The utility of wax replicas as a measure of crab attack frequency in the rocky intertidal

Abstract

Crabs are thought to play a vital role in structuring gastropod populations. Studies quantifying the frequencies with which crabs attack gastropods in natural settings are, however, scarce. Although a wide variety of techniques exist with which predator -prey interactions can be investigated (e.g. laboratory experiments, exclusion caging, tethering and population surveys), there is a need for methods that can provide large amounts of quantitative data, particularly documenting the frequency with which crabs attack gastropods. This study examines the utility of using wax replicas of gastropods to determine crab attack frequencies. Replicas of Chlorostoma funebralis, Nucella ostrina and Nucella lamellosa were bolted to mesh screens and deployed in the rocky intertidal. Crabs attacked wax replicas of gastropods, leaving characteristic marks in the wax. In most cases, the appendage used in the attack could be identified from the marks (i.e. chelae vs walking legs). The effectiveness of this technique was verified using surveys of repair scar frequencies of the gastropod populations; patterns in attack frequency, determined from the number of marked wax replicas, were consistent with those of repair frequency, in that both were greater at the wave protected, quiet water locality. This study confirms the value of wax replicas in investigations of crab predation to determine the frequency and type of attack, and illustrates the potential of this method for quantifying predation intensity. The development of techniques that quantify the magnitude and exact nature of the effects of crab predation on intertidal communities is pivotal, given the intensity of commercial fishing of some species of crabs.

Can crabs kill like a keystone predator? A field-test of the effects of crab predation on mussel mortality on a northeast Pacific rocky shore

“…To confirm whether crab predation was responsible for the loss of uncaged mussels at each tidal elevation, during experiment 2, we deployed wax snail replicas near our experimental blocks at both low and high elevations. Wax snail replicas were used because they have been shown to be an effective method to quantify crab predation, as the frequency of attacks on wax replicas are easily quantified and attributed to crabs [ 35 ] and also strongly correlated with crab scar repair frequency on the shells of living snails [ 35 ]. Four wax snail replicas (modeled after the whelk Nucella ostrina , a common inhabitant of our site) were bolted to each of twenty 8.5 x 8.5 cm plastic mesh screens, each of which were then bolted to the substrate in the vicinity of the caging experiment blocks at both low and high elevations for a total of 80 wax snail replicas.…”