24 July 2008

Which came first, the snail or the egg?

The tiny land snails of the genus Vertigo have appeared on this blog before (here and here). The shells of most Vertigo grow to be about 2.5 mm in length. They reproduce by laying eggs that are, out of obvious necessity, even smaller. But, as discussed here, there is a limit to how small an egg can get relative to its mother.

According to Myzyk (2005), who measured about Vertigo 3200 eggs, the mean egg diameters of 7 species of Vertigo varied from about 0.54 mm to about 0.74 mm. The largest eggs belonged to Vertigo moulinsiana.

Vertigo moulinsiana and its egg. Figs. 2-3 from Myzyk (2005).

As you can also see in the picture of V. moulinsiana on this page, the aperture of that species and those of most other Vertigos are rather cluttered by the various folds and lamellae inside, usually referred to as the teeth.

How do then the relatively large eggs pass thru those barriers? Myzyk reported that the eggs of the Vertigo species he examined were not spherical but slightly flattened and, furthermore, they had gelatinous covers rather than hard shells. They were somewhat flexible, in other words. Myzyk summarizes it thus: "All the studied species have toothed apertures; the flattening of the eggs and the absence of calcified envelopes make it possible for the eggs to get out of the shell."

This leads to the question posed in the title: did the genus Vertigo evolve partially blocked apertures first or eggs with gelatinous covers first? They probably couldn’t have had the fully developed teeth in their apertures while trying to lay hard-shelled eggs. So, I suspect evolution followed either of the 2 possible pathways: 1. Their eggs lost their hard shells first (or, perhaps they never had hard-shelled eggs) and then the apertural teeth came; 2. The apertures and the softening of the egg covers evolved concurrently.

Myzyk, S. 2005. Egg structure of some vertiginid species (Gastropoda: Pulmonata: Vertiginidae). Folia Malacologica 13:169. pdf (open access but registration required)

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