Estimating cod egg developmental stage based on DNA concentration

Sigurd Heiberg Espeland,Hanne Sannæs,W Stewart Grant

doi:10.1093/icesjms/fsx172

Abstract

Abstract Pelagic egg surveys are commonly used to map Atlantic cod spawning areas in the northeastern Atlantic. However, a sampling location may not necessarily indicate a spawning location, because more developed eggs may have drifted long distances in coastal currents. Newly spawned eggs have only a few embryonic cells, whereas eggs in later developmental stages have progressively larger numbers of cells and hence greater amounts of DNA. The progression through developmental stages largely depends on temperature, which influences cell division and growth. Preservation of the eggs for later analysis presents logistical dilemmas. Preservation in ethanol is suitable for DNA extraction but obscures developmental stage, as the eggs shrink and turn opaque. On the other hand, preservation in formaldehyde retains visual characteristics, but this method limits the ability to extract intact DNA. Here we report a method to estimate developmental stages in Atlantic cod eggs by assaying the amount of DNA from eggs preserved in ethanol. We used a correlation between the amount of DNA in an egg and its embryonic developmental stage in samples from western Norway and used this correlation to estimate developmental stages of eggs sampled in northern Norway.

Highlights

Sampling newly spawned pelagic eggs is widely used to map spawning areas to predict future recruitment and to study the dynamics of larval development among stocks (Ciannelli et al, 2010; Borjesson et al, 2013; Espeland et al, 2015)
Atlantic cod (Gadus morhua) spawn large numbers of pelagic eggs that have a relatively long pelagic period that depends in part on water temperature
We show how measures of DNA concentration after standardized extraction protocols can be used to estimate the developmental stage of an egg

Summary

Introduction

Sampling newly spawned pelagic eggs is widely used to map spawning areas to predict future recruitment and to study the dynamics of larval development among stocks (Ciannelli et al, 2010; Borjesson et al, 2013; Espeland et al, 2015). At 4C, egg development progresses from zygotes in newly spawned eggs to hatched pelagic larvae in 20–24 days ($500 h), whereas in colder waters (near 0C), development times may extend beyond 40 days (Westernhagen, 1970). During this long pelagic period, developing eggs often experience a large amount of variation in oceanographic processes and may be advected far from the spawning area (Myksvoll et al, 2014; Espeland et al, 2015). In marine protected areas it may be more acceptable to sample eggs from highly fecund species, as eggs have a high expected mortality, while fishing mature adults may be undesirable and unnecessary

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Results

Conclusion