12 November 2010

A whole lotta Donax—Part 2

In the previous post I wrote about the 436 Donax shells I had once collected in Puerto Rico. In her comment, our regular reader Susan Hewitt asked if any of the shells had been drilled by the predatory naticid snails. As opposed to the irregular holes drilled by octopuses in the shells of their molluscan prey, the naticids (family Naticidae, moon snails) eat up their victims thru almost perfectly round holes.

A few of the Donax shells in my sample indeed have what appear to be naticid holes.

The following figure from British Prosobranch Molluscs by Fretter & Graham shows holes drilled in bivalve shells by the snails Natica (B) and Nucella (D). The former is a naticid, while the latter is in the family Muricidae.

Many more shells in the sample have chips at their edges. Here is an example.

I suspect those clams were preyed on by crabs.


Anonymous said...

I am wondering if these holes were in fact drilled by naticids. I can't see the one on the left very well, but the one on the right does not look typical.

Usually a naticid drill hole is not only perfectly round, but also perfectly countersunk, in other words the typical naticid drilling hole has a beautifully chamfered or beveled outer edge. In an incomplete naticid drilling, there is even a central raised point so the whole thing thing is shaped sort of like an impact crater, except of course it does not have a raised edge!

Muricids on the other hand drill neat round holes that are not beveled. But muricids usually live on rocks rather than in the sand like a naticid does.

I would love to know exactly how the naticid drilling process works in order to produce a precision-engineered drilling hole with chamfered edges! Presumably the snail rotates the clam while rasping with its large and presumably rounded-edged radula, and simultaneously secreting a weak acid? I wonder if anyone has been able to study this process?

I have emailed my extraordinarily knowledgeable friend Harry G. Lee of the Jacksonville Shell Club in Florida to see what he thinks of the valves in your images Aydin.

Susan J. Hewitt

forestwalk/laura k said...

interesting! i will be sure to come back and see what becomes of the 'mystery of the holes'...