It's snail week on the Daily Bucket!
The Daily Bucket is a regular feature of the Backyard Science group. It is a place to note of any observations you have made of the world around you. Insects, weather, meteorites, climate, birds and/or flowers. All are worthy additions to the bucket. Please let us know what is going on around you in a comment. Include, as close as is comfortable for you, where you are located. Each note is a record that we can refer to in the future as we try to understand the patterns that are quietly unwinding around us.
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Florida is known as a haven for introduced species. In fact, southern Florida is second only to the Hawaiian Islands (within the U.S.) in the extent of its non-native biota. The state is overrun with invading plants, reptiles, birds, frogs, fishes, and so on. One type of invasive that may not have occurred to you are invasive snails.
Among the more impressive of the gastropods, invasive or otherwise, are the apple snails. Apple snails are members of the family
Ampullariidae, a group of large freshwater snails that are widespread in the tropical regions of world. They are frequently kept in freshwater aquaria and many introductions are unwanted former aquarium animals. They are herbivores and are potentially capable of causing extensive damage to aquatic vegetation. Interestingly Apple Snails are more closely related to marine snails such as conchs and whelks than they are to the typical small freshwater snails we all know (see box at the end for more details if you are interested). Unlike the Bubble Snails of Oceandiver's excellent diary, the apple snails have separate male and female individuals (they are dioecious if you want to use some jargon).
Two genera of Apple Snails occur in Florida. The giant ramshorn snail, Marisa cornuarietus, is native to South America. As its name suggests it resembles the small freshwater ramshorn snails but is much larger. It is widespread in south Florida. It's not certain if its introduction was accidental or a deliberate attempt to control aquatic vegetation.
Most of the Apple Snails introductions in Florida and, indeed, around the world, involve species in the large genus Pomacea, native to the New World tropics. These are large globular snails, approximately the size of a small apple. Unfortunately the taxonomy of Pomacea is a mess with a huge number of described species, many, if not most, of which are simply minor color variations of other species. It seems likely that five species either occur in Florida or may have occurred here in the recent past. There have been some recent taxonomic name changes but I will stick to the 'traditional' names to avoid confusion.
One species of Pomacea is native to Florida, P. paludosa, the Florida Apple Snail. This species is primarily found in peninsular Florida and nearby islands in the Caribbean. It occurs spottily somewhat further north where the water stays warm enough in the winter.
The other four species have all been introduced from South America. Two, P. haustrum and P. diffuse remain restricted to southern Florida. A third, P. insularum, the Island Apple Snail, has spread through much of the state. A fourth, P. caniculata, the Channeled Apple Snail is the species of greatest concern. This species has caused widespread destruction of rice fields in regions of Asia where it has invaded. A suspected invasion in the Jacksonville area was presumed contained a few years ago but now it is uncertain if those individuals were actually Channeled Apple Snails.
As large, and relatively slow moving, hunks of protein, apple snails are potentially valuable food items. Two bird species, the Snail Kite and the Limpkin are specialists on apple snails and are widespread in the New World tropics. Snail Kites have a limited distribution in southern Florida and appear to be declining in the U.S. Limpkins are found through most of Florida but are definitely more abundant in general further south.
The local Apple Snail Story.
Although there is a plethora of general information on Pomacea snails in Florida it is difficult to find much in the way of detailed documentation of population sizes, damage to vegetation, etc.
It is thought that the native species declined here in Tallahassee region, due to the invasion of Hydrilla a highly invasive submerged aquatic plant. The story is that the native snails don't like Hydrilla and thus began to decline when the Hydrilla pushed out much of the native aquatic plant life. The departure of Limpkins from the Wakulla Springs area is attributed to the rise of Hydrilla and loss of Apple Snails. It's a compelling story and it could be true. But I couldn't find much in the way of detailed data.
The other side of the story is the spread of the Island Apple Snail. This species is willing to feed on a wider range of plant species than the Florida Apple Snail including invasive species such as Hydrilla and Water Hyacinth. The Island Apple Snail (and the Channeled Apple Snail) can be clearly distinguished from the native Florida Apple Snail by the color of their eggs. The native species has relatively large, white eggs. The non-native species have eggs that are bright bubble gum pink. The eggs are deposited on emergent vegetation or other objects above the waterline. The snails emerge from the water at night to lay the eggs.
Island Snail eggs have been visible on emergent plants in Lake Jackson ever since I moved close to this north Florida Lake almost five years ago. Among other things the lake is known for its diverse and plentiful aquatic vegetation. Snail numbers seemed reasonably constant until this year. Now the number of pink masses has skyrocketed. I took advantage of the high water to explore Meginnis Arm, the most southerly extension of the lake and look for snails, eggs, etc.
