Gamete interactions and fertilization behavior in the larvacean, Oikopleura dioica

Abstract

Summary Oikopleura dioica are often cultured in the laboratory but for several reasons are clearly not in conditions similar to those of the open sea. Despite this impediment, some interesting behaviors of the spawning of adults and gametes in 3-ml well plates were observed from April and May 1992–1994 using the circulation devices of Fenaux and Cie and Gorsky and Fenaux (2002). Testis sac size measurements and observations of swimming behavior of numerous individual males were made during sperm release. We calculated the intervals between initiation of sperm release and its cessation, the sinking rates of individual eggs, the potential for re-fertilization of unfertilized eggs up to 24 h post release, and the effects of sperm concentration on fertilization success. We found that in a group of 74 males the total minutes of spawning time was 1,076 min with an average time to completion of spawning by all males at 14.54 min. Individual eggs sink in a 10-ml graduated cylinder at the rate of 2.35 mm/s in still water at 16°C. During sperm shedding, most males made a series of from one to five small horizontal loops every few seconds along the upper edge of a 3-ml well. The number of loops made increased during spawning. We suggest that this behavior is a decreasing regular spiral. Most males make clockwise loops while swimming against the inner edge of the wells but a few individuals rotated in the counterclockwise direction. Sperm aging was studied using sperm 100 min or older prior to the addition of newly spawned eggs. After fresh sperm were added to small groups of 24-h-old eggs incubated in 3-ml wells, several eggs cleaved properly and developed to the twitching larval stage. Fertilization success curves obtained on three different days using separate batches of eggs and sperm from each male produced nearly identical curves. However, sperm left standing at 16°C for longer than 100 min rapidly lost potency. Addition of higher sperm concentrations dramatically increased the fertilization rate.