Dry Down Impacts on Apple Snail Recruitment

This section focuses on the effects of dry downs on apple snail recruitment into a population.  Most of the information in this section is from Darby et al. (2008) and an older published study.  The laboratory experiments described below are those discussed in the Dry Down Survival section.

 

Apple Snail Reproduction occurs in the Dry Season             

The production of apple snail egg clusters shows an obvious seasonal pattern (Darby et al. 1999; Darby et al. 2003; Darby et al. 2008).  This pattern is consistent among locations and years (see Figure 1a), assuming there are no dry down conditions.  The peak month of production is April, May, or June (assuming there is not a dry down or extreme high water).  These three months account for ~80% of the annual egg production; they also coincide with the steepest decline in water levels (see Figure 1b).  A similar pattern of egg cluster production was observed in a laboratory setting (Darby et al. 2008).  Snails do not lay eggs in dry down conditions (also see Darby et al. 2002). 

Figure 1.  a) Patterns of apple snail egg cluster production presented as the percent of total annual cluster production per month (~80% of annual production within grey box) (Data from Odum, Hanning, and Darby, as cited in Darby et al. 2008).  Part (b) shows standardized water stage values ranging from 0 to 1 that reflect minimum and maximum 20-year average daily water stage for six representative wetlands (Lake Okeechobee [LOKEE], Blue Cypress Water Management Area [BCWMA], Lake Kissimmee [LKISS], and Water Conservation Areas [WCAs] 1, 2B, and 3A). Figure adapted from Darby et al. (2008).

The Effect of Dry Down Timing on Recruitment

The timing of a dry down may be as influential on snail demography as the duration (Darby et al. 2008).  As noted above, the peak period of egg production generally coincides with the lowest annual water levels in the late dry season (Figure 1b); in some years, this may result in water levels below ground level.  If the dry season ends with a drying event, then recruitment will be cut short; how much it will be shortened depends on the timing.  For example, a 2.5-3 month dry down may result in minor impacts on reproduction if it occurs after peak reproduction.  However, if a dry down of the same duration coincided with peak egg production, recruitment would be significantly reduced.  Such a situation occurred on Lake Kissimmee in 1996 during a January-June dry down; the result was near-elimination of apple snail egg production and subsequent population declines (Darby et al. 2004). 

Dry downs also impact apple snails by stranding hatchlings and juveniles (Darby et al. 2008).  The amount of time between hatching and a dry down event determines the window of opportunity for hatchling/juvenile growth.  Hatchlings (initially 3-5 mm) grow at a rate of approximately 10-13 mm per month for their first 1-2 months (Hanning 1979; Glass and Darby 2008); apple snails can reach adult size (~ 25-30 mm) in approximately 2 months.  Snails hatched in early April would be almost adult size by June, and would, therefore, have a relatively high capacity for surviving an ensuing dry down.  However, if a dry down occurred in April, these same snails would be much smaller and survival would be much lower (see section on Dry Down Survival).  Relatively rapid growth rates appear critical to the young-of-the-year apple snail, for whom 2-4 weeks of growth significantly increases survival in an ensuing dry down (Darby et al. 2008). 

In summary, because periodic dry downs are important for maintaining suitable snail habitat, management that allows such drying events should be considered (see Management Issues), but consideration of the timing and duration is critical to avoid significant demographic impacts (Darby et al. 2008).  Based on the best available data to date, a dry down that occurs every 2-3 years would have little impact on snail survival and recruitment, particularly if the lowest water levels did not occur until late May or June and/or dry down duration did not exceed 6-8 weeks (Darby et al. 2008).  As the duration of the drying event increases, and the more it coincides with the peak period of egg production, the greater the impact will be to the apple snail population. 

 

Literature Cited:

• Darby, P.C.  1998.  Florida apple snail (Pomacea paludosa SAY) life history in the context of a hydrologically fluctuating environment.  Doctoral dissertation, University of Florida, Gainesville, Florida.  154 pp. 

• Darby, P.C., R.E. Bennetts, J.D. Croop, P.L. Valentine-Darby, and W.M. Kitchens.  1999.  A comparison of sampling techniques for quantifying abundance of the Florida apple snail (Pomacea paludosa Say).  Journal of Molluscan Studies 65:195-208.

• 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 (Pomacea paludosa Say) population.  Malacologia 45(1):179-184. 

• Darby, P.C., P.L. Valentine-Darby, H. Franklin Percival, and W.M. Kitchens.  2004.  Florida apple snail (Pomacea paludosa SAY) responses to lake habitat restoration activity.  Arch. Hydrobiol. 161 (4):561-575.

• 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.

• Glass, N.H. and P.C. Darby.  2008.  Calcium and pH effects on the growth and shell integrity of Florida apple snails.  Aquatic Ecology (accepted for publication).

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

• Odum, H.T.  1957.  Trophic structure and productivity of Silver Springs, Florida.  Ecological Monographs 27:55-112.