358 Diet of Dinocras cephalotes and Perla marginata (Plecoptera: Perlidae) in an Apennine stream (northwestern Italy) Tiziano Bo, Stefano Fenoglio,1 Giorgio Malacarne University of Piemonte Orientale, Department of Life and Environmental Science, Via Bellini n. 25, 15100 Alessandria, Italy Abstract—The feeding habits of nymphs of Perla marginata (Panzer) and Dinocras cephalotes (Curtis) were investigated in the Rio Orbarina (northwestern Italy). These species are among the largest European carnivorous freshwater invertebrates and they play an important role in the trophic structure of small, fishless Apennine streams. We examined the gut contents of 60 P. marginata and 60 D. cephalotes nymphs to characterize the diets and evaluate possible feeding differences between the species. In both of these predaceous stoneflies, the diet included vegeta- ble detritus, mainly in the smaller instars. Both species showed trophic preferences, since only a few taxa constituted most of the ingested prey items, independently of their availability in the substratum. Interestingly, there were no clear differences in prey selection between nymphs of the two species. Résumé—Nous avons étudié les habitudes alimentaires des larves de Perla marginata (Panzer) et de Dinocras cephalotes (Curtis) dans le Rio Orbarina (nord-ouest de l’Italie). Ces espèces sont parmi les invertébrés d’eau douce carnivores les plus grands d’Europe et elles jouent un rôle im- portant dans la structure trophique des petits cours d’eau sans poissons des Apennins. Notre tra- vail examine le contenu stomacal de 60 larves de P. marginata et de 60 de D. cephalotes.Lebut de l’étude est de définir le régime alimentaire et d’évaluer les différences dans l’alimentation des deux espèces. Le régime alimentaire de ces deux plécoptères prédateurs inclut du détritus végé- tal, particulièrement chez les plus petits stades. Il existe de nettes préférences trophiques dans le régime des deux espèces : un petit nombre de taxons constitue la majeure partie des proies ingé- rées, indépendamment de leur disponibilité dans le substrat. Il est intéressant de noter que nous n’avons décelé aucune différence claire dans la sélection des proies entre les larves de ces deux espèces. [Traduit par la Rédaction] Bo364et al. have been studied to investigate leaf breakdown Introduction and allochthonous input processes in lotic sys- Aquatic insects predominate in the trophic tems (Gessner et al. 1999; Fenoglio et al. structure of streams (Wallace et al. 1987; Allan 2005a), while predaceous stoneflies are a good 1995) and their feeding habits have been in- model for predator–prey studies in streams creasingly studied in the last few decades (Tikkanen et al. 1997). (Tierno de Figueroa and Sanchez-Ortega 1999; In many small, fishless streams, Plecoptera Monakov 2003). belonging to the suborder Systellognatha are Plecoptera is an important and often domi- the dominant predator group, with important nant order in stream ecosystems (Zwick 2000). roles as top-down control elements of inverte- The feeding habits of stonefly larvae are varied, brate communities (Wipfli and Gregovich 2002) as reflected by the great variation in the struc- and as “ecological engineers” (Zanetell and ture of the mouthparts of different Plecoptera Peckarsky 1996). Therefore, recent studies have species. Plecopteran detritivorous shredders evaluated the trophic roles of these organisms Received 19 January 2006. Accepted 18 December 2006. 1Corresponding author (e-mail: [email protected]). Can. Entomol. 139: 358–364 (2007) © 2007 Entomological Society of Canada Bo et al. 359 in the field (Bo and Fenoglio 2005; Fenoglio et Lumbriculidae and early instars of some al. 2005b) and in the laboratory (Elliott 2004). Diptera and Trichoptera, which were identified Gut contents analysis is the most commonly to the family level. The total length of used technique in field studies. It is based on P. marginata and D. cephalotes nymphs was the assessment of undigested and sclerotized measured (0.1 mm accuracy). Nymphs were prey parts in the stomach of predators and com- later processed to assess food consumption by parison between the prey eaten and the prey means of gut contents analysis. Guts were re- available in the natural environment. moved and the contents of the alimentary canal The “optimal foraging theory” (Krebs 1978) were analysed by the transparency method for states that predators include the most profitable slides (Faure’s fluid). Identification of prey was prey in their diet in terms of energy content, en- based on sclerotized body parts, particularly counter rate, prey density, handling time, and head capsules, mouthparts, and leg fragments. other factors. Among Perlidae, some species Stewart and Stark (2002) stated that the count seem to be selective, feeding mainly on a few of sclerotized fragments (i.e., head capsules