The Role of Predation in the Diet of Niphargus (Amphipoda: Niphargidae)
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Fišer et al. The role of predation in the diet of Niphargus (Amphipoda: Niphargidae) Cene Fišer, Žana Kovačec, Mateja Pustovrh, and Peter Trontelj Oddelek za biologijo, Biotehniška fakulteta, Univerza v Ljubljani, Večna pot 111, Ljubljana, Slovenia. Emails: [email protected], [email protected], [email protected]; [email protected] Key Words: Niphargus balcanicus, Niphargus timavi, Dinaric Karst, Vjetrenica, feeding, foraging. Niphargus (Amphipoda: Niphargidae) is the largest genus of freshwater amphipods1, with over 300 described species. Recently, it was identified as important for addressing ecological and evolutionary questions2. Nevertheless, the exploration of these issues critically depends on understanding the biology of individual species, which is mostly unknown. For instance, utilization of food sources has rarely been studied. Laboratory observations3,4 acknowledged a broad spectrum of foraging behaviors in N. virei Chevreux 1896 and N. fontanus Bate 1859, ranging from limnivory, detritivory, predation on oligochaetes and arthropods, feeding on decayed leaves and carrion, and fish flakes. Sket5 briefly mentioned how N. krameri Schellenberg 1935 fed on dead conspecifics. Finally, Mathieu et al. speculated that adult N. rhenorhodanesis Schellenberg 1937 could prey upon their own conspecifics6. Here we report on dietary data of two species from the Dinarc Karst. In 2001, we held several individuals of the troglobiotic Niphargus balcanicus Absolon 1927 in an aquarium. Unlike many other amphipods, this large (about 30 mm) species retains an upright position during activity (swimming, walking on ground) and rest. The animals were kept in the speleological laboratory in permanent darkness at 8 ºC. After a few days, we dropped a live isopod Asellus aquaticus (about 10 mm long) into the tank. The isopod never reached the bottom: the niphargid intercepted it, vigorously seized it with its gnathopods, smashed and ate it. All other Asellus occasionally fed to the amphipods later on were seized and eaten immediately. Elements of active predatory behavior were also observed in this species in its natural environment in Vjetrenica Cave, Herzegovina. Observed in a pool, a passing cave shrimp (Troglocaris sp.) attracted the attention of a N. balcanicus individual. The amphipod turned toward the shrimp, following it for about half a meter but failed to catch it because it was swimming too fast. It is hard to imagine what would have happen if it did catch the shrimp, because both animals were approximately of the same length, the shrimp being much bulkier than the amphipod. 2010 Speleobiology Notes 2: 4-6 4 Fišer et al. The other species, N. timavi Karaman 1954 is endemic to two small karstic drainages in the northwestern Dinaric Karst7. In 2004, several hundred animals were found feeding on a dead grass snake (Natrix natrix) in the spring of Podstenjšek in southwestern Slovenia. In 2007, we collected several samples of N. timavi from a locality nearby (spring of Kolaški potok8). At that site, N. timavi dwells in sympatry with the surface amphipod Gammarus fossarum Koch 1835. In order to find out whether the two species select different food, we examined stomach contents of 14 specimens of each species. All specimens were measured and partially dissected to obtain the anterior part of the digestive tract (cardial stomach). The stomachs were subsequently macerated in glycerin on microscopic slides. Their contents were examined under a light microscope and described qualitatively. In both species we identified four types of food particles, namely debris of vascular plants (well preserved plant cells), filamentous plant material (colorless filaments with thick, walls, resembling fine roots), algal cells, and arthropod remnants. The G. fossarum specimens measured between 9-14 mm, the N. timavi specimens 7-15 mm. The food composition of both species was essentially the same. It consisted mainly of plant material, where plant debris was most commonly found, while arthropod remnants were found in only three specimens of each species (Figure 1). It is unclear whether arthropod remnants originated from carrion or from prey. Figure 1. Frequency of food types observed in stomachs examined in a surface (Gamarus fossarum) and a subterranean (Niphargus timavi) amphipod species at a site of sympatric occurrence. Fourteen specimens of each species were examined. 2010 Speleobiology Notes 2: 4-6 5 Fišer et al. In short, data on N. timavi support the hypothesis that niphargids are generalists in foraging behavior. Moreover, the observations on G. fossarum and N. timavi do not contradict the notion that predation is an important feeding strategy in freshwater amphipods9. Observations on N. balcanicus even reinforce this view, implying that specialized subterranean amphipods may prey on animals up to their own size. Literature Cited: 1. Väinölä, R., Witt, J. D. S., Grabowski, M., Bradbury, J. H., Jazdzewski, K. & Sket, B.. Global diversity of amphipods (Amphipoda; Crustacea) in freshwater. Hydrobiologia 595, 241–255 (2008). 2. Fišer, C., Sket, B. & Trontelj, P. A phylogenetic perspective on 160 years of troubled taxonomy of Niphargus (Crustacea: Amphipoda). Zoologica Scripta 37, 665–668(2008). 3. Ginet, R. Écologie, éthologie et biologie de Niphargus. Sommaire général. Annales de spéléologie 15, 127–237 (1960). 4. Rumm, P. Zur Ökologie des Grundwasseramphipoden Niphargus fontans (Bate 1859). Deutsche Gesellschaft für Limnologie e. V. Tagungbericht, 384–388 (2000). 5. Sket, B. Prispevek k poznavanju naših amfipodov. Biološki vestnik 6, 67–75 (1958). 6. Mathieu, J., Debouzie, D. & Martin, D. Influence des conditions hydrologiques sur la dynamique dʼune population phreatique de Niphargus rhenorhodanensis (Amphipode souterrain). Vie et Milieu 37, 193–200 (1987). 7. Fišer, C., Sket, B. & Stoch, F. Distribution of four narrowly endemic Niphargus species (Crustacea: Amphipoda) in the western Dinaric region with a description of a new species. Zoologischer Anzeiger 245, 77–97 (2006). 8. Fišer, C., Keber, R., Kereži, V., Moškrič, A., Palandančić, A., Petkovska, V., Potočnik, H. & Sket, B. Coexistence of species of two amphipod genera: Niphargus timavi (Niphargidae) and Gammarus fossarum (Gammaridae), Journal of Natural History 41, 2641–2651 (2007). 9. Macneil, C., Dick, J. T. A. & Elwood, R. W. The trophic ecology of freshwater gammarus spp. (Crustacea: Amphipoda): problems and perspectives concerning the functional feeding group concept. Biological Reviews 72, 349–364 (1997). 2010 Speleobiology Notes 2: 4-6 6 .