Cocoon deposition of Rhyacodrilus hiemalis Ohtaka (Tubificidae) in Lake Biwa, Japan

Abstract

The breeding ecology of the tubificid worm Rhyacodrilus hiemalis was studied in Lake Biwa with special reference to timing and duration of cocoon deposition, number of eggs in a cocoon, and the substrate type and the position used for cocoon attachment. Cocoon deposition lasted from the beginning of January to the end of March, and peaked between mid-January and mid-February. Laboratory experiments showed that most R. hiemalis breed only once during their lifetime. In its natural habitat, most cocoons contained from 5 to 15 eggs (mean = 10.82, SD = 3.55; range = 0–33, n = 378), and the mean number of eggs per cocoon decreased as the deposition season progressed. The average number of eggs per cocoon of R. hiemalis is the greatest known among the tubificids. In Lake Biwa, R. hiemalis positioned its cocoons beneath the sediment, attached to the shell surface of bivalves and gastropods and to rock substrates; laboratory experiments showed that most cocoons were located less than 2 cm beneath the sediment surface. Under laboratory conditions, R. hiemalis produces a single cocoon during its lifetime, with a large number of eggs in midwinter, a period when potential predation on the eggs is supposed to be at its lowest during the year, in Lake Biwa. The site of cocoon attachment near the sediment surface guarantees a supply of well-oxygenated water. This may enhance embryonic development and shorten the embryonic stages, thereby achieving rapid preparing growth prior to summer aestivation and minimizing predation risk during the vulnerable early instar stages.