Reproductive goldilocks: The Florida apple snail reproduction across water depths

Blog by: Nathan Barrus
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    In the classic children’s story, Goldilocks and the Three Bears, Goldilocks walks into a house owned by a family of bears and eats the bears’ porridge, uses the bears’ chairs, and sleeps in the bears’ beds. But when trying out each of the bear’s possessions, the condition of each possession - whether the heat of the porridge, or softness of the chairs and beds – starts at one extreme and goes to the other.  In the end, Goldilocks chooses the intermediate condition that is “just right”.  Historically, the Everglades was a flowing system; water flowed from Lake Okeechobee south to Florida Bay. In the early 1900s, the Everglades were divided and impounded by levees, and then drained by canals to create space for people to live and farm. Impoundments resulted in “ponding” in some areas while drainage reduced water levels and flooding duration in others. Overall, the current Everglades has altered hydrologic conditions and may have created extremes in depths on either side of the spectrum. These changes affect plants and animals adapted to life in the water, and understanding what hydrologic conditions are “just right” for these plants and animals can help us predict how they will respond to Everglades restoration.

    The Florida apple snail (Pomacea paludosa) is a species of conservation concern because it was historically the sole prey for the federally endangered bird, the Snail Kite (Figure 1). In the 2000s, there was a substantial population of Kites within South Florida with most kites concentrated in the Everglades, but in 2002 and again in 2007, Snail Kites experienced dramatic declines. Since Snail Kites exclusively eat apple snails, it is thought that their declines in 2002 and 2007 were a result of declines in the Florida apple snail population. There just weren’t enough snails for kites to breed and survive within the Everglades. Interestingly, in 2007 the non-native Giant apple snail (P. maculata) – a south American cousin of the Florida apple snail – invaded the lakes and canals north of the Everglades and provided Kites with an alternative food source (Figure 1). The Kite populations have rebounded since this invasion, but the major concentration of Kites switched from within the Everglades to the wetlands and lakes north of the Everglades. For some unknown reason, Giant apple snails in the Everglades do not reach the same high population sizes that they do in the lakes and wetlands north of the Everglades. It is a hope that restoring the water conditions of the Everglades will help the Florida apple snail populations rebound and support snail kites within the Everglades again. But it is unknown what hydrologic conditions are “just right” for the Florida apple snail.

Figure 1: Photos of apple snail egg clutches (right) and adult apple Florida apple snails (left). Bright pink apple snails egg clutches are from the giant apple snail (P. maculata) while the white egg clutches are from the Florida apple snail (P. paludosa).

    The Florida apple snail and its cousin lay eggs on plants, trees, and other substrates that are partly submerged in water. They climb out of the water onto these substrates to lay their eggs just far enough that if the water levels rise during rain, the eggs won’t get flooded. The eggs can then incubate in a place safe from aquatic predators, and after hatching the snails fall back into the water. The Florida apple snail lays most of their eggs in the dry season when water levels are dropping, particularly in March through May. The cues that trigger snails to lay their eggs in the dry season aren’t well known, but it has been hypothesized that egg laying is tied to water depths. If egg laying is tied to water levels, we can assess how restoring the Everglades water dynamics will affect apple snail reproduction. Because apple snails lay their eggs above the water, it is relatively easy for us to count the number of clutches that apples snail lay each year. Therefore for 3 years, we counted apple snail clutch numbers in four wetlands. Then, we tested what environmental cues were triggering snails to reproduce. 

    The Florida apple snail reproduction was “Goldilocks-like” because reproduction was highest in the intermediate depths; water depths that were too shallow and water depths that were too deep were poor for reproduction (Figure 2). Shallow water likely hinders reproduction because the Florida apple snail has adapted to survive periods of dry conditions by burrowing into the sediment, then retracting into their shells. Although there is an ecological explanation for why shallow water is poor for reproduction, it is less clear why deeper water is poor for reproduction. The current hypothesis is that living in deeper water may be more energetically costly, so Florida apple snail’s might not be able to allocate as much energy to producing eggs. Alternatively, it may be that male Florida apple snails might not be able to find females as easily in deeper conditions. Experimentation will be necessary to determine the true cause.

Figure 2: redrawn relationship of number of Florida apple snail egg clutches across water depths.  This relationship most accurately represents the dry season (most productive) reproduction curve when temperatures are cooler.

    The “Goldilocks” response presented an apparent contradiction between the historical depths of the Everglades and the ideal depths for reproduction of the Florida apple snail. In the book, Landscapes and Hydrology of the Predrainage Everglades, Chris McVoy used historical records from expeditions into the Everglades as well as records from canal building crews to estimate what historical water levels were like within apple snail habitat, which is the sloughs of the Everglades. These records seem to indicate that historical water levels in the sloughs were typically deeper in the dry season than are favorable for apple snail reproduction.  This contradiction suggests that increasing depths in the Everglades could make conditions poor for reproduction by the Florida apple snail. But this also raises the question, where did apple snails live in the Everglades before drainage? Was it in the shallower places on the periphery of the current extent of the Everglades that are now populated and farmed? Did the Florida apple snail colonize the current Everglades after drainage made some areas shallower? We likely won’t be able to answer these questions, but they are important to ponder when considering restoration, because trying to make conditions beneficial for all animals in a place that is nearly half the size of the historical extent may be impossible. Besides, there are still a myriad of other unanswered questions unknown about apple snail populations that could be more important. Thus, although one aspect of the ecology of the Florida apple snail suggests that increasing depths may be a negative for the Florida apple snail, the real answer to how the Florida apple snail will respond to restoration is likely more nuanced (Box 1).

Relevant Citations

McVoy, C. W., W. P. Said, J. Obeysekera, J. A. VanArman & T. W. Dreschel, 2011. Landscapes and Hydrology of the Predrainage Everglades, University Press of Florida, Gainesville, FL.

Barrus, N. T., M. I. Cook, & N. J. Dorn. Life history responses of tow co-occurring congeneric Apple Snails (Pomacea maculata and P. paludosa) to variation in water depth and metaphyton total phosphorus.