Predator ontogeny affects expression of inducible defense morphology in rotifers.

Many prey organisms show induced morphological responses to predators including changes in protective spine length, such as in rotifers, although previous studies have mainly focused on how prey become larger than the predator gape-size optimum. Here we show that a large-sized predator makes prey rotifers escape below the gape-size optimum of the predator by reducing spine length. In experiments and field studies we show that during part of their ontogeny fish larvae feed intensively on the common rotifer Keratella cochlearis, and that larval fish predation reduces rotifer spine length both through induction of shorter spines and selective predation on long-spined individuals. We also describe a global scale pattern in spine length of K. cochlearis, showing an increasing variance in spine length with latitude. This pattern may be explained by differences in fish reproduction from once per year at high latitudes to several times per year at lower latitudes. That spine length is adaptively adjusted to the ontogeny of a dominant predator taxa provides a novel view on our understanding of factors affecting temporal and spatial variations in prey defense morphology.

Ecological speciation in European whitefish is driven by a large-gaped predator.

Lake-dwelling fish that form species pairs/flocks characterized by body size divergence are important model systems for speciation research. Although several sources of divergent selection have been identified in these systems, their importance for driving the speciation process remains elusive. A major problem is that in retrospect, we cannot distinguish selection pressures that initiated divergence from those acting later in the process. To address this issue, we studied the initial stages of speciation in European whitefish (Coregonus lavaretus) using data from 358 populations of varying age (26-10,000 years). We find that whitefish speciation is driven by a large-growing predator, the northern pike (Esox lucius). Pike initiates divergence by causing a largely plastic differentiation into benthic giants and pelagic dwarfs: ecotypes that will subsequently develop partial reproductive isolation and heritable differences in gill raker number. Using an eco-evolutionary model, we demonstrate how pike's habitat specificity and large gape size are critical for imposing a between-habitat trade-off, causing prey to mature in a safer place or at a safer size. Thereby, we propose a novel mechanism for how predators may cause dwarf/giant speciation in lake-dwelling fish species.

The behavioral response of prey fish to predators: the role of predator size.

Predation is one of the key factors governing patterns in natural systems, and adjustments of prey behaviors in response to a predator stimulus can have important ecological implications for wild fish. To investigate the effects of predators on the behavior of prey fish and to test whether the possible effects varied with predator size, black carp (Mylopharyngodon piceus) and snakehead (Channa argus) (a size-matched predator treatment with a similar body size to prey fish and a larger predator treatment with approximately 2.7 times of the body mass of prey fish) were selected to function as prey and predator, respectively. Their spontaneous activities were videorecorded in a central circular arena surrounded by a ring holding the stimulus fish. The distance between prey and predator fish was approximately 200% of the distance between two prey fish, which suggested that black carp can distinguish their conspecifics from heterospecifics and probably recognize the snakehead as a potential predator. The prey fish spent substantially less time moving and exhibited an overall shorter total distance of movement after the size-matched or large predator was introduced, which possibly occurred due to increased vigilance or efforts to reduce the possibility of detection by potential predators. However, there was no significant difference in either distance or spontaneous activities between two predator treatments. These findings suggested that (1) an anti-predator strategy in black carp might involve maintaining a safe distance, decreasing activity and possibly increased vigilance and that (2) the behaviors of prey response to predators were not influenced by their relative size difference.