Egg Cluster Counts

Egg cluster counts take advantage of the conspicuous white eggs deposited above the water line by female apple snails (the process is called oviposition).  Egg clusters are a sign that snails are present, but cluster counts do not necessarily reflect adult snail abundance or density.  

What they are:

• Apple snails lay eggs in clusters of 20-30 eggs on emergent vegetation above the water line.
  

  

• Although the sampling unit can vary, a 1 m x 2.5 m polyvinylchloride (PVC) quadrat is often used (Darby pers. comm.).

• The quadrat is placed over emergent vegetation and the number of clusters is counted.  

  

 

 What the method is used for:

• Counts of egg clusters over time in a given area can be used to monitor the temporal pattern of cluster production (e.g., see figure below from Darby et al. 2008 [citations for Silver Springs and LOKEE data are in Darby et al. 2008]).

   



• Egg cluster counts have been used to indicate specific vegetation and general habitat preferences that may be affected by water and habitat management (Darby, pers. comm.). 

• Assessments so far indicate that egg clusters are not reliable indices of relative abundance (see Limitations below).  However, their potential use for this purpose continues to be explored. 

 

Sampling Protocol:

• Apple snail egg cluster counts are most often conducted along transects established along an ecotone defined by emergent vegetation (e.g., sawgrass, cattail, pickerelweed) and wet prairie or slough habitats (e.g., spikerush, lily pads and/or open water) (see picture below).

  

• To sample one transect, the 1-m x 2.5-m quadrat is flipped end over end 20 times (total transect length of approximately 50 m if you are working along a contiguous patch of vegetation).  Typically, 20-30 quadrats yield data with reasonable precision as indicated by coefficients of varation (Darby, pers. comm.).  This requires approximately 20-30 minutes with a team of two. 

• The resulting data are the number of clusters per quadrat, from which mean clusters per 2.5m² and standard error (based on n=20 to 30) are estimated. 

• To estimate egg density in a large area, randomly deploy 100 pvc quadrats in the available emergent vegetation types on which eggs may be found (Darby, pers. comm.). Extrapolate the clusters counted per total area of available vegetation type based on the sample of 100 quadrats.

Their limitations:

• Oviposition is affected by various factors, including the snail’s annual cycle, temperature, vegetation type, and water level (Hanning 1979; Turner 1996; Darby unpublished data).  Consequently, snails may be relatively abundant, but egg cluster counts may be low or zero (e.g., Darby et al. 2009 has observed no egg production despite snail densities >1.0 snail / m² because of unusually high water levels in the dry season).

• Estimates of annual egg production based on monthly counts were positively associated with snail density (Darby and Ren 2004); however, density only explained about 30% of the variation in total egg production.  The researchers are working on a revised protocol to control for some of the environmental variation in an effort to use egg cluster counts as an index of relative abundance, at least in some contexts.     

 

Literature Cited:

• 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. and A.K. Ren.  2004.  The potential utility of apple snail egg clusters in the context of an Everglades restoration performance measure.  Abstract, p. 101, First National Conference on Ecosystem Restoration, December 6-10, 2004, Lake Buena Vista, FL.

• Darby, P.C., N.H. Glass, D.J. Mellow, S.E. Kell, R.E. Eckert, and P.L. Valentine-Darby.  2009.  Apple snail demography in the Everglades: 2005-2007.  Draft Final Report to U.S. Fish and Wildlife Service, Vero Beach, FL.

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

• Turner, R.L.  1996.  Use of stems of emergent vegetation for oviposition by the Florida apple snail (Pomacea paludosa), and implications for marsh management.  Florida Scientist 59(1): 34-49.

 

Related Publications:  

• Darby. P.C., L.B. Karunaratne, and R.E. Bennetts.  2005.  The influence of hydrology and associated habitat structure on spatial and temporal patterns of apple snail abundance and recruitment.  Final Report submitted to the U.S. Geological Survey.  72 pp.