Meeting Abstract
California grunion spawn on sandy beaches during spring high tides. Embryos develop within the sand enclosed in the chorion, which must be strong enough to protect the embryo but also allow hatching. Grunion embryos are competent to hatch at 8 days post-fertilization (dpf) at 20°C, but require an environmental trigger, agitation by waves, to hatch. If the first spring high tides after fertilization do not reach them, embryos can extend incubation and hatch during a subsequent spring tide. We hypothesized that chorions of unfertilized grunion eggs would have lower compressive strength than chorions of fertilized eggs because of hardening of the chorion at fertilization, and that chorions would have higher compressive strength during normal incubation (10 dpf) than after extended incubation (28 dpf) due to degradation over time. Gametes were collected from adult grunion in March-July 2014. Eggs were fertilized, developing embryos were incubated in the laboratory at 20°C for up to 30 dpf, and compressive strength was measured with a Kistler force transducer. Our first hypothesis was refuted because mean compressive strength (N/m2) either did not differ significantly between unfertilized and fertilized eggs (7 of 9 collection dates), or was significantly greater in unfertilized eggs (2 of 9 dates). Mean compressive strength was greater in chorions at 10 dpf than at 28 dpf in only some clutches of eggs. The values of crushing force measured in grunion are within the range for other species, including salmonids and plaice. The strength of grunion chorions apparently does not depend on hardening as a result of the cortical granule reaction at fertilization, and instead may be determined during oogenesis.