or items (Fenoglio and Bo 2004), while others legs) can give a reasonably accurate count of seem to be more opportunistic (Dudgeon 2000). prey consumed. Each head was counted as an In this study we examined the diet of the imma- individual, as was each leg of the same type ture stages of two Perlidae species: Perla margi- (e.g., first right leg of a mayfly). nata (Panzer, 1799) and Dinocras cephalotes Gut contents were also compared with the (Curtis, 1827). These species attain a final size natural composition and abundance of macro- that places them among the largest European invertebrate communities in the riverbed. More- carnivorous freshwater invertebrates, and they over, to analyse a possible shift in food play an important role in the trophic structure preference according to nymph size, we sepa- of small, fishless Apennine streams. The aim of rated the nymphs into three length classes this study was to analyse the diet of P. margi- (P. marginata: small, <5.0 mm, n = 15; interme- nata and D. cephalotes nymphs of different diate, 5–15 mm, n = 21; large, >15 mm, n = 24; sizes to test the hypothesis that these species D. cephalotes: small, <5.0 mm, n = 9; interme- have feeding preferences and to evaluate possi- diate, 5–15 mm, n = 30; large, >15 mm, n = ble differences in their diets. 21). Feeding preferences were quantified using the trophic electivity index of Ivlev (1961), Materials and methods rp− E = ii Dinocras cephalotes and P. marginata nymphs + were collected in the Rio Orbarina (570 m a.s.l.) rpii on 24–25 October 2005. This second-order stream in the Regional Natural Park of Beigua where ri is the proportion of ingested species is a typical Apennine lotic system with good and pi is the relative abundance in the benthic environmental quality; it is classified as First community, and the electivity index of Class in the Italian Extended Biotic Index McCormick (1991), (Ghetti 1997), corresponding to an environment WN−1/ with no trace of human-induced alteration. We E* = i + examined 60 D. cephalotes and 60 P. marginata WNi 1/ nymphs collected in a 200 m riffle. All samplings were conducted early in the morning where because Systellognatha are considered to be rp/ = ii chiefly nocturnal feeders (Vaught and Stewart Wi 1974). Also, using a Surber net (20 cm × ∑ rpii/ 20 cm, 255 μm mesh), we collected samples in the same reach to assess the presence and abun- and N is the number of food items. Both in- dance of the taxa of the natural benthic inverte- dexes range from –1 to 1. A value of –1 indi- brate population. Samples were preserved in cates total avoidance, 1 indicates preference, 95% ethanol. and 0 indicates indifference. In the laboratory, all organisms were counted Analysis of variance (ANOVA) was per- and identified to the genus level except for formed with Systat 8.0 (Wilkinson 1992). © 2007 Entomological Society of Canada 360 Can. Entomol. Vol. 139, 2007 Table 1. Percent relative abundance (% value in the Table 1 (concluded). community) of macroinvertebrates collected in the riverbed of the Rio Orbarina (northwestern Italy). Relative Taxon abundance (%) FFG* Relative Annelida Taxon abundance (%) FFG* Lumbricidae 0.12 Cg Plecoptera Lumbriculidae 0.48 Cg Leuctra sp. 5.75 Sh Tricladida Nemoura sp. 1.56 Sh Dugesia sp. 2.16 P Amphinemura sp. 1.20 Sh Protonemura sp. 0.36 Sh Gastropoda Isoperla sp. 2.51 P Ancylus fluviatilis 0.24 Sc Perla marginata 7.19 P Nematomorpha Dinocras cephalotes 7.19 P Gordius sp. 0.36 P Ephemeroptera Arachnida Ecdyonurus sp. 4.31 Sc Hydracarina 0.72 P Torleya major 2.75 Cg Paraleptophlebia sp. 3.95 Cg *FFG, functional feeding groups: Cg, collectors–gather- Epeorus sylvicola 8.26 Sc ers; F, filterers; P, predators; Sc, scrapers; and Sh, shred- ders (see Merritt and Cummins 1996). Baetis sp. 8.38 Cg Ephemera danica 0.12 Cg Trichoptera Tinodes sp. 0.12 Sh Results Sericostoma sp. 7.31 Sh In total, we collected 835 stream inverte- Hydropsyche sp. 8.02 F brates belonging to 44 taxa (see Table 1). We Philopotamus sp. 6.71 F examined the gut contents of 60 D. cephalotes Hyporhyacophila sp. 0.96 P and 60 P. marginata nymphs. Rhyacophila sp. 0.36 P Most of the food ingested consisted of insect Odontocerum albicorne 0.48 Sh larvae, but algae and vegetable fragments were Photamophilax cingulatus 0.36 Sh found in a large number of specimens of both Limnephilidae 0.12 Sh species. Figure 1 shows the percentages of guts Diptera with prey fragments, with detritus only, and Chironomidae 4.07 Cg with detritus and prey fragments for the differ- Atherix sp. 3.71 P ent size classes of the two species. There were Tipula sp. 0.60 Sh no significant differences between the two spe- Limoniidae 0.24 P cies in the number of prey fragments ingested Simuliidae 0.24 F (ANOVA: F1,118 = 0.034, P = 0.56) or the num- Odonata ber of prey taxa (ANOVA: F1,118 = 1.12, P = Onychogomphus forcipatus 0.48 P 0.29).