Genus: Conus
Cone shells are remarkable among gastropods in that they exhibit astonishing variation. There are over 700 recognized species within the family Conidae, and over 500 species just within the genus Conus (Holmes, 2014).
So, why are there so many types of cone snails?
(1) Adaptive Radiation
Why are there so many individual species? This is an example of a phenomenon known as adaptive radiation, or the rapid diversification of a species often resulting from an environmental change.
The specific habitat and food source of an organism is known as its ecological niche, and when new ecological niches arise, organisms tend to speciate (divide into new species) to fill those niches and reduce competition. As each proto-species evolves to fill a given ecological niche, it gains helpful structural and chemical adaptations while losing traits that reduce its evolutionary fitness.
This explains the wide variety of shell patterns, physical structures and toxins found among cone shells. Each variation between species is an adaptation to a slightly different environment or food source. It’s likely that cone shells experienced adaptive radiation after the Mesozoic extinction killed previously dominant species of predatory snails (Olivera et. al 2006).
(2) Genetic mutations
While the theory of adaptive radiation explains cone shell diversity on the macro-level, what specific, microbiological process drives cone shell diversity?
The answer lies in their conotoxin genes, which have one of the highest rates of gene duplication and substitution observed among multicellular organisms (Phuong et. al 2016). Scientists hypothesize that exogenous polypeptide genes (i.e genes that code for proteins that are released from the body) are subject to accelerated evolution compared to endogenous polypeptide genes (genes that code for proteins that function inside the body) (Biggs et. al 2010). The mature toxin sequence of conotoxin genes were found to have an abnormally excessive mutation rate, especially compared to the stable, highly conserved signal region of the gene (Olivera et. al 2006). This high rate of mutation would allow cone shells to adapt complex venom cocktails relatively quickly, increasing their evolutionary fitness in changing environments.
Since exogenous polypeptides– like conotoxins– mediate an organisms’ relationship with its environment, a high rate of mutation among those genes would allow species to adapt quickly to environmental changes. As one author put it, “venom is … a biochemical distillate of the interactions of a particular cone snail species with its biotic environment” (Olivera et. al 2006, p. 224). It’s even possible that stressing factors could trigger an accelerated mutation rate (Olivera et. al 2006).
Consider this: analysis of the species-level complexity of cone shell venom reveals that venom complexity is directly related to diet breadth– that is, cone shells that feed on multiple types of prey have more complex venom than specialists that feed on one type of prey (Phuong et. al 2016). Conotoxin composition’s relation to prey choice could offer one explanation for how conotoxin gene divergence drives speciation events among cone shells. Changes in prey could result in changes in behavior and conotoxin composition, further reinforcing behavioral reproductive barriers between groups of snails.
