27 August 2006

One more snail at the edge: Melampus bullaoides


Melampus bullaoides and the other snails in the family Ellobiidae always live near the sea in mangrove areas, salt marshes and on rocky coasts. Their lives, especially their reproduction, are so closely tied to the sea that they are usually not considered to be land snails. They are not exactly aquatic snails either; they have evolved to live at the boundary of the sea and the land.

I found this specimen among stranded piles of rotting seaweed on a rocky coast near Tampa, Florida.


Like their more terrestrial distant relatives, ellobiid snails have lungs and are hermaphrodites, but unlike the former, they have their eyes not at the tips but at the bases of their tentacles. This latter characteristic makes them look more like the marine prosobranch snails. Moreover, as Martins1 noted, the species in the subfamily Melampinae, including M. bullaoides, have veliger larvae that start out as plankton in the sea before settling down to more terrestrial lives. The larvae also have opercula, which they lose at an early age; adult ellobiids don't have opercula. In comparison, many marine snail species retain their opercula in adulthood.

Taken together, these and other pieces of evidence unequivocally demonstrate that ellobiid snails evolved from marine ancestors.

The previous post in this series of snails that live at the edge of the sea (or land, depending on which way evolution is taking them) was about Cerithidea scalariformis.

1. Martins, A. M. de F., 1996. Anatomy and systematics of the western Atlantic Ellobiidae (Gastropoda: Pulmonata). Malacologia 37:163-332.

1 comment:

pascal said...

That's interesting - I've always thought that to be fully terrestrial, there needed to be some kind of peramorphosis in the marine larvae to achieve terrestriality. The fact that this snail has a veliger state is pretty cool.