The varix: evolution, distribution, and phylogenetic clumping of a repeated gastropod innovation

Abstract

A recurrent theme in evolution is the repeated, independent origin of broadly adaptive, architecturally and functionally similar traits and structures. One such is the varix, a shell-sculpture innovation in gastropods. This periodic shell thickening functions mainly to defend the animal against shell crushing and peeling predators. Varices can be highly elaborate, forming broad wings or spines, and are often aligned in synchronous patterns. Here we define the different types of varices, explore their function and morphological variation, document the recent and fossil distribution of varicate taxa, and discuss emergent patterns of evolution. We conservatively found 41 separate origins of varices, which were concentrated in the more derived gastropod clades and generally arose since the mid-Mesozoic. Varices are more prevalent among marine, warm, and shallow waters, where predation is intense, on high-spired shells and in clades with collabral ribs. Diversification rates were correlated in a few cases with the presence of varices, especially in the Muricidae and Tonnoidea, but more than half of the origins are represented by three or fewer genera. Varices arose many times in many forms, but generally in a phylogenetically clumped manner (more frequently in particular higher taxa), a pattern common to many adaptations.ADDITIONAL KEYWORDS: evolutionary trends -fossil record -Gastropoda -shell sculpture -snails.

How do gastropods grow synchronized shell sculpture? Effect of experimental varix manipulations on shell growth byCeratostoma foliatum(Muricidae: Ocenebrinae)

“…Although such varices are formed in a regular pattern, they are not aligned between adjacent whorls, so one varix cannot act as a direct cue for positioning a new varix. Second, juvenile snails often lack varices (Webster & Vermeij, ), so the first varices must be positioned without reference to any prior varix. For example, juveniles of C. foliatum initially produce many narrow axial ribs that are gradually spaced further apart and increase in size until they achieve the adult pattern of three varices per whorl, with intervarical nodes (smaller axial sculpture elements; Webster & Vermeij, ) placed mid‐way between each varix (Spight & Lyons, ).…”

“…Second, juvenile snails often lack varices (Webster & Vermeij, ), so the first varices must be positioned without reference to any prior varix. For example, juveniles of C. foliatum initially produce many narrow axial ribs that are gradually spaced further apart and increase in size until they achieve the adult pattern of three varices per whorl, with intervarical nodes (smaller axial sculpture elements; Webster & Vermeij, ) placed mid‐way between each varix (Spight & Lyons, ). The synchrony seen in adults is not fully established until shells reach around 4–5 whorls (~30 mm shell length), although this is highly variable (N. B. Webster, unpubl.…”

“…Inset: Anatomy of a varix. 1, intervarical region; 2, uplifting; 3, filling in; 4, new lip (modified from Webster & Vermeij, 2017). Scale bar = 5 mm the aperture of the same shell or from other recently empty shells, were added to a shell by gluing them in the appropriate position with cyanoacrylate glue.…”

“…Many gastropods and ammonoids exhibit a morphogenetic countdown, in which a specific temporal series of structures form (e.g., changes to the sculpture or coiling axis) prior to the terminal aperture (Seilacher & Gunji, 1993;Webster & Vermeij, 2017). Examples include the unusual coiling of heteromorph ammonites ( Figure 6A; Seilacher & Gunji, 1993), the recurved "sinistroid" tuba of some diplommatinid land snails ( Figure 6C; Liew & Schilthuizen, 2016;Liew et al, 2014), and the nine separate origins of subterminal varices (Webster & Vermeij, 2017). The precise positioning during growth of these countdown structures requires the same level of control as synchronized varices.…”

“…Inset: Anatomy of a varix. 1, intervarical region; 2, uplifting; 3, filling in; 4, new lip (modified from Webster & Vermeij, ). Scale bar = 5 mm…”