The following material is adopted, with permission, from the web pages of

Genevieve Anderson, Biological Sciences Department, Santa Barbara City College: http://www.marinebio.net/marinescience/03ecology/tpindex.htm

Index Species of the Middle Tide Zone

Gooseneck Barnacles 

G. Anderson

Gooseneck barnacles, scientific name Pollicipes polymerus, have the characteristics of the more common volcano-type barnacles (like the buckshot and Balanus), in that they are also filter feeders, capable of living rather high in the intertidal, being hermaphroditic, cross fertilizers, and having numerous planktonic larvae. They differ in that their bodies are on top of a permanently attached stalk covered with a thick skin. This stalk is a wonderful seafood (no guts inside) and tastes much like clam when steamed.

They tend to reproduce in summer with 3-7 broods per year. A unique thing about the planktonic larva of this species is that when it settles out of the plankton it generally crawls around, hunting for adults of its species before adhering to a solid substrate. This explains why single goosenecks are rarely seen--even when there is empty space on the rock there are always hundreds of goosenecks crowded together in rounded hummocks.

Gooseneck Barnacle clumps (G. Anderson)

They can live 20 years, or more. As they grow they make new calcareous plates protecting their bodies. When first secreted, these plates are shiny and pearlescent. After repeated high tides and battering waves (with sand and rocks), the pearly shells become pitted and dull.

Gooseneck Barnacle with a new shiny calcium plate (above left), Gooseneck Barnacle with dull and pitted calcium plates (above right) G. Anderson

It is interesting to look through these masses of gooseneck barnacles in our tidepools and see newly secreted shells. This means that the barnacle is happy, healthy and growing.

Mussels

The California mussel, scientific name Mytilus californianus, dominates southern California rocky shorelines exposed to waves. Mussels and gooseneck barnacles often occur in great numbers together.

Gooseneck Barnacles and Mussels (K. Meldahl)

Mussel showing byssal threads and their attachment pad to the rock (G. Anderson)

The mussel is a space dominator - attaching itself with an array of exceptionally strong byssal threads. Each of these is laid down individually by the animal's soft and flexible foot, which protrudes from between the two shells at high tide until it touches a solid surface. A special liquid is secreted inside that runs down a groove in this foot and out the end. This liquid is like super glue and hardens as a small pad at the bottom attached to the surface. As the mussel withdraws its foot, the liquid continues to harden, producing a strong thread attached to the inside of the animal's body. Numerous byssal threads are laid down by each mussel to keep it attached.  

When they are young they can loosen their threads and move about a bit, but when they are older most of them stay put. They do not hesitate to grow on top of each other and other species - resulting in interesting mussel clumps that are themselves a habitat harboring over 100 species of marine organisms.

Mussel, its shells being used as a substrate by Buckshot Barnacles (G. Anderson)

Mussels open their shells just a crack at high tide to feed, which allows them to circulate 2-3 quarts of seawater through their shells each hour - mucus on their gills traps plankton for their food. When there is a lot of plankton mussels can grow up to three inches each year and will overgrow most other species. But they rarely get a chance to live lower in the intertidal because their main predator, the sea star, consumes them. Studies done by scientists who remove the sea stars from rocky intertidal areas show that mussels will prevail all the way to 20 feet below sea level as giant clumps if left unchecked by their natural predator. Other factors also affect them, like big waves tearing off clumps, that get too large, or parasites, but it is the sea star that has the most influence.

Aggregating Anemones

The aggregating anemone, scientific name Anthopleura elegantissima, dominates southern California rocky shorelines from the lower part of the middle tide zone down to the upper part of the low tide zone.

Each anemone is round, with a mouth in the middle. The mouth is surrounded by feeding tentacles that have stinging cells capable of capturing small crustaceans, fish and anything that happens to touch them that cannot get away. The tentacles feel sticky to humans, but we are only feeling their 'sticky' cells. The real stinging cells of the aggregating anemone cannot penetrate our hands so it is safe to touch them.

Transition from California Mussels of the Middle Tide Zone to Aggregating Anemones of the Low Tide Zone (G. Anderson)

Aggregating Anemones with their tentacles pulled in at low tide (G. Anderson)

Food that is gathered by the tentacles is pulled to the mouth where it is ingested and digested. They are very simple animals, without a complete digestive tract, and thus there is no anus. So, ingested material that is not digested (like shells and bones) must be regurgitated back out the mouth. Sounds rather unpleasant, but this explains why sometimes the tidepool anemones look like they are turning inside out. Wayward periwinkle snails, if they have toppled from their high perch, may be swallowed by these anemones, but the snail will usually keep its trap door operculum closed until the anemone tires of its presence. Then, when the anemone spits it out, the periwinkle snail starts its long trip back up the rock to its preferred (and safe) Splash Zone.

At low tide these anemones pull in their tentacles and become a lump on the rock. The sides of their bodies are covered with adhesive structures that attach bits of shell, rock and seaweed. When closed up at low tide the attached material causes the anemone to look like a bed of crushed shell, but when you touch it, the anemone (whose body can be up to 80 percent water) releases water and feels gushy. The bits of debris probably reflect light to keep the anemone cooler and reduce water loss at low tide since these critters do not have a protective shell. The debris could also be dispersing wave action as the tide ebbs and flows each day.

At high tide, this species can split down the middle, pull apart and reform, resulting in two identical (but smaller) individuals. Each of these divides repeatedly until there are hundreds of aggregating anemones, all crowded together (thus the name aggregating). Each animal in the group is genetically identical, a clone.

Aggregating Anemone clone, lower half under water (open) and upper half above water (closed) pictured above (G. Anderson)

An exciting thing happens when two clones meet. Members of the same clone extend their tentacles at high tide and do not mind touching members of their own clone … but, should a member of another clone be touched, they fight until one moves or dies. There are special fighting tentacles (that are deflated and not visible until a clone war starts) tucked just under the regular tentacles and the outside of the body. These special fighting tentacles are called acrorhagi and during a clone war they are inflated. They look different than the regular tentacles, being shorter, rounder, and very white. These acrorhagi are fully illustrated later in this lesson when discussing the starburst anemone.

The clone war is a slow motion confrontation with the clonal adversaries stretching toward their enemy to touch the acrorhagi. The acrorhagi have nasty stinging cells that damage the tissue of whatever they touch. Back and forth for hours these anemone enemies fight until one moves or dies. This behavior leaves wonderfully obvious anemone-free areas wherever two clones meet. We don't have to do DNA analysis to know there are two (or more) clones present on the intertidal rocks. Other critters may use these anemone-free areas to travel.

Anemone free area between two aggregating anemone clones (above left), Close up of anemone free area (above right) (G. Anderson)

The aggregating anemone not only asexually reproduces to form clones, but once a year it releases eggs or sperm into the water as a broadcast spawner. Its planktonic larva is the source of the original anemone on the rocky shore that forms each clone. These anemones are separate-sexed so each clone is either all male or all female.