Ontogenetic dietary shift in the larvae of Cybister japonicus (Coleoptera: Dytiscidae) in Japanese rice fields

Description of the larva of Cybisterlewisianus Sharp, 1873 (Coleoptera, Dytiscidae, Cybistrinae)

We describe for the first time, the larvae of Cybister (Cybister) lewisianus Sharp, 1873, an endangered species of diving beetle in Japan, emphasizing the chaetotaxy of the cephalic capsule, head appendages, legs, last abdominal segment, and urogomphi. Cybisterlewisianus larvae are characterized by a longer third article of antenna 3 than the sum of the first and second articles; rounded apex of parietal setae 1-3; labium seta 8 absent; elongated trochanter seta 4, not multi-branched; rounded apex of abdominal setae 1, 12, and 13 (instar I); narrow lateral projections of the frontoclypeus; pronotum without two dark-brown longitudinal stripes dorsally (instar III); and the base of the thick row of small setae on the inner edge of the mandible angulate and projecting medially (all instars).

Fundamental Niche Narrows through Larval Stages of a Filter-Feeding Marine Invertebrate

AbstractOntogenetic niche theory predicts that resource use should change across complex life histories. To date, studies of ontogenetic shifts in food niches have mainly focused on a few systems (e.g., fish), with less attention on organisms with filter-feeding larval stages (e.g., marine invertebrates). Recent studies suggest that filter-feeding organisms can select specific particles, but our understanding of whether niche theory applies to this group is limited. We characterized the fundamental niche (i.e., feeding proficiency) by examining how niche breadth changes across the larval stages of the filter-feeding marine polychaete Galeolaria caespitosa. Using a no-choice experimental design, we measured feeding rates of trochophore, intermediate-stage, and metatrochophore larvae on the prey phytoplankton species Nannochloropsis oculata, Tisochrysis lutea, Dunaliella tertiolecta, and Rhodomonas salina, which vary 10-fold in size, from the smallest to the largest. We formally estimated Levins's niche breadth index to determine the relative proportions of each species in the diet of the three larval stages and also tested how feeding rates vary with algal species and stage. We found that early stages eat all four algal species in roughly equal proportions, but niche breadth narrows during ontogeny, such that metatrochophores are feeding specialists relative to early stages. We also found that feeding rates differed across phytoplankton species: the medium-sized cells (Tisochrysis and Dunaliella) were eaten most, and the smallest species (Nannochloropsis) was eaten the least. Our results demonstrate that ontogenetic niche theory describes changes in fundamental niche in filter feeders. An important next step is to test whether the realized niche (i.e., preference) changes during the larval phase as well.

The ontogenetic dietary shift from non-dangerous to dangerous prey in predator-eating predators under capture risk

Evaluating the patterns and generality of ontogenetic dietary shifts (ODSs) contributes to understanding prey-predator interactions and food web dynamics. Numerous studies have focused on predators that target distinctively lower trophic-level organisms. However, the ODS of predators that routinely prey on organisms at similar trophic levels (i.e., predator-eating predators) have been neglected in ODS research. The ODS patterns of predator eaters may not fit into conventional frameworks owing to constraints of potential capture risk (e.g., deadly counterattack from prey) and body size. We aimed to reveal the ODS patterns of predator eaters and determine whether the patterns were affected by body size and capture risk. Assuming that capture risk is a significant factor in ODS patterns, we expected: (1) juvenile araneophagic spiders to forage on non-dangerous prey (insects) and capture larger non-dangerous prey more frequently than dangerous prey (spiders); and (2) as they grow, their prey types will shift from non-dangerous to dangerous prey because larger predators will be able to capture dangerous prey as the optimal food. As a result of field observations, we revealed that the major ODS pattern in these spiders changed from a mixed (both insect and spider) to a spider-dominant diet. The model selection approach showed that this diet shift was partly due to predator size, and the relative importance of predator size was higher than the life stage per se and almost equal to species identity. In these spiders, the body size of spider prey tended to be smaller than that of insects when the predators were small, suggesting that capture risk may be a critical factor in determining the ODS patterns of these predators. Therefore, our study adds to the evidence that the capture risk is crucial in comprehensively understanding the mechanisms determining ODS patterns in natural systems.

Hydrophobic-hydrophilic crown-like structure enables aquatic insects to reside effectively beneath the water surface

Various insects utilise hydrophobic biological surfaces to live on the surface of water, while other organisms possess hydrophilic properties that enable them to live within a water column. Dixidae larvae reside, without being submerged, just below the water surface. However, little is known about how these larvae live in such an ecological niche. Herein, we use larvae of Dixa longistyla (Diptera: Dixidae) as experimental specimens and reveal their characteristics. A complex crown-like structure on the abdomen consists of hydrophobic and hydrophilic elements. The combination of these contrasting features enables the larvae to maintain their position as well as to move unidirectionally. Their hydrophobic region leverages water surface tension to function as an adhesive disc. By using the resistance of water, the hydrophilic region serves as a rudder during locomotion.

Suzuki, Takaku, Hariyama and colleagues report on a crown-like structure found on the heads of midge larvae. This structure, analysed using scanning electron microscopy and experimental methods, enables subsurface adhesion and aids in control of locomotion in this region of the water column.

