Siphonaria sipho, copyright G. & Ph. Poppe.

Belongs within: Heterobranchia.

The Siphonarioidea are a group of gastropods uniting the living Siphonariidae with the tentatively assigned fossil Acroreia. The Siphonariidae, false limpets, are a group of air-breathing limpet-like gastropods found in marine and brackish waters. A lateral pneumostome is present, its position marked by an inner groove on the shell and an interruption in the ring of muscle scars. Acroreia is a genus of smooth, thin-shelled limpets known from the Cretaceous and early Tertiary.

Air-breathing limpets
Published 30 May 2020

For many people, the most familiar members of the gastropods are the terrestrial snails. Gastropods started their evolution as marine animals, breathing through gills, but members of one lineage would instead evolve their own version of a lung, a large hollow in the mantle cavity opening through a hole alongside the head called the pneumostome. Possession of this lung cavity would enable slugs and snails to thrive in the terrestrial environment but the structure had originally evolved in a marine context, and even today one may find marine lung-bearers occupying habitats along the coast. One such group is the siphon limpets or ‘false limpets’* of the Siphonarioidea.

*You know, normally I don’t overly concern myself with vernacular names. They are not regulated and not obliged to follow reason. But even so, the name ‘false limpet’ makes me grit my teeth. The name is presumably inspired by the fact that siphonariids are not direct relatives of the ‘true’ limpets of the Patellogastropoda. But the limpet morphotype, where the typical spiral gastropod shell is reduced to a simple cap, has evolved on multiple occasions. As well as the siphonariids and patellogastropods, there are the keyhole limpets of the Fissurellidae, the freshwater limpets of the Ancylini, and many others, all consistently referred to as ‘limpets’. The name refers to a morphology, not to a clade, and by that measure the siphonariids are no more ‘false’ than any other limpets.

Flat siphon limpets Siphonaria atra, copyright Ria Tan.

Living siphonarioids are placed within a single family, the Siphonariidae, whose members with their cap-shaped, often radially ribbed shells are found in littoral environments in temperate and tropical regions of the world. A second family, the Acroreiidae, is recognised from the Cretaceous and early Tertiary; the inclusion of these smooth, thin shells in the Siphonarioidea is somewhat tentative (classification of limpets in the fossil record is always a challenge because their simple shell form renders them light on distinguishing characters). Siphonariids are readily distinguished from other living limpets by the presence of a groove on the underside of the right side of the shell marking the position of the pneumostome. In dorsal view, this groove is often indicated by an asymmetry in the outline of the shell with one side produced. The pneumostome is also associated with a broad gap in the ring of muscle holding the shell in place; the ring is more complete in other limpets. Seemingly as a result of this lower extent of muscle, siphonariids cling to their home rocks with less tenacity than other limpets and are mostly restricted to more sheltered locations (Simone & Seabra 2017). On the other hand, they do have a more flexible foot than their competitors, allowing them to potentially move more quickly. Like other limpets, siphonariids are grazers, scraping microalgae as they crawl about. Siphonariids have a weaker radula than patellogastropods and so scrape somewhat less forcefully; when members of the two clades occupy the same habitats, patellogastropods are generally the more abundant. The majority of siphonariids (where known) have planktonic larvae but some species are known to be direct developers.

Siphonaria lessonii, copyright Mikelzubi.

Obviously, the marine but lung-possessing siphonariids are potentially of great interest in understanding how the gastropod lung evolved. Many earlier researchers thought that the siphonariids may have evolved from terrestrial ancestors who had returned to the seashore but this is no longer thought likely to be the case. In most lunged gastropods, gas exchange is effected in the mantle cavity via dense blood vessels in the cavity wall but in siphonariids a gill structure is present within the lung (this lung-gill combination makes the siphonariids particularly well suited for moving freely both above and below the water surface). The gill of siphonariids is quite similar to that of the sacoglossans, a group of herbivorous sea-slugs. Though it was long presumed that the lung-bearing gastropods belonged to a single clade, more recent molecular phylogenies have confused the issue (Kocot et al. 2013). The sacoglossans are likely to be close to the ancestry of lunged gastropods as a whole, but it is possible that siphonariids are more closely related to the sacoglossans than the other lung-bearers. It remains an open question whether the siphonariid combination of lung and gill represents an intermediate stage towards the vascular lung of the terrestrial forms, or whether siphonariids and other lung-bearers each evolved their pneumostome from close but distinct ancestors.

Systematics of Siphonarioidea
<==Siphonarioidea [Divasibranchia, Siphonariacea, Siphonariida, Siphonarimorpha]
    |--Acroreia Cossmann 1885 [=Acroria Cossmann 1889; Acroreiidae, Acroriidae]BR05
    |    |--*A. baylei (Cossmann 1882) [=Nacella baylei]BR17
    |    `--A. (Vasculum) obliqua (Cossmann 1885)TTE93
    `--Siphonariidae [Siphonariadae]BR05
         |--Siphonacmea Habe 1958 [Siphonacmaeidae, Siphonacmeidae]BR05
         |    `--*S. oblongata (Yokoyama 1926) [=Acmaea oblongata]BR17
         |--Liriola Dall 1870 [Liriolidae, Liriolinae]BR05
         |    `--*L. thersites (Carpenter 1864) [=Siphonaria thersites]BR17
         |--Benhamina Finlay 1927P61
         |    `--*B. obliquata (Sowerby 1825)P61 [=Siphonaria obliquataP61, Kerguelenia obliquataF27]
         |--Berlieria deLoriol 1903TTE93, KC60
         |    `--*B. ledonica de Loriol 1903KC60
         |--Williamia Monterosato 1884O27
         |    |--W. peltoides Carpenter 1864O27
         |    `--W. vernalis Dall 1870O27
         |--Anisomyon Meek & Hayden 1860 [Anisomyonidae]BR05
         |    |--*A. borealis (Morton 1842)BR17 [=Hipponix borealisBR17; incl. Helcion carinatus Meek & Hayden 1856W77]
         |    |--A. centrale Meek 1870W77
         |    |--A. depressus Etheridge 1907F71
         |    |--A. haydeni Shumard 1861BS00
         |    `--A. patelliformis Meek & Hayden 1860BS00
         |--Kerguelenella Powell 1946P61
         |    |--*K. redimiculum [=Siphonaria redimiculum]P61
         |    |--K. flemingi Powell 1955P61
         |    |--K. lateralisP61
         |    |    |--K. l. lateralisP61
         |    |    `--K. l. macquariensis Powell 1939P61
         |    `--K. stewartiana Powell 1939P61
         `--Siphonaria Sowerby 1823 [Siphonariinae]BR05
              |  i. s.: S. aequilirata [=S. aequilorata; incl. S. leviuscula]C64
              |         S. algesirae Quoy & Gaimard 1833TMT96
              |         S. amaraC64
              |         S. atra Quoy & Gaimard 1833H09
              |         S. capensisE96
              |         S. characteristica Reeve 1842R42 [=S. gigas f. characteristicaEO64]
              |         S. denticulata Quoy & Gaimard 1833H09
              |         S. depressa Pease 1863K65
              |         S. gigasEO64
              |         S. kurracheensis Reeve 1856BD86
              |         S. lecanium [incl. S. ferruginea, S. lecanium var. maura, S. lecanium var. palmata]C64
              |         S. normalis Gould 1846WBS-S15
              |         S. pectinataGO06
              |         S. picaC64
              |         S. propriaHS01
              |         S. siquijorensis Reeve 1856H09
              |         S. tristensisC64
              |         S. virgulataH15
              |         S. wieseri (Wade 1926)KB01
              |--S. (Siphonaria)P61
              |    |--*S. (S.) sipho Sowerby 1823BR17
              |    |--S. (S.) australis Quoy & Gaimard 1833P61
              |    |--S. (S.) laciniosa (Linnaeus 1758) [=Patella laciniosa]KB01
              |    `--S. (S.) zelandica Quoy & Gaimard 1833P61
              |--S. (‘Kerguelenia’ Rochebrune & Mabille 1889 non Stebbing 1888)O27
              |    |--S. (K.) brannani Stearns 1872O27
              |    |--‘Kerguelenia’ innominata Iredale 1915F27
              |    |--S. (K.) lateralisC64, F27
              |    `--‘Kerguelenia’ redimiculumF27
              `--S. (Simplisiphonaria Hubendick 1945)P61
                   `--S. (*S.) colliana Suter 1909P61

*Type species of generic name indicated


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