Prey species: The isopod I. balthica is a colour polymorphic, generalist herbivore. In our study area, the Northern Baltic Sea (locational details below), the isopod has a lifespan of 13 to 15 months (Salemaa 1979). In earlier studies we have shown that isopod activity can be considered as a personality trait, with consistency of the behaviour over time (Yli-Renko et al. 2015). Moreover, personality is related to the overwintering survival in the laboratory (without predator presence) but also to the survival in the presence of native predators (Yli-Renko et al. 2015; 2018). This makes I. balthica an optimal model species to explore how personality can influence the survival of a native prey species in the presence of an invasive predator.
Invasive and native predator species: The North American white-fingered mud crab (R. harrisii) is a small omnivorous benthic crustacean, native to the Atlantic coast of North America (Williams 1984), from where it has expanded around the globe, mostly via ballast waters (Roche and Torchin 2007). In the Finnish Archipelago Sea (Northern Baltic Sea), where this study was conducted, it was first found in 2009 (Fowler et al. 2013) and has since been expanding its local range, occupying multiple habitats (Fowler et al. 2013; Kotta et al. 2018). Within the Finnish Archipelago, the crab not only forages on mussels and snails but also on amphipods and isopods (Forsström et al. 2015), which provoked a shift in the invertebrate community (Forsström et al. 2015; Jormalainen et al. 2016). Recently, it was shown that the crab prefers the shelter of rocky bottoms independent of food availability (Riipinen et al. 2017), and also that they are likely to negatively impact nest-building fishes by taking over their occupied nests (Lehtonen et al. 2018).
The European perch (Perca fluviatilis) is a common native predatory fish and forages on I. balthica. The fish hunts visually within littoral environments of the Northern Baltic Sea and was used successfully in former studies as a model predator under experimental conditions (see e.g., Engström-Öst et al. 2009; Yli-Renko et al. 2018).
Field collections of test species
In May 2012, we sampled all species used herein for experimentation within the Finnish Archipelago Sea. All experiments were conducted at the Archipelago Research Institute (ARI: 60º14′N, 21º58 E) from the University of Turku. The isopod and the native fish species were collected in the vicinity of ARI, while the invasive mud crab was collected in its invasive range within the Finnish Archipelago Sea (see Fowler et al. 2013).
A total of 116 adult isopods (77 males, 39 females) were collected by hand from bushes of Fucus vesiculosus, while the crabs (a total of 72) were collected using habitat traps. Both sexes (42 males and 30 females) were used in the experiments. The native fish predator (N = 5) was caught via nets and traps that were installed at the same sites where isopods were collected. All species were transported in buckets and quickly brought to the laboratory at ARI and kept in plastic aquaria.
Prior to experimentation, all test species were maintained in a temperature-controlled laboratory room at ~13ºC, which corresponds to the in situ water temperatures during early spring. The light/dark rhythm inside the laboratory was adjusted to natural conditions (14 h day, 10 h night). Sexes of the crabs and the isopods were determined, and their carapace width (crabs) and weight (isopods) were measured, respectively.
A total of N = 116 isopods were kept individually in plastic aquaria (dimensions 23 cm x 13 cm x 14 cm; without aeration 2.7 L) for 12 h to ensure that we measured their activity rather than their exploration behaviour in a novel environment. Activity which has been shown to be a personality trait (Yli-Renko et al. 2015) was measured for each isopod by observing their movements and locations repeatedly every 20 minutes following the protocol developed and used by Yli-Renko et al. (2015). Each aquarium had sand and a small rock on the bottom to mimic their natural habitat. In addition, an apical piece and a basal piece of F. vesiculosus were placed at the opposite ends of the aquaria to provide shelter and food. This decoration (sand-rock-apical piece/basal piece) also made the observations of the isopod location straightforward. We observed the location of each isopod over one day (from 8:00 am to 8:20 pm local time) with a total of 37 observations per individual. The isopod could be in one of the following positions: interacting with the ‘apical’ or ‘basal’ piece of the alga; resting on the sand near the ‘apical’ or ‘basal’ piece of the alga; buried in the sand; moving along the sides of the aquaria; or resting on the rock. We quantified the individual activity as the number of changes of location that the isopod made during the 37 observations. Hence, we obtained the activity of each isopod, which was then used for experimentation.
Cue of invasive and native predators
For the anti-predator response experiment, we selected a total of 72 isopods with 43 males and 29 females. For I. balthica, it has been reported that inactivity is an anti-predator response (Jormalainen and Tuomi 1989). Therefore, we measured their anti-predator response cues by comparing their movements in the same aquaria (see above) but now with predator cues. The cues were either 35 ml of water added from tanks with either the invasive crab, the native fish P. fluviatilis or control water (without any predator cue). After the water was added carefully, we waited for 30 seconds before their activity was measured. For each isopod we observed its number of movements repeatedly every 20 minutes. These measurements were conducted over 4 h (from 4:40 pm to 8:40 pm local time) with a total of 12 observations per individual. In every trial, i.e., at 20 min intervals, new water from the predator tanks was added.
Predation experiment with the invasive crab
To study the survival of isopods with different activity levels (= personality), we continued the experiment and added into each of the aquaria (N = 72) one R. harrissii individual of varying size. We then measured the isopod survival over four days. Within the first two days of experimentation, isopod survival was checked twice per day and afterwards only once per day, which was sufficient to gather the survival data. Crabs used in the predation experiment were not fed for 12 h before the experiment.
In order to study whether the cue of invasive and native predators or control water affected the activity of isopods, we ran a generalized linear mixed model (GLMM) with negative binomial distribution and log link function. The different cues (native, invasive, and control), sex, and personality (activity level = resting on rock/sand, moving, interacting with algae) were treated as fixed factors. Isopod body size (weight) was included as a covariate. To avoid multicollinearity in our analyses we first calculated Spearman’s rank correlation coefficient for body size and activity as these two are continuous factors. We did not find any correlation between these factors and thus both can be used as a fixed factor in the analysis (Spearman rank correlation: rS = −0.07; p = 0.54). First, we also included in the model the interaction sex x cue as we were interested in whether isopod sexes responded differently to the predator cues. However, based on p-values, the interaction was not significant and therefore we simplified the model and removed the interaction from the final analysis.
To compare the probability of survival within the isopod personality types and sexes in the presence of the invasive crab species, we also ran a GLMM, but this time with a binomial error distribution (alive-dead) and logit link-function (a total of N = 72 isopods from which N = 19 were found dead, and not eaten, which were excluded from the analyses). We treated isopod personality (activity) and sex as fixed factors. Earlier laboratory studies showed that the size of the crab affected the impact on prey items (Forsström et al. 2015), while the size of the isopods affected their survival in the presence of a predatory fish (Jormalainen and Tuomi 1985). Thus, the body size of crabs and isopods were included as covariates (weight for isopods and carapace width for crabs). All analyses were carried out using SAS Enterprise Guide (6.1, Cary, NC, USA).