Diodora aspera
rough keyhole limpet

The Rough Keyhole Limpet can be found anywhere in coastal regions from Afognak, Alaska, to Baja Calafornia. It is found primarily in low intertidal areas, and has been seen up to 40 feet subtidally in the south (Morris et al. 1980).

The Keyhole Limpet clings tenaciously on and under large rocks in the sub to low intertidal. They can also be found on large kelp stipes. Their strong foot allows them to thrive in some intertidal areas where turbulent wave action is prevalent.

This gastropod has a unique and diagnostic hole in the apex of its shell (about 1/10 the shell length, and slightly anterior to the center) which functions as an exhalant opening. Water is drawn up under the shell from the forward end and over the gills, where it is then forced out of the hole. Because the limpet anus lies near the gills, (a result of the torsion process) this method of respiration is essential in maintaining mantle cavity sanitation, as Diodora aspera is constantly receiving clean, oxygen rich seawater. Other limpets lack this unique flow-through system, but have developed different methods to seperate clean incurrent seawater from waste-laden excurrent flow.

The Keyhole Limpet can reach lengths of up to 70 mm. Its shell is thick and has a triangular shape in profile. This organism has coarse exterior, with numerous rough radial ribs (every forth rib larger) crossed by concentric threads, creating somewhat of a lattice effect. The shell color is greyish white, with brownish rays radiating from the apex. The interior of Diodora aspera is white and the edge of the shell is crenulate (Morris et al. 1980).

These animals are most active, in terms of feeding, during high tides. When the tide recedes they return instinctively to the same spot occupied previously (a smooth, protected rock surface often scraped out to snugly contour their shells) to await the return of high water (Morton 1958).

Diodora aspera has developed a fascinating defensive response to combat predation by seastars. When the keyhole limpet senses an attack, it extends its foot and raises its shell. Its mantle then moves out from under the shell and covers both the side of the exposed foot and the shell. The mantle margin lining the keyhole at the apex of the shell also protrudes upward and outward. As a result, the tube feet of the seastar can't grip the limpet, and it moves on to other prey (Morris et al. 1980).

This particular limpet is an omnivorous grazer. It feeds by scraping rocks with its radula. Various bryozoans are its food of choice, but it also consumes algae, as well as some sponge species (Morton 1958).

The Keyhole limpet has an extremely unique feature associated with its blood. The blue respiratory pigment, hemocyanin, shows no change in its affinity for oxygen when pH levels are altered. In studying this feature, scientists are working to determine how this respiratory pigment differs from our own from a molecular structure standpoint. In this manner, this organism may help provide new information in defining the structure and function of human blood components (Morris et al. 1980).

Limpets play a key role in the intertidal ecosystems where they thrive. As algeal grazers, they help maintain the delicate and complex balance essential for such a diverse group of organisms to survive. They clear rocks of algae, allowing space for other organisms such as Mytilus (mussel) and Chthalamus (barnacle) species to attach to the substrate. Additionally, they are an important food source for the keystone predator Pisaster Ochraceus (seastar).

Recent studies suggest that this animal's unique apical opening is not only associated with sanitation, but also plays an important role in inducing passive flow through the mantle cavity. In studies where the keyhole of Diodora aspera was blocked (either naturally or experimentally) no evidence of damage to the mantle cavity or associated organs was found. In these experimental trials, water entered ventro-posteriorly with respect to the gill tips, and exited over the head region (near the anus). The apical opening proved unnecessary as a means of waste removal. Researchers did find however, that the keyhole played an essential role in allowing water to flow passively through the mantle cavity. Thus, it is thought that this function of the apical opening may have been just as significant as its role in sanitation in terms of limpet evolution (Voltzow et al. 1995).

The polychaete Arctonoe vittata is an extremely common commensal organism found within the mantle cavity of the Keyhole Limpet. The worm, sometimes as long or longer than the limpet itself, often bites predatory seastars that attach themselves to its host, causing them to retreat (Morton 1958).