The hitchhiking snails were so tiny it was impossible to confirm that they were male, but a recent photograph taken by another researcher clearly shows a larger snail, presumably male, associated with the float of a large female.
Was Recluzia truly the transitional form the researchers were seeking? Or might the transitional form be a species of Janthina, whose juveniles build their own floats, rather than hitchhiking on females' floats---a life history more in line with the juvenile droguing hypothesis? To answer that question, Churchill compared physical characteristics of Recluzia and Janthina with those of the ancestral epitoniids. Recluzia, she found, shares six
characteristics with epitoniids; Janthina has none. This finding points to Recluzia as the transitional form, strongly supporting the egg mass hypothesis.
Churchill and colleagues went on to reconstruct the path that led from egg mass to bubble raft. In the scenario they propose, the ancestors of janthinids lived on the ocean floor, and females formed tethered egg masses with associated males, just as a number of present-day epitoniids do. The egg mass then became modified for buoyancy, resembling a typical Recluzia float, which serves as raft, egg-storage area and platform for juveniles. In the next step, all individuals began making their own floats, so the hitchhikers were lost, but the floats continued to serve as rafts and (in females) egg mass carriers. The present-day species Janthina cf. prolongata and Janthina exigua exemplify this lifestyle.
Finally, the rafts lost their egg-carrying function altogether and came to serve only as floatation dev
|Contact: Jim Erickson|
University of Michigan