Hatching Events in the California Grunion, Leuresthes tenuis
This study provides the first detailed description of the events at hatching for an anamniotic vertebrate egg. California Grunion are ideally suited for this study, because these fish eggs incubate fully terrestrially and delay hatching until presented with an environmental trigger. Grunion eggs can be induced to hatch on demand within a few minutes, by mechanical agitation in seawater. We suggest that the process of hatching in grunion involves an enzymatic weakening of the chorion followed by the embryo's active efforts to escape. Using a dissecting microscope, microscope camera, and digital video camera, we compared embryonic movements within the chorion prior to hatching. Then, we recorded the exact moment of hatching with digital video to identify events that occur consistently. Prior to exposure to the hatching trigger, Grunion embryos move minimally within the egg. After the hatching trigger, embryos significantly increase the amount and types of movements within the egg. The chorion distorts in a location superior to the caudal region, and fluid escapes from the egg. Embryonic activity usually culminates in a vigorous tail lash that splits the chorion open. Ultimately, usually within two minutes, the Grunion larva emerges from the chorion into its new aquatic environment.Abstract
Comparison of movements of Grunion embryos before and after agitation but prior to hatching, by paired t-test. Movement categories are twitch (TW, t = −1.761, P = 0.0439), body torsion (BT, t = −1.569, P = 0.0633), body torsion and rotation (TR, t = −2.315, P = 0.0136), and body torsion with tail lash (TL, t = −6.064, P < 0.0001) against the chorion, followed by emergence. Asterisks indicate significant differences
Quadrant divisions for a Grunion egg; indicating the site of chorionic outpocketing by percentage in 80 eggs
Representative photographs of events during the hatching process. (A) Immediately following agitation, fluid is visible at arrow. (B) Chorionic outpocketing at arrow. (C) Upper body torsion, continued in (D). (E) Tail protrudes from chorion at arrow, signaling emergence. (F–G) Thrashing movements to escape the chorion. (H) Larva emerges and hatching is complete
Contributor Notes
Department of Biology, Pepperdine University, 24255 Pacific Coast Highway, Malibu, California 90263-4321. Present address: (TS-B) Biology Department, University of North Texas, P.O. Box 305220, Denton, Texas 76203. (KLM)karen.martin@pepperdine.edu Send reprint requests to KLM.