The owl limpet, Lottia gigantea, lives on North America’s Pacific coast, from Baja California, Mexico, to Washington in the United States. Predation and exploitation have reduced the extent of the species' northern range, whereas populations in the southern range are relatively stable. The greatest abundance of this species is found in the middle of its range, offshore of San Diego, California. (Fenberg and Rivadeneira, 2011)
Lottia gigantea resides in mussel beds and on bare rock in the middle to high rocky intertidal zone. This narrow, vertical zone is covered by high tide. During the day, a limpet clamps down within its home scar to prevent desiccation and overheating. Its adhesion is so strong that regardless of its orientation on the substratum, the limpet is not dislodged by high-energy wave impacts. (Denny and Blanchette, 2000; Lindberg, et al., 1998)
Lottia gigantea gets its name from the brown outline found on the inner surface of its shell which resembles the silhouette of the tufted Great Horned Owl. Lottia gigantea is North America’s largest limpet, and adults average 8 to 10 cm in length. As with all prosobranchs, limpets have a mantle, shell, foot and radula. The mantle secretes a calcium carbonate shell to protect its soft, visceral tissues. The low, rounded univalve is oval shaped, and its apex is off center and closer to the anterior end. The rough shell is mottled with light grey and brown colors which camouflage it within its rocky intertidal habitat. Attached atop the shell, the smaller rough limpet Collisella scabra can often be found, as well as bluish scars indicating locations of prior hitchhikers. (Cowles, 2012; Denny and Blanchette, 2000; Hinton, 1987; Karleskint, Jr., et al., 2010; Kido and Murray, 2003)
The muscular foot is its means of locomotion and is light orange in color. The radula is a ribbon of teeth used to scrape algae off of rocks and other hard substrata. As the anterior teeth are worn down, new ones are replaced from the posterior end of the radula. (Karleskint, Jr., et al., 2010)
During high tide in January or February, Lottia gigantea annually broadcast spawn directly into the water. Large amounts of gametes are released from the intertidal zone and swept out by the tide into the open ocean where they unite with gametes from conspecifics in the process of external fertilization. The developing larvae are part of the zooplankton which feed on phytoplankton. Scientists have not determined the length of time required to develop from gametes into veliger larvae, a ciliated free-swimming stage, because the microscopic larvae are indistinguishable from those of other limpets. In similar Lottia species, Lottia digitalis and Lottia asmi, each take five days to reach metamorphic competence, transforming to the settlement stage. (Fenberg, et al., 2010; Fenberg, 2008)
Tides, longshore, and Davidson currents return mollusk larvae to shore where they adhere to mussel shells or hard substrate cracks and crevices within the mussel bed community. Due to the long stretches of unsuitable sandy beach habitat along the California coast, Lottia gigantea larvae can settle only within the rocky intertidal coastline where they can continue to grow. Owl limpets are indistinguishable from other limpets when shell length is less than 15 mm. Between two to three years old, males transition into females. (Fenberg, et al., 2010; Kido and Murray, 2003)
Growth rate is slower and highly varied in mussel bed communities. On bare rock, the majority of growth takes place in spring and summer, and minimal growth occurs in autumn and winter when limpets allocate resources to gamete development. (Kido and Murray, 2003)
During high tides in January or February, owl limpets annually broadcast spawn. (Fenberg, 2008)
Lottia gigantea is a protandric sequential hermaphrodite, in that an individual begins life as male. A subordinate male grows well into adulthood within an area of densely aggregated conspecifics, but after an adult male acquires personal territory, his behavior changes to that of a solitary, dominant, territorial limpet, thus triggering sex change from male to female. (Wright, 1989; Wright, 1989)
Adult males are physically smaller and substantially outnumber the larger, solitary females, yet males produce far less gonadal mass. Less available male gonadal production leads to lower reproduction within the population. Of potentially greater reproductive stress is human predation. (Kido and Murray, 2003)
Humans collect larger individuals for the shell, the muscular foot, and for fish bait. Size-selective harvesting targets sexually mature females, reducing their overall body size and lowering the population's reproductive potential. In addition, human selection has driven the evolution of different life history characteristics in this species. Owl limpets in harvested areas change sex at a smaller size, and grow more slowly than L. gigantea in Marine Protected Areas where sex change occurs in larger, faster growing individuals. (Fenberg, 2008; Pombo and Escofet, 1996)
There is no parental involvement after releasing gametes.
