Patellogastropoda

Patellogastropoda was proposed by David R. Lindberg, 1986, as an order, and was later included in the subclass Eogastropoda Ponder & Lindberg, 1996.[5]

Ventral view of Patella rustica showing its foot and head with tentacles.

Patellogastropoda have flattened, cone-shaped shells, and the majority of species are commonly found adhering strongly to rocks or other hard substrates. Many limpet shells are covered in microscopic growths of green marine algae, which can make them even harder to see, as they can closely resemble the rock surface itself.

The substance making up the teeth in the radula of limpets is among the strongest biological materials known, with a tensile strength about five times stronger than that of spider silk. The teeth are composed of goethite, an iron-based mineral, woven in a particular way into grouped 1μ thick bundles.[8][9]

Many limpets create a home "scar" on the rock to which they always return between tides, the scar provides excellent protection from predators as well as helping to prevent dehydration during low tides. They adhere to the substratum via the adhesion/ suction of the stiffened foot against the rock surface to which it bonds each time with a layer of pedal mucus.[10]

The majority of limpet species have shells that are less than 3 in (8 cm) in maximum length and many are much smaller.

Internal anatomy without most musculature or circulatory information

The true limpets have an internal structure much like that of other members of Mollusca. Their diffuse nervous system is oriented around three principal pairs of ganglia—the cerebral, pleural (which are hypoathroid), and pedal—located in the animal's snout and surrounding its esophagus in a ring. The pleural and pedal ganglia each send a nerve cord back through the rest of the body, the pleural nerve cords and the pedal or ventral nerve cords (the latter are embedded in the foot musculature in Patellagastropoda). Just outside the pedal ganglia are each of the two statocysts (though see Bathyacmaea secunda as an exception to this rule). Like the keyhole limpets, the true limpets have retained both kidneys though in Patellagastropoda the kidneys both lie on the animal's right side and the further right of the two— the "right" kidney— is much larger than the other. The right kidney also has a sponge-like texture whereas the left kidney is essentially a small sac into which hang folds from the sac's walls.[11] They do not have ctenidia, instead obtaining oxygen through a ring of gill lamellae that encircle the mantle just inside the shell edge and from the surface of the roof of the nuchal cavity which is exposed to air when the animal is no longer under water and which is covered in a network of blood vessels all of which eventually carry oxygenated blood and connect to the auricle through a series of veinlets on the animal's left side. Vestigial ctenidia have been adapted into osphradial patches (one on each side of the mantle cavity) with which the animal can "smell". Their low dome-shaped shell is able to withstand the forces of turbulent intertidal water. Inside, the head bears two tentacles, each with a tiny black "eye spot" at its base (limpets can sense light but cannot see images with these eyes). The heart lies within a pericardium and is composed of a single (morphologically left) auricle, a single ventricle, and bulbous aorta which sends blood to both the anterior and posterior aortae. It lies near the surface of shell on the left, and opposite it on the right are three tubules or "papillae" in a row: that of the left kidney, the anus, and that of the right kidney: all three exit near the same place on the right posterior side inside the mantle cavity.

Between these papillae and the heart lies the neural "visceral twist", a nervous condition called streptoneury or chiastoneury, which characterizes many molluscs and all gastropods whose ancient ancestor had an anus located posterior to its head but which now have it positioned much closer because of a change in the arrangement of the shell. In the evolutionary course of the relocation of the anus, the various ganglia posterior to the pleural and pedal ganglia had to conduct a twist— this means, for example, that the osphradium on the animal's left side is innervated through the right side of its body and vice versa. The condition is called streptoneury, but the phenomenon is known as torsion. In the Patellogastropoda, the twist is located directly behind (i.e., posterior to) the pleural ganglia; in other closely related groups (e.g., Zeugobranchia, Neritopsina, and Ampullariidae) the twist stretches backwards well into the visceral mass (digestive glands, intestines, gonad, etc.).

The digestive gland and interweaving intestine occupy most of the visceral mass behind the head. At the posterior ventral end is the large gonad organ which, when ripe, bursts and empties its gametes into the right kidney from which they are then expelled directly into the surrounding water. One theory of the function of the osphradia is to sense the release of such gametes by other nearby patellogastropods, triggering a corresponding release in any proximate opposite-sex animals of the same species (see diagram for additional anatomic information).

Representatives of the true limpets are common inhabitants of rocky shores of all oceans, from tropic to polar regions.[12]

Some true limpets live throughout the intertidal zone, from the high zone (upper littoral zone) to the shallow subtidal, but other species live in deep sea and their habitat include hydrothermal vents, whalebone (baleen), whale-fall[13] and sulphide seeps.[14]

They attach themselves to the substrate using pedal mucus and a foot. They locomote using wave-like muscular contractions of the foot when conditions are suitable for them to graze. They can also "clamp down" against the rock surface with very considerable force when necessary, and this ability enables them to remain safely attached, despite the dangerous wave action on exposed rocky shores. The ability to clamp down also seals the shell edge against the rock surface, protecting them from desiccation during low tide, despite their being in full sunlight.

When true limpets are fully clamped down, it is impossible to remove them from the rock using brute force alone, and the limpet will allow itself to be destroyed rather than stop clinging to its rock. This survival strategy has led to the limpet being used as a metaphor for obstinacy or stubbornness.

Ttagaebi-bap (limpet rice)

Ttagaebi-kal-guksu (limpet noodle soup)

Larger limpet species are, or were historically, cooked and eaten in many different parts of the world. For example, in Hawaii, limpets (Cellana species) are commonly known as ‘opihi,[17] and are considered a delicacy; the meat sells for $25 - $42 a pound (454g). In Portugal, limpets are known as lapas and are also considered to be a delicacy. In Chile they are also called "lapas" but are so abundant that it's just considered a regular dish. Within Gaelic Scotland and Ireland, a limpet is known as a báirnach, and Martin Martin recorded (on Jura) limpets being boiled to use in a substitute for breast milk. In Ulleungdo, a Korean island, limpets are called ttagaebi (따개비) and are used to make ttagaebi-bap (limpet rice) and ttagaebi-kal-guksu (limpet noodle soup)