Dry Down Survival

This section focuses on the ability of the Florida apple snail to survive dry down conditions.  For other aspects of apple snail dry season ecology, see the sections on Movements (which includes movements during decreasing water levels), Basic Biology (which discusses the apple snail’s life span and annual die-off), and Dry Down Impacts on Recruitment. Information in this section is based primarily on Darby et al. 2003 and Darby et al. 2008.  

Survival in Pomacea and Pila Snails  

Other apple snail species, found in both the Pomacea and Pila genera, can survive dry marsh conditions for 3-25 months by aestivating (Cowie 2002).  Aestivation is a state of inactivity and reduced metabolic activity that enables an animal to conserve moisture and energy reserves.  For more detailed information, consult Cowie (2002) and the original research cited therein. 

When faced with a dry down, some snail species bury themselves in the substrate only partially (typical for the New World genus Pomacea), while others burrow down as much as 1 meter (Old World genus, Pila) (Cowie 2002).  Some species are able to survive a substantial loss of soft-tissue weight during aestivation; reports of weight loss for three species ranged from 5-62%.  The shell and sealed operculum provide a barrier to water loss.  Metabolism during aestivation is aerobic in some species and anaerobic in others. 

Survival in the Florida Apple Snail 

Darby et al. (2003) discussed problems with past studies on the Florida apple snail that concluded the species has little tolerance to dry down conditions.  After collecting information to support earlier conclusions by Hanning (1979) and Ferrer et al. (1990) that P. paludosa has a 1 to 1.5-year life span, Darby et al. (2008) surmised that the past studies on dry down tolerance were confounded by an annual adult die-off.  The purpose of the most recent dry down survival work by Darby et al. was to determine the capacity of the Florida apple snail to survive dry downs without being confounded by the adult die-off.    

Apple snails do not actively seek deep water refuge as water levels within a lake or wetland recede (Darby et al. 2002).  Snails tend to stop moving when water levels drop to 10 cm.  If water levels continue to drop, some proportion of apple snails will become stranded and must subsequently contend with dry down conditions.  [See pictures below of a marsh during a dry down, and of a snail found alive and aestivating in that marsh].  As discussed in greater detail below, an individual’s ability to survive these conditions depends on its size and reproductive status.  If a dry down occurs in the reproductive season, and a snail has already expended energy on mating and egg production, it may be less likely to survive an extended dry down.

The Experiments (Darby et al. 2008):   Three laboratory experiments were conducted using adult-size snails (~30 mm shell width) prior to, during the peak, and at the end of the reproductive season (Darby et al. 2008).  The two treatments were stable water levels throughout (~15 cm; control tanks), and dry down tanks, in which water levels started at 15 cm and dropped to substrate level over the course of seven days.  A similar experiment was conducted using juvenile snails.  Three size classes of snails were tested for drought tolerance—  3-5 mm (newly hatched snails), 6-9 mm (1-2 weeks' growth after hatching), and 10-15 mm (starting as 3-5 mm hatchlings and growing for 3-4 weeks) (see  Figure 1 below for snail size range).  Survival of snails smaller than 10 mm in dry down conditions could not be assessed due to their fragile shells.  Therefore, snails had to be removed from the experiment and placed in water to see if they revived and were alive.  This was considered removal sampling because these snails were not reintroduced into the experiment.

Data from the first experiment (with pre-reproductive adults that had over-wintered) and the latter part of the third experiment (with pre-reproductive adults that had hatched out that same calendar year) indicated that pre-reproductive adult-sized snails can survive dry marsh conditions for several weeks to months.  After 6 weeks of dry down conditions, 94% of the pre-reproductive snails survived; after 12 weeks 71% survived; and after 18 weeks 27% survived (see Figure 2 below).

Juvenile apple snails exhibited a lower capacity to survive dry down conditions than adult snails.  The juvenile experiment was stopped after 8 weeks mainly due to high mortality.  Because removal sampling was used for the 3-5 and 6-9 mm snails, and survival was assessed on a different group of snails each week, survival from week to week could actually increase.  After 4 weeks in dry conditions, 75% of the 6-9 mm and 10-15 mm snails survived.  Survival for the smallest size class was more erratic and after 4 weeks was between 0 and 50%.  In dry down tanks, juvenile snail survival fell below 50% after 8 weeks in dry down conditions (Figure 2).      

Figure 2.  Survival for experimental snails (a, control; b, dry down) after water levels dropped below substrate level in dry down tanks.  Gray shaded area reflects the post-reproductive die-off.  Figure from Darby et al. (2008).  

Darby et al. (2008) observed that the Florida apple snails appeared to survive by aestivating.  The "dry down" snails withdrew into their shells and tightly sealed their opercula.  The qualitative assessment was that they became lighter in weight and appeared desiccated as the experiment progressed.  The snails that died likely perished from desiccation and depletion of energy stores, as reported for other snails in dry conditions (Burky et al. 1972; Storey and Storey 1990; Thomas and Agard 1992).

Literature Cited: 

• Burky, A.J., J. Pacheco and E. Pereyra.  1972.  Temperature, water, and respiratory regimes of an amphibious snail, Pomacea urceus (Muller), from the Venezuelan savanna.  Biol. Bull. 143:304-316.

• Cowie, R.H.  2002.  Apple snails (Ampullariidae) as agricultural pests:  their biology, impacts, and management.  P. 145-192, In G.M. Barker (ed.) Molluscs as Crop Pests.  CABI Publishing, Wallingford, UK.

• Darby, P.C., R.E. Bennetts, S.J. Miller, and H. Franklin Percival.  2002.  Movements of Florida apple snails in relation to water levels and drying events.  Wetlands 22(3):489-498.

• Darby, P.C., P. L. Valentine-Darby, and H. F. Percival.  2003.  Dry season survival in a Florida apple snail population (Pomacea paludosa Say).  Malacologia 45:179-184.

• Darby, P.C., R.E. Bennetts, and H.F. Percival.  2008.  Dry down impacts on apple snail (Pomacea paludosa) demography: Implications for wetlands water management.  Wetlands 28(1):204-214.

• Ferrer, J.R., G. Perera and M. Yong.  1990.  Life tables of Pomacea paludosa (Say) in natural conditions.  Florida Scientist  53(supplement):15.

• Hanning, G.W. 1979.  Aspects of reproduction in Pomacea paludosa  (Mesogastropoda:Pilidae).  M.S. Thesis, Florida State University. Tallahassee. 138pp.

• Storey, K.B. and J.M. Storey.  1990.  Metabolic rate depression and biochemical adaptation in anaerobiosis, hibernation and estivation. Quart. Rev. Biol. 65: 145-174.

• Thomas, M.A., and J.B.R. Agard.  1992.  Metabolic rate depression in the ampullariid snail Pomacea urceus (Muller) during aestivation and anaerobiosis.  Comp. Biochem. Physiol.  102A:675-678.