Monday, September 28, 2015

Just Making a Living in the Miocene ~ Defense of the Moon Snail

In which the blogger takes undue offense and turns that into a post.
Gastropods are a challenge, raising the question, How could an animal that looks like that create a shell that looks like that?  Put bluntly, I enjoy the shells of gastropods for their aesthetically pleasing geometry.  Snails themselves, sans their shells (e.g., slugs), may markedly less aesthetically appealing, though that should be no strike against them.  They are still fascinatingly complex and marvels of evolution.

Gastropods account for the vast majority of mollusks that have lived, or are living, on Earth.  They are, as paleontologist Donald Prothero put it, “remarkably successful,” expanding into vastly different environments, from the marine waters where they originated over 500 million years ago in the Cambrian Period, to fresh water, and then to widespread terrestrial niches, evolving lungs in the process of making that move onto land.  Although elaborate shells have evolved in some species, in others, shells have been dramatically reduced or, indeed, forsaken entirely.  Their underlying body plan “is least modified from the ancestral molluscan body plan.”  (Prothero, Bringing Fossils to Life, 1998, p. 279.)  The first gastropods were grazers, dining on algae.  (Carolyn H. Declerck, The Evolution of Suspension Feeding in Gastropods, Biological Reviews, Volume 70, Issue 4, November, 1995, p. 549.)  Over time, they evolved ways “to exploit almost every possible mode of life – as herbivores, as deposit feeders, as plankton feeders, as scavengers, as parasites, and as predacious carnivores.”  ( R. D. Purchon, The Biology of the Mollusca, 1977, p. 41.)

Recently, a friend invited me to collect fossils on the western shore of the Chesapeake Bay at a spot abounding in fossil mollusk shells, including those from a wide variety of gastropods.  Here the Little Cove Point member of the St. Marys Formation, a Miocene Epoch formation, is exposed; the fossils eroding out of the cliff sides date from perhaps 10 to 9 million years ago.

On this excursion, we found, among other shells, a wealth of moon snails, members of the family Naticidae.  Two specimens are pictured below:  I believe the first is from Neverita duplicata (among previous names for this species is Polinices duplicatus), the second is from Euspira heros (previously known as Lunatia heros).  These particular species are still with us.

This snail shell is a favorite of mine, although, until this hunting trip, I'd never found ones despite their abundance.  Those in my collection came by other means.  In the process of identifying the wide variety of shells we'd collected on this trip, I was somewhat taken aback by the treatment of the moon snail by Harold E. Vokes, John D. Glaser, and Robert D. Conkwright in the Maryland Geological Survey’s Miocene Fossils of Maryland (Bulletin 20,  2nd Edition, 1999, available on CD from the MGS.)  This edition was prepared by Glaser and Conkwright based on the 1957 edition written by Vokes.

I offer this post as a defense of the moon snail.

Let me be clear, Bulletin 20 is a useful guide for fossil collectors searching along the shores of the Chesapeake Bay.  The volume features descriptions and drawings of most of the large fossil species found here, and “at least one representative of the genera that occur less abundantly.”  (p. 2.)  It also delineates in some detail the characteristics of the three marine formations of the Chesapeake Group – the Calvert, Choptank, and St. Marys – as well as the members of those formations.  Altogether, a valuable resource . . . except for the authors' gratuitous slap at the moon snail's feeding habits.

