Predator-prey interactions between shell-boring beetle larvae and rock-dwelling land snails

Abstract

Drilus beetle larvae (Coleoptera: Elateridae) are specialized predators of land snails. Here, we describe various aspects of the predator-prey interactions between multiple Drilus species attacking multiple Albinaria (Gastropoda: Clausiliidae) species in Greece. We observe that Drilus species may be facultative or obligate Albinaria-specialists. We map geographically varying predation rates in Crete, where on average 24% of empty shells carry fatal Drilus bore holes. We also provide first-hand observations and video-footage of prey entry and exit strategies of the Drilus larvae, and evaluate the potential mutual evolutionary impacts. We find limited evidence for an effect of shell features and snail behavioral traits on inter- and intra-specifically differing predation rates. We also find that Drilus predators adjust their predation behavior based on specific shell traits of the prey. In conclusion, we suggest that, with these baseline data, this interesting predator-prey system will be available for further, detailed more evolutionary ecology studies.

Figure 1. Greek Albinaria snails and their Drilus predators.

A, Albinaria hippolyti from Crete (photo: V. Wiese). B and C, the clausilium, shown in the shell aperture after removal of the left lateral shell wall (B shows a less-obstructing, N-type clausilium, C shows a more obstructing, G-type clausilium). D and E, a male and a female, respectively, of a yet undescribed Drilus species from Crete (scale: 2 mm). F, a full-grown larva of Drilus “L” from the Peloponnese (same scale as D and E). G, an estivating A. discolor from the Peloponnese, with a Drilus exit bore hole. H, a Drilus “L” exiting from its prey, an A. menelaus from the Peloponnese.

Figure 2. Map of Crete and surrounding islands, showing contours of regionally varying Drilus predation rates (given as percentages attacked shells per sample) in Albinaria, derived from bore-hole frequencies in 1,160 museum samples from Naturalis Biodiversity Center, Leiden, The Netherlands, Haus der Natur, Cismar, Germany, and Natural History Museum, Budapest, Hungary.

Maps were drawn using inverse kriging distance calculation in R v.2.15.2 , with packages gstat , maptools (R v.0.8-27), rgdal (R v.0.8-12) and rgeos (R v.0.3-2).

Figure 3. A, five Drilus species from the Peloponnese have different specificities for Albinaria as prey, and concomitant propensities to bore holes in the shell (calculated as the number of bore holes divided by the total number of prey).

B, difference (P<0.05, Fisher's exact test) in prey specificity between Drilus “D” (more Albinaria-specific) and “E” (less Albinaria specific) in two localities where both species occur syntopically (these data are not included in fig. 3A). C, differences in Drilus predation rate (dark portion of the bar) between solitarily (“sol”) and group-wise (“agg”) estivating snails of A. caerulea in four 5 m² plots in the islands of Paros, Naxos, and Thira (voucher numbers RMNH.MOL.84354-84363, RMNH.MOL.85192, and RMNH.MOL.85193). An aggregate was defined as a cluster of >20 snails, with distances of <2 cm separating them. A snail was considered solitary if it was >20 cm distance from a conspecific (significance tested with Chi-square test). D, positions of entrance holes in shells of species with an N-type clausilium compared with those in shells of species with a G-type clausilium, shown for the entire eastern Peloponnese as well as for the site Monemvasia, where both clausilium types occur microsympatrically. (P-values are derived from Fisher's exact test.)

Figure 4. Still from Video S1, showing a Drilus “L” larva (in the lab) creating an exit bore-hole from within a prey Albinaria meleaus, followed by the lengthy procedure by which the larva emerges from the shell.