User:User53184/Siphonal canal

Article body
The size and shape of the siphonal canal are important in taxonomic identification of mollusks. Sealed siphonal canals are a morphological feature of Muricidae subfamilies Ocenebrinae and Typhinae, a feature not found in other subfamilies from this taxonomic group. One gastropod whose shell has an exceptionally long siphonal canal is the Venus comb murex. Some gastropods have a simple siphonal notch at the edge of the aperture instead of a canal. A short siphonal canal is preferred by some species of obligate commensal bivalves. Curvemysella paula is specially adapted to live inside snail shells occupied by hermit crabs. The crescent shaped shell of C. paula facilitates entry into the narrow space inside snail shells where they attach to the columellae and siphonal canal via byssal threads. The position and location of attachment anteriorly near the final whorls of the shell is an evolutionary behavior of adult individuals which prevents removal by the host.

Evolution

Siphonal indentations have evolved multiple times in gastropods and are widespread among many clades. Euomphanilae gastropods in the genus Scalites developed siphons in the early Ordovician period (448-443 MYA); however, they are not observed in any other members of the clade. 22 of an estimated >23 instances of siphonal indentations evolved in Murchosinoniinae gastropods - the two major clades in this group are Vetigastropoda (1 siphonate group: Tylozone) and Apogastropoda. Apodastropoda contains Caenogastropida (14 instances of siphon evolution) and Heterobranchia (3 instances of siphone evolution).

The evolution of siphonal indentations and related parts are not associated with species diversification and thus are likely not key innovations. Diversification is observed in only 17% of siphonate clades and primarily occurred during the Ordovician, late Paleozoic, and Mesozoic eras and was restricted to clades with few genera. Active predation only occurs in 1 siphonate clade (~4.5%) and is not considered a driver of evolution.