottom of the channel, and many
chitons, snails, and worms. Look closely. Many exciting animals may
be cryptically colored, which protects them from predation.
Even cryptic color, however, is sometimes not enough. Imagine having a rogue wave of huge suckered feet bearing down on you as you rest in a crevice between the tides. This terrifying situation faces many species in Barb's Wet Channel, where the sunflower star resides. Although it has an innocent enough name, the sunflower star's large size and even bigger appetite makes it a threat to most invertebrates in the surge channel.
Many species have devised defenses against this death star, which can grow to a diameter of more than four feet (1.3 meters), with more than 15,000 tube feet aligning its arms! For example, the sea cucumber flexes it normally stiff body, so that it can wiggle away from the asteroid storm bearing down upon it. Sea urchins lay down their spines, as if in submission to the large sea star. But the urchin's test, or shell, actually is lined with small beaklike claspers, that bite at the feet of the sunflower star, causing it to retract an arm. Even a clam, usually still as a mountain, will violently thrust out its muscular foot when it feels the tube feet probing its shell.
The tube feet of the sea star move through the coordination of a complex water-vascular system using muscular contractions. Tube feet on the underside, or oral side, of sea stars allow the animals to move slowly along surfaces. Water is taken in through the madreporite, a circular pore near the center of the upperside, or aboral side of the sea star, and then pumped through a series of canals, which lead to the tube feet through a one-way valve. Water has to be constantly replenished because of seepage through the tissues into the surrounding environment, so if you handle a sea star, be careful not to keep it out of water for long. Contraction of muscles above the tube foot push water into the foot to elongate it. Muscles within the tube foot allow for side-to-side motion and retraction. Adhesive mucous attaches the sea star to the substrate and provides the necessary friction for locomotion.
If you look in the crevice above the surge channel, groups of feather duster worms (Eudistylia spp.) can be found, where they take advantage of the constant water flow created by the channel. These worms feed by collecting particles suspended in the water column, a method known as suspension feeding. The anterior portion of the worm, the feather duster, is replete with a whorl of tentacles. The stalk of each tentacle is called the radiole, and the branching feathers are called pinnules. Each pinnule is covered with cilia that beat in a controlled fashion, directing currents of water through the feathers. The current generally flows from the outside of the whorl of tentacles to the center and out the open top. The tentacles catch particles of food floating in the water column and then transport them down the ciliated grooves to the open mouth, found at the center of the whorl.
Some species likely to be found in the channel: