SEDIMENTOLOGICAL STUDIES ON MATERIAL INCORPORATED BY NATICID GASTROPODS IN EGG MASS CONSTRUCTION

Naticid gastropods are the only molluscs that incorporate large amounts of local sediment in the construction and sealing of their unique egg masses, called egg collars. Phylogenetic analysis suggests the use of sediment is the basal condition for naticids and might have contributed to their diversification in the early Cretaceous. The function of sediment in egg collars is unknown. Several functional hypotheses were proposed for this use of sediment by maternal snails including 1) camouflage from visual predators, 2) internal support for the large egg mass, and 3) protection against dessication. These hypotheses were tested using comparative analyses of incorporated grains through comparison of egg mass sediment and sediment samples taken directly beneath each egg mass. Paired samples were obtained for several naticid taxa from various locations in the Pacific Ocean. Most collars were found in their original position, though some collars had evidently been transported.

Comparisons of egg collars and sediment-free gelatinous collars show similar average size, disproving the hypothesis that incorporated sediment is necessary as an infrastructure for larger egg masses. The location of incorporated grains within the collar precludes any anti-dessication function. Paired t-tests of sediment samples from non-transported collars revealed no significant differences in grain size, angularity, or sorting between incorporated and local sediment. This result suggests maternal naticids do not select specific grains for their egg masses. Analysis of the orientation of incorporated material revealed no preferred arrangement of grains. The lack of selectivity and arrangement of incorporated grains suggests sediment might function as camouflage from visual predators.