Swamp Coffee
Lately the weather is promising to turn severely mild and it is enticing me out for lunchtime walks again. The boardwalk I take loops through the mangroves, allowing one to walk a short way under the canopy without getting wet. The tide seemed exceptionally high the other day, inundating areas that I usually find dry. But even during lower high-tides, it’s hard not to notice clumps of little brown snails clustered together on the roots and trunks. I’ve often wondered what species they might be and the reasons for this crowding behavior. Why do they seem to be so “afraid” of the water? Why do they cluster together like this? What is the benefit? Do the same snails always crawl to the same spot? Or are they apt to move freely from place to place? Why don’t they go higher up on the roots? How do they know when to stop?
Asking questions like these is one of the lessons I’ve learned from reading Aydin Örstan’s blog, The Snail’s Tales. There is wisdom in these little mollusks. They can tell us so many things about an environment just by their presence (or absence) there. So I finally decided to stop wondering and dig a little deeper for some answers.
It turns out these are likely a species called the coffee bean snail (Melampus coffeus). There is another species that occurs in the area (M. bidatentus), but it is more often found in salt marshes. Both are pulmonates, which means they have a lung and not gills and would likely drown if they stayed underwater too long. This partly explains the reason for not wanting to get their feet wet. It’s also thought that they avoid predation from aquatic predators (crabs and fish) by hanging out in the high and dry. However, they are vulnerable to terrestrial predators like ibis.
The coffee bean snail, Melampus coffeus, is a small intertidal snail. The shell is ovate, tapering toward the base. Most snails are brown with three horizontal light bands (Kaplan 1988), but the background color in some individuals may range from shades of gray or tan to yellowish brown. The aperture is narrow and long, with a sharp outer lip and an inner margin bearing two teeth (Kaplan 1988). The pale brown inner lip is turned slightly backwards, and the spire is conical. Unlike many other snails, M. coffeus lacks an operculum (Ruppert & Barnes 1994).
-From the Indian River Lagoon Species Inventory, Smithsonian Marine Station at Fort Pierce
Ungrounded Coffee Beans
To figure out if these animals always returned to the same spot, I imagined doing some kind of experiment where enough would be tagged and multiple observations made to see if, how and when they returned. Indeed, I soon discovered this is precisely what Proffitt and Devlin did in a mark–recapture study of their own in Boca Ciega Bay, part of the nearby Tampa Bay estuary system. During high-tide they marked all of the snails on three of the black mangrove trees in their study site. They used a different colored paint on the snails for each of the trees. During the subsequent observations they noted the numbers and locations of their marked snails. After about a month, almost half the snails were recorded on the original tree where they had been marked. But some of the 195 original marked snails had moved up to ~6 meters away from their starting tree, expanding to 33 new trees in the area.
The researchers also looked at the effect these snails have on the mangrove leaf productivity. Through a series of experiments with tethered, bagged and caged leaves, they determined that 40.5% of leaf litterfall was consumed by M. coffeus in their study site. As the authors note, this makes the coffee bean snail “a dominant littervore,” converting large portions of the mangrove leaf material into snail biomass and larvae.
The authors also note the snails would maintain their cyclical migrations even when kept in aquariums for a number of days without any tidal flux. So there is probably some chronobiology occurring here. This makes sense, because if they just waited until things started to get wet, it might be too late to make it to high ground (at a snail’s pace). However, I was unable to find any information on what makes them stop. Presumably there is no benefit to migrating higher in the trees, as they would just have to expend the energy to climb back down. They’d also likely be more spread out, exposed to predators (safety in numbers) and drying conditions. If anyone has ideas or knows, please leave a comment.
- C. E. Proffitt, D. J. Devlin, Marine Ecology Progress Series 296, 209 (2005).
- S. B. Rothschild, Beachcomber’s Guide to Gulf Coast Marine Life: Texas, Louisiana, Mississippi, Alabama, and Florida (Taylor Pub, 2004).
Proffitt, C., & Devlin, D. (2005). Grazing by the intertidal gastropod Melampus coffeus greatly increases mangrove leaf litter degradation rates Marine Ecology Progress Series, 296, 209-218 DOI: 10.3354/meps296209
What The Shell is That? is a recurring series on this blog, featuring some of the flotsam and jetsam, flora and fauna that wash up on the beaches of southwest Florida. I am a recreational taxonomist and amateur naturalist. As such, all identifications are speculative unless noted otherwise. If you have additional information on the organisms included here, or have questions about critters you’ve found on the beach, please contact me or leave a comment below.