The female Lottia gigantea vigilantly guards its algae garden. Adjacent territories are separated by a narrow neutral zone, a strip of purposefully ungrazed algae. Wet or dry, invaders are quickly rammed and bulldozed with the limpet’s plow-shaped anterior shell shoved under the intruder’s foot (ramming speed of 1.07 mm/s as opposed to grazing speed of 0.05 mm/s). Mushrooming (raising and forcefully lowering the shell onto an intruder’s foot) effectively forces retreat, too. The method of territorial defense employed by a female depends on whether the intruder is a predator, or is competing for space or food. Average response time to an intruder is 57 s, and average intruder evasion response time is 22 s. (Schroeder, 2011; Shanks, 2002)
Space competitors (such as the recently-settled larvae of the bivalves Mytilus californianus, M. edulis, & Septifer bifurcatus), are bulldozed to the garden perimeter while the clonal sea anemone, Anthopleura sola, is bulldozed off the substratum and into the waves. Acorn barnacles (Balanus glandula and Chthamalus fissus) and the gooseneck barnacle Pollicipes polymerus are rasped off the rock face entirely. (Stimson, 1970)
Smaller or slower food competitors like the volcano keyhole limpet are overrun and bulldozed while larger or faster individuals like the chiton may escape the pursuing resident. Other species of limpets, including Lottia digitalis, L. scabra, L. paradigitalis, L. persona, and L. fenestrata, are bulldozed into the waves as are the gastropod snails Tegula funebralis, Littorina planaxis, L. scutulata, Thais emarginata, and Acanthina spirata. (Stimson, 1970)
Before the morning low tide, a territorial limpet returns to its home scar, a depression worn into the rock that the shell margin exactly fits into. The shell margin shape is no accident for it grows and conforms to the particular contours of its home scar, forming an effective seal that reduces thermal and desiccation stress resulting from the animal's exposure until the next high tide immerses the hungry limpet. (Miller, et al., 2009; Schroeder, 2011)
The female owl limpet maintains a range, proportional to its body size, of up to 1000 square centimeters around its home scar. (Stimson, 1970)
A female Lottia gigantea must come in tactile contact with an intruder before the trespasser is perceived. Approximately 70% of the time, trespassers are detected. Scientists do not know how the mucus trails of a female acts as a territory marker; however, female L. gigantea avoid mucus of conspecific females, indicating chemoreception/chemical sensory ability. (Schroeder, 2011; Shanks, 2002)
Lottia gigantea can distinguish between grazers, predators, and benign foreign objects. In experiments, objects such as a human finger, pencil erasers, and steel pegs elicited no response and were likely perceived by the limpets as obstructions. (Stimson, 1970)
The female Lottia gigantea is a territorial herbivore that alters its environment by gardening. The limpet maintains a 1 mm thick algae mat of nearly 1000 square centimeters around its home scar. Garden size is proportional to shell size. After exposure to approximately 20 minutes of incoming tidal splash, the limpets begin to graze. While eating, glands on the foot secrete a sticky pedal mucus trail that acts like fertilizer, trapping bacteria and stimulating algae growth for later consumption. This pedal or provendering mucus is a nutritional adaptation which also aids in locomotion. (Connor, 1986; Stimson, 1970)
Provendering mucus lasts significantly longer and differs chemically and physically from stationary mucus. Stationary mucus has a high water content (93.5%) to reduce desiccation while resting on its home scar between high tides. Provendering mucus contains nourishing proteins and carbohydrates. Before L. gigantea forages, it encircles its home scar, depositing a lot of provendering mucus which it retraces on its return trip home. (Connor, 1986)
The female limpet uses its radula to scrape back and forth on rock, removing the surface layer of algae. Distinctive, parallel scraping marks remain after the day’s feeding. On subsequent days, adjacent areas within the algae patch are grazed. The female moderately grazes all of its territory in patches over 3-4 days. Careful management promotes long-term benefits, and a competent female can maintain the same garden for at least 4 years. Removal of a mature female limpet from its home range can have devastating effects, as trespassing juveniles, non-territorial males, and interspecific species raze the algal meadow within 2 weeks, leaving nothing to farm. If transplanted to a barren area, Lottia will generate another 1000 square cm algae garden within approximately 3 weeks. (Shanks, 2002; Stimson, 1970)
In contrast, a non-territorial neighboring male does not create a garden; therefore, he has neither foraging strategy nor resources to protect. Instead, the male is an opportunistic feeder that raids female gardens, eating indiscriminately and disregarding recent grazing trails. The intrusive male eats in the garden until detected, then hastily retreats from the alerted female. (Shanks, 2002)
Carbon dating on Lottia gigantea shells found on the California Channel Islands revealed human harvesting for over 10,000 years. In modern history, California commercial fishing occurred during the majority of the 1900s until harvest levels declined in the 1990s. Since L. gigantea has no effective defenses against human predation, people have significantly impacted the larger, predominantly female population. To compensate for the loss of females, males utilize the plasticity of their genes to mature faster and transform into females. (Erlandson, et al., 2011; Fenberg and Roy, 2012)
The American black oystercatcher preys on giant owl limpets. Approximately 45% of the adult bird’s diet is chiefly limpets. An adult bird cannot pry a mature female from its home scar due to the limpet’s formidable adhesive strength, so smaller male limpets are routinely removed. Oystercatchers are sensitive to human presence and avoid human populated areas. In a remote setting, a single wintering flock can remove hundreds of limpets during a single low tide, and one study reported that L. gigantea represented 78% of the limpets that parents fed to young chicks. (Denny and Blanchette, 2000; Lindberg, et al., 1998; Lindberg, et al., 1987)
