09 March 2010

Stenacme floridana: what was Pilsbry thinking?

In 1945, the great Henry Pilsbry, who was then about 83 years old and still very active, described a new family, a new genus and a new species of what he thought was a pulmonate marine snail from Florida: Stenacme floridana, family Stenacmidae.

This was a small, about 6 mm long, snail. Because it had an operculum, Pilsbry first thought it was prosobranch, or a gill-breathing snail. Then, he changed his mind.

It was not until I examined the radula that it became clear that we had a member of the Pulmonata. It is one of the very few pulmonates, like Amphibola, having an operculum.
Pulmonate snails breathe by means of a lung and the majority of them do not have an operculum; they lost it during their evolution from an ancestor that, nevertheless, had one. We know that the ancestor of the pulmonates had an operculum, because the adults in only one pulmonate genus, the amphibious Amphibola carry an operculum and the embryos in the pulmonate family Ellobiidae, also amphibious, go thru an operculated stage during their development. Based solely on the radula morphology of his specimens, Pilsbry thought he had a new family of operculated pulmonates.

But when Pilsbry dissected specimens of his new snail, he saw that they had a well-developed gill in their mantle cavity. His paper even includes a drawing of Stenacme's gill.

Pilsbry's drawings of a live
Stenacme floridana and its opened mantle cavity showing the gill.

Pilsbry was confronted with clear evidence that something was amiss with his taxonomic interpretation. But he still believed he had a pulmonate snail. He called the snail's mantle cavity the "lung cavity" and its gill a "secondarily developed gill".

All pulmonate gastropods, including the operculated Amphibola (see, Pilkington et al. 1984), have a pneumostome, the opening to the otherwise sealed lung inside the mantle cavity. The pneumostome of a live snail is relatively easy to observe, because it periodically opens and closes for gas exchange with the atmosphere.

If Stenacme floridana was a pulmonate, it should have had a pneumostome. This little fact seems to have slipped Pilsbry's mind; he makes no mention of seeing a pneumostome either in live or dissected snails.

Stenacme floridana is in fact a gill-breathing marine snail. This was established in 1958, a year after Pilsbry died, by Robertson & Oyama. The species, now called Alexania floridana, is in the family Epitoniidae.

How did the master Pilsbry, who had ruled the world of malacology for many decades, make such a gaffe? Had old age clouded his mind and his eyes? Had he grown too powerful and peerless and unjustifiably moved beyond criticism? Perhaps, the answer is all of the above and more.

In his long obituary of Pilsbry, H.B. Baker wrote:
...he positively was pained by the exposure of occasional real errors, such as those caused by faulty optics (inadequate microscopes or, when he grew oldest, his own tired eyes).
Baker also noted that Pilsbry's co-workers sometimes let his errors remain uncorrected, because, otherwise, "his feelings would have been hurt".

It seems that in the world of scientific research it is never too safe to leave everyone else behind.

Baker, H.B. 1958. Nautilus 71:73.
Pilkington et al. 1984. Journal of the Royal Society of New Zealand 14:327.
Pilsbry, H.A. 1945.
Nautilus 58:112.
Robertson, R. & Oyama, K. 1958.
Nautilus 72:68.


Anonymous said...

I think back then that radula studies were really the hot thing! The radula was considered to be the Latest Greatest Way of seeing "beneath the lid" in terms of taxonomic affinities. The radula was really thought to be a Rosetta Stone, and was held in very high esteem, in the same way that molecular studies are now.

I have to assume that the radula formula of this species, or maybe that of epitoniids in general (being that they are ectoparasites) is a bit atypical and somewhat resembles the radula of pulmonates?


Susan J. Hewitt


I don't know much about radulae & I couldn't tell a pulmonate from an epitoniid using only its radula. Then again, neither could Pilsbry.