Here's what I found.
First in the main body of the lake the water was siltier than usual, perhaps due to the rainy summer. Mats of Hydrilla are usually obvious at or right below the surface but were not visible. This is probably due to the recent rise in the lake level, which, combined with the cloudy water, makes it difficult to see how much Hydrilla is actually present
Second, even in open water, some distance from shore, there were signs of the apple snails. There were a few egg masses on lotus and water lily leaves, meaning the snail (size of an apple, remember) had to crawl up the long narrow stalk, across the underside of the leaf, and onto the top. More commonly there were egg masses on the remains of former lotus flowers.
The same objects were also valued as perches by dragonflies.
The lower part of Meginnis Arm (i.e. the part closer to the rest of the lake) is a narrow channel with extensive areas of emergent vegetation on both sides. Among the larger vegetation is a small leguminous tree. I'll try and figure out the species before my next diary. As you can see they were very popular with the snails. One of my discoveries on these trips is that it is very difficult to take distance shots of the eggs. They always overexpose and the bright pink, so obvious when viewed in person, gets lost.
The channel itself was a good place for spotting snail eggs as the kayak was always close to vegetation. It was also good for spotting limpkins, a peculiar bird that resembles an Ibis but is actually more closely related to rails and cranes. Lake Jackson is close to the northern edge of the range of both the limpkin and the native Apple Snail. There were no limpkins on the lake when we moved in to our house, in late 2008. I saw the first ones in the summer of 2010. During 2011, the last time the water was high enough to allow exploration of Meginnis Arm, limpkins were mostly found in the Arm and in other weedy backwaters of the lake. Today limpkins can be heard calling from the lake at virtually any time of the day or year or year. I also see them flying across the main body of the lake more frequently.
I heard Limpkins quite a bit on my excursions but mostly caught fleeting glimpses.
More easily seen were the remains of the victims of the limpkins insatiable appetite for snails. Also much more easily picked up and photographed.
I only saw a single living snail, which I chanced upon as it was crossing the channel.
I was impressed with its speed and agility. It quickly righted itself after crawling onto a too small piece of debris. These pictures are bit silty because I had to hang onto the channel bottom with one hand to hold the kayak in place for the photos.
This in hand photo shows the snail's operculum, which protects the snail's body when it is withdrawn into the shell. It allows the snails to survive periods of drying.
Just beyond the snail spotting location the channel opens into a long, narrow pond.
Although there is more development in this area relative to the channel there is still quite a lot of aquatic and emergent vegetation. However I could see no sign of Apple Snails in this part of the lake. I didn't make an exhaustive search but the snail egg cases are quite conspicuous and if they are present in any number they are hard to miss.
I also didn't see or hear any Limpkins in this area although they had been quite conspicuous here in the past as seen in this 2011 photo.
So at this point I have more questions than answers. Did native apple snails formerly live in Lake Jackson? If so, are there any left? Why are there no apple snails (or at least very few) in the upper part of Meginnis Arm? Will Limpkin numbers keep pace with Apple Snail numbers? Will the snails have a negative effect on the lake's vegetation? The Island Apple snails apparently eat almost any kind of aquatic plant except for really tough ones like these reeds.
Ideally I would survey other parts of the lake and make carefully observations of number of egg masses, shells, living snails and limpkins. Also make quantitative measures of the amount of vegetation. And I would do this regularly. Unfortunately I have a full time job so, while I am going to continue to observe, I will have to do so fairly informally.
While researching this I discovered that my knowledge of gastropod evolution was out of date. The Gastropoda are the snails and slugs and are the largest class within the phylum Mollusca. They are the only mollusks to live on land. Classically the Gastropods were split into three groups: Prosobranchs (mostly marine snails), Opisthobranchs (sea slugs, nudibranchs, and relatives), and Pulmonates (mostly freshwater and terrestrial snails and slugs). It turns out that modern molecular biology reveals that these groups are not really valid. One thing this revealed to me is that the freshwater and terrestrial ways of life in gastropods have evolved many times. Seems like an excellent subject for some comparative genomics.
"Green Diary Rescue" is Back!
Meteor Blades has revived his excellent series. As MB explained, this weekly diary is a "round-up with excerpts and links... of {diaries} bringing matters of environmental concern to the community... "
"Green Diary Rescue" will be posted every Saturday at 1:00 pm Pacific Time on the Daily Kos front page. Be sure to recommend and comment in the diary.