According to the Vokes and company, three species of moon snails lived here during the Miocene – Polinices duplicatus, Lunatia heros, and Lunatia hemicrypta.  (As noted above, the accepted scientific names for these species have changed.  Well, that’s true for the first two.  Tracking down the current name for the third continues to challenge me.)  Here’s the complete first paragraph of the Bulletin’s description of the family:
The species of the family Naticidae are carnivorous animals that prey upon other mollusks.  They seek out another shell and, using a peculiar tooth ribbon that is common to almost all snails, drill a small round hole through it and suck out the juices of the animal inside.  These holes, usually about one-eighth inch in diameter, are often seen in the shells of bivalves and gastropods in the Maryland Miocene fauna and are mute reminders of the unpleasant death suffered by the animal that formed the shell.  (p. 22)
I understand fully why the authors drew attention to these holes.  They are easily and widely found on fossil mollusk shells of the Chesapeake Group.  According to paleontologist Sally E. Walker, "The best trace fossil evidence of gastropod predatory activity are drill holes preserved in molluscan prey."  (Traces of Gastropod Predation on Molluscan Prey in Tropical Reef Environments, in Trace Fossils:  Concepts, Problems, Prospects, edited by William Miller, III, 2007, p. 339.  A portion of this chapter can be found on Google Books.)  More specifically, she noted that the members of the Naticidae family, in particular, "leave an excellent trace fossil record."  (Walker, p. 329.)

I don't have any quarrel with the attention Vokes and his colleagues gave to drill holes nor with their focus on moon snails in this regard.  But this is the only place where the authors provided any detail about the feeding habits of Maryland Miocene gastropods.  As a result, they seem to be suggesting that only moon snails use this method of predation.  Further, I think they, quite inappropriately, evinced a degree of revulsion with the naticid means of gaining sustenance.

My unease with this paragraph prompted me to consider how the moon snails fit into the broader context of the rest of the gastropod taxa Vokes et al. cited in Bulletin 20.  I asked and tried to answer several questions:  (1) How common were carnivorous gastropod genera in the Maryland Miocene?  (2) Among these taxa, how unique were the moon snails in this mode of attack?  (3) What other methods of predation might these taxa have practiced that could offend some sensibilities?

Prevalence of Carnivorous Gastropod Genera

Vokes and company cited a total of 55 gastropod genera.  I assume that 29 of them are extant because they appear in zoologist Harald A. Rehder’s National Audubon Society Field Guide to North American Seashells (1981).  For each family to which these genera belong, Rehder’s volume describes, often in wonderful detail, how its members typically feed.  In general, I ascribed the method for the family to any genera in that family.  When, for a very few families, Rehder described multiple, different approaches to feeding, or an unknown feeding habit, I relied on the Paleobiology Database (accessed through the Fossilworks, a portal to the database) and went with whatever it stated in its brief listing of feeding habits for specific genera under "Ecology."  I also crosschecked Rehder's family feeding description with the genera feeding data in the Database.  There are some significant limitations inherent in both of these sources:  e.g., information only to the family level in the Field Guide, only scant description at the genus level in the Database.  So, whatever I've come up with probably only gets me into the ballpark.

That said, I found that almost 82 percent (45) of the 55 gastropod genera probably are, or were, carnivores (this includes 3 parasitic genera).  Given that the vast majority of snails in the Maryland Miocene were carnivorous, the moon shells clearly weren’t alone in their choice of food.

Who Drilled?

Interestingly enough, moon snails were not the only drillers included in the gastropod taxa that Vokes and his crew listed.  Though the Naticidae are considered a "major" drilling taxon among gastropods, they are joined in that designation by the Muricidae.  (Walker, p. 325.)  Members of that latter family are notorious drillers.  Indeed, today’s Urosalpinx cinerea carries the common name Atlantic Oyster Drill because it’s the scourge of oyster beds, drilling through the shells and feeding on the soft tissue inside.  Several genera belonging to the Muricidae family graced the waters during the Miocene here, such as Ecphora and Urosalpinx.

Perhaps Vokes' sole focus on the moon snails reflected the research available (then and now) on gastropod drilling in this time and place which has focused almost exclusively on the naticids.  I cannot believe that the Muricidae weren't drilling; that's one of the things they do.  Have I just overlooked the relevant research?

