Shrinking pupal cocoons of Rhyacophila lezeyi (Trichoptera, Rhyacophilidae) in a highly acidic stream during the summer season

By

Aoya, Kokichi
Hayakawa, Atsushi
Iwata, Tomoya
Tanida, Kazumi

Type

Article

Date of Publication

2023-9-29

Original Publication

Beiträge zur Entomologie

Volume

73

Series / Issue

Issue: 2

Pages

131--136

Subjects

cocoon shrinkage , membrane semi-permeability , osmolarity , pH , Water temperature

Contributed by

Pensoft Publishers

Language

English

Published by

Berlin Akademie-Verlag 1951-1997

Identifiers

DOI: https://doi.org/10.3897/contrib.entomol.73.e107479

 

Abstract

Shrinking pupal cocoons of Rhyacophila lezeyi were often found during summer in Shibukuro Stream, a highly acidic mountain stream in northern Japan (pH = 2.82 on average). We performed both field surveys and laboratory rearing experiments to clarify the mechanisms of R. lezeyi cocoon shrinkage. The R. lezeyi cocoon shrinkage proportion increased in years with high stream water temperatures and was related to water temperatures before and after pupation at the study site. Approximately 90% of the prepupae and pupae inside the shrinking cocoons died during the rearing experiment, implying that cocoon shrinkage caused by high water temperature strongly influenced R. lezeyi pupal survival. Laboratory experiments showed that R. lezeyi’s pupal cocoon membranes were semi-permeable and that the cocoon fluids were always hyperosmotic, indicating that water molecules can continuously enter the cocoon fluids from the stream water until the turgor of the cocoon wall is reached. However, the shrinking cocoons showed lower fluid volume and higher osmolarity than the normal turgescent cocoons. The reduction of osmotic gradient across the membrane during decreased stream flow due to less precipitation and/or the damage to the cocoon membrane and pupal body from high and fluctuating water temperatures and low pH are possible mechanisms for R. lezeyi pupal cocoon shrinkage.