Jun-ichi Okano

The effects of particle surface texture on silk secretion by the caddisfly Goera japonica during case construction

Caddisfly larvae secrete silk to line the inner walls of their portable cases to create smooth surfaces that enhance their respiratory efficiency. In this study, Goera japonica Banks larvae were allowed to repair their portable cases using artificial mineral particles with different surface textures to compare the conformation and quantity of silk secreted on the inner case surface. Three types of particles with different surface roughness were used in the experiments; surface roughness was determined by confocal laser scanning microscopy. Particle selection experiments were conducted for every possible pairing of the three particle types. Thick silk layers were secreted on cases built using rough particles, hardly any silk layers on cases built using smooth particles and brindled thin silk layers were deposited over intermediate particles. The mean silk secretion rate was much higher (244%) with rough particles than with smooth ones, while the rate was somewhat higher (60%) with intermediate particles. In addition, smooth particles were strongly preferred to rough particles, while smooth particles were slightly more preferable than intermediate particles. These results suggest that G. japonica larvae preferred particles with smoother surfaces as case-building materials, because silk production can be decreased as surface texture becomes smoother. Also, there may be a trade-off between the costs of material selection and silk secretion for case construction by caddisfly larvae; saving energy by secreting less silk on smoother particles allows more energy to be spent on selecting smoother particles or on other activities.

Effect of current velocity and case adaptations on the distribution of caddisfly larvae (Glossosoma, Trichoptera)

In aquatic systems, oxygen availability is often a limiting factor affecting animal distribution. We documented the distribution of case-bearing caddisfly larvae (Glossosoma) in a riffle of the Hirose River, Japan, and found that larval density increased with increasing current velocity. We also experimentally measured the effect of flow and case morphology on oxygen consumption and mortality. Larvae were divided into “perforated” and “closed” case treatment groups where gaps in the upper side of the dome-shaped case were left open or closed. They were placed in either lotic or lentic conditions. Oxygen consumption was higher in the lotic compared to the lentic environment for both groups, and was higher in individuals of the perforated-case group than in those belonging to the closed-case group in both environments. Larval mortality was lower under lotic conditions than lentic conditions for the perforated-case group, and higher in the closed-case group in both environments. Our results suggest that larvae prefer areas with high current velocity as they are otherwise unable to obtain sufficient oxygen because of their lack of undulatory behavior. Furthermore, gaps in the larval case appear to enhance passive gaseous exchange via water flow. We also discuss the functional significance of the glossosomatid case.

The effects of surface roughness of sediment particles on the respiration of case-bearing caddisfly larvae

For aquatic tube dwellers, the ventilation system is an essential trait because the external structure impedes gas exchange. Their gas-exchange efficiency is determined by various environmental factors. We assessed the effect of microscale surface roughness (texture) of the inner wall of the case on the respiration of caddisfly larvae Psilotreta kisoensis Iwata (Odontoceridae:Trichoptera). We compared the O2 consumption of larvae in 2 types of cases constructed from smooth and rough artificial particles at 2 water temperatures (15 and 23°C). Larvae in smooth cases consumed more O2 than those in rough cases at 23 but not at 15°C. Larvae in rough cases exhibited no significant respiration response to water temperature. In contrast, larvae in smooth cases increased their O2 consumption as the temperature increased. In addition, the increasing degree of O2 consumption tended to be higher in lightweight immature larvae. These results indicate that a rougher surface depresses larval respiration activity, probably because of friction between the larval body and case wall, which may lead to high mortality and low growth. The roughness of sediment particles is determined by local geology and causes plasticity or intraspecific local variation in case-making behavior. Therefore, the roughness of sediment particles is not only a limiting factor for animal distribution but also exerts a locally different evolutionary pressure on animal respiration systems.

Mineralogical composition of sediment determines the preference for smooth particles by caddisfly larvae during case construction

1. The mineralogical/petrological composition of the substratum influences aquatic organisms in several ways. However, the actual mechanisms are often unclear. Some caddisfly larvae actively concentrate smooth quartz particles in their portable cases thus producing a smooth inner surface of the case wall.

Differential Responses of Two Ecologically Similar Case-Bearing Caddisfly Species to a Fish Chemical Cue: Implications for a Coexistence Mechanism

The mechanisms for the coexistence of multiple species occupying the same ecological niche are often puzzling. Predator effects on competitively superior species is one possible mechanism. In this study, we tested whether the presence of size-selective predators (fishes) acts as a mechanism for the coexistence of two species of case-bearing caddisfly larvae, Perissoneura paradoxa and Psilotreta kisoensis (Odontoceridae, Trichoptera). The larvae of these two species have similar ecological and life history traits except their body size, and they have been found to coexist only in habitats shared with predatory fishes. Experiments on intra and interspecific competition revealed that the larger Pe. paradoxa always outcompeted the smaller Ps. kisoensis in the absence of predatory fishes, suggesting that Pe. paradoxa performed intra-guild predation on Ps. kisoensis. We also conducted experiments to examine how strongly each of these species responded in terms of case repair with/without a predator chemical cue after their cases were partly dismantled. Perissoneura paradoxa exhibited a stronger case repair response in the presence of a predator chemical cue than that exhibited by Ps. kisoensis, suggesting that Pe. paradoxa is more vulnerable to fish predation, probably because their body size is in the preferred prey range of fishes. We suggest that the presence of predators works in the favor of smaller, subordinate species through size-selective predator effects, enabling these two competitive species to coexist in the same habitat.

Integrated trophic position decreases in more diverse communities of stream food webs

The relationship between biodiversity and ecosystem functioning is an important theme in environmental sciences. We propose a new index for configuration of the biomass pyramid in an ecosystem, named integrated trophic position (iTP). The iTP is defined as a sum of trophic positions (i.e. the average number of steps involved in biomass transfer) of all the animals in a food web integrated by their individual biomass. The observed iTP for stream macroinvertebrates ranged from 2.39 to 2.79 and was negatively correlated with the species density and the Shannon–Wiener diversity index of the local community. The results indicate a lower efficiency of biomass transfer in more diverse communities, which may be explained by the variance in edibility hypothesis and/or the trophic omnivory hypothesis. We found a negative effect of biodiversity on ecosystem functioning.

Effects of resource-dependent cannibalism on population size distribution and individual life history in a case-bearing caddisfly

Resource availability often determines the intensity of cannibalism, which has a considerable effect on population size distribution and individual life history. Larvae of the caddisfly Psilotreta kisoensis build portable cases from sedimentary sands and often display cannibalism. For this species, the availability of preferable case material is a critical factor that affects larval fitness, and material is locally variable depending on the underlying geology. In this study, we investigated how sand quality as a case material determines cannibalism frequency among larvae and, in turn, how the differential cannibalism frequency affects the body-size distribution and voltinism. Rearing experiments within a cohort revealed that a bimodal size distribution developed regardless of material quality. However, as the preferable material became abundant, the proportion of larger to smaller individuals increased. Consecutive experiments suggested that smaller larvae were more frequently cannibalized by larger ones and excluded from the population when preferable smooth material was abundant. This frequent cannibalism resulted in a bimodal size distribution with a significantly higher proportion of larger compared to smaller individuals. The size-dependent cannibalism was significantly suppressed when the larvae were raised in an environment with a scarcity of the preferable case material. This is probably because larvae cannot enjoy the benefit of rapid growth by cannibalism due to the difficulties in enlarging their case. At low cannibalism the growth of smaller individuals was stunted, and this was probably due to risk of cannibalism by larger individuals. This growth reduction in small individuals led to a bimodal size-distribution but with a lower proportion of larger to smaller individuals compared to at high cannibalism. A field study in two streams showed a similar size distribution of larvae as was found in the rearing experiment. The bimodal ratio has consequences for life history, since a size-bimodal population causes a cohort splitting: only larvae that were fully grown at 1 year had a univoltine life cycle, whereas larvae with a stunted growth continued their larval life for another year (semivoltine). This study suggests that availability of preferable case building material is an important factor that affects cannibalism, which in turn affects larval population size structure and cohort splitting.

The effect of human activities on benthic macroinvertebrate diversity in tributary lagoons surrounding Lake Biwa

In aquatic ecosystems, tributaries play an important role in maintaining the populations and diversity of aquatic organisms throughout the drainage basin, but because of their small catchments, these ecosystems are often more susceptible to the effect of land-use changes and flow-regime alterations. Here, we examined anthropogenic effects on benthic macroinvertebrate diversity in the tributary lagoons, collectively called “Naiko,” surrounding Lake Biwa. We sampled macroinvertebrates and determined the environmental characteristics of 20 tributary lagoons. We identified the environmental factors determining the diversity of macroinvertebrates and found that turbidity significantly diminished species diversity. We assessed the anthropogenic stressors that contributed to the increase in turbidity and found that human population density and the proportion of paddy fields in the watershed area were positively correlated with turbidity, most likely caused by erosion of terrigenous organic matter from the paddy fields and urban areas. In addition, the presence of sluice gates and the lengths of channels connecting to the main lake were positively correlated with turbidity, suggesting that these factors lowered hydrologic connectivity and retained organic matter. We concluded that changes to the landscape and decreased hydrologic connectivity caused by human activity increased turbidity, which in turn decreased macroinvertebrate diversity. The identification of these factors in tributaries is vital for developing a strategy for habitat restoration to conserve the entire ecosystem of the Lake Biwa basin.