08 April 2008

Who preys on Truncatella?

Occasionally, while examining a high-resolution picture on my computer monitor, I may see details in the picture that I didn’t notice while taking the picture. An example was discussed in this post. This happened again recently. When I was in Florida near the end of March, I photographed a crawling Truncatella in its habitat within a pile of seaweed stranded behind a beach. Later at home, I was disappointed that the picture of the crawling Truncatella itself wasn’t very revealing, but in the same frame I saw something else that was more interesting and that had escaped my attention in the field: a peeled empty Truncatella shell (near the right hand corner).


If I had noticed that shell in the field I would have taken it. But a day later, there was no hope of finding a particular shell that was only a few millimeters long among seaweeds even if I had remembered the exact spot where I had taken the picture. Luckily, however, I have a few similarly peeled Truncatella shells in my collection. Here is one of Truncatella caribaeensis (shell length=~4 mm).


Crabs are known to peel the shells of marine gastropods that they prey on. Could it be that tiny juvenile crabs practice their skills on the equally tiny Truncatella before moving on to larger snails? Carabid beetles, which are known predators of land snails, are another possibility. So when I found this beetle (~11 mm long) on the same beach under a pile of seaweed where there were also lots of Truncatella, I got excited. I thought maybe this was the Truncatella predator. I must admit, though, that I don’t know if this was a carabid* or not.


I took the beetle and some live Truncatella home, put them in a petri dish with some damp sand and seaweed. For about a day and a half I checked the dish several times in hopes of finding peeled shells. But, alas, there were none. The Truncatella kept crawling around while the beetle hid under the sand. Eventually, I returned them all to the beach.

The mystery peeler of Truncatella remains at large.

*Note added later: Since I posted the picture of the beach beetle on BugGuide.net, it has been identified as a Scarites sp. in the family Carabidae as I had suspected.


Anonymous said...

I think you're on the right track with arthropods due to the mechanics required for peeling. Hard to say exactly, but some of the really small hermit crabs and juveniles of other types have to eat, too. :)

Chad Ferguson

Cephalopodcast said...

Found a beetle similar (same?) to that one on a mangrove island in Tampa Bay during a coastal clean up. Picked it up to show someone. Must of sprayed me with something, because there was the most noxious smell on my hand after that. Replusive.

Wanderin' Weeta said...

Always something more to learn here!

I have seen peeled snails, but I just thought they had been broken by wave action.

How big a snail can a crab peel?


Landon Ross, whose opinion I had seeked, e-mailed these comments:

"I fear that I have no revealing information on the question of the Truncatella predator. I, too, have seen the peeled shells, and I agree that carabids seem likely to be the tigers in question. Of course, various other arthropods often share the same seaweed drifts, especially young fiddler crabs and occasionally myriapods, as well as other sorts of beetles.

I seem to recall that one sees this phenomenon mostly, perhaps entirely, in the more smooth, thin-shelled (and typically larger) species such as T. caribaeensis and T. pulchella, rather than in the more heavily ribbed types: T. scalaris or T. bahamensis and such. Indeed, perhaps the beetles (if those are the predators) are a driving evolutionary force for the ribbing. I would also note that the reflected apertural ridge is usually very robust in the heavily ribbed taxa (not surprisingly), and this, alone, might offer significant protection against "peelers"."


Wanderin' Weeta: I don't know the maximum size threshold beyond which snails become immune to crab predation. Protection depends not only on size but also on shell thickness & any knobs, ridges that may be present on the outside of the shell & also within the aperture.

Wanderin' Weeta said...

Thanks, Aydin. BTW, I discovered some of my own peeled snails, thanks to you. I blogged about it (linking to you) here.