SWANSON, C.A.: Consequences of communal egg-laying in the apple murex snail, Phyllonotus pomum

Communal egg-laying, the sharing of hard substrates by conspecifics for depositing clutches of egg capsules, is common among gastropod molluscs. The apple murex snail, Phyllonotus pomum, lays globular communal egg masses composed of clutches simultaneously deposited by 2-52 females. As mass volume increases, the surface area to volume ratio decreases. This may create a cost-benefit tradeoff between edge effects such as fouling, UV light, and predation, and the potential for low water flow and decreased oxygen in mass centers. Movement patterns of individuals and the availability of suitable substrates will influence communal egg-laying behavior. These organisms may deposit eggs communally because of a limitation of suitable deposition sites or because it provides increased fitness. In a caged field experiment, females contributed to a single communal mass even when an excess of substrate was available suggesting that substrate limitation is not driving communal behavior. To document the effect of increasing mass size on offspring, juvenile numbers and sizes per capsule just prior to hatching were recorded for a variety of mass volumes. The placement of capsules within a mass had a significant effect on juvenile number, with the center having the most, bottom fewer, and top the least number of juveniles. As mass volume increases, juvenile size decreases. Although not significantly different from top and bottom capsules, this relationship is stronger for center capsules. Because larger masses have more center capsules, females gain greater numbers of offspring but these offspring are slightly smaller. Communal egg-laying in Phyllonotus pomum alters offspring variation. This has potential fitness consequences in environments where size-number tradeoffs may be important.