Limpets

Allison Merritt

Limpets are found in marine environments, mainly within the mid-intertidal and subtidal zones. They are a member of the phylum Mollusca, class Gastropoda, subclass Opisthobranchia, and the order Archeogastropoda. Limpets are the most primitive of all the gastropods (Lutz, 1986).

Limpets have the same main structure as other molluscs. They are divided into three distinct regions: The head-foot which is basically concerned with locomotion and sensory reception, the visceral mass which contains organs of digestion, circulation, excretion, and reproduction, and the mantle which surrounds the visceral mass and secretes the shell (Lutz, 1986). The shell of the limpet has become secondarily symmetrical, as opposed to its asymmetrical juvenile shape, and looks like a Chinese straw hat. Especially adapted for clinging to rocks and shells, the limpet has a low broad shell that can be pulled down tightly and offers less resistance to waves and currents in its environment. Its large mucus covered foot functions as an adhesive organ as well as for movement and its skirt-like mantle margin may have tentacles, which are an important sense organ (Barnes and Ruppert, 1994).

Limpets move by creeping on the broad flat ventral foot. They achieve this by deforming their body. Small transverse waves of contraction sweep along the foot of the limpet from back to front. The area of the foot in the contracted region is lifted and after relaxation it is replaced on the substratum a little in front of the point at which it was raised. During each wave of contraction a small section of the foot performs a step and all the steps combined give the appearance that the limpet is gliding (Barnes et al., 1973).

To feed, limpets use their radula, with the exception of some patellacean limpets. The radula is a ribbon-like tongue that has many teeth, at least twelve in each transverse row, that act as a broad scraper. When the radula is retracted the marginal fan-like teeth direct ingested particles into the center of the gutter of the limpet, which is produced by lateral folding of the radula ribbon. A food-laden mucous string is formed within the gutter and pulled into the esophagus and then into the stomach by rotating a mucous mass within the style sac. With the exception of the patellacean limpets, the stomach is like that of a generalized mollusc. Most molluscs have one pair of salivary glands that open into an anterior dorsal wall buccal cavity. These glands secrete mucus that lubricates the radula and entangles the ingested food particles. The food in the mucus string moves from the buccal cavity into the tubular esophagus and then to the stomach. The stomach of a generalized mollusc has the shape of an ice cream cone, with a broad anterior end where the esophagus opens and a tapered posterior end, which leads to the intestine. The stomach is lined with chitin except for a ciliated region where two ducts make a pair of digestive glands. The posterior region of the stomach is ciliated and called the style sac. Cilia of the style sac are responsible for rotating the contents of the stomach. Digestion is partly extracellular by enzymes released from glands in the esophageal region and partly intracellular within the digestive diverticula.

Intertidal limpets homing ability, the ability to return to their home site, may be related to their feeding. Homing depends on chemical cues laid by the limpet on feeding excursions. Homing may reduce intraspecific competition, by establishing a grazing territory and may also reduce desiccation and predation of the limpet because of the tight fit it has to its home site. A remarkable discovery made by Connor and Quinn (1984) found that the mucous of some of the homing species, but not of the nonterritorial migratory species, stimulates algal growth in their territory (Barnes and Ruppert, 1994).

The only limpet that has been found to live in the photic zone of the western North Atlantic is the Acmaea testudinalis. This limpet appears to have an affinity for grazing on crustose corallines. The limpets in Steneck’s (1982) experiment were never observed eating macrophytes even though the abundance of macrophytes increased during the experiment. Corallines that lacked crustal branches were the only types of crustose corallines that the limpets grazed on. This is probably because it is the most nearly planar surface in the intertidal and subtidal zones and limpets need a close fit on a smooth surface to feed. It seems that the radula of A. testudinalis has evolved to work particularly well on coralline crustose. They have short, stout teeth with a wide base of attachment, which gives them greater excavating abilities on the hard coralline crustose. Rather than feeding on high quality food such as diatoms or filamentous algae, the limpets have found it beneficial to use coralline because it doesn’t vary seasonally, as the others do. The limpets also preferred to graze on coralline at night and away from their home site. The relationship between the limpet and the coralline was found to be mutually beneficial. The coralline crustose that had limpets grazing on it was healthier than those that did not have any limpets grazing on it. Probably because the limpets prevented the coralline from becoming overgrown (Steneck, 1982). Other types of limpets also eat fine algae, sponges, and other organisms growing on kelp (Ruppert and Barnes, 1994).

Respiration in limpets is achieved by the shell overhang, which produces a grove on each side between the foot and the mantel edge. The inhalant ventilating current, produced by the action of lateral cilia of the gills, enters the mantel cavity anteriorly on the left side. It then flows through the gills, only present on the left side, up through the gill filaments, then continues upward, and exits on the right. The patellacean limpet lacks a gill in the mantel cavity. Instead the mantle folds forming secondary gills, which project into the groove on each side of the body. Some species’ exiting current or part of it may flow posteriorly in the mantel grooves (Ruppert and Barnes, 1994).

Limpet reproduction is external. Both of their kidneys are functional. The right kidney provides the outlet for either the sperm or eggs. The eggs are provided with gelatinous envelopes, which are produced by the ovary or the terminal part of the kidney. Gametes pass though a short duct that extends from the gonad and opens into the distal part of the kidney, the gametes are then directed by the kidney into the mantle cavity through the nephridiopore. The genital duct is comprised of the gonoduct proper and the right kidney. Fertilization takes place in the seawater after the eggs are swept out of the mantel cavity (Ruppert and Barnes, 1994).

In conclusion, the limpet has evolved over time to find its niche in the mid-intertidal and subtidal zones of the marine ecosystem. It has formed a mutualistic relationship with food preference so that they may both coexist in the same habitat.

Works Cited

Barnes, R. D., and E. E. Ruppert. 1994. Invertebrate Zoology. 6th ed. New York: Saunders College Publishing Harcourt Brace College Publishing.

Barnes, R. D., C. A. Villee, and W. F. Walker Jr. 1973. General Zoology. 4th ed. Philadelphia: W. B. Saunders Co.

Lutz, P. E. Invertebrate Zoology. 1986. Reading: Addison-Wesley Publishing Co.

Steneck, R. D. 1982. "A Limpet-Coralline Alga Association: Adaptations and Defenses between a Selective Herbivore and its Prey." Ecology 63(2): 507-22.

Connor, V.M., and Quinn, J.F. 1984. "Stimulation of Food Species Growth by Limpet Mucus." Science. 225: 1843-844.