When confronted by the predatory sea star Pisaster ochraceus, L. gigantea will "mushroom" by lifting its shell off the rock to reduce drag and allow for speedier retreat. When fleeing, L. gigantea hastily withdraws, crawling upwards on the substratum at a rate comparable to territorial ramming speed. The sea star cannot crawl out of water to follow, because its locomotory system functions only when the animal is submerged. (Denny and Blanchette, 2000; Sagarin, et al., 2007)
Lottia gigantea are herbivores whose behaviors affect nearby conspecifics as well as other species within the middle to upper intertidal community. The shells of female owl limpets serve as substrata to commensal organisms such as small barnacles and limpets including the rough limpet, Lottia digitalis, and L. pelta. Females are territorial algae gardeners. Preferential removal of a larger female affects the species composition of the local microhabitat. Smaller limpets consume the unguarded microalgae while mussels, barnacles, and anemones move in to compete for the newly unoccupied space. (Lindberg, et al., 1998)
Owl limpets are an important food source for shorebirds such as the American black oystercatcher and are collected by humans for subsistence and for fishing bait. (Kido and Murray, 2003; Pombo and Escofet, 1996)
In the 1980s, owl limpet commercial harvesting peaked and then declined in the 1990s, although numerous incidents of contemporary poaching indicate that Lottia gigantea is still a desirable harvested species. Unfortunately, large scale poaching occurs regardless of site protection. In September 2004 at Rancho Marino Reserve in Cambria, California, a full-time resident manager noted four major harvesting occurrences, including the illegal collection of 2500 limpets. On a smaller scale, in 2005 at White’s Point near Los Angeles, a family collected approximately 75 owl limpets. (Sagarin, et al., 2007)
There are no known adverse effects of Lottia gigantea on humans.
Marine Protected Areas, or MPAs, offer incidental poaching protection so males and females can grow larger and more slowly at a more natural rate. In 1999, commercial fishing of Lottia gigantea was prohibited in California; however, recreational harvesting is permitted with a daily limit of 35 individuals per person. The resilient intertidal limpet population is a human-stressed species. Shifting baselines of reduced shell length and decreasing population size show the need for effective monitoring, enforcement, and habitat management; these can be accomplished not only by extending MPAs in California for intertidal organisms such as L. gigantea, but also for countless marine species worldwide. (Erlandson, et al., 2011; Kido and Murray, 2003)
Lottia gigantea was the first lophotrochozoan selected for whole genome sequencing by the Joint Genome Institute in order to shed light on the origin of the owl limpet's evolutionary success. The genome was sequenced in 2007, and the sequence data are available from The Genome Portal of the Department of Energy Joint Genome Institute website. (Grigoriev, et al., 2012)
Olivia A. Turner (author), San Diego Mesa College, Sarah M. Turner (author), San Diego Mesa College, Paul Detwiler (editor), San Diego Mesa College, Renee Mulcrone (editor), Special Projects.
living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.
body of water between the southern ocean (above 60 degrees south latitude), Australia, Asia, and the western hemisphere. This is the world's largest ocean, covering about 28% of the world's surface.
having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.
uses smells or other chemicals to communicate
the nearshore aquatic habitats near a coast, or shoreline.
active at dawn and dusk
having markings, coloration, shapes, or other features that cause an animal to be camouflaged in its natural environment; being difficult to see or otherwise detect.
animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature
fertilization takes place outside the female's body
union of egg and spermatozoan
A substance that provides both nutrients and energy to a living thing.
An animal that eats mainly plants or parts of plants.
having a body temperature that fluctuates with that of the immediate environment; having no mechanism or a poorly developed mechanism for regulating internal body temperature.
Animals with indeterminate growth continue to grow throughout their lives.
the area of shoreline influenced mainly by the tides, between the highest and lowest reaches of the tide. An aquatic habitat.
offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).
A large change in the shape or structure of an animal that happens as the animal grows. In insects, "incomplete metamorphosis" is when young animals are similar to adults and change gradually into the adult form, and "complete metamorphosis" is when there is a profound change between larval and adult forms. Butterflies have complete metamorphosis, grasshoppers have incomplete metamorphosis.
having the capacity to move from one place to another.
the area in which the animal is naturally found, the region in which it is endemic.
active during the night
an animal that mainly eats plankton
condition of hermaphroditic animals (and plants) in which the male organs and their products appear before the female organs and their products
mainly lives in oceans, seas, or other bodies of salt water.
breeding is confined to a particular season
remains in the same area
uses touch to communicate
that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).
defends an area within the home range, occupied by a single animals or group of animals of the same species and held through overt defense, display, or advertisement
animal constituent of plankton; mainly small crustaceans and fish larvae. (Compare to phytoplankton.)
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