I thought maybe the fossil shells I have collected from the Chesapeake Bay could shed some light on this Naticidae and Muricidae disconnect.  The handiwork of these two families can be told apart:  holes drilled by Naticidae are beveled, wider on the exterior surface and narrower on the shell's interior, while those of the Muricidae are nearly straight.  (Alan R. Kabat, Predatory Ecology of Naticid Gastropods With a View of Shell Boring Predation, Malacologia, Volume 32, Number 1, 1990.)  The picture below of a bivalve shell from the St. Marys found on this same trip shows clearly that it fell victim to a moon snail.  The configuration of the hole tells it all.  This shell is from the species Dallarca idonea (cited in Vokes et al. as Anadara idonea.).

But, when I looked at my fossil shells for drilled holes that might have been made by Muricidae snails, there were none to be found, at least, none that I would, with confidence, attribute to them.  That's troubling because members of this family are drillers and they would have drilled here.  I have a thought about what might be partly at a work.  The Naticidae are infaunal feeders, which means they are operating partly or completely covered by bottom sediments, whereas the Muricidae are epifaunal, doing their thing on the sediments.  This makes a world of difference for the kinds of mollusks each type of snail is likely to encounter and feed upon.  Might that also influence the kinds of mollusk fossils commonly collected?  It's a whole new line of research for me (for another time and place).  In the end, I still come back to my conviction that the Muricidae were drilling here.

Other Dining Styles That Might Offend

I guess I was particularly dismayed by Vokes’ closing description of the drilled holes:  mute reminders of the unpleasant death suffered by the animal that formed the shell.  Is there some moral judgement here?  Some disgust at how the moon snail made, and makes, its living?  As far as I'm concerned, at that moment the authors stepped out of character, departing dramatically from the tone and the content of the rest of the volume.

What’s telling is that the Naticidae and other drillers certainly weren’t the only taxa likely to have engaged in feeding habits during the Maryland Miocene that might offend some sensibilities.  This came clear from some of Rehder’s descriptions of how those 29 extant genera included in his guide caught and consumed their food.  (And it doesn't excuse Vokes' closing words that these other feeding processes were unlikely to leave fossil traces.)

For instance, here’s what Rehder had to say about the Epitoniidae family (the Wentletraps), “[They] are carnivorous, preying on sea anemones, corals, and probably other coelenterates.  They feed by tearing off big pieces of tissues with their large, paired, filelike jaws.”  (p. 449.)

Or consider the Oliva genus which lived in these Miocene waters.  Still with us, it's part of the Olive Shell family (Olividae).  How do these family members gain sustenance in marine waters?  “Olive shells are scavengers or carnivores; they feed on bivalves and crabs, enveloping their prey with the foot and then carrying it under the sand to digest at leisure.”  (p. 579.)

One of my favorite approaches to feeding is practiced by the Conus genus whose fossil shells are found in the St. Marys formation (albeit rarely); the genus is part of the Conidae family (Cone Shell).  These gastropods have evolved a neat method of securing their next meal.  Their radula teeth have become harpoon-like spears that the animals launch at prey to capture them; then, through these teeth, the gastropods deliver a powerful, killing toxin, a poison that is quite dangerous for humans.  I wont even get into the practices of parasitic gastropods.

Sure, I may feel some twinges of revulsion at some of these methods of feeding, but I recognize how parochial that is of me.  After all, these are simply ways of making a living which have evolved because they enhance the ability of organisms to survive and reproduce.  They are the result of natural selection at work on the variation in living organisms.  As biologist Jerry A. Coyne has noted, “Selection is not a mechanism imposed on a population from outside.  Rather, it is a process, a description of how genes that produce better adaptations become more frequent over time.”  (Why Evolution is True, 2009, p. 117.)  So, there’s no morality to be imposed or found here, this behavior of the moon snails and other carnivorous gastropods just is.  Evolutionary biologist Stephen Jay Gould summed it up quite well, “[N]ature contains no moral messages framed in human terms.”  (Nonmoral Nature, Hen’s Teeth and Horse’s Toes, 1994, as reprinted in the Unofficial Stephen Jay Gould Archive.)
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