Ultrastructure and morphology of antennal sensilla of the adult diving beetle Cybister japonicus Sharp

The morphology and distribution of the antennal sensilla of adult diving beetle Cybister japonicus Sharp (Dytiscidae, Coleoptera), have been examined. Five types of sensilla on the antennae were identified by scanning electron microscope (SEM) and transmission electron microscope (TEM). Sensilla placodea and elongated s. placodea are the most abundant types of sensilla, distributing only on the flagellum. Both these types of sensilla carry multiple pore systems with a typical function as chemoreceptors. Three types of s. coeloconica (Type I–III) were also identified, with the characterization of the pit-in-pit style, and carrying pegs externally different from each other. Our data indicated that both type I and type II of s. coleconica contain two bipolar neurons, while the type III of s. coleconica contains three dendrites in the peg. Two sensory dendrites in the former two sensilla are tightly embedded inside the dendrite sheath, with no space left for sensilla lymph. There are no specific morphological differences in the antennal sensilla observed between males and females, except that the males have longer antennae and more sensilla than the females.

Abiotic predictors of faunal communities in an ombrotrophic peatland lagg and an open peat bog

Most ecological research has hitherto focused more on sea and lake ecosystems than on peatland habitats. The primary objectives of this paper were to analyse the ciliate, rotifer, cladoceran, copepod and insect assemblages in a horizontal lagg and an open peat bog, and to assess the influence of physical and chemical parameters on their communities. Sampling was done in a transitional bog from May to October 2012 in a transect comprising the lagg and the open peatbog. The first two axes of a principal component analysis accounted for 49.8% of the total variance in the composition of the faunal communities studied. The distribution of samples in ordination space suggested that the habitats are distributed along the gradient of water level and the gradients of total organic carbon and nutrients. Assemblages of all groups investigated showed a strong compositional gradient correlated with surface water and phosphates. However, species composition of ciliates and rotifers was explained by conductivity and/or chlorophyll-a concentration. The results suggest that the lagg zone of a bog can function as an ecotone, with significantly greater species richness and abundance of faunal communities.

Abundance, diversity, and seasonal population dynamics of aquatic Coleoptera and Heteroptera in rice fields: effects of direct seeding management

Recent introduction of modern drainage systems has produced intensely dry conditions in rice farmlands and has degraded habitats for aquatic animals. In this study, we compared water beetle (Coleoptera) and water bug (Heteroptera) communities within rice fields cultivated under different management regimes: V-furrow no-till direct-seeding (DS) and conventional regimes. In DS fields, rice is sown in well-drained fields, and flooding is performed a month later than in conventional rice fields. DS fields are then continuously flooded until harvesting; unlike in conventional fields, where midseason drainage is performed in summer. We observed that DS fields supported higher densities of water beetles and water bugs than conventional fields, probably because of the high compatibility between the flooding period and the reproductive season of the insects. The species richness of water beetles was higher in DS fields than in conventional fields. Overall, DS fields showed higher water beetle and water bug abundance, but the effect was variable for individual species: seven species were more abundant in DS than in conventional fields, whereas two species showed opposite tendencies. Considering the differential responses among species to the management regimes, a mosaic of DS and conventional fields is preferable to either field alone for the conservation of aquatic insects in rice agroecosystems.

Density-Dependent Effects of Amphibian Prey on the Growth and Survival of an Endangered Giant Water Bug

Amphibian predator-insect prey relationships are common in terrestrial habitats, but amphibian larvae are preyed upon by a variety of aquatic hemipterans in aquatic habitats. This paper suggests that the survival of the nymphs of the endangered aquatic hemipteran Kirkaldyia (=Lethocerus) deyrolli (Belostomatidae: Heteroptera) is directly and indirectly affected by the abundance of their amphibian larval prey (tadpoles). Young nymphs of K. deyrolli mainly feed on tadpoles, regardless of differences in prey availability. Nymphs provided with tadpoles grow faster than nymphs provided with invertebrate prey. Therefore, tadpole consumption seems to be required to allow the nymphs to complete their larval development. In addition, the survival of K. deyrolli nymphs was greater during the period of highest tadpole density (June) than during a period of low tadpole density (July). Higher tadpole density moderates predation pressure from the water scorpion Laccotrephes japonensis (Nepidae: Heteroptera) on K. deyrolli nymphs; i.e., it has a density-mediated indirect effect. These results suggest that an abundance of tadpoles in June provides food for K. deyrolli nymphs (a direct bottom-up effect) and moderates the predation pressure from L. japonensis (an indirect bottom-up effect). An abundance of amphibian prey is indispensable for the conservation of this endangered giant water bug species.

Predatory ability of adult diving beetles on the Japanese encephalitis vector Culex tritaeniorhynchus

The predatory ability of adult Japanese diving beetles on 4th instars of the Japanese encephalitis vector mosquito, Culex tritaeniorhynchus, was assessed under laboratory conditions. To determine the differences in the predatory ability among 14 beetle species inhabiting rice fields, the following species were introduced to 10 Cx. tritaeniorhynchus 4th instars in a plastic cup: 5 small-bodied species (< 9 mm in body length) comprising Hydroglyphus japonicus, Noterus japonicus, Laccophilus difficilis, Hyphydrus japonicus, and Agabusjaponicus; 7 medium-bodied species (9-20 mm in body length) comprising Hydaticus rhantoides, Hydaticus grammicus, Rhantus suturalis, Eretes griseus, Hydaticus bowringii, Agabus conspicuous, and Graphoderus adamsii; and 2 large-bodied species (> 20 mm) comprising Cybister brevis and C. japonicus. The average 24-h predation rate was highest in medium-bodied species (> 90%), followed by small-bodied species (31%) and large-bodied species (19%). The functional responses to Cx. tritaeniorhynchus larvae of 3 medium-bodied species (H. grammicus, R. suturalis, and E. griseus) were estimated. Eretes griseus exhibited the highest attach rate and shortest prey-handling time, suggesting that medium-bodied diving beetles, especially E. griseus, may be efficient predators of mosquito larvae in rice fields.