Feeding at Cobble Beach
Suspension Feeding
Some of the main suspension feeders at Cobble Beach are Terebratalia transversa (Brachiopoda), Ophlitaspongia (Demospongiae), Mytilus (Bivalvia), Chlamys hastata (Gastropoda), Petrolisthes (Crustacea) and Cucumaria miniata (Holothuroidea). Most of the suspension feeders are found in the lower zones of the tide, due to the fact that they require water to feed. Typically a suspension feeder's diet will consist of bacteria, phytoplankton, zooplankton and detritus. Many filter feeders are sessile, but there are mobile filter feeders such as Petrolisthes (below).
Deposit Feeding
Many polychaete worms make a living as direct deposit feeders, which ingest fine sediment and absorb organic material on the sediment particles. However, this lifestyle was rare at Cobble Beach. Possibly this is related to the relative scarcity of fine sediment, except relatively deep below the cobble layer (we did not excavate this layer and don't know whether more such worms might be found there).
A selective deposit feeder found on the cobble beach is the sipunculan or peanut worm (Phascolosoma sp.). These animals are often found wedged between cobbles; they use a crown of short tentacles to sort out the digestible material from the small amounts of sediment that accumulate around the bottoms of rocks.
Herbivory
A large but relatively rare herbivore at Cobble Beach is the green sea urchin, Strongylocentrotus droebachiensis . They use a complex structure for chewing or grinding, called Aristotle's lattern (below), which is a set of 5 teeth and associated bones and muscles. These are used to project the teeth out of the mouth, grab pieces of algae (or animal material, if they can reach it), and tear off pieces that are then swallowed whole.
The herbivore grazers at Cobble Beach include the two Littorina species (L. scutulata and L. sitkana ) below. Like the limpets and chitons, they develop a specialized rasping tongue called a radula, which they use to scrap algae from the rocks.
Carnivory and Scavenging
Large predators were relatively scarce at Cobble Beach. One that we did see, though not found in our transects, is the sunflower star (Pycnopodia helianthoides). Pycnopodia is an omnivorous carnivore; it will eat almost anything it can capture, and many other animals have extreme defensive responses to this large sea star. You are likely to see this predator only at extremely low tides.
Predation by carnivores can have a profound effect on other organisms of Cobble Beach. For example, given sharp divisions between the tidal zone distributions of shore crabs (Hemigrapsus, right) and porcelain crabs (Petrolisthes eriomerus ), we hypothesize that porcelain crabs may avoid the higher zones where shore crabs are abundant. |
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Focus on Ocenebra
lurida
The predatory snail Ocenebra lurida is an uncommon but interesting inhabitant of Cobble Beach. Unlike most other predatory snails that drill into hard-shelled prey, Ocenebra lurida will readily attack limpets by crawling on top of the limpet shell and rasping a hole in it. This feeding strategy requires some interesting decisions by the snail about what to eat within a single prey item. After drilling a hole, the snail selectively eats first the gonads and then the digestive gland, but not the foot of the limpet. The assumed reasons for this are two fold. First, these soft organs are located close to the point of drilling. Second, and more importantly, Ocenebra may avoid attacking the muscular foot tissue of the limpet because this is the structure that is keeping both the limpet and its attacker attached to the rocky substrate. Larger Ocenebra might be better able to keep a grip on both the limpet and the substrate when they attack relatively small limpets (and could eat the prey foot tissue). If this were an important consideration (for example, if the foot is an especially nutritious tissue that is risky for small snails to eat), then one might expect to see a greater difference between the size of the snail and the size of its limpet prey as Ocenebra grow larger. In fact, larger Ocenebra attack larger (not smaller) limpets. Possibly these larger snails gain more nutritional benefits from the soft organs of larger limpets than they would from eating both the soft organs and the foot of smaller limpets.