Complexity of the prey spectrum of Agaronia propatula (Caenogastropoda: Olividae), a dominant predator in sandy beach ecosystems of Pacific Central America

Abstract

Olivid gastropods of the genus Agaronia are dominant predators within invertebrate communities on sandy beaches throughout Pacific Central America. At Playa Grande, on the Pacific Coast of Costa Rica, we observed 327 natural predation events by Agaronia propatula. For each predation event, we documented prey taxa and body size of both predator and prey. The relationship between predator and prey size differed for each of the four main prey taxa: bivalves, crustaceans, heterospecific gastropods, and conspecific gastropods (representing cannibalism). For bivalve prey, there was increased variance in prey size with increasing predator size. Crustaceans were likely subdued only if injured or otherwise incapacitated. Heterospecific gastropods (mostly Olivella semistriata) constituted half of all prey items, but were only captured by small and intermediately sized A. propatula. Large O. semistriata appeared capable of avoiding predation by A. propatula. Cannibalism was more prevalent among large A. propatula than previously estimated. Our findings suggested ontogenetic niche shifts in A. propatula and a significant role of cannibalism in its population dynamics. Also indicated were size-dependent defensive behavior in some prey taxa and a dynamic, fine-scale zonation of the beach. The unexpected complexity of the trophic relations of A. propatula was only revealed though analysis of individual predation events. This highlights the need for detailed investigations into the trophic ecology of marine invertebrates to understand the factors driving ecosystem structuring in sandy beaches.

Keywords: Agaronia; Cannibalism; Olividae; Ontogenetic niche shift; Predator–prey interaction; Sandy beach; Trophic network; Tropical Eastern Pacific.

Figure 1. Example of a predation event recorded in the wild.

A large Agaronia propatula with filled metapodial pouch is observed sailing with the backwash (A) before burrowing into the sediment (B). The snail is captured and the prey item removed from the pouch (C); it is a smaller conspecific (D). Photo credit: Winfried S. Peters.

Figure 2. Size distribution of Agaronia propatula (4 mm intervals from 17–21 to 57–61 mm) found with prey in their metapodial pouches at Playa Grande (n = 327; white bars).

For comparison, the size distribution of actively foraging A. propatula on the intertidal sediment surface at the same location reported by Cyrus et al. (2015) is shown (gray bars in background; n = 313).

Figure 3. Prey spectra for size classes of Agaronia propatula.

Prey was categorized as B, bivalves (light gray); G, gastropods (of which over 98% were Olivella semistriata; white); C, crustaceans (dark gray); and A, A. propatula (cannibalism; black). The largest size class (57–61 mm) is omitted since it included only one case, a bivalve. Sample sizes are given on top of the columns.

Figure 4. Relationship between shell lengths of Agaronia propatula and its bivalve prey.

(A) Size distribution of A. propatula with bivalves in their metapodial pouches (white bars). The size distribution of A. propatula carrying prey of any taxon is shown in the background. (B) Predator size plotted vs prey size. Gray circles represent predation on Donax sp., white circles represent predation on other bivalves. The coefficient of determination (r²) and the geometric mean functional relationship (GMFR) for the entire bivalve dataset are shown. The dashed line indicates a predator-to-prey size ratio of 1; interactions in which the prey was larger than the predator lie to the right of this line. Here, as well as in Figs. 5 and 6, zones of interest are marked by Roman numerals to facilitate discussion in the main text.

Figure 5. Relationship between shell length of Agaronia propatula and body length of its crustacean prey.

(A) Size distribution of A. propatula with captured crustaceans (white bars); the size distribution of A. propatula carrying any prey is shown in the background. (B) Plot of predator vs prey size. Predation events observed at Playa Grande are given as circles; white circles indicate predation on Emerita sp., while gray ones represent predation on other crustaceans. r² for the subset of observations from Playa Grande with prey sizes below 20 mm and the corresponding GMFR are indicated. Observations of successful predation on Emerita sp. made at El Cuco are shown as black diamonds. The dashed line marks the predator-to-prey size ratio of 1.

Figure 6. Relationship of the shell lengths of Agaronia propatula and its heterospecific gastropod prey.

(A) Size distribution of A. propatula with other gastropods in their metapodial pouches (white bars). The size distribution of predators carrying any prey is given in the background. (B) Plot of predator size vs prey size. Gray circles and white squares represent predation on Olivella semistriata and Mazatlania fulgurata, respectively. r² is given for the entire gastropod dataset. The predator-to-prey size ratio of 1 (dashed line) and the postulated threshold ratio of 1.45 for predation on gastropods (solid line) are highlighted.

Figure 7. Relationship of predator and prey shell lengths in cannibalistic interactions in Agaronia propatula.

(A) Size distribution of A. propatula carrying conspecifics in their metapodial pouches (white bars). In the background, the size distribution for all A. propatula carrying prey is shown. (B) Predator size plotted versus prey size. r² and the geometric mean functional relationship (GMFR) are given, and the predator-to-prey size ratio of 1 (dashed line) as well as the postulated threshold ratio for gastropod predation (1.45) are marked. Arrows point at three outliers on which the correlation depended.

Figure 8. Comparison of the expected and observed distributions of successful cannibalistic predators across the size classes of Agaronia propatula.

Gray bars represent the expected distribution, calculated assuming that individual A. propatula meet randomly on the beach, and given the size distribution of all successful predators (see Fig. 2) and the size-dependence of success in cannibalism determined by Cyrus et al. (2015). White bars show the observed distribution of successful cannibals (compare Fig. 7A).