Air-breathing Limpets

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.

REFERENCES

Bouchet, P., J.-P. Rocroi, B. Hausdorf, A. Kaim, Y. Kano, A. Nützel, P. Parkhaev, M. Schrödl & E. E. Strong. 2017. Revised classification, nomenclator and typification of gastropod and monoplacophoran families. Malacologia 61 (1–2): 1–526.

Simone, L. R. L., & M. I. G. L. Seabra. 2017. Shell and body structure of the plesiomorphic pulmonate marine limpet Siphonaria pectinata (Linnaeus, 1758) from Portugal (Gastropoda: Heterobranchia: Siphonariidae). Folia Malacologia 25 (3): 147–164.