The foraging ecology of Ranatra montezuma (Heteroptera): an optimal forager in Montezuma Well, Arizona

Ranatra montezuma (Heteroptera: Nepidae) are size selective predators in Montezuma Well and our data suggest an energetic advantage when feeding on larger Hyalella montezuma prey (Crustacea: Amphipoda). We studied prey-size preference and the effect of prey size on daily energy ingestion and development of R. montezuma. We also examined the foraging behavior of R. montezuma with respect to the optimal foraging theory. The average processing time for adult and juvenile amphipod prey was 41.2 min and 14.4 min, respectively. Daily energy ingestion (joules) was measured for R. montezuma reared at 21 degrees C in the laboratory. Calorimetry of whole amphipods yielded 14.4 joules mg⁻¹ dry weight; R. montezuma removed 47% of this energy from H. montezuma during feeding. Ranatra montezuma ingested significantly more energy day⁻¹ and had shorter instar durations when fed adult amphipods. First, second, third, fourth, fifth instars, and adult R. montezuma feeding on adult Hyalella ingested 2.3, 3.8, 5.2, 8.0, 11.8, and 43.1 joules per day⁻¹ respectively, with a total instar duration of 68.2 d. In contrast, first, second, third, fourth, fifth instars, and adult R. montezuma feeding on juvenile Hyalella ingested 1.2, 1.9, 2.3, 3.7, 7.8, and 8.8 joules day⁻¹ respectively, with a total instar duration of 78.1 d. Energy ingestion mg⁻¹ body weight decreased with successive instars of R. montezuma. A list of publications on the biology and geology of Montezuma Well is provided.