Yu Isobe

Reach-Scale Distribution Dynamics of a Grazing Stream Insect, Micrasema quadriloba Martynov (Brachycentridae, Trichoptera), in Relation to Current Velocity and Periphyton Abundance

Abstract Reach-scale temporal shifts in the distribution of larvae of a grazing caddisfly, Micrasema quadriloba (Brachycentridae), were investigated in a Japanese mountain stream. The larvae showed an aggregated distribution within the reach at the beginning of the immigration, then became randomly dispersed throughout the reach as the immigration progressed. The abundance of periphyton in the reach decreased dramatically with increasing dispersal of the larvae. Simple regression analyses revealed that the streams flow regime was the most important environmental factor that determined the reach-scale distribution of the larvae and that the relationship between the flow regime and the distribution of the larvae shifted temporally. In addition, our results suggest that only this species of grazing insect, which was dominant in the study reach, controlled the reach-scale abundance of the periphyton.

Changes in periphyton abundance and community structure with the dispersal of a caddisfly grazer, Micrasema quadriloba

We examined the larval population densities and biomass of a caddisfly grazer, Micrasema quadriloba, and the abundance and community structures of periphyton at a segment scale (7.4 km with four study sites), along a second-to fourth-order Japanese mountain stream throughout the grazer’s life cycle. In the uppermost riffle of the study segment (site 1), periphyton abundance was kept at low levels when the larvae occurred. The larval distribution spread downstream as larvae developed from first instars in May to fifth instars in January. We performed multiple regression analyses to test the effects of environmental variables and larval biomass on periphyton abundance in both the riffle of site 1 and the study segment; the results revealed that the larval biomass was significantly negatively correlated with periphyton abundance similarly in both the riffle and the study segment. In addition, both the correlation and community analyses showed that the larval biomass was significantly negatively correlated to the relative abundance of large and/or filamentous microalgae, which appeared in the uppermost layer of the periphyton mat, and that larval biomass was significantly positively correlated to the relative abundance of small diatoms, which strongly adhered to the substrate. Thus, the present study implied that the grazing of M. quadriloba larvae would regulate the abundance of periphyton in a riffle and also regulate the abundance and community structure of periphyton at the segment scale with the expansion of their longitudinal distribution.

Comparison of egg period, hatching rate, and first-instar nymphs among three species of stonefly

Egg period was compared among several temperature conditions (11°C, 16°C, 20°C, 23°C) in Sweltsa sp., Stavsolus japonicus, and Isoperla aizuana (Plecoptera). The shortest mean egg incubation period was 27.8 days at 20°C in Sweltsa sp., 118.1 days at 16°C in Stavsolus japonicus, and 162.0 days at 20°C in Isoperla aizuana on average. Egg hatching rate was also the highest at the water temperature that provided the shortest egg incubation period. Based on laboratory data, eggs of Sweltsa sp. were considered to be deposited in May and hatched in June in the field. Thus, they must have spent the summer as nymphs in the field. Eggs of Stavsolus japonicus and Isoperla aizuana were considered to be deposited in April to May and hatched in September to October in the field. Visible eyes of Stavsolus japonicus and Isoperla aizuana appeared in August. It is likely that the long egg period of Stavsolus japonicus and Isoperla aizuana reflects that these two species spend the summer as dormant eggs in the field.

Japanese species of the genus Oyamia (Plecoptera: Perlidae), with notes on O. nigribasis from Korea

A new species of Oyamia, O. cryptomeria, is described from western Honshu, Japan. The other two Japanese species, Oyamia lugubris (McLachlan) and O. seminigra (Klapálek), are redescribed. The new species is distinguished from the other species of Oyamia by the male genitalia and punctures on the egg. The O. seminigra complex is proposed for the new species and O. seminigra. The two species in the complex and the other Japanese species, O. lugubris, share some characteristics of larvae and male genitalia. An only single representative of the genus from the continent, O. nigribasis Banks, has different characteristics from the Japanese species. An additional characteristic of the genus is a feature of the mouthparts; i.e. a row of setae on the inner side of the larval lacinia.

Distribution and drift dispersal dynamics of a caddisfly grazer in response to resource abundance and its ontogeny

Stream grazers have a major impact on food web structure and the productivity of stream ecosystems; however, studies on the longitudinal (upstream versus downstream) and temporal changes in their drift dynamics and resulting distributions remain limited. Here, we investigated the longitudinal and temporal distributions and drift propensity of a trichopteran grazer, the caddisfly, Micrasema quadriloba, during its life cycle in a Japanese stream. The distribution of larvae significantly shifted downstream during the fifth instar larval stage during late winter; with periphyton abundance (i.e. their food source) showing similar shifts downstream. Therefore, our results show that the drift dispersal the caddisfly occurs in response to decline in available food resources (i.e. food-resource scarcity) and an increase in food requirements by growing individuals. Furthermore, our results show that this observed longitudinal shift in larval distribution varies through their life cycle, because the drift dispersal of fifth instar larvae was greater than that of immature larvae. The correlation between periphyton abundance and drift propensity of fourth instar larvae was not statistically significant, whereas that of fifth instar larvae was significantly negative. In conclusion, we detected an ontogenetic shift in drift propensity, which might explain the longitudinal and temporal distributions of this species.

A comparative study of the morphology and distribution of setae and hairs on the larval legs of two families (Perlodidae, Perlidae) of Japanese Plecoptera

A scanning electron microscope (SEM) was used to investigate the morphology and distribution of setae and hairs on the larval mid leg femur, tibia, and third tarsomere in representatives of six genera of Perlodidae and Perlidae. On the ventral surfaces of these leg segments we observed previously described clothing hairs, spine-like setae, and pinnate setae. We describe for the first time a new type of seta that we refer to as candle-like seta. Distinct differences in the distribution of hairs and setae are exhibited between the two families. Perlidae have hairy legs and on the femur the percentage of setae as compared to that of the clothing hairs is very high. Perlodidae are less hairy and on the femur the percentage of setae as compared to clothing hairs is much lower. The distribution of setae and hairs is similar among genera in the Perlidae, but not between genera of the Perlodidae. Pinnate setae are observed only in Neoperla geniculata. We discuss the types of setae and their distribution on the larval legs that characterise the families in relation to their habitats as well as to their behaviours observed in an aquarium using a video camera.