Life History And Multiple Antipredator Defenses Of An Invertebrate Pelagic Predator, Bythotrephes Longimanus

Ecology, Jan, 2000 by Dietmar Straile, Astrid Halbich

DIETMAR STRAILE [1]

Limnological Institute, University of Konstanz, P.O. Box 5560, D-78457 Konstanz, Germany

Abstract. Multiple antipredator defense strategies and their interactions were examined in a field study of the predatory planktonic waterflea Bythotrephes longimanus in Lake Constance, at the northern fringe of the European Alps. Because of its large body size and conspicuousness, Bythotrephes is a preferred prey of freshwater fish. We observed seasonal changes in life history and morphology and diel vertical migration, all best understood as a response to fish predation. Bythotrephes population dynamics were characterized by pronounced population growth in late spring, maximum abundances in June, and a steady decline toward the end of the season. In late spring, high population growth rates were achieved by means of large clutches, low investment in individual offspring, and small size at first reproduction. While the population was still increasing, a marked life history shift occurred. The reproductive strategy of females switched toward high per-offspring allocation at the expense of clutch size. This c hange in reproductive behavior resulted in an increase in the size of neonates and was accompanied by an increase in the size at first reproduction. Such a life history shift is typically observed in the presence of gape-limited predators, which points to the importance of juvenile fish as the principal vertebrate planktivores in Lake Constance. The length of Bythotrephes defensive spina increased throughout the season, reflecting increasing predation pressure, probably owing to seasonally increasing mouth gape size of juvenile fish. In contrast to the predictions of the predator-avoidance theory, the migration amplitude of large and conspicuous Bythotrephes was small as compared to other zooplankton species. We argue that theory and data can be reconciled if the migration behavior of Bythotrephes is considered as a result of an interaction of predator defenses in the presence of predominantly juvenile, gape-limited fish. As Bythotrephes achieved protection because of its life history and spina, the costs of large did migrations may outweigh the benefits.

Key words: alternative antipredator defenses; Bythotrephes; diel vertical migration; gape-limited predation; juvenile fish; Lake Constance, European Alps; life history, morphological defense, offspring allocation; reproductive investment.

INTRODUCTION

Predation is known to influence the biology of prey species in numerous respects, including their morphology (Harvell 1984, Bronmark and Miner 1992, Tollrian 1995), behavior (Lima and Dill 1990, Lampert 1993, Sih 1994), and life history traits (Growl and Covich 1990, Wellborn 1994). The intensity of a predator-prey interaction, the direction of the selection pressure imposed on the prey population, and the suitability of a certain defensive trait depend on the type of predator (Polis 1988, Wilbur 1988, Persson et al. 1996). Non-gape-limited predators (NGLPs) whose prey size windows encompass the size spectra of a prey population generally select the largest individuals of a population (Brooks and Dodson 1965, Wellborn 1994). Feasible defensive responses to NGLPs include life history shifts (Wellborn 1994, Rodd and Reznick 1997) and behavioral defenses (Lampert 1993, Endler 1995, Persson et al. 1996). The prey size window of a gape-limited predator (GLP) does not include the large individuals of a prey popula tion (Huangh and Sib 1991, Wellborn 1994). In response to GLPs' life history shifts (Growl and Covich 1990, Tollrian 1995), behavioral defenses (Kvam and Cleiven 1995) and morphological defenses are common (Stemberger and Gilbert 1987, Wicklow 1988, Bronmark and Miner 1992, Tollrian 1995). Some species are known to exhibit several defensive traits. For example, daphnids may react to increased predation pressure with life history shifts (Taylor and Gabriel 1992, Tollrian 1995) and changes in behavior (Lampert 1993, Kvam and Cleiven 1995) and morphology (Tollrian 1995, Swaffar and O'Brien 1996). Likewise, increased predation pressure on guppies is associated with changes in life history, behavior, body form, and color (Endler 1995). While the predation effect on individual defensive traits is well studied, relatively little is known concerning the effects of predation on the interactions between different traits (Sparkes 1996, De Meester et al. 1995).

The freshwater onychopod cladoceran genus Bythotrephes longimanus (Cladocera, Cercopagidae) is native to lakes throughout the Palearetic (Berg and Garton 1994) and reproduces parthenogenetically during the summer and gametogenically during autumn (Mordukhai-Boltovskaya 1957, Ketelaars et al. 1995). Because of its large body size, Bythotrephes is highly positively selected by adult fish, i.e., NGLPs, and thus is important if not dominant in the diet of fish in many temperate lakes,including Lake Constance (Giussani 1974, Nilsson 1974, Langeland 1978, Fitzmaurice 1979, Becker 1992). However, not adult fish but juveniles are regarded as the most important planktivorous fish in many lakes (Mehner and Winfield 1997). As mouth gape size increases with growth, juvenile fish switch progressively to larger prey items and finally start to ingest Bythotrephes around midsummer (Hartmann 1983, Jachner 1991, Wang 1994) but remain GLPs in respect to Bythotrephes (Barnhisel and Harvey 1995). Hence, Bythotrephes is exposed at the same time to GLPs and NGLPs, i.e., juvenile and adult fish. Many studies focus on the effects of a single type of predator, and relatively few have compared the effects of different types of predators on prey populations (Wilbur and Fauth 1990, Kurzava and Morin 1998, Sih et al. 1998). The exposure of prey species to different size classes, species, and types of predators is probably the norm rather than the exception (Hall and Raffaelli 1991, Polis 1991) and may have effects that cannot be predicted simply by summing the effects of single predator types (Sih et al. 1998).