Bubbly

A common question in marine biology is how do life histories evolve?  Less common is how do life histories contribute to new evolutionary innovations?

Intermediate species with a bubble raft and egg masses (pink below the bubbles). photograph by Denis Riek

It has long been a wonder how snails walk on (or more accurately, under) the surface of water. These snails float in the neuston (at the air-water interface) on rafts made of bubbles resembling the bubble wrap you'll soon be pulling out of holiday packages. How the snails transitioned from a bottom dwelling lifestyle to one closer to the sun has remained a mystery. The key is determining how the bubble rafts evolved. Churchill and her colleagues unraveled the solution by using DNA evidence to piece together a family/evolutionary tree (phylogenetics). Like a court room, the DNA revealed the source of the commotion.  The bubble-rafting snails' closest relatives were bottom-dwelling snails which made egg masses encapsulated in mucus. Bubble-rafters have modified this mucus to be quick hardening in order to trap air for floats.  Intermediate species (see picture) have maintained both the bubble floats and egg masses; however some of the most derived snails have transitioned to brooding instead producing egg masses.

Reference: Females floated first in bubble-rafting snails. CKC Churchill, DO Fioghil, EE Strong and A Gittenberger. Current Biology. 2011.

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