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Families Patellidae, Nacellidae, Acmaeidae, Lepetidae, Lottiidae
The limpet shape is one of the most successful of shell forms. A broad foot adhering to the substrate by suction, covered completely with a protective shell, is a body form that has evolved independently in many different groups of molluscs. The order Patellogastropoda, the true limpets, is a group that separated from gastropods early in molluscan evolution, and is quite distinct from latter evolved groups that have taken up the limpet shell shape. The Patellogastropoda are the most primitive gastropod molluscs, although that is not to say they are unsuccessful animals. They are among the most common molluscs on intertidal rocky shores worldwide.
Most true limpets live intertidally or in the shallow subtidal, down to the depth for which light penetration allows algal growth. Some feed by scraping the fine film of microalgae from what appear to be bare rock surfaces, others crop the thin layer of encrusting algae. Deep-water species probably live on detritus. All true limpets broadcast their eggs and sperm into the sea where fertilisation occurs. After a short larval life, intertidal species settle out onto the shore depending on variation of wind, tide and currents.
The true limpets were previously divided into three families, based on the gill type; Acmaeidae, with one true molluscan gill, or ctenidium, in the mantle cavity over the head; Patellidae, where the ctenidium has been lost and been replaced with a secondary gill of a circle of gill leaflets surrounding the foot in the space between the foot and the shell; and Lepetidae, with no gills at all, respiring through the body surface. But research in the 1990s showed that there were more basic characters, including shell structure and alimentary canal (radula, jaws and gut) which showed the evolutionary history of the group. Shell microstructure is a particularly useful character, with up to six shell layers varying in crystal structure, thickness and sometimes colour. The order is now divided into five families.
The family Patellidae is defined by having nine chromosomes, and on shell microstructure and radula characters. The main external feature of the animal is the secondary circle of gill leaflets completely surrounding the foot. Patellids occur in the eastern Atlantic and southern Africa as well as Australia. In NSW there are two species, both large and common on rocky shores.
The family Nacellidae is defined by chromosomal, shell microstructure and radula characters. As with the Patellidae, the main external feature of the animal is the secondary circle of gill leaflets surrounding the foot. Most species occur in rocky, intertidal habitats. There is only one species in NSW, Cellana tramoserica, the most prominent limpet on rocky shores.
The family Acmaeidae is again defined by shell microstructure and radula characters. In this family there is a true ctenidium gill over the head. Only one species of this family occurs in NSW, the rare, deepwater Pectinodonta kapalae.
The family Lepetidae is distinguished by radula characters and loss of the osphradium, gill, and reduction of mantle tentacles. All species are subtidal. The only species which occurs in NSW is Propilidium tasmanicum, a minute shell up to about 5 mm in length.
The family Lottiidae contains most of the species previously classified in the Acmaeidae. Lottiids occur worldwide and are intertidal, rarely occurring deeper than 30 m. In NSW there are seven members of this family, most of which are common on rocky shores. The gills in this family may be a primary gill or secondary leaflets.
The NSW fauna of true limpets is now fairly well understood at the species level, particularly the intertidal species. Due to the variation in shell form and colouration there are many synonyms for most species in these families. It was not until work based on anatomy rather than the shells alone was published (Ponder & Creese, 1980) that the species were well defined. The intertidal limpets are convenient animals for ecological research, and over the last 30 years they have been subject to intensive study (Underwood, 1974), particularly Cellana tramoserica (see description of that species for details). Knowledge of the deep water and minute species is still poor in terms of distribution, habitat and ecology.
Powell, A.W.B. 1973. The Patellid Limpets of the World (Patellidae). Indo-Pacific Mollusca, 3(15):75-205.
Ponder, W.F. & Creese, R.G. 1980. A Revision of the Australian species of Notoacmea, Collisella and Patelloida (Mollusca: Gastropoda: Acmaeidae). Journal of the Malacological Society of Australia 4(4): 167-208.
Underwood, A.J. & Chapman, M.G. (eds). 1995. Coastal Marine Ecology of Temperate Australia. University of NSW Press: Sydney
All the patellogastropod limpets known from NSW are treated in detail here, with the exception of the following two southern species. These are known from NSW by only a single specimen each in the Australian Museum collection.
Patelloida profunda calamus (Crosse & Fischer, 1864). Twofold Bay, NSW, to WA, including Tasmania.
Notoacmea alta Oliver, 1926. Nadgee, NSW, to Spencer Gulf, SA, including eastern Tasmania.
Some terminology specific to limpets:
The shells of the true limpets in NSW vary widely in shape and colour. Unworn, adult shells of the larger species can usually be identified, but it is almost impossible to identify worn shells, particularly juveniles, and the smaller species. Each species is restricted to a well defined habitat, and this provides a primary character for identification; for example, Notoacmea petterdi occurs on vertical surfaces at high tide level, Patelloida mimula lives on oysters in estuaries, Patelloida mufria lives on other molluscs.
As noted above the limpet shape has been achieved by many other molluscan families with anatomical structures markedly different to the true limpets. The main families are:
Siphonariidae: Air breathing pulmonates closely related to the land snails, with a lung rather than gills. Two species are common on NSW rocky shores. Examination of the underside of the shell shows a groove on the right side at the opening into the lung.
Trimusculidae: Air breathing pulmonates with small, cap-shaped shells with the apex pointed backwards. They have radial sculpture and a siphonal furrow internally on the right side.
Fissurellidae: The keyhole limpets, with a hole at the apex of the shell or a slit in the margin at the front. One species, Montfortula rugosa, is common on NSW rocky shores; it shows only a weak slit in the margin anteriorly.
Capulidae: These shells have a coiled apex, often overlapping the posterior edge.
Hipponicidae: These limpets have a foliaceous surface and live attached to other shells, on to which they secrete a calcareous basal plate. They have a U-shaped muscle scar internally.
Phenacolepadidae: These shells are radially sculptured, with a periostracum, a coiled apex and a U-shaped muscle scar internally.
Cocculinidae: Rare, deep sea limpets, with a thin shell of which the protoconch is usually visible.
Copyright Des Beechey 2005