Estimating cod egg developmental stage based on DNA concentration

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.

Fecundity of the langoustine Metanephrops mozambicus Macpherson, 1990 (Decapoda, Nephropidae) in eastern South Africa

The fecundity of the langoustine Metanephrops mozambicus Macpherson, 1990 was investigated using egg-bearing females caught in commercial trawls off eastern South Africa. A generalized linear model (GLM) framework was used to assess fecundity and egg diameter relative to langoustine carapace length (CL) and egg developmental stage. Fecundity ranged from 407 to 1606 eggs per clutch, increased with increasing CL and decreased between egg stages 1 and 4. Egg diameter increased significantly between stages 1 and 4 and larger females carried larger eggs. Egg loss during incubation was estimated at 10%. Four egg stages used during field sampling were re-examined and illustrated. A late stage embryo removed from an egg about to hatch appeared to be adapted to a benthic, rather than natatory existence.

Brood Size and Offspring Age Affect Risk-Taking and Aggression in Nest-Guarding Common Gobies

The importance of brood size, offspring age, and male size for parental care behaviour was studied in the common goby, Pomatoschistus microps. In field observations, the aggression of nest guarding males was measured as attacks towards a finger when disturbing the nest. Attacking males had larger and more developed clutches compared to non-attacking males, but did not differ in body size. In another set of observations nest guarding males were exposed to a predator (eelpout, Zoarces viviparus) and subsequently chased away from their nests. Time away from the nest decreased significantly with egg developmental stage, i.e. with the time the male had spent guarding a particular brood. However, no correlations with male body length or numbers of eggs in the nest were found. We conclude that male common gobies evaluate future reproductive success by using brood age and brood size as cues for making decisions about risk-taking and aggressive behaviour during parental care.