Maternal versus environmental constraints on the oocyte size of a marine pelagophil fish

Abstract

We investigated maternal body size and environmental effects on the oocyte size of the Atlantic sardine, Sardina pilchardus, a marine pelagophil, using a novel, oocyte stage-specific approach which takes into account the spawning lag. In particular, we focused on anatomical constraints imposed by the maternal body cavity to the growth of the ovary and its oocytes during the course of vitellogenesis (VIT) and oocyte maturation (OM). We analyzed variability in oocyte size in VIT and OM ovaries as a factor of maternal size, ambient temperature (SST) and preservation medium (formalin vs. ethanol) using the spawning lag of recent and imminent spawners, respectively, as a covariate. We showed that oocyte size correlates with maternal size in OM ovaries but not in VIT ovaries. More specifically, 10 cm difference in maternal body length corresponds to 0.112 mm2 (or 12 %) difference in the cross section of fully hydrated oocytes just previous to ovulation. On the other hand there was no inverse relationship between OM oocyte size at spawning and batch fecundity. Ambient SST was not an important determinant of hydrated oocyte size since the same 12 % difference in oocyte size was estimated for the entire SST range of the study. We suggest that these stage-specific differences in the effect of maternal size on oocyte size are due to a functional relationship between the abdominal volume and ovarian size.