Does Pomacea’s Aerial Respiration Requirement Determine Environmental Impact?

Aerial respiration of Pomacea via siphon (Jess Van Dyke)

Though Apple Snails are freshwater inhabitants, they are clearly amphibious. Pomacea possess combed, gill-like structures (ctenidium) for aquatic respiration and lung-like pulmonary sacks for aerial respiration, as well as buoyancy regulation. When dissolved oxygen levels are high (5-6 ppm), the snails remain mostly underwater, but when low (1-2 ppm), they rely on their siphons and “lungs” to breathe fresh air (San Martins et al., 2009). In any case, all Pomacea regularly come to the surface to ventilate their “lungs.” Such behavior is obligatory.  Preventing aerial respiration negatively affects activity, feeding, and survival (Seuffert and Martín, 2009). They must occasionally take a breather!

Darby et al. (2002) reported that Apple Snails prefer to inhabit shallow water areas ( < 50 cm) because of the need to breathe atmospheric air without expending a large amount of energy to move to the surface. Darby (1998) also suggested that unconsolidated organic material may restrict movement into deep water. However, recent observations of healthy Pomacea as deep as 14.6 m (48’) in Apopka Spring indicate that depth alone is not a deterrent to the snails (Bernatis, 2010). Seuffert and Martín (2009) have concluded that Pomacea are unevenly distributed relative to the access to air – – concentrated less than 2–4 m from the nearest emergent substrate. Simply put, Apple Snails need easy access to emergent plants (or other structure), so they can crawl up and catch a breath of fresh air.

According to the annual survey by FWCC, Pomacea insularum is now present in 22% of Florida’s public lakes and rivers (see below). The mystery in Florida is why this invasive, exotic snail will strip one lake and leave the vegetation relatively unaffected in another. Perhaps, three physical features of lakes play important roles: shoreline development, average depth, and mean slope. Shoreline development (SD) is simply the ratio of the length of the shoreline of a lake to the circumference of a circle with the same area as that lake. The higher the SD ratio is the more complex the shape of the lake. Average depth is self-explanatory, and mean slope is the proximity of bathymetric contours to one another. Taken together, these parameters determine the extent of the littoral zone and the abundance of emergent vegetation.

A relatively small, shallow lake with a gradual bathymetry but a complicated shoreline would seem to provide the best habitat for Pomacea insularum. In such a lake, on a per area basis, food would be more abundant; emergent vegetation would always be near for aerial respiration (and egg deposition); and dissolved oxygen would tend to be high, unlike in a deeper lake with an anaerobic hypolimnion. In short, a greater percentage of the lake would be within 2-4 m of substrate that Seuffert and Martín (2009) suggest is Pomacea’s preferred habitat. The greater the density of Pomacea insularum, the more likely the snail population could consume all of the aquatic vegetation in a given lake. Let’s see how this theory plays out. Posted by Jess Van Dyke

Annual Survey of Florida’s Public Waters  for Pomacea insularum by the FWC:
               Year

Number

        Area (ac)

2006

4

4779

2007

45

680301

2008

50

691045

2009

75

752529

2010

86

736263

2011

